The Additional Planets

Mike Brown and his colleagues, Chad Trujillo of Gemini Observatory and David Rabinowitz of Yale University, have shaken the world of astronomy right down to that solar system poster, finding orbital objects as far as 10 billion miles from the sun in a ring of debris called the Kuiper belt. They nicknamed those icy bodies Quaoar, Sedna, Santa, Easter Bunny, and Xena.

Xena, recently confirmed by a team of German scientists as being at least 30 percent larger than Pluto. Xena prompted an ongoing debate in the International Astronomical Union as to whether it should be formally recognized as the 10th planet, or whether Pluto should be demoted and the number of planets reduced to eight.

Quaoar is about half the size of Pluto. Everybody was really excited and wanted to hear about it. This was June of 2002.

Sedna was completely unexpected. It's 8 billion miles from the sun—Pluto is 3.6 billion—and in 2004 we had no idea that things in that very outer region of the solar system existed...Sedna shouldn't be there. There's no way to put Sedna where it is. It never comes close enough to be affected by the sun, but it never goes far enough away from the sun to be affected by other stars, which is the case with comets that have been observed in the Kuiper belt. But it's in a very elliptical orbit, and there's no way to form anything in an elliptical orbit like that...Sedna is incredibly far away, and we never would have seen it if it weren't as close as it gets on its orbit. In fact, there's about a 200-year period when we can see it, and it has a 12,000-year orbit. So what does that mean? If we see it for 200 years out of 12,000, that means there's only a 1 in 60 chance that we could've seen it, which means to me that there may be 60 of these things out there. And if there are 60 of these things, then there are probably 20 of these things just a little bigger and maybe a couple the size of Mercury or Mars.

Xena is the most distant object ever seen in orbit around the sun—10 billion miles away. And it turned out to be 1,800 miles in diameter, about 400 more than Pluto.

The true outer limit is about 5 trillion miles beyond Pluto, where the sun looks like nothing more than a bright star and temperatures hover just a few degrees above absolute zero. Astronomers believe this region, called the Oort cloud, contains a vast collection of icy debris left over from material that came together to form the sun, Earth, and the other major planets 4.6 billion years ago.
Nothing in the Oort cloud is visible directly through telescopes, but astronomers infer its existence because it occasionally spits out objects that plunge toward the sun, where they sprout long, vaporous tails and become comets...everything in the Oort cloud added together might not outweigh Earth...The inner Oort cloud, which stretches from a few tens of billions of miles to a few hundreds of billions of miles from the sun, is much more stable than the edge of the region. If any objects orbit there—nobody knows—they would stay in place indefinitely. Brian Marsden of the Harvard-Smithsonian Center for Astrophysics speculates that there could be full-fledged Earth-size planets in this zone, circling unseen in the dark...Today's best telescopes can penetrate only to the nearest part of the solar system's outer regions, known as the Kuiper belt.
"The Man Who Finds Planets," By Cal Fussman, Discover Magazine, May 2006, Vol. 27, No 5