As a boy, I recall paging through an old, nine-volume encyclopedia we kept on a livingroom shelf. Published some time in the 1920s, it was hopelessly out of date from a science standpoint, and I remember reading its entry on the planets and feeling, with the smug certainty of youth, that my modern world (this was the late 1950s) had now figured out most of the puzzles still unsolved by the editors. The editors, after all, hadn't known about Pluto, but we did, and Pluto's discovery surely meant that the mapping of the Solar System was complete. Youthful smugness has a way of being brought up short by events, and so do astronomical depictions of things we've seen imperfectly. Today we know that Pluto itself is one of what may be a vast number of 'ice dwarfs,' a kind of planet probably numerous in the Kuiper Belt. Sedna and Quaoar are members of this class, as is, evidently, Neptune's moon Triton and the newly discovered 10th planet, still known officially as 2003 UB313. Indeed,...

Peering through Titan's murky atmosphere with radar, the Cassini orbiter has sent back evidence of what may be a large shoreline on the moon, one where liquids flowed not long ago and may still be present. As shown in the image below, the shoreline divides a bright from a darker, smoother area where liquids seem to be involved. "This is the area where liquid or a wet surface has most likely been present, now or in the recent past, said Steve Wall, radar deputy team leader from NASA's Jet Propulsion Laboratory. "Titan probably has episodic periods of rainfall or massive seepages of liquid from the ground." The evidence mounts up as scientists study bay-like features in the images, and networks of channels that may show where liquid hydrocarbons have flowed. Intriguingly, two types of channel formation seem to be present. The most recent Cassini pass shows long and deep channels indicating flow over large areas, while other images from previous radar passes show a dense network of...

The largest known asteroid, 1 Ceres, is 930 kilometers (580 miles) across, and represents about 25 percent of the asteroid belt's total mass. Recent work using the Hubble Space Telescope's Advanced Camera for Surveys now shows that the asteroid is nearly round, leading to the belief that it has, like terrestrial planets, a differentiated interior: a dense silicate core surrounded by lighter minerals and covered by a crust of carbon-rich compounds. But more extrordinary still is the suggestion that water ice is buried beneath the asteroid's crust. The evidence is tantalizing -- Ceres' density is less than that of Earth's crust, and its surface gives signs of water-bearing minerals. Wrapped in a solid ice mantle surrounding the asteroid's core, Ceres could actually possess more water than all the fresh water found on Earth. Image: NASA's Hubble Space Telescope took these images of the asteroid 1 Ceres over a 2-hour and 20-minute span, the time it takes the Texas-sized object to...

Centauri Dreams has talked often about what may be the main driver for deep space exploration -- the need to protect a vulnerable Earth from catastrophic asteroid or comet strikes. Ongoing work to understand how such impacts fit into the history of the Solar System is revealing much about their nature. Now a team of University of Arizona and Japanese scientists has uncovered the culprits in the massive bombardment of the early Solar System some 3.9 billion years ago: main belt asteroids like those now found between Mars and Jupiter. And here's what's intriguing. The objects that have been cratering various planets since that era of the so-called Late Heavy Bombardment are a different species. Now it is the Near-Earth Asteroids (NEAs) that pose the greatest threat. UA Professor Emeritus Robert Strom and colleagues report their findings in the article, "The Origin of Planetary Impactors in the Inner Solar System," which runs in the September 16 issue of Science. From a University of...

"Santa," "Easterbunny," and "Xena" may be odd names, but they beat the official designations given these objects by the International Astronomical Union -- 2003 EL61, 2005 FY9, and 2003 UB313. All three are Kuiper Belt Objects (KBOs) discovered with the 48-inch Samuel Oschin Telescope at Palomar Observatory. The last of the three is the now famous 10th planet, but the other two KBOs are close to Pluto-size themselves, and like that world, are in elliptical orbits that take them out of the plane of the Solar System. Did we say '10th planet?' Centauri Dreams realizes the designation is controversial, especially at the IAU, but cannot resist the urge to editorialize (if only obliquely) on behalf of a planetary designation for Xena. The rule seems simple: Pluto-size and up means planet. So how do such orbits happen? Mike Brown, a professor of planetary astronomy at Caltech and leader of the discovery team, says these exotic objects may have actually formed in a much warmer environment....