Rather than have a Solar System with 20 planets, it was decided to redefine "planets" and the sub-category of "dwarf planet' was created to account for this new growing number of smaller objects in the outer reached of the Suns influence.
This week Pluto has regained its status as king of the dwarfs. Eris has turned out to be smaller than Pluto after all.
November 5, a chance alignment provided new data. As Eris crawled along its orbit, some 14 billion km from Earth, it passed in front of a distant star from Earth's vantage point, casting a small shadow across our planet, an event known as an occultation. By timing the duration of the occultation at multiple sites, researchers can estimate the size of the shadow and hence the size of an object.
Combining results from several observatories able to observe the occultation yields a diameter that is "almost certainly" less than the 2340km of Pluto. However, this opens the door on another mystery.
Eris's mass, determined from the orbit of its moon Dysnomia, is about 25% greater than that of Pluto. The newly calculated diameter doesn't change that fact. So if these results hold true, the density of Eris is even higher than we thought. Its albedo (reflectivity) is greater than estimated before too, since it is reflecting light to us from a now reduced surface area. Not long ago this would have thrown serious doubt on the results, but earlier this year astronomers found that the large Kuiper Belt object 50000 Quaoar is essentially a dense rock.
Obviously, these interesting new members of the solar system have plenty of secrets to reveal, and as we find more of them Pluto's status may again change in relation to them, but for today anyway, Pluto is again king of the Kuiper Belt.
Hail, Pluto. Long live the King.