Chris Chyba
Carl Sagan Chair for the Study of Life in the Universe
SETI Institute & Stanford University

I would say that the way to think about a planet is's a collection of rock and liquid and gas that's gotten big enough to be spherical, to be in the shape of a sphere.

If you're the size of a small asteroid, you don't have to look anything like a globe, like a sphere. You can look like a potato or a dumbbell or anything else, because the gravity's not strong enough to sort of smooth out the high spots, to pull the high spots in and compress you down into a globe.

Mercury, named for the mesenger of the gods--where speed is of the essence--revolves once around the Sun every 88 days in a very elliptical orbit.

Second smallest of the planets, Mercury's temperatures vary from 400C to -170C, depending on which heavily cratered face looks toward the Sun.

Venus, second planet from the Sun, named for the Goddess of love and beauty, and seen from Earth as the third brightest object in the sky, after the Sun and Moon.

Venus is about the same size as Earth, but it's a hot hellish world, with the thick clouds of sulphuric acid, and surface temperatures of over 450C.

A runaway greenhouse effect makes Venus hotter than Mercury even though it's twice as far from the Sun.

The Magellan spacecraft used radar to peer through the clouds, revealing more than 100,00 volcanoes, and a mountain taller than Mount Everest.

Earth, third rock from the Sun, is the densest body in the solar system, with a core of rock and iron.

Unique in the solar system for plentiful liquid water on its surface and, as yet, the only planet where we know for sure that life began and evolved.

Perhaps by understanding the planetary processes that have shaped our neighbors, we'll be able to keep our home world habitable...

Mars, one out from Earth, the Red Planet... now locked in an icy deep-freeze.

Half the size of Earth, Mars has giant volcanoes...

A vast canyon that would stretch clear across North America...

Evidence of running water from long ago...

And perhaps, still, liquid water deep underground.

Jupiter, King of the Planets, is the second most massive object in our solar system after the Sun.

Perhaps there's a solid core of rock and liquid metal, but here the surface is all clouds in motion, including a swirling hurricane--the "Great Red Spot"--a storm that has lasted more than 300 years.

Voyager showed us that Jupiter's moons are as exciting and dynamic as the planet itself.

In all Jupiter has more than 17 moons--stay tuned for more!!!--and a faint ring system.

Saturn, the largest and most finely-patterned rings in our solar system.

Also a gas giant, Saturn is squashed, smaller from pole to pole than around its equator.

Voyager 2 used Saturn's gravity to boost it on to Uranus.

Uranus, is the only planet whose pole rather than equator faces the Sun, for reasons we still don't know.

Like the other gas giants, Uranus has rings... and more than 20 moons--the most of any planet in the solar system until the recent discovery of Saturn's 4 new moons announced October 26, 2000.

Neptune, last gas giant, is named for the god of the sea and- appropriately, blue-ish in color.

Neptune has the fastest winds in the solar system, more than 2400 kilometers per hour, and a "Great Dark Spot seen by voyager, now seems to have disappeared.

Pluto, is usually the outermost, and certainly the smallest planet in our solar system... the only world never visited, so far, by a spacecraft from Earth.

Pluto is mysterious, but we do know it has a moon, Charon, about a third its size, the largest satellite, relative to its parent planet, in the solar system.

Chris Chyba
Carl Sagan Chair for the Study of Life in the Universe
SETI Institute & Stanford University

There are two reasons why you need to explore the solar system if you want to understand the Earth. The first reason is very straightforward. If you want to understand a particular region of the Earth, you need to understand the surrounding region to understand its environment. If you want to understand a planet in our solar system, you have to understand the solar system. That's what sets the conditions for that planet.

The second reason we need to understand the other planets to understand the Earth is because we can't do experiments with the Earth. There's a sense in which we are doing a huge uncontrolled experiment with the Earth right now by pumping greenhouse gases into our atmosphere, but there's no control for that experiment. We don't know how it's going to come out. We can get insight into those questions by seeing how other worlds evolved.

We can look at Venus and see what happens when you have a runaway greenhouse effect.

We can look at Mars and see what happens when you have too little greenhouse effect.

Even though we can't do experiments with the Earth, we can do comparative planetology and learn about what might happen or what could happen on a world like the Earth by looking at other worlds where those things have already happened.