Chapter 2 – Other Methods: Probing our progress

We began using probes to help us learn about planets almost as soon as we started to launch things into space. Naturally, we started with the nearest planet – the Earth. Sputnik didn’t include any instruments; the USSR had launched it more as a publicity stunt than as a serious scientific endeavor. But by measuring the drag of the upper atmosphere on Sputnik and by measuring how its radio signal was changed by the ionosphere, we were able to learn quite a bit about the Earth’s atmosphere. The first satellite from the USA, Explorer 1, carried four different instruments and was the first satellite to detect the Van Allen radiation belts that protect us from cosmic rays. Within a few years, the skies around Earth would be crowded with satellites measuring everything from cloud cover to gravity to ground cover to fluctuations in the magnetosphere. And we continue to launch Earth-observing satellites even today; NASA currently has more than two dozen satellites in orbit around Earth, along with probes sent from more than a dozen other nations and several private companies.

But you can stare at the Earth for only so long when there are other planets out there to discover. And we’ve done our best to send probes to each of the major planets, starting with Luna 2 sent out to the Moon by the USSR in 1959. Since then, more than 200 probes have been sent to planets throughout the Solar System. Most of those were sent before 1989, when the USA and the USSR were engaged in a long, drawn-out game of “one-upsmanship”. We’d send a probe to Mercury and the USSR would send a bigger probe to Venus. They’d send a probe to Mars and we’d send two bigger probes. And so on. After the fall of communism, Russia didn’t have the money to keep playing the game and we decided that it wasn’t as much fun.


As a result, we’ve sent fewer probes out since 1989. But what we lack in quantity, we’ve more than made up in quality[1] . The probes we sent out in the early 1960s could measure just a few things and had to be directed every step of the way. Those that are sent out today can measure many different things and perform a complicated set of instructions before needing to get help from the mission directors back on Earth.

For example, the first spacecraft to visit Mercury was Mariner 10. It carried seven different instruments (imaging system, infrared radiometer, ultraviolet airglow spectrometer, ultraviolet occultation spectrometer, two magnetometers, charged-particle telescope, and plasma analyzer) and gave us about 7,000 pictures of a p[ortion of the planet’s surface. Contrast that with the MESSENGER spacecraft that is currently orbiting Mercury. It has but six instruments (Mercury Dual Imaging System, Gamma-Ray and Neutron Spectrometer, Magnetometer, Mercury Laser Altimeter, Mercury Atmospheric and Surface Composition Spectrometer, Energetic Particle and Plasma Spectrometer) but has imaged the entire surface and has taken more than 700,000 images and millions of measurements.

There are currently more than a dozen space probe missions being run by NASA alone. We’ve got probes orbiting Mercury, Mars, Saturn, and the Moon. We’ve got probes running around on Mars, drilling holes and taking measurements. And we’ve got probes on their way to Pluto, Jupiter, Ceres, and the Moon. And we’ve probes planned for Mars, Titan, and the Moon.

Dust tracks in aerogel from the Stardust probe (Image courtesy NASA)

Dust tracks in aerogel from the Stardust probe (Image courtesy NASA)

But perhaps the most exiting thing about probes is that they can do more than just take pictures and make measurements; they can also bring back samples. The first probe to bring back a sample from a rock in space was Stardust, which flew to the comet Wild 2, grabbed some of the dust being thrown off of the comet, and came back to Earth with the samples. The dust grains are currently being studied by scientists in an effort to improve our understanding of how the Solar System formed2. Since then, we have also retrieved samples from the asteroid 25143 Itokawa using the Hyabusa probe, and plan to send probes to Mars and the Moon to retrieve samples.


Some space wonks will scoff at my assertion, and cite examples such as the Mars Climate Orbiter which crashed because the engineers used English units and the scientists used metric. But that was a rare exception and not the norm. By and large, we’ve gotten very good at sending probes where we want them to go.


Chapter 2: Seeing Lines

In the 1850s, physicists had developed a primitive sort of solar panel; they were shining light onto metals and getting electrons in return. What was most interesting to them was that changing the color of the light didn’t change the number of electrons but it did change the kinetic energy each electron had. Using shorter and short wavelength light, they were able to get faster and faster electrons. But if they used too short or too long a wavelength of light, then they would get no electrons at all!

They tried several explanations for the phenomenon but, as was the case with the black body, none of them worked well. Fortunately, Planck’s black body solution pointed the way to a true solution. In 1905, Einstein discovered that photons were not smeared across the spectrum, but were instead individual, indivisible bits of energy that he called “quanta”[1]. He showed that the energy of a photon was directly linked to its wavelength. If a photon had too little energy, then it couldn’t break an electron free. But if it had more than was needed, then that extra energy couldn’t go into another electron; it could only be used to add kinetic energy to the one electron that was freed.

But Einstein’s work did more than point the way to better solar panels; it also described how we could use light to determine an object’s composition. The key was realizing that it wasn’t just the photons that were quantized; the orbits of the electrons around the atoms[2] were quantized as well!That meant that it took a specific amount of energy to move the electron from one orbit to the next and only photons with that amount of energy would be nabbed by the electron. Those with too much energy or too little, which is to say those with the wrong color, would simply pass on by. As a result, atoms and molecules would only absorb certain colors of light and would also emit those same colors. These spikes in of light were known as absorption lines and the emission lines.


Though these lines had been observed as early as 1802 and had been linked to composition in 1860[3], it took Einstein’s paper to explain how the absorption and emission lines worked. Once the theory caught up with the practice, spectroscopy took off. The spectra of common rocks and minerals on Earth were analyzed and compared to those of comets, asteroids, and planets with surprising results. Comets were shown to contain many rocks similar to those on earth, along with water, ammonia, and even organic material!

Asteroids were divided into three groups based on their spectra. The rare M-type asteroids had spectra indicating that they were mostly iron, with trace amounts of nickle and other metals. The common C-type asteroids had spectra indicating large amounts of carbon and trace amounts of water. And the S-type asteroids had spectra similar to those for basalt and granite. This became very handy when we started examining the bits of asteroids that crashed to Earth as meteorites; thanks to spectroscopy, we’ve been able to match the meteorite with the asteroid that it was knocked off of (what planetologists call the “parent body”). As we will see, that has helped us to understand how the asteroids and the planets formed and changed over time.

Object Atmospheric composition (in order of abundance)
Sun Hydrogen, Helium, Oxygen, Carbon, Iron, Neon, Nitrogen
Jupiter Hydrogen, Helium, Ammonia, Methane
Saturn Hydrogen, Helium, Ammonia, Methane
Uranus Hydrogen, Helium, Methane
Neptune Hydrogen, Helium, Methane
Earth Nitrogen, Oxygen, Carbon dioxide, Argon, Helium
Venus Carbon dioxide, Nitrogen, Helium, Neon, Argon
Mars Carbon dioxide, Nitrogen, Argon

Similarly, the spectroscopy of the planets showed some interesting and amazing things. We have discovered that Jupiter, Saturn, Uranus, and Neptune are surrounded by atmospheres that are very similar to that of the Sun, while the Earth, Venus, and Mars have atmospheres that are very different from the Sun. Even better, we have been able to show that there are a very few meteorites that are actually pieces of the Moon that were knocked off by a large impact; after spending some time orbiting in space, they crashed to Earth. And the Moon isn’t the only planet that has given us samples; we also have some meteorites that are known to have come from Mars[4].

These compositions give further support to the groupings that were developed based on just the planetary mass and diameters in the previous section. If the groupings were just a random artifact, then we would expect that the atmospheres of the planets in each group could be very different; instead, with the sole exception of Earth, each group has similar atmospheres. And that trend continues with the smaller planets and remaining junk (asteroids and comets) in the Solar System. Based on the spectral characteristics, each group has a composition that is very similar and so probably evolved in the same way. These results would only become more certain once we started sending probes out to the other planets.

[1] It is one of the great historical ironies that in his Nobel Prize winning paper, Einstein laid the foundations for quantum mechanics, which he always felt was incorrect. In the same paper, he also described how to build lasers and explained why CO2 is a greenhouse gas.

[2] This is a bit of nomenclature that causes more confusion than it clears up. When a scientist speaks of orbiting electrons, many people think of a little tiny electron circling the center of the atom much as the Earth circles the Sun. This is natural and wrong. The electron’s orbit is more like a group of deformed jelly beans than a simple circle, and the electron can be anywhere on the surface of those jelly beans at any time. Indeed, in some interpretations of quantum mechanics, the electron is at all places on that surface at the same time.

[3] Indeed, in 1868 a pair of astronomers noticed an unusual yellow line in the Sun’s spectrum and decided that it represented a new element that they called “helium” which meant “from the Sun”. It wasn’t until 1882 that helium was found on Earth. The clever reader will have noticed that this happened before Einstein explained why spectroscopy worked. This is fairly common in science; we often notice that something works and then spend decades trying to explain why it does. In the meantime, we just enjoy the fact that it does.

[4] The martian meteorites are most famous for their inclusion of what some have interpreted as evidence for life on Mars. However, this claim is contentious to say the least; there are many in the astrobiology community who think that the structures seen in the martian meteorites could have been formed through purely chemical means.

Chapter 2: Seeing the light – Blue hot, red cool

The other primary tool of planetologists is light. As you might expect, we use light to make images. But we don’t limit ourselves to visible light and we don’t just look at the images; we also look at the light itself. We do that because the light can tell us some fairly important things about the planet, including what it (or at least its outer layer) is made of and what temperature it is. To understand how light can tell us that, we need to step back to the turn of the 20th century when Einstein and his pals were turning everything that we knew about physics on end.

In the late part of the 19th century, scientists had measured the energy radiated by a black body[1] and had come up with an equation that matched the observations for long wavelengths of light[2] fairly well. However, the values that the equation gave when it was used to calculate the energy that should be given off at the shorter wavelengths were ridiculously high. The equation suggested that a black body would give off enough gamma radiation and x-rays to kill everyone in the area. Since the scientists weren’t dying from radiation[3], there had to be something more going on.

In 1900, Planck figured out what it was. Instead of allowing just any wavelength of light to be created, he posited that black bodies could only generate specific wavelengths; he quantized the problem. This solved the problem of physicists not being killed by radiation by showing that they were never in danger and provided them with an unexpected benefit: Planck’s equation also showed that the maximum intensity of the radiation was proportional to the temperature. Put more simply, though all black bodies radiate energy across a broad spectrum of colors, hotter black bodies would emit more energy at smaller frequencies.

As the temperature of a black body increases, the color of light that it gives off changes.

This effect is actually one that is fairly common but not well-recognized by non-physicists. If you have ever seen an incandescent light bulb, you’ve seen this effect in action. As the filament heats up, it glows red then blue; it is the mixture of colors that gives the “white” light. Similarly, a bar of metal that is heated starts to glow a dull red, then a brighter blue, and finally becomes white hot. But it does more than just give light; it tells us the temperature.

If you were to find the wavelength where the most energy was given off for a planet, then you could find the temperature of that planet. And if you have ever used one of the thermometers that you just place on a person’s forehead (instead of some other part of a cranky child’s anatomy), then you have used this exact technique to find their temperature[4]. We can also take the equations one step further to find out what temperature a planet should be if it were a black body, after adjusting to take the planet’s albedo[5] into account. And when we do, we get some fairly surprising results; few of the values are right!

Deciphering the reason that the values aren’t right is one of the most interesting areas in planetology. It turns out that the terrestrial planets tend to have higher temperatures than expected thanks to the greenhouse effect; without it, the Earth would be a cold, frozen ball in space. And jovian planets tend to have higher than expected temperatures because they are still shrinking, which generates heat. We’ll get into the two processes in more detail later; first we need to rejoin the 20th century physicists as they solve another problem.

[1] A “black body” is one of those physics concepts that make the math simpler and the problem easier to solve. It is basically an object of any shape that perfectly absorbs all light, no matter what wavelength it has or angle it comes in at. And because the laws of optics work both ways a black body is also a perfect radiator; it will give off light in proportion to its temperature.

[2] Remember that the “color” of light is inextricably linked to its wavelength, which is related to its frequency by the speed of light. A wavelength of 700 Å corresponds to red light, and a wavelength of 400 Å is blue. However, light doesn’t stop there. A wavelength of 1000 Å is equal to infrared light and is emitted by “heat lamps”. A wavelength of 100,000 Å corresponds to microwaves, just like the ones in your kitchen. And a wavelength of 1,000,000,000 Å is what carries radio and television signals. At the other end of the spectrum, a wavelength of 10 Å is what tanning beds give off as ultraviolent radiation. X-rays come in at 0.01 Å, and gamma rays have a wavelength of about 0.0001 Å.

[3] With the unfortunate exception of those who actually worked with radioactive materials, that is.

[4] Those thermometers were originally developed by NASA for use in planetary probes, and then for use with astronauts. This is just one of the innumerable spinoffs from the space program.

[5] Albedo was introduced to take the purely hypothetical situation of a perfect absorber/radiator and make it a little more realistic; it reflects the amount of light that bounces off of the black body without ever being absorbed. An albedo of 0.10 means that only 90% of the incoming light is absorbed by the black body and 10% caroms off. If you’ve ever seen a picture of Earth from space, it was taken using the 30% of the Sun’s light that the Earth reflects.

Chapter 2: Gravity is your friend – A peek inside

Gravity is what creates the forces that make planets round. But gravity also does more than that; it provides us with a tool for looking inside those planets and for measuring the amount of stuff that makes up the planets. In order to understand how gravity can do that for us, we need to step back to 1687, when Newton showed that gravity explained planetary orbits. Basically, Newton was able to show that gravity somehow[1] caused planets to orbit in certain ways, called conic sections, and that the gravity an object created was proportional to its mass. An object with more mass had more gravity and made things orbit faster around it than an object with less mass would.

The planet’s gravity creates the moon’s orbit and the moon’s gravity causes the planet to wobble.

The practical effect of Newton’s work was that planetologists were able to use a planet’s moons to determine how much mass the planet had. Even better, since Newton also showed that the moon’s mass tugged on the planet, they were able to use tiny perturbations in the planet’s motions to determine how much mass the moon had[2]. But knowing the mass was only the first step in their logic chain. They also knew how large the planet and moons were, which gave them the volumes. And when the mass is divided by the volume, that gives the density of an object[3]. And that turns out to be very enlightening indeed.

A select group of planets, arranged by density and size.

When the densities of the planets were calculated[4], they tended to make four distinct groups. This is made even more evident when the size of the planet is plotted against the density. There is the group of big things with low densities, such as Jupiter and the Sun; with the obvious exception of the Sun, these are the jovian planets. Then there is the group of small things with low densities, such as most of the moons and Pluto. Next there’s the group of small things with a medium density, such as Mars and our Moon. Then there are the small things with very high density, such as Earth, Venus, and Mercury; these are the terrestrial planets[5].

If we assume that the objects in each group are similar in composition, then we can say that Mercury and Venus have a composition that is similar to that of the Earth. We can also say that the only difference between Jupiter, Saturn and the Sun is size; had Jupiter or Saturn been a little larger, we’d be living in a solar system with two suns. Similarly, we can say that Pluto is very similar in composition to Titan and Rhea.

Material Density (g/cc)
Ice 0.920
Water 1.000
Iron 7.800
Air 0.001
Granite 2.700
Basalt 3.100
Wood 0.750
Aluminum 2.700
Diamond 3.500

But what is that composition?Looking at common materials, we can see that Jupiter has a density close to water. And Saturn is actually less dense than liquid water; if we could find a bathtub big enough, the planet would float in it like some gaudy giant rubber ducky. Meanwhile, Pluto is about twice as dense as water which implies that it has some ice and a lot of rock. Similarly, Earth is denser than granite or basalt (which are plentiful at the surface) but less dense than iron (which we know to be plentiful in the center). So density can only give us the most general idea of a planet’s composition; we’ll need some other tool to learn more.

But we shouldn’t give up on gravity just yet, because it can tell us one last thing. It can tell us how the mass is distributed inside the planet. To understand how gravity tells us this, you need to consider a spinning top. As the top spins, it wobbles from side to side. That wobble is caused by slight anomalies in the way that the mass is distributed around the spin axis; the parts with more mass get pulled down slightly more by gravity, leading to a wobble as the top spins. By observing how the top wobbles, we can therefore deduce how the mass is distributed inside of it.

We do exactly the same thing with planets. The planets do not all orbit in a plane, nor do the moons orbit in a plane about each planet. As a result, the gravity of the Sun and the other planets is never perfectly aligned with the spin axis of any body. This then causes the planets and the Sun to wobble ever so slightly[6]. By measuring those wobbles, we can determine how the mass is distributed inside of each planet. Because planetologists like to keep things simple, we usually divide the measured moment of inertia by the mass of the planet and the square of its radius; this allows us to compare all of the planets on an apples-to-apples basis.

Body Moment of inertia
Ring 1.00
Hollow sphere 0.67
Homogenous sphere 0.40
Moon 0.39
Sphere with ½ core 0.37
Mars 0.37
Earth 0.33
Neptune 0.29
Jupiter 0.26
Uranus 0.23
Saturn 0.20
Sun 0.06
All mass on axis 0.00

Looking at the moments of inertia for the planets, it is obvious that none of them are hollow inside, no matter what Burroughs has written. It is also obvious that the moment of inertia goes down as more stuff gets concentrated near the center. It is also clear that, though the Moon appears to be fairly homogenous (at least with respect to the mass distribution), the terrestrial planets have cores with most of the mass. Similarly, the jovian planets are likely to have rocky cores covered with a lot of gas and high-pressure ices[7]. And the Sun has almost all of its mass at the center, with very little in the outer regions.

Gravity can also tell us how the mass is distributed near the surface of a planet. The parts with extra mass, such as mountains, have higher gravity than the parts with less mass, like oceans. That means that satellites will speed up slightly as they come closer to the parts with extra mass and slow down as they leave them. By tracking the satellites very carefully, we can map out the distribution of mass near the surface of a planet. This technique was pioneered by the Luna-10 probe, which seemed to show that some of the impact craters on the Moon were gravity highs (indicating extra mass) instead of gravity lows (as would be expected for a basin). These regions with mass concentrations, or masscons for short, had to be mapped out carefully before we could send men to the Moon. The same technique has been applied on Earth. Modern gravity-measuring satellites are sensitive enough to track submarines under water and to detect the change in groundwater levels caused by spring rains. With the recent MESSENGER mission to Mercury and the Mars Global Surveyor mission, we are also developing detailed information about the gravity fields of other planets and learning much about their interiors from that.

Earth’s normalized gravity field, known as the geoid. Image courtesy of NASA.

So, thanks to gravity, we know how much mass each planet has, and how that mass is distributed. We can even get a rough idea of what that mass must be made of. But gravity is far from being our only tool, even if it is the most versatile.

[1] That “somehow” is still being debated. Newton didn’t like the idea of “action at a distance”, which is how gravity works. Einstein liked it even less, and circumvented the problem by saying that gravity was inherent in the shape of space-time. That solved the problem for big, slow things such as planets but didn’t work too well when the object was either very small or very fast. Currently, scientists are looking for the Higgs boson, which may provide particles with mass and so would bridge the gap between space-time curvature and quantum effects.

[2] That technique is alive and well today; it is being used by NASA and the ESA to determine the masses of exoplanets by looking at how the star’s motion changes.

[3] Technically, it gives the “bulk density” or density of the thing as a whole. To understand the difference between bulk density and density, think of someone riding in a hot air balloon. If the balloon is going up, then the bulk density of the hot air, the gondola, and the person is less than the density of the air around it even though the person has a density that is higher than air.

[4] Technically, what has been calculated is the compressed bulk density. Because the stuff above compresses the stuff below (remember the bulk modulus?), it allows us to pack more stuff in less volume which gives a higher density that we would get if everything were spread out. Fortunately, we can apply a little math to the problem and get the uncompressed bulk densities, which are anywhere from 2% lower (for Mercury) to 38% lower (for Earth).

[5] The lumpers in the planetological community place Mars and the Moon in the group of terrestrial planets. And the splitters divide the jovian planets into the ice giants and the gas giants. For now, we’ll go with the happy medium of the four groups outlined above, but will revisit the question later.

[6] One practical effect of this wobble is that the Earth’s tilt changes slowly over a period of 26,000 years. When the pyramids were built, the “North star” was not Polaris in Ursa Major but Thuban in Draco. And the ancient Greeks had no North star as the Earth’s spin axis pointed to a blank patch of sky!

[7] You should know that when a planetologist says “ice” it doesn’t mean “that stuff you put in your drink”. Most gases turn into a solid when the temperature is low enough or the pressure is high enough. Methane ice, carbon dioxide ice, ammonia ice – all of them are just “ice” to a planetologist.

Chapter 2: Gravity – Getting hot

Gravity doesn’t just shape the planets and stars into balls; it also heats them up. About 1/3 of the heat escaping the Earth’s surface is “fossil heat” from the planet’s formation. Gravity heats the planets in two ways. First, it smashes rocks together to make the planets. And then it squeezes those rocks into a tighter mass.

The first method of heating is probably the simplest to understand. If you drop a rock from the top of your house to the ground, you’ll see that it accelerates, going faster and faster until it hits the ground. The same thing is true when those rocks fall from the edge of the Solar System to the planet’s surface; they move faster and faster until they finally smash into the planet [1]. And it turns out that the speed that the rock hits at depends only on the size of the planet [2]. A rock made up of one atom will fall just as quickly as one as big as the Moon. That speed is known as the escape velocity, because it turns out that the speed a rock hits with when it falls from the edge of the Solar System is exactly the same as the speed you’d have to throw it in order to toss it back out to the edge of the Solar System [3].

Object Mass (kg) Escape Velocity (km/s)
The Sun 1.99×10^30 617.7
Mercury 3.30×10^23 4.3
Venus 4.87×10^24 10.3
Earth 5.97×10^24 11.2
The Moon 7.35×10^22 2.4
Mars 6.42×10^23 5
Ceres 9.43×10^20 0.5
Jupiter 1.90×10^27 59.5
Saturn 5.68×10^26 35.6
Uranus 8.68×10^25 21.2
Neptune 1.02×10^26 23.6
Pluto 1.35×10^22 1.2
Eris 1.67×10^22 1.4

When the rock speeds up, it is converting what scientists call potential energy, or energy of position, into kinetic energy, or energy of motion. And the interesting thing about heat is that, looked at one way, it is nothing more than random motion on a molecular scale. So, as you might guess, adding kinetic energy in a random fashion is very similar to just adding heat.

The second way that gravity heats up planets is by squeezing them. As we saw in the previous section, rocks are deformed under pressure. As we know from the bulk modulus, adding pressure to rocks makes them just a little bit smaller. And adding pressure to gasses makes them a lot smaller. What makes this fascinating is that compressing things, be they rocks, liquids, or gasses, also heats them up through adiabatic heating.

As materials are compressed, they heat up because the stuff inside has less room to move

As materials are compressed, they heat up because the stuff inside has less room to move

The way to think about this is to consider a group of your friends. Let’s suppose that you have 100 friends and you all decide to meet in a stadium. Because the area is so big, you can bounce around quit a bit without ever meeting one of your friends. So you all decide to meet in a night club. The area is smaller, so you interact with many of your friends fairly often. Now you decide to take everyone to your living room. Unless you have a really big living room, there are now a lot of people in a very small space and everyone is always bumping into each other.

The same thing happens when you compress the parts of a planet. The smaller the space that the rock or atmosphere takes up, the less room the molecules have to move in. They therefore interact more often which leads to more random motion which, was you know, is just another way of saying “the temperature goes up”. Though this effect is small for planets that are mostly solid, like the Earth, it is huge for planets that are mostly gaseous, like Jupiter. And the bigger that the planet is, the more important the effect.

That happens partly because being large just means that there is more stuff that goes in. If you’ve ever made a pie, then you are familiar with this problem. Bigger pie pans need a lot more apples to fill them than small ones do. But the amount of crust you have to roll out doesn’t increase as fast as the amount of filling does.

Practically, what this means is that if you double the diameter of a planet, then it will hold eight times more matter. That’s why even though Jupiter is just eleven times as wide as Earth, it has 1,300 times the volume. And that means that 1,300 times the stuff fell into Jupiter, which means 1,300 times the heat. But while the volume increases as the cube of the radius, the surface area only goes up as the square. As a result, that 1,300 times the heat is trying to escape through just 121 times the surface area. As a result, bigger things get hotter and stay hot longer.

This is most notable for stars. If we stole 5% of the Sun’s matter, we’d have a ball about fifty times as big as Jupiter. We’d make an object that was about four times the width of Jupiter but was hot enough to start fusion. It would be a star. And it couldn’t help becoming one simply because gravity had heated it up enough through compression and collisions. If we stole just 1% of the Sun’s mass, we’d have a ball only about ten times larger than Jupiter. When we allowed that to collapse, it would get hot enough to glow but it wouldn’t get hot enough to start fusion. We’d have a brown dwarf.

So there is a spectrum of objects, ranging from big enough to glow to big enough to fuse; as with the boundary between “planets” and “junk”, where we draw the line is purely arbitrary. Planetologists choose to draw it at “big enough for fusion” because that’s when stars start making new matter out of old instead of just cramming stuff together. And it all happens thanks to gravity.

[1] Or, as we like to say in the planetology biz, “they accelerate until the impactor creates an astrobleme”

[2] Or at least, mostly just on the size of the planet. If the rock is already moving through space, then that velocity can also add to the speed of the falling rock. For our purposes, we’ll pretend that everything started from a standstill.

[3] Geeks and sharp-eyed editors will complain that I’ve ignored the role of air friction. After all, throwing something through the atmosphere slows it down a bit. That is true, but it doesn’t add more than 10% to the total speed so we’ll ignore it for now.

Chapter 2: Gravity is your friend – Why are planets round?

As we saw in the last chapter, if we base the definition of “planet” on historical or planetological grounds, then a planet is anything that is naturally occurring and big enough to be round but not so big as to start fusion. But what do we mean by “big enough to be round”? Why should a big thing be round? Why can’t a planet be shaped like a cube or a pretzel or Marilyn Monroe? The answer lies in the materials that are used to make the planets.

Every material thing that exists can be described in any number of ways. You could describe a candy bar using its weight, its nutritional content, its color, or the number of kids that like the way it tastes. Similarly, the stuff that goes into making a planet can be described chemically (i.e., by its elements and molecules), optically (i.e., how it looks using different colors of light), or by its physical properties (i.e., how it responds to being squeezed or distorted). Though all of those ways of describing a planet have their places in planetology, the description that concerns us now is the physical properties. Specifically, we are interested in the property that describes how a planet responds to being crushed and the property that describes how a planet responds to being squeezed, because these two properties also tell us why planets are round.

The first physical property is how a planet or anything else responds to being crushed. When you sit on a chair, the cushion underneath you is crushed by your weight and responds by getting smaller. When you stand up, the cushion comes back into shape. If you were to sit on a rock, it too would get just a little smaller and then get just a little larger when you stood up. And if you were to sit on a marshmallow, then it would get a lot smaller and then get a lot larger when you took your weight off of it. The physical property that describes just how much a substance resists getting smaller when you sit on it is called the bulk modulus.

Some things, such as water, have a very high bulk modulus; that is, they do not change size very much under pressure. A cup of water from the bottom of the ocean, where it is under a lot of pressure, has almost exactly the same amount of water in it as a cup of water from the top of the ocean, where it is under very little pressure. Other things, such as air, have a very low bulk modulus so that they change size quite a bit. If you were to take a cup of air to the bottom of the ocean, it would fit into a teaspoon.

A large bulk modulus means that it takes a large pressure to squash an object. Steel has a large bulk modulus.

A small bulk modulus means that it takes a small pressure to squash an object. Marshmallows have a small bulk modulus.

As an example of the bulk modulus, imagine that you have a marshmallow in one palm and a rock in the other. If you close your hand and make a fist, the marshmallow gets much, much smaller. The rock, not so much. The marshmallow has a much smaller bulk modulus than the rock and is compressed more by the same amount of pressure. So things with a small bulk modulus get small under pressure, and those with a big bulk modulus stay big.

The bulk modulus does a good job of explaining what happens when something is compressed evenly on all sides; that is, when it is in what planetologists call hydrostatic equilibrium. But what happens when things are not in equilibrium? To understand this, imagine that you are holding a marshmallow between your finger and thumb. As you squeeze your fingers together, the marshmallow compresses in one direction but expands in the others; it changes shape.

A large shear modulus means that it takes a large pressure to change an object’s shape. Steel has a large shear modulus.

A small shear modulus means that it takes very little pressure to deform an object. Peanut butter has a small shear modulus.

When something changes shape, it undergoes shear; that is, rather than simply becoming smaller, the sides move relative to each other [1]. And, just as was the case with the bulk modulus, things with a low shear modulus are easy to force into new shapes and things with a high shear modulus are very hard to change into a new shape. Water has a shear modulus of zero. As a result, when you pour water from one container to the next, it takes on the shape of the new container. Solid steel has a high shear modulus. No matter where you put and I-beam, it stays the shape of an I-beam. And glaciers are made of ice, which has an intermediate bulk modulus. Though you may pile snow up at one place, it will slowly ooze downslope over the course of centuries [2].

And it is that slow oozing that forces planets to be round. As you pile more and more rock in one place, the rocks on the bottom become compressed. But the amount of force on the side of the pile is not the same as the amount of force on the bottom. As a result, the rock pile is pushed more from the top than the sides and ends up squeezing out, just like the marshmallow. As a result, any pile of rock that is more than 400 km across will flow into a round shape in less than a million years.

The bottom rock is under more pressure from the top two rocks, and spreads out.

Of course, there are round objects and really round objects. For example, the largest depression on Earth is nearly 11 km deep and the highest spot on Earth stands nearly 9 km high [4]. Thus, the Earth has a maximum height difference of about 20 km. Though this is very much smaller than the 400 km difference that is needed to force an object to flow, it does show that the Earth is not perfectly round [3]. All told, the Earth is just 0.1% away from being perfectly round. Venus is almost as round as the Earth, but other planets are bumpier. For example, the topography of Mars extends from the deeps of Valles Marineris (7 km) to the heights of Olympus Mons (22 km) making the planet 0.3% away from perfect sphericity. And Mimas, the “Death Star” moon of Saturn that is the smallest known planet, has a depression 10 km deep that takes it 2.5% away from being a perfect sphere. From these observations, we can say that any object that is more than 5% away from being a perfect sphere (after taking into account secondary effects such as spinning) is not in hydrostatic equilibrium and therefore is not a planet.

[1] The sharp-eyed science nerd will have noticed that I am actually describing “pure shear” and not “simple shear”. The reason for that is two-fold: first, simple shear is not simple to describe, and second, pure shear is what happens here.

[2] Though many textbooks (and would be science reporters) claim that the stained glass in medieval cathedrals is thicker at one end than the other because glass has a low shear modulus, this is simply not true. We have many Roman goblets than show no appreciable change in thickness, despite being older than the medieval windows. Those stained glass windows have glass that is thicker on one end because of the way that the glass was made and not because it slumped over the years.

[3] True science nerds will recognize that the tallest point above sea level is not the same as the world’s tallest mountain. Mt. Everest is the world’s highest point at 8,848 m. But at 10,207 m the world’s tallest mountain is Mauna Kea. Because most of the mountain is below sea level, very few are aware of the actual height.

[4] We will ignore the fact that the Earth also bulges out at the equator, due to its rotation. This effect is also seen on other planets, with the most egregious example being Haumea which spins around so quickly that it has deformed into a large olive-shaped body.

Chapter 3: How did we get here? – Everything goes “crash!”

So it is the death of earlier stars that gave this Solar System life. The vaporized remnants of earlier stars spread throughout the nearby region, creating a thin molecular miasma that is nonetheless far denser than the surrounding space. In typical intragalactic space, there is about one molecule in every ten cubic centimeters. If you were to capture that space in a two liter bottle, you would have 200 molecules; for comparison, there are about 6×1022 molecules in two liters of air. There would be 148 hydrogen molecules, 48 helium atoms, and four molecules of something else (most likely carbon monoxide). But in a region seeded by supernovae, you would capture 200,000 molecules; hydrogen would again make up the majority of the matter, but simple compounds such as carbon monoxide, water, and even amino acids would also be present.

The relatively dense clouds of simple molecules are called molecular clouds by astronomers. They make up perhaps one half of one percent of the entire galaxy by volume but are responsible for all stellar formation. Astronomers estimate that there are some 6,000 molecular clouds in the Milky Way galaxy alone. Each of these clouds holds enough matter to form 100,000 stars the size of our Sun and can be as much as 100 light years across. Perhaps the best known of these is the nebula located in Orion’s sword, which is located some 1300 light years away and can be seen with the naked eye. Only slightly less well-known is the Eagle Nebula, which was photographed by the Hubble Space Telescope. In both of these cases, the molecular cloud has been disturbed by some outside force.

Close up of the Eagle Nebula where stars are being formed; photo from NASA.

And it is that force that is important. Until they are disturbed, the molecular clouds float along in the galaxy, diffuse and unimportant. Though the individual molecules are cold and far apart, they nevertheless are moving fast enough and interact often enough to counter-act the pull of gravity. But when the density in the molecular cloud reaches a critical threshold, gravity takes over and the mass begins to collapse. And when the mass is large enough, the process becomes unstoppable and a new solar system is formed.

Astronomers do not know for certain what causes the density to increase in any given molecular cloud. Local turbulence, like eddies in a stream, is considered to be one possibility; turbulence is also thought to be the main reason that the molecular cloud breaks into hundreds of smaller volumes instead of collapsing as one large system. As the molecular cloud condenses, its natural rotation causes some parts to constrict more rapidly than others. This then causes the parts to separate and form individual pockets of material, all with about the same rotation and orientation. When turbulence is a factor, only a few solar systems at a time will form and they will be clustered in a region as turbulence is inherently localized.

A second possibility is the effect of density waves. If you have ever driven in traffic, you’ve experienced these; they are simply places where the density, be it cars or hydrogen, is slightly higher. They happen because not everything is moving at the same speed or in the same direction. In traffic, the faster cars are catching up and the slower cars are falling behind. If the slower cars are in front, then the effect of the faster cars catching up is to increase the number of cars in the area, which then leads to a traffic jam. The same thing happens in a molecular cloud, with one vital difference; it is the gravity of the cloud itself that causes things to speed up and slow down. The cloud as a whole is pulsating with a beat that lasts a hundred thousand years. And, in each beat, there are places where the density increases; if it increases enough, then a solar system is born. As is the case with turbulence, only a few solar systems at a time will form. Unlike turbulence, these will be widely scattered as the density waves are broadly separated.

A third popular suspect is the effect of a nearby supernova. The light pressure from a supernova can compress the molecular cloud across a large region, leading to the formation of multiple solar systems over a wide region simultaneously. In addition, supernovae emit many high energy protons which can transmute 25Mg (a non-radioactive isotope of magnesium) into 26Al (a radioactive isotope of aluminum). The 26Al then decays into 26Mg, releasing x-rays and heat. Why is this important? First, because the heat can be used to melt the interior of planets as they form. And secondly and more importantly because this provides a very accurate “clock” that we can use to determine how long it takes planets to form. It takes just 7,150 years for 26Al to decay. By measuring the amount of 26Mg in a meteorite, we can then estimate how soon it formed after the light from the supernova hit it.

Though all three effects played some part in the formation of this solar system, it was the supernova that pushed us over the edge and into formation. We know this because of the abundance of 26Mg in the meteorite fragments that have fallen to Earth. We are thus lucky enough to be able to tell not just how we formed but how quickly. And it turns out that we formed very quickly indeed.

Chapter 3: How did we get here? – Inside the stars

At 300,000 years after the Big Bang, the Universe is primarily made up of three things [1]: atoms of hydrogen, atoms of helium, and light. Of those three things, it was hydrogen that dominated and continues to dominate. When the Universe finally cooled enough for atoms to form, 73% of them were hydrogen and 26% of them were helium. And that was a good thing, as it allowed for the creation of all the other atoms in the Universe.

It did that by the same process of fusion that makes our Sun shine. Inside each star, atoms are compressed and heated to a white hot incandescent plasma; temperatures are so high that they strip the electrons from the atoms, effectively rolling the clock back to 300,000 years after the Big Bang [2]. The naked nuclei would normally repel each other, fly apart, and cool allowing the electrons to rejoin the atoms, but they are forced together by their own gravity. Thus, it is the interaction of the different forces that allows fusion to happen – and that prevents it from happening everywhere.

This process can be thought of as being like skating over a hill into a valley. It is easy to roll into the valley, but first you have to have enough energy to get over the hill. Without that energy, you would not spontaneously fall into the valley. In fusion, the hill is the electromagnetic repulsion of the protons. Without it, fusion would occur spontaneously as gravity pulled neutrons together. However, pressure and temperature can supply energy to protons; the more extreme the conditions, the more protons that will have enough energy to make it “over the hill”. Because the necessary temperature and pressure is so high, fusion can only take place in the innermost 30% of the Sun. And, because the rate of cooling increases as things get smaller, pressures and temperatures are too low and fusion cannot happen in objects less than 1/10th the Sun’s mass [3].

Though the details of fusion in a star are fairly complex and only poorly understood, the broad outlines are well known and are based in the nuclear interactions of the protons and neutrons in the elements. In order to understand how fusion works, we must first understand how atomic nuclei are put together.

Inside of each nucleus are protons and neutrons. Each element is defined by the number of protons that it has in its atomic nucleus; it is the number of protons that determines the number of electrons that an atom can hold, and so determines how that atom with chemically react with other atoms. For example, every hydrogen atom has just one proton, every helium atom has two protons, and every oxygen atom has eight protons. But the number of neutrons in a given element can differ; each different type with a unique number of neutrons is called an isotope [4]. For example, there are three types of hydrogen. One has no neutrons, and is commonly called “hydrogen”. One has one neutron, and is commonly called “deuterium”. And one variety of hydrogen has two neutrons, and is commonly called “tritium”. However, because it would be too complex to give each element a unique name for each isotope, scientists have simplified the system and refer to the isotopes by the total number of protons and neutrons, called the atomic weight, in the nucleus. Thus, hydrogen is hydrogen-1, deuterium is hydrogen-2, and tritium is hydrogen-3. And, because scientists get very tired of writing things like hydrogen-1 and uranium-235 all of the time, this gets further simplified to the element’s symbol with the atomic weight as a superscript on the left, so hydrogen-1 becomes 1 and uranium-235 becomes 1235U [5].

The fusion reaction inside the Sun
(Image from Windows on the Universe)

The number of protons and neutrons in a nucleus is important because that is what fusion changes. In the simplest kind of fusion, two 1 nuclei combine to create a 2H nucleus. This changes one of the protons into a neutron; because the neutron weighs slightly less than a proton, the extra weight is given off as a positron [6] and a neutrino [7]. Next, two 2H nuclei fuse to create a 3He (helium-3) nucleus and a gamma ray. Finally, two 3He nuclei join together to make a 4He nucleus, two 1 nuclei, and just a little energy.

Though each individual fusion reaction only produces a microscopic amount of energy, a vast number of them take pace inside a typical star. In the Sun, about 4 x 1038 hydrogen nuclei are consumed each second, releasing the equivalent of one million times the total world consumption of energy. This process has been going on since the Solar System formed, 4.5 billion years ago and has in that time has converted forty times the Earth’s mass from hydrogen to helium. Obviously, this cannot go on forever. As the hydrogen in an area is used up, the reaction proceeds outward. Because the core of a star is the first part to reach the temperatures and pressures necessary for the reaction to proceed, hydrogen burning begins there and moves outward as a spherical shell. Fortunately, hydrogen is not the only element that releases energy when fused; however, those with more protons (and hence more electromagnetic repulsion) require higher temperatures to initiate fusion. For example, though hydrogen will fuse at a mere ten million degrees kelvin [8], temperatures must reach a billion degrees kelvin before carbon will fuse. In addition, elements with atomic numbers higher than iron do not produce energy when fused. Thus, there is a limit to the number of possible fusion reactions and to lifetime of stars.

Fusion inside a star happens in shells; as the fuel for one type of fusion runs out, the ashes become the fuel for the next type.
(Image from Wikipedia)

And it is the death of those stars that most interests the planetologists, because it is the death that creates the more massive elements and scatters them across the Universe. When a star that is less than half the size of the Sun runs out of hydrogen, it cannot reach the temperatures needed to fuse higher elements. So, like a campfire that has run out of fuel, it slowly dies and cools. The vast majority of stars in the Universe die this way. But when a start that is between half the size and ten times the size of the Sun runs out of hydrogen in a shell, it starts to burn higher and higher elements at higher and higher temperatures. The higher temperatures cause the star to expand, becoming a red giant and then finally collapsing. As it collapses, it warms the interior back up to the point where fusion is possible. As a result, these stars pulsate creating strong shock waves each time that fusion turns back on. Those shock waves propel the star’s outer shell outward as a planetary nebula [9]. This nebula is rich in heavier elements up to iron and serves as a rich source of materials for new solar systems.

But it is the death of the largest stars, those that are more than ten times the size of the Sun, that most interest us. As the center of one of these massive stars runs out of fuel, it collapses suddenly and all at once, instead of in the herky-jerky manner of the smaller stars. Like adding gasoline to a fire, this sudden collapse ignites a flash of fusion that reaches temperatures and pressures high enough to create elements higher than iron on the periodic table. It also generates strong gamma rays and other high-energy photons; these will become important when we look at the creation of this Solar System. The force of the explosion is great enough to propel the newly created matter at immense speeds away from the star, enriching the interstellar medium and setting the stage for the next round of stellar evolution [10].

Because we know how long a star of a given size can burn before running out of fuel, we even know how many rounds of stellar evolution there have been since the Big Bang. After the Big Bang, the early stars formed. They were almost pure hydrogen, with some helium and just a trace of lithium. They formed somewhere between 150 million and 1,000 million years after the Big Bang; the uncertainty is a measure of just how difficult it is to see things that far back. Indeed, no images of the individual stars from that time have ever been recorded (though we do have images of early galaxies). From computer models, we know that these early stars were probably very large and so were very hot and burned very, very fast. These stars then exploded in supernovae that seeded the Universe with the elements needed for the next round of stellar formation, including carbon, iron, and the rest of the periodic table.

Because the second round of stars included those higher elements, they were smaller and burned more slowly. Though the early stars ran through their fuel in less than a hundred million years, the second set of stars lived for nearly ten billion years or a thousand times as long as their parents. But eventually, one by one, these stars also became supernovae and created yet more higher elements that were spread out to form the third set of stars and planets. That second population were the parents of our Sun and the planets in this Solar System. And the same elements that created the Solar System are in you, me, and every living thing on this planet; thus, we are all children of the stars – and the great grandchildren of the Big Bang.

[1] The discerning science fan will ask “What about dark matter and dark energy?” The honest cosmologist will reply “We’re not sure. Ask again after we’ve gathered some more data.” It is likely that the dark matter, which is the “missing mass” of the Universe that is required to make our observations of stellar motion match those predicted by Einstein and Newton’s theories of gravity, was created at the same time as all of the other matter in the Universe. However, some have suggested that the dark matter arises out of the dark energy in much the same way that matter was supposed to be created by the expansion of the Universe in the Steady State hypothesis. Without more observations, we simply cannot answer the question.

[2] Indeed, this is how all particle physics experiments work; we simply add enough energy to make conditions like those at some time after the Big Bang. But, because temperatures (and therefore energy levels) get exponentially higher as you get closer to the Big Bang, it takes exponentially more powerful (and therefore bigger and more expensive) “atom smashers” to simulate those conditions. The prototype of the van de Graff generator, an early particle accelerator, was built in the scientist’s kitchen using two aluminum bowls and his wife’s silk stockings; it could simulate conditions at about 100,000 years after the Big Bang. The Large Hadron Collider that has recently found evidence for the Higgs Boson, took ten years to build and cost $9 billion. It can simulate conditions less than a second after the Big Bang.

[3] The exceptions to this being the rare man-made cases where something else, such as an atomic bomb or fusion chamber, supplies the necessary pressure and temperature.

[4] “Isotope” means “in the same place” in Greek (Scientists are very fond of Greek), and refers to the fact that oxygen falls in the same place on the periodic chart and undergoes the same chemical reactions, no matter how many neutrons it has.

[5] To make things even more confusing, the name is pronounced as if the superscript were on the right; i.e., we say “U-235” and not “235-U” (which could sound very rude in some circles).

[6]A positron is the antimatter equivalent of an electron. It has exactly the same mass as an electron but the opposite charge. Put a positron in the same room as an electron, and all you’ll see is a big flash of light.

[7] The “little neutral one”, or neutrino, was the subject of one of the greatest investigations in the history of science. We knew from the math exactly how many neutrinos should be generated from fusion in the Sun. But we detected only 1/3rd the amount that we should have. There were three possible answers to the conundrum Either our math was wrong, or the Sun was dying, or something unexpected was happening. After many experiments confirming the math, we finally discovered that there were actually three types of neutrino and our detectors could only measure one of them. The best part was that this discovery then led to more insight into how sub-atomic particles work.

[8] Kelvin degrees are the same as Celsius degrees, but start at absolute zero (-273.15°C) instead of at the freezing point of water. And Celsius degrees are exactly the same as centigrade degrees, only with a new name.

[9] These are called “planetary nebulae” because astronomers thought that they looked like the planet Uranus, not because planets form in them.

[10] Astronomers are probably out burning copies of this book even as we speak, as I have completely ignored complicating factors such as the star’s composition, the presence of binary companions, and the intricacies of fusion in stellar interiors. Then again, astronomers call everything that isn’t helium or hydrogen a metal, so we’re about even in the oversimplification race.

Chapter 3: How did we get here? – A Cosmic Roadtrip

A cosmic roadtrip

The story of this solar system and every living thing on it begins 13.75 billion years ago. It is a simple tale, filled with wonder and sights beyond imagining. As is always the case, it is a saga  filled with life that springs from the ashes of death and death that is born in the very creation of life. The story of us is both complex in its interactions and simple in its causes. But to understand it, we must go back to the beginning, 13.75 billion years ago.

Because many people have difficulty in understanding what educators call “deep time”, allow me to offer this analogy. Let’s pretend that you have the ability to move in time by walking and that every inch that you walk takes you ten years back in time. If you walk one foot, then you have moved 120 years back in time to when your great-grandparents were born. Walking seventeen feet takes you back to when Julius Caesar was trying to make a name for himself by splitting Gaul into three parts. Walking a mile takes you back 633,600 years, when mankind was first moving out of Africa. Walking eight and a half miles back puts you in the Messinian, just in time to see the Mediterranean sea form as a waterfall a mile high, ten miles wide, and one hundred miles long poured through the Straights of Gibraltar. Walking one hundred miles puts you on the spot as an asteroid hits the Yucatán Peninsula and seals the fate of the dinosaurs 65 million years ago. Walk a thousand miles, or roughly the distance from New York City to Miami, and you are on a planet that is covered with glaciers during the “Snowball Earth” period. Walk the length of the Appalachian trail, some 2,184 miles, and you have reached the supercontinent Columbia which is covered by volcanic ash and barren of any life more advanced than a bacteria.  Walk the 7200 miles from Portland, Oregon, to Ushuaia, Argentina, and there will no longer be an Earth to stand on as you arrive 4.5 billion years ago just as the flash of a supernova triggers the birth of our solar system. And, if you walk the entire circumference of the Earth, you will arrive 13.75 billion years ago, at the Big Bang.

Everything goes “Boom!”
Everything in this universe started with the Big Bang [1]. At the time that the Universe began, everything was squashed into a tiny space no larger than the head of a pin. As you might guess, it was both very hot and very dense in that pinhead. Because of the heat, no protons, no neutrons, and no electrons could exist; the only matter was in the form of quarks; any higher form of matter would be torn apart by the energies in the early universe. For some reason [2], the universe began to expand rapidly [3]. This took energy, which allowed the quarks to cool. After just one millionth of a second, the Universe was about the size of our Solar System and things were cool enough for elementary particles, such as protons, neutrons, and (a little later) electrons to be stable.

It took another three minutes for the Universe to expand to 1,000 times the size of the Solar System. During that three minutes, the neutrons and protons cooled enough for the strong nuclear force to bind them together into atomic nuclei. And it takes another 300,000 years (or 4/10ths of a mile on our walking tour) before the Universe has cooled enough for the weak nuclear force to bind the electrons to the nuclei, forming atoms.

At this point, the Universe is about 1/1000th of its current size. But the binding of the electrons allows photons to move freely without being captured; the Universe becomes “transparent” and the Cosmic Microwave Background radiation is born [4]. More importantly, the atoms begin to clump together, forming stars, which clump together to form galaxies. Without these two steps, there would be no Solar System and no us.

[1] One of the most amusing things about the Big Bang is that it got its name from the people who didn’t think that it happened. Fred Hoyle, who supported the “steady state”model (which held that the universe has always existed and Olbers was an idiot), used the term as a way of distinguishing the upstart idea from his view. The steady state is now a historical footnote and the Big Bang is widely recognized as the best explanation of how we got here.

[2] Cosmologists are still arguing about exactly why this happened.

[3] Cosmologists’ two favorite questions are “Expand into what?” and “What was there before the Big Bang?” We still don’t know the answer to the second question. The answer to the first is basically “Itself”. There’s a lot of math behind the second answer, but even the cosmologists don’t agree on what it really means.

[4] It is that same CMB that offers us our clearest view of the early universe. Amusingly, it was discovered by accident. Two Bell Laboratory scientists were attempting to send signals using balloons in orbit (early precursors of modern telecommunications satellites). They were puzzled by the noise that they saw in their signals until a friendly neighborhood cosmologist said “Hey! That looks like the microwaves that we’ve been looking for!” The Bell Labs scientists won the Nobel Prize for their accident, and the cosmologists had to settle for lots and lots of lovely data.

Appendix 2: A Timeline of Discoveries

Year Notes
1581 BC The Babylonians describe Venus as ‘Bright Queen of the Night Sky’
1534 BC Egyptian astronomers note the retrograde (backward) motion of Mars.
1400 BC Babylonian astronomers see Mercury and name it ‘the jumping star’ due to its odd behavior of appearing in both the evening and night-time skies.
600 BC The Greek realize that Hesperus and Phosphorus are the same wandering star and rename it Aphrodite.
430 BC Anaxagoris discovers that the Moon shines by reflected sunlight.
400 BC The Greeks finally catch up to the Babylonians and realize that the planet they see at night is the same one that they see in the morning.
360 BC Heraclides Pinticus suggests that the Earth rotates on an axis, making the stars appear to move through the sky. His friends suggest that he cut down on his drinking instead.
350 BC Aristotle proposes a geocentric universe; combined with his other philosophical inventions, it will dominate Greek and then Christianthought until the Renaissance two thousand years later.
250 BC Aristarchus of Samos measures the relative sizes of the Sun and Moon. Shocked by the Sun’s much larger size, he proposes the helicentric universe. Shocked by his impeity, he is threatened with death if he doesn’t recant.
145 AD Ptolemy (aka Claudius Ptolemaeus) writes Tetrabiblios (“Four books”) describing a geocentric universe and explaining the retrograde motion of Mars, Jupiter, and Saturn as “epicycles”. This work would constrain astronomy and philosophy for more than a thousand years.
1543 Copernicus posthumously publishes a book detailing the characteristics of a heliocentric universe and how it differs from a geocentric one.
1572 Brahe observes a ‘new star’ (nova) and helps to overthrown Aristotle’s grip on modern thought.
1576 Brahe publishes a geo-heliocentric model that tries to reconcile Galileo’s observations with Ptolemy’s ideas. Though it becomes popular after the Church declares the heliocentric model anathema, it nevertheless is shown to be wrong within the decade.
1600 Bruno is burned at the stake for heresy, which included his support of a heliocentric universe.
1605 After much trial and error, Kepler discovers that planetary orbits can be described as ellipses and lays out his laws of planetary motion.
1609 Galileo improves the spyglass and turns his new “far seer” (telescope) onto the heavens.
1610 Galileo discovers the first two “Medicean planets” Callisto and Ganymede.
1610 Galileo discovers the second two “Medicean Planets’ Europa and Io.
1611 Kepler posthumously publishes the first true science fiction story ever written. His footnotes describing the science behind the story are several times longer than the story itself.
1625 Galileo makes the first compound lens for magnifying small things; it is called a “little seer” (microscope) in recognition of his work.
1629 Isaac Bekman tries to measure the speed of light using the flash of a cannon. He fails.
1655 Huygens discovers the largest moon of Saturn and names it Titan (“Really big”).
1668 Newton builds the first reflecting telescope. His design will form the basis of modern telescopes.
1671 Cassini discovers Iapetus, Saturn’s second-largest moon.
1671 Cassini publishes an ephemerides of Galilean moon orbits, split up by longitude of the Observer on Earth. This allows the first accurate determination of longitudes while at sea.
1672 Cassini makes the first measurement of the orbit of Mars.
1676 Rømer makes the first measurement of the speed of light, based on eclipses of Jupiter’s moons.
1687 Newton publishes Philosophiæ Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”) which uses gravity to explain why planets move in the way that Kepler described.
1734 Swedenborg proposes that the Solar System formed from a nebula of gas and dust.
1764 Lagrange solves the 3 body problem which explains why asteroids clump near Jupiter and Earth.
1766 Johann Titius proposes a missing planet between Jupiter and Mars.
1772 Johann Bode proposes a missing planet between Mars and Jupiter. Because the two came up with their ideas independently, it is given the name of the Titius-Bode Law.
1781 Herschel discovers Uranus. Originally he thought it was a comet, but changed his mind based on its regular motion. Interestingly, it had been seen but not recognized as a planet by Galileo
1783 John Mitchell proposes the existence of black holes.
1787 Herschel discovers the two largest moons of Uranus. He shows off his erudition by naming them after characters in Shakespear’es play A Midsummer’s Nights Dream: Oberon and Titania.
1801 Piazzi discovers the planet Ceres. Originally hailed as the ‘missing planet’ predicted by Titius-Bode, it is later the cause of much confusion.
1802 Oblers discovers the planet Pallas, which is also in the area predicted by Titius-Bode. He attempts to rescue the Ttius-Bode law by sugesting that the two asteroids are actually part of a planet that broke apart.
1804 Harding confuses matters still more by discovering a third planet (Juno) in the area predicted by Titius-Bode.
1807 Olbers discovers one more planet (Vesta)  in the Titius-Bode region.
1838 Bessel makes the first observation of stellar parallax and puts the final nail in the coffin of the geocentric universe.
1845 After a gap of 38 years, Hencke discovers yet another planet in the region that Titius-Bode predicted would hold one.
1846 Naming Neptune was a subject of intense debate and indirectly led to the formation of the IAU
1868 More than 100 asteroids have been discovered.
1877 Hall discovers Mars’ two moons.
1902 More than 500 asteroids have been discovered.
1921 More than 1,000 asteroids have been discovered.
1926 Goddard launches the first liquid fueled rocket and sets the stage for Apollo. The New York Times publishes an editorial denouncing Goddard; they will retract it in 1969, when Apollo 11 lands on the Moon.
1930 Tombaugh discovers “Planet X”. It is named Pluto despite the intense lobbying of Lowell’s widow to name it after herself.
1951 Kuiper predicts the existence of a large region on the fringes of the Solar System, filled with the left-overs of planetary formation
1957 The USSR launches the first artificial satellite. They name it Sputnik (“Little moon”).
1958 Pioneer 1 is launched; this is the first probe to another planet. Though it misses its target (the Moon), it heralds a new age of planetology, with direct and up-close information taking the place of telescopic observations.
1972 Pioneer 10 is launched; it is the first space probe to reach escape velocity for the Solar System.
1981 More than 10,000 asteroids have been discovered.
1982 The Infra Red Astronomical Satellite (IRAS) discovers a circumstellar disk around Beta Pictoris. This is the first direct evidence of planet building around other stars.
1988 Campbell, Walker, and Yang discover a planet orbiting another star (an exoplanet).
1992 The first Kuiper Belt object is discovered. Though the discoverers wanted to name it “Smiley”, the IAU demurred and the object still remains unnamed.
2000 More than 100,000 asteroids have been discovered.
2003 More than 100 exoplanets have been discovered.
2004 Brown et al. discover the first Kuiper Belt Object larger than Pluto. Faced with the possibility of having hundreds of planets, the IAU panics and redefines the word so that they won’t have to think about it.
2010 More than 500 exoplanets have been discovered.
2012 Nearly 700 exoplanets and more than 400 planets and 300,000 asteroids in this solar system have been discovered.