How Planets Work

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Congratulations!

You are now the proud owner of an Acme planet, the finest planets in the galaxy. Whether it's a rocky terrestrial planet, a gas giant, or an ice planet, your Acme planet is guaranteed to give you billions of years of satisfaction.

Your planet comes with a durable factory finish. You can keep the finish pristine for eons by following a few simple rules. Protect the surface from harsh abrasives like large impacting asteroids. Avoid sources of excessive heat, like exploding supernovae. Melting or vaporizing your planet voids the warranty.

If your planet is stored in a moderate temperature setting, it may become coated with a thin film of liquid water. This is normal and does not constitute a malfunction. Cooler parts of your planet may become coated with frozen water, this too is normal and does not constitute a malfunction. Under some circumstances, your planet may become covered with a thin greenish film called life. Connoisseurs consider this the best part.

Bulk Density

Even if we don't have any samples of a planet, we can tell a lot about it from its bulk density. Bulk density is the mass of the planet divided by its volume. We know the mass by observing the planet's effects on other bodies or - best of all - spacecraft tracking. We determine the volume from the size and shape of the body. Earth-based measurements will do if we don't have spacecraft data, but spacecraft data are far better.

Shape

Why Are Planets Round?

Planets are round because of gravity. Gravity pulls everything inward toward the center of a planet.

The pressure inside a planet is simply due to the weight of the overlying material, pulled inward by the planet's gravity. The stronger the gravity, the greater the pressure. A meter thick layer of rock exerts more pressure on earth than it would on the Moon, because of the earth's greater gravity. If the pressure deep in a planet is greater than the strength of the rocks, the rocks can flow. High temperatures make the rocks flow more easily. If we have a large, irregular body, the pressures deep in the interior will cause rocks to flow out from under high spots (where the pressure is greater) and into low spots (where the pressure is less). High bulges will sag inward, and the floors of depressions will flow outward.

In the animation above, let's pretend we're trying to build a square planet. If the planet ends up as big as the earth (blue circle), the original square will be 11,300 km on a side. The corners of the square are mountains over a thousand kilometers high and the sides in between are hundreds of kilometers below the eventual surface of the planet.

The pressures under the corners are so enormous that they sink into the planet (yellow arrows) and squeeze material outward in between. The process finally stops when the differences in pressure inside the planet are small enough that the rocks of the planet can withstand them.

Equatorial Bulges

Fast rotating planets are not spherical, but bulge at their equators because of "centrifugal force." The earth's equatorial bulge is too small to be seen in spacecraft images, and indeed many commercial globes are more out of shape than the real earth. The earth's equatorial bulge is 1/298 of its diameter, so the radius at the equator is about 11 kilometers more than at the poles. One implication of this difference is that the Ecuadorian volcano Chimborazo and the Peruvian peak Huascaran are the farthest points from the center of the Earth, not Mount Everest.

Faster-rotating planets have larger equatorial bulges. Jupiter and Saturn are about 10% greater in equatorial diameter than polar diameter.

Gravity

Studies of a planet's gravity can furnish important clues about the interior of the planet. The path of a satellite around a planet is slightly affected by masses within the planet. Satellite tracking has been used to study crustal thickness and deep rock masses for the Moon, Mars, and Venus and even Saturn's moon Titan, and of course this information is of critical importance for accurate satellite navigation on Earth (also missile targeting).

There's another clue we can derive from spacecraft tracking, called Moment of Inertia. If you've ever turned a playground carousel with children on it, you know it's easier to start and stop if the children are bunched in toward the center. The quantity that describes how rotating objects behave is called moment of inertia. If we can determine the moment of inertia of a planet, we can tell a lot about how mass is distributed within the planet.

But how do we determine the moment of inertia? If the body precesses, like the earth, we can determine the moment of inertia from the precession. Otherwise, moment of inertia affects the shape and size of a planet's equatorial bulge, and it can also be determined if we have enough measurements of the planet's gravity from spacecraft observations.

Atmospheres

High in the fringes of a planet's atmosphere, individual molecules obey the same laws of physics as spacecraft. If they're moving faster than the escape velocity of the planet, they can escape.

So two things dictate whether atoms can escape from a planet:

The more massive a planet is, the more likely it is to retain an atmosphere. However, Mercury, with a mass 0.055 that of earth and an escape velocity of 4.25 km/sec, lacks an atmosphere, while Saturn's huge moon Titan has a mass 0.0225 that of earth, an escape velocity of 2.64 km/sec, and a very significant atmosphere, actually denser than that of earth. Obviously mass alone can't be the sole determining factor.

Temperature is also important. The average velocity of molecules in a gas at is 145.5 ˆš(T/m) where T is temperature (in formulas, T is almost always Kelvin) and m is the molecular mass. For nitrogen at room temperature, m = 29 and T = 298 K, and the velocity is about 466 meters per second. On Mercury, where maximum temperature is about 700 K, the average velocity of a hydrogen molecule (m=2) is 2700 meters per second or 2.7 kilometers per second. Now that's less than Mercury's escape velocity, but still about half. And a significant number of molecules are moving much faster than average - about 2% are moving twice as fast and one in 10,000 moving 3 times as fast. A pretty substantial number will be moving fast enough to escape. Once they're gone, they're gone, and the remaining molecules, also heated by the sun up to 700 K, will still have some that exceed escape velocity, and so on. Mercury can't hold on to hydrogen very long. For carbon dioxide (m = 44), the average velocity is only 580 meters per second.

For Titan, T = 94 K. For hydrogen, average velocity is about 1000 meters per second, close enough to Titan's escape velocity not to hang around all that long. For nitrogen (m = 29), the dominant component of Titan's atmosphere, the average velocity is a sluggish 261 meters per second, and for carbon dioxide (m = 44) it's only 212. So Titan can hold on to these heavier gases.

But there's more to it than that. The earth's outermost atmosphere has temperatures above 1000 K even though the average surface temperature is about 290 K. High energy light from the Sun and charged particles (the "solar wind") accelerate these atoms to high speeds. Fortunately, since Earth's escape velocity is a hefty 11 km/sec, it's still not enough. But on Mercury the sun would accelerate molecules near the top of any atmosphere to very high speeds, and a lot would escape. So even if Mercury had a carbon dioxide atmosphere to begin with, and a high enough escape velocity to hold it, the Sun would still strip it away eventually. On Titan, much farther from the Sun, the Sun would be far less effective at stripping off an atmosphere.

(Since hydrogen is so light and escapes so easily from planets, do we have anything to fear from replacing fossil fuels with a hydrogen economy? Not much. First, the hydrogen will be kept in containers and very quickly burned to create water vapor, returning to its original state. Any that does escape will move at the low velocities typical of the lower atmosphere, and will quickly combine with oxygen in the air (see Hindenburg, 1937). So effectively none of it will get high enough to be accelerated to escape velocity.)

Early in their life cycles, stars go through a phase called the T Tauri phase (after a star that is in that phase now) where it blasts off tremendous amounts of particles at very high speeds. If the inner planets early in the history of the solar system had dense atmospheres, most or all of it would have been torn off by the powerful T Tauri solar wind. But planets in the outer solar system would have been much less affected.

Oceans

On earth, oceans are made of water, but there's no natural law that says an ocean must be made of water. On Saturn's moon Titan there is good evidence for large lakes of the hydrocarbon ethane. We could imagine a very cold planet with seas of liquid nitrogen. But regardless of the liquid, there are physical laws that govern whether a planet can have an ocean.

People who have camped at high altitudes know that water boils at lower temperatures. At altitudes around 20 kilometers, it boils at body temperature, so pilots of high altitude aircraft must wear pressure suits. And in a vacuum, water would boil away entirely.

An ocean on a planet without an atmosphere would evaporate. The vapor would remain around the planet as a temporary atmosphere, but pretty soon whatever caused the planet to lack an atmosphere - low mass, high temperature, or stripping by solar wind - would cause the vapor to leak into space, and the ocean would be gone completely. So the first rule for an ocean is that the planet must have an atmosphere, meaning the planet must be massive and cool enough to retain an atmosphere.

Also, liquids are an "in between" state between solids and gases, and only exist in a certain temperature range. Solids can never be too cold to be solid and gases can never be too hot to be gases, but liquids can both freeze and evaporate. So a planet has to have a temperature that significantly overlaps the liquid range. Temperatures can occasionally fall below the freezing point (as on earth) or perhaps exceed the boiling point, but the planet must remain mostly within the liquid range. So we can expect seas, lakes, and oceans on planetary surfaces to be fairly uncommon in the universe.

But another possible way for a planet to have an ocean is to have an internal ocean. Most small bodies in the outer solar system have rocky cores and icy surfaces. The rocky cores have small amounts of the radioactive elements uranium and thorium, and ice is a pretty good insulator. It doesn't take much heat production from a small rocky core to melt ice below the surface. It is widely believed that Jupiter's satellite Europa has a subsurface ocean, possibly shallow enough to probe. Subsurface oceans could be common on icy worlds. Lack of an atmosphere won't affect an ocean that's sealed beneath a solid shell, though there might be a tiny loss if liquid ever leaks to the surface occasionally.

Even if a planet has an ocean, it may not be able to retain it. Stars brighten with age, and a planet that may have an ocean early in its star's lifetime could lose it later on. If evaporated molecules from the ocean make it high into the atmosphere, they may be broken apart by a process called photodissociation, where ultraviolet light knocks atoms off the molecule. For example, water molecules can be split into hydrogen and oxygen, and since hydrogen is so light, it can easily escape. If the planet gets so hot that evaporation is very extensive, a planet can lose an ocean pretty quickly. Many astronomers think Venus may once have had an ocean but lost it this way. Present theories of solar evolution predict the Earth will become too hot to hold oceans in one or two billion years. Freezing, on the other hand, is no threat.

Liquid nitrogen boils at 77 K. If it forms an ocean on a planet with a nitrogen atmosphere (probably from evaporating the ocean), the average velocity of the molecules in the atmosphere will be a sluggardly 237 meters per second. A body with an escape velocity of a kilometer a second should be able to hold that atmosphere, that is, an icy body roughly 2000 kilometers in diameter. Such an object would be very far from its parent star, so even though liquid nitrogen has a small liquid temperature range, temperatures on the object might not vary all that much.

Names and Places

What is Sea Level?

Even on Earth, defining sea level is harder than you might think. Ocean currents and weather distort the sea surface in ways having nothing to do with the overall shape of the earth. When we say Mount Everest is 29,028 feet above sea level, how do we know where the sea would be under Mount Everest? Not only is the sea surface distorted into a bulging ellipsoid by the earth's rotation, but masses within the earth create bumps and hollows. The shape the sea would have if the earth were totally covered with water is called the geoid. Its shape can be calculated from accurate measurements of the earth's gravity.

For planets without water, elevation is measured from the surface of a sphere with radius equal to the average radius of the planet. The planet will probably also have an equatorial bulge, so the actual shape of the planet is approximated with a flattened sphere called an ellipsoid. On the earth, the average elevation is actually 2.5 kilometers below sea level.

Mapping lumpy and irregular bodies is an evolving field.

Places, Everyone

Every known body in the solar system rotates, so latitude and longitude are defined just the way they are on the Earth. The poles are the points the planet rotates around, and the equator is the circle midway between the poles.

For independently rotating planets, zero longitude, like zero longitude on the earth, is arbitrary. Usually it is defined as the longitude that was facing the earth or the sun at some specified time. But all satellites are locked to their home planets, and zero longitude on these bodies is not arbitrary. Zero longitude is the longitude facing the planet. East longitude is considered positive, west longitude is negative.

What's in a Name?

A commission of the International Astronomical Union oversees naming, and the names (so far) are recognized by the various spacefaring nations. The rules are:

Sooner or later colonists on the Moon or Mars will want a Mount Putin or Mount Reagan, and we will probably let them name things as they see fit. We can deal with that issue when it happens.

Planetary geographical features have Latin names. Latin is traditional, apolitical, and the closest thing to a universal language in history. Here are a few of the most important, with the literal Latin translation in parentheses, followed by the geographical meaning.

What Is A Planet?

In ancient times there were seven planets, counting the Sun and Moon and not counting the earth. After Copernicus and Kepler laid out the true nature of planets, the number of planets dropped to six; the Earth was now in and the Sun and Moon out. Uranus was discovered in 1781, raising the count to seven. The huge gap between Mars and Jupiter inspired speculation that there were unknown planets hidden there, and the first one, Ceres, was found in 1801. Three others followed in the next six years, and the number of planets climbed to 11. But by the mid-19th century minor planets were being discovered at an accelerating rate. All were small, and it was obvious they weren't in the same league even as Mercury, so a new category was created: minor planets. The known asteroids were demoted and the planet count dropped back to 7. The discovery of Neptune in 1846 raised it to 8.

Neptune had been discovered because of its gravitational effect on Uranus. Could there be more planets waiting to be discovered this way? Percival Lowell, advocate of the "canals" on Mars, thought so, and launched a long campaign to find one. Finally, in 1930, his assistant, Clyde Tombaugh, found a distant object. It was called Pluto (the name was suggested by a ten-year old girl, Venetia Burney) and raised the planet tally to nine.

Right from the git-go, Pluto was a troublemaker. It was tiny, much too small to affect Neptune's orbit. But it could perhaps be as large as Earth, or at least Mercury. But a near miss with a star as seen from Earth in the early 1970's showed Pluto must be smaller than the earth's moon. The discovery of Pluto's satellite Charon in 1978 paradoxically strengthened and weakened Pluto's status. Having a satellite strengthened the case for calling Pluto a planet, but having a satellite also made it possible to calculate the combined mass of the two objects, and together they turned out to be much smaller than even Mercury.

As astronomers discovered there were a host of objects in the outer solar system, some rivaling Pluto in size, and that even minor planets have satellites, the case for demoting Pluto gained momentum. In 2006 the International Astronomical Union created a new category of planet, "dwarf planet," defined as a body large enough for gravity to pull it into a sphere. This condition is called "hydrostatic equilibrium," that is, the shape of the planet is due mostly to gravity and rotation. Pluto, a couple of other distant objects, and the asteroid Ceres were designated "dwarf planets."

The current definition of a true planet is, first, that it has to be massive enough for gravity to pull it into a sphere (it can, of course, have an equatorial bulge) and second, it has to be massive enough to "clear" a zone around it. An asteroid orbiting a million miles inward or outward from the earth's orbit would very soon have its orbit perturbed by close passages with the Earth. But Pluto and Ceres aren't massive enough to prevent other objects from having nearby orbits.

The diagram at left is a snapshot of the Solar System at a particular time. The green dots are asteroids. Note that the asteroid belt has a fairly sharp outer edge halfway between Mars and Jupiter and a sharp inner edge just outside the orbit of Mars. Anything that tries to orbit closer to either planet will soon have its orbit disturbed by the gravitational pull of Mars or Jupiter. This is what it means for a planet to "clear" its vicinity.

The purple dots are comets and the red dots are asteroids that enter the inner Solar System. Neither group will stay in those orbits very long. They will either hit a planet or have a close encounter that radically changes their orbits.

This is not over yet. There are numerous large asteroids big enough to be somewhat round that will challenge the definition of roundness as a criterion for dwarf planets. And as we learn about solar systems outside our own, we will surely discover newly formed systems with large objects that are planets by any reasonable criterion but have not yet cleared the space around them. Even something as small as Ceres or Pluto would disturb the orbits of objects very close by, so the definition of how wide and clear a "cleared zone" must be will need to be - ahem - cleared up. But since the definition of planets has changed several times already, it should be no surprise that the definition may change in the future.


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Created 30 July 2008, Last Update 15 April 2011

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