Are astronomers searching for the 10th planet?

Astronomers are always on the look out for new members of the solar system. Not all astronomers, mind you. Just those who are interested in the outer solar system. The most active programs involve searches for new comets, asteroids and satellites of the outer planets, but there is an active program, which has been operating for many years to search for trans-Plutonian objects. Part of the difficulty is in deciding what defines a planet. If Pluto is a planet, how about something two or three times smaller making it just a little bigger than the asteroid Ceres? Three recently discovered trans-Neptunian objects, Varuna and AW197 are about 550 miles across, and Quaoar is 780 miles across making them as much as 1/8 the size of Pluto.

Quaoar is about 42 AU from the Sun, and 2 AU farther out than Pluto, which works out to be about 180 million miles. So far, there are over 600 known objects with sizes near 60 miles in diameter, which have been identified beyond the orbit of Neptune. Most of these have been discovered since detailed searches began in 1992. All of them seem to be very, very dark bodies, probably covered with a crust that is rich in organic compounds.

Astronomers can pretty much rule out an object as big as Jupiter orbiting a little further out than Pluto, since it’s gravitational tugs on Pluto's orbit would be pretty obvious. But a planet as large as Earth at, for instance, twice Pluto's distance from the Sun would be almost undetectable. So, the answer is that we simply don't know if a bonifide 10th planet exists beyond the orbit of Pluto. There is so little sunlight out there that finding them would be a major technological accomplishment especially if its surface is as dark as the objects we are seeing such as Varuna and Quaoar.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011

Return to Ask the Astronomer