After a seven-year voyage, the Cassini spacecraft recently beamed back to Earth the first close-up images of Saturn's largest moon, Titan. In January, a probe will touch down on the moon's surface, making it the farthest human-made object to land on a celestial body. But Titan is just the beginning of our quest to understand what's out there, both close to home and far, far away.
Europa
The best place to find life in our solar system is Europa, one of Jupiter's four large moons. It has a nearly planetwide saltwater ocean buried beneath several miles of ice and kept warm by gravitational tidal flow. If life is found swimming here, our notion of evolution and extraterrestrial life could change.
The moon
Scientists would like to build a manned optical observatory within the Orientale crater basin. The site straddles the moon's near and far sides, an ideal position for listening to extraterrestrial signals without interference from Earth's radio noise.
Triton
The size and makeup of Neptune's largest moon suggest that it once independently orbited the sun until it was captured by Neptune. Figuring out the circumstances of Triton's adoption would provide clues about how our solar system took shape.
Alpha Centauri A
This star is a near twin of Earth's sun, with similar age, brightness, and temperature. The main difference is that it has a companion star. Discovery of a planet orbiting Alpha Centauri A, just 4.4 light-years away, would result in instant funding for warp drives.
Mars
The Mangala Valles basin shows evidence of a once-thriving lake, complete with flow channel and delta. Since Earth's deltas are often rich with nutrients, the possibility of finding life, fossilized life, or the chemicals necessary for life on Mars are greatest here.
Tau Ceti
While there's hope that this star has Earth-like planets, recent telescopic observations suggest that it is surrounded instead by a disc of asteroids and comets. Since Tau Ceti is about twice as old as our sun, it raises the frightening question, Is this our future?
55 Cancri
55 Cancri is the most diverse solar system after ours. Of its four planets, one circles the central star a mere 3.5 million miles away. Scientists are interested in how this planet got there and how it remains intact.
Mercury
Frozen water naturally collects on Earth, Mars, the moon, and, er, Mercury. Yes, radar images show water ice on the planet closest to the sun. Astronomers would like to know if the water comes from deep within the planet or if it arrived on comets and meteorites.
47 Ursae Majoris
This sunlike star, 43 light-years from Earth, is the best candidate to have a solar system like ours. Two giant gas planets, similar to Jupiter and Saturn, orbit the star at some distance, leaving its habitable zone free to develop terrestrial planets like Earth in stable orbits.
16 Cygni
Seventy light-years from Earth, 16 Cygni is a wide double star. So far, only one planet has been discovered in the system, orbiting the dimmer 16 Cygni B. Which brings us to the prime question: Why did a large planet form around one star but not the other?
90377 Sedna
This red spherical body may be our link to the stars. About two-thirds the size of Pluto, it orbits the sun once every 11,500 years in an elliptical path that takes it to the ends of the solar system. Hitching a ride on Sedna could be our ticket out of the solar system.
Pluto
In the next few years, Pluto's barely-there atmosphere will freeze, settling into a crunchy surface frost for at least 200 years. Visiting Pluto and its nearby asteroids would be like inspecting a vault where the origin of the solar system is kept on ice.
Titan
The nitrogen-rich atmosphere on Saturn's largest moon is thick with petrochemical clouds that rain complex organic molecules. While no one has observed hydrocarbon-based life in space, some think Titan might offer the best imitation of a prebiotic Earth.
