Sedna? Quaoar? Planet X?

I got a question via email regarding a previous post I made on planet-like bodies beyond Pluto. Luke asks, "What's the deal on these discoveries of Sedna and Quaoar? Is it just Planet X repackaged?" In short, the answer is no. Sedna and Quaoar are quite real. For those who don't know, Planet X was first proposed back in the mid-19th century to explain the perturbations in the motion of Uranus -- then Neptune. Percival Lowell, an American astronomer, named it Planet X, and conducted extensive searches for it in the early 20th century -- to no avail -- although his images did record a faint image of Pluto, which wasn't recognized until Pluto's discovery in 1930. Some astronomers still hold out hope for a large planet, possibly several times larger than Earth, with a highly elliptical orbit that has taken it out to the farthest reaches of our solar system. This planet they suggest, can explain the appearance of long period comets that we see. Sedna, Quaoar and their ilk are far from Planet X however. They're more like Pluto -- not a planet. Their mass and orbit suggest that they may be the norm for the environment beyond Neptune. What this all goes to show us is, just when we, the general public, are getting comfortable with our own solar system, we realize that we really know just a little about it. It's huge, and possibly quite stranger than we thought it was. For more on this topic, check out the following:

The Size and Distribution of Trans-Neptunian Bodies -- Trilling, Allen, Brown, Holman and Malhotra [PDF]

The Planet X Saga -- from Bad Astronomy