ered in own solar system was Pluto, first glimpsed by Clyde Tombaugh in 1930. (Or Neptune, discovered in 1846 by Johann Galle, if you're one of those semantic nitpickers who'd rather see Pluto classified as a Really Big Comet, with moon.) But indeed, this new object may be large enough to qualify as a brown dwarf, which is a kind of very dim star. That would be exciting, since it would put our nearest stellar neighbor only half a light-year away, one-tenth the distance of Proxima Centauri, the next closest contender.

Let's talk distance for a minute. The Earth orbits 150 million kilometers from the sun, a distance referred to as one Astronomical Unit, or A.U. At its very farthest, Pluto reaches 39 A.U., or about 5.4 light-hours. This distance also marks the lower edge of the Kuiper Belt, which extends roughly 1,000 A.U. higher still, and is home to all our short-period comets. Way above these, in a region from 10,000 to 50,000 A.U., are the countless billions of long-period comets that form the Oort cloud, and it's here that our mystery object resides. By one estimate, the object is a Jupiter mass (1 MJ) planet circling at 32,000 A.U. By another, it's a 3 MJ brown dwarf orbiting at 25,000 A.U. in the opposite direction. If this uncertainty makes you wonder, you're not alone; the findings will need to be refined, confirmed, cross-checked and reconfirmed in the coming years before astronomers will generally accept them.

Can we take a look?

You may also wonder why no one has ever spotted this beast with a telescope. The answer may be that it's simply too dim and too far away. Even if the object is a brown dwarf, glowing like a hot coal at the bottom of campfire, at a distance of more than two trillion miles it would take a sophisticated infrared telescope and a determined search to pick it up. The Hubble Telescope might manage it, or possibly the Space Infrared Telescope Facility (SIRTF) NASA is hoping to launch in the early aughts. Another possible answer is that the object doesn't exist, and that the disrupted cometary orbits that point to it are caused by something else, such as galactic tides.

Another question you may have, if you've been following astronomy for any period of time, is whether this is the fabled "death star." Back in the 1980s, scientists like Daniel P. Whitmire speculated about Nemesis, a dark star on a highly elliptical, 30-million-year orbit whose swings around the sun would rain comets down on the inner solar system, causing the periodic mass extinctions we see in the Earth's fossil record. Well, there may or may not be a Nemesis out there--the latest evidence is that brown dwarfs are far more common than previously thought--but with an orbit only 4 to 6 million years long, this latest Planet X isn't it.

A final question you may have--actually several questions--is if Planet X is a brown dwarf, could it have warm moons? If so, could they support the liquid water necessary for life? And could we send a probe up there to beam back pictures within a human lifetime, and eventually send humans directly?

The answers are: maybe, probably not, and absolutely.

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Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots, and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog, Asimov's, SF Age and other major markets, and his novel-length works include Aggressor Six, the New York Times Notable Bloom, and upcoming The Collapsium.