The Outer Planets
Pluto's status in the solar system has declined in recent years. Much smaller than any of the other planets and not much bigger than the largest of the asteroids, astronomers began to question whether Pluto should be considered a planet at all. As the power of telescopes rapidly increased during the 1990s, more trans-Neptunian objects (TNOs) of a similar size to Pluto were discovered just beyond its orbit in what is known as the Kui-per Belt.
What is the Kuiper Belt?
Named after astrophysicist Gerard Kuiper (rhymes with “viper”), the Kuiper Belt is an area of space on the edge of the solar system near the ecliptic plane. The Kuiper Belt extends some 5 billion miles from the sun, a little more than fifty times the distance between Earth and the sun. More than 800 Kuiper Belt objects have been found.
The Tenth Planet
In 2005, Caltech astronomer Mike Brown discovered the biggest of all of the objects found in the Kuiper Belt so far. The object, initially called 2003 UB313, was named Eris after the Greek goddess of chaos and discord, because of its extremely eccentric orbit. Eris is slightly bigger than Pluto. A NASA press release announced her discovery as the finding of a tenth planet, but this was never formally confirmed.
In reality, Eris presented a further challenge to Pluto's continuing status as a planet. In 2006, the International Astronomical Union controversially reclassified Pluto, Eris, and their largest neighbors as dwarf planets. The new 2006 definition of a planet defined them as bodies that have “cleared their neighborhoods.” Both Eris and Pluto still have significant debris in their orbits. Not all astronomers agree with this and there has been a public campaign to restore Pluto to full planet status.
Since the International Astronomical Union changed the rules about what constitutes a planet in 2006, there are now only officially eight planets in the solar system. However, there also eight TNOs of such size that they have now been designated dwarf planets. These are, in descending order of size, Eris, Pluto, Makemake, Haumea, Sedna, Orcus, Quaoar, and Varuna.
The Discovery of Sedna
Over the last fifteen years, the Kuiper Belt has been quite extensively charted, though it is possible that several more large objects remain to be found, possibly including something as large as Pluto. It was long thought that beyond the Kuiper Belt lay largely empty space. Then, in March 2004, Brown's team at Caltech found a body at least half as large as Pluto much further out. They named it Sedna, after the Inuit sea goddess. The discovery of this object has changed astronomers' theories about the composition of the outer solar system. Sedna's extremely elongated orbit ranges 76–1,000 astronomical units (one AU is equivalent to the distance between the sun and Earth). This 10,500-year orbit is so eccentric that Sedna spends only a small fraction of its orbital period near the sun, where it can be easily observed.
Brown believes this area will now be a rich hunting ground for new planetary bodies. He noted in his 2007 Lowell lecture, “Sedna is about three-quarters the size of Pluto. If there are sixty objects three-quarters the size of Pluto, then there are probably forty objects the size of Pluto. . . . If there are forty objects the size of Pluto, then there are probably ten that are twice the size of Pluto. There are probably three or four that are three times the size of Pluto, and the biggest of these objects . . . is probably the size of Mars or the size of the Earth.” Discoveries in the outer solar system in the near future may completely rewrite the map of our local neighborhood. According to Brown, there may be fifty or more objects that once would have been considered big enough to be classified as planets.
Could there be another Earth-sized planet yet to be found in our solar system?
Astronomers have not excluded the possibility that an Earth-like planet could be located further out than 100 AU with an eccentric and inclined orbit. Computer simulations have suggested that a body roughly the size of Earth was ejected outward by Neptune early in the solar system's formation and may currently be in an elongated orbit between eighty and 170 AU.