Jupiter
Jupiter, named rightly for the greatest of the Roman gods, is very close to the dividing line between planets and stars.  Many, many times larger than the Earth (and containing more mass than all of the other planets, moons, etc except for the sun), Jupiter is so massive that it has been able to hold onto anextremely thick atmosphere, including massive quantities of hydrogen, which is light enough to escape from Earth relatively easily.  By calculating the density of matter needed to initiate fusion, it can be seen that Jupiter is nearly large enough, although it would only be a small, weak star, many times dimmer than the Sun.  Nevertheless, it can be shown that Jupiter does have some internal energy of its own, as it gives off more energy than it receives from the sun (the only planet to do so).  Unfortunately, the dense atmosphere, severe storms, and awesome gravity of Jupiter make a landing impossible even for unmanned craft, and in fact even makes it impossible to determine if there is actually a solid surface (although one is believed to exist based on other evidence).  Far more interesting about Jupiter are its moons.  In total, there are 16, but the four largest, known as the Gallilean Moons after their discoverer 390 years ago, are by far the most intersesting.  The smallest, Io, is about the size of our Moon, and is extremely volcanically active, with volcanoes spewing sulfuric ash across the whole of the planet.  Ganymede, the largest, is larger than Mercury and Pluto.  While having the dry, cratered look of our moon, density and other indicators point to the surface being made largely of water ice, and recent studies by the Gallileo probe have indicated that it might have a vast salty ocean under the surface.  Like Ganymede, although smaller, Callisto is largely ice with a rocky core.  For biologists, Europa is by far the most promising body in the solar system.  With a surface of water ice and extremely strong evidence of a salty ocean beneath, many believe life could spawn near thermal vents or other sources of energy.  In the cases of all of the water-bearing moons, the possibilities for human uses are almost unlimited, as they supply water, oxygen, and rocket fuel in almost unlimited quantities.

Saturn
Saturn, the second largest planet, follows Jupiter.  Known mostly for its vast system of rings, it also holds the title for the most moons, 22 (although this number is changing with Cassini's closer observations).  Like Jupiter, the atmosphere is too thick and the gravity too strong to allow landing on Saturn.  More distant than Jupiter, it is also much less well known, although that is changing greatly with the arrival of the Cassini spacecraft.  Unlike the Gallileo probe to Jupiter, which dropped a probe into the great planet, Cassini carries a probe which it dropped onto Saturn's largest moon, Titan.  Shrouded in a thick nitrogen atomosphere and believed to have liquid methane and other organic molecules on the surface, Titan is often compared to the primordial soup from which life arose on Earth.  Thus, Titan could be very important both for resources for our life-support and in the search for life.

Uranus/Neptune
Uranus is the next planet, followed by Neptune.  Both are farther out than can be readily studied from Earth, and have each only received one fly-by, in both cases by Voyager 2.  Unfortunately, little is known about these two planets, except that they are both somewhat smaller than Saturn (and much larger than Earth), both have thick atmospheres (although they are thin enough that we can detect liquid oceans beneath), and both are very cold.  Each has a number of moons, but even less is known about them.

Pluto
Currently farthest out of the planets, Pluto is the strangest.  Tiny by planetary standards, it is the farthest planet out for most of its 248-year orbit of the sun.  Due to an extremely eccentric orbit, however, it spends part of its orbit closer to the Sun than Neptune.  With its small size and great distance, Pluto was not discovered until 1930, is beyond the range even of the Hubble Space Telescope to see clearly, and has never been visited by a probe.  Of interest is the fact that it has a moon, Charon, which is roughly one-half its own size, which questions whether it really is a planet-moons system at all.  A probe was, in fact, scheduled for this decade, but the bloated cost of ISS, among other programs, sealed the doom for such a mission, and it was cut from NASA funding.

Kuiper Belt
At and beyond the orbit of Pluto is the Kuiper Belt, made up of comets and small planet-like bodies very similar to Pluto.  Once again, like Pluto, these objects are too small and too far away to really study.  Thousands have been identified, some nearly as large as Pluto (which is why some astronomers question Pluto's status as a planet). 

Oort Cloud
Beyond the Kuiper Belt, the Oort cloud is made up of gas, dust, and small bodies which are only weakly held to our solar system.  Well beyond the travels of any probe yet launched, we have very little concrete data on its composition.  Our best data comes from observations of comets, most of which originated in the Oort cloud before being jostled out of their normal orbit to fall towards the sun.

Inner System (Sun-Asteroid Belt)

Last Updated 2/26/05