By Dave Trapani
2003UB313 The 10th Planet
This new planet is the largest object found in orbit around the sun since the
discovery of Neptune and its moon Triton in 1846. It is larger than Pluto, discovered
in 1930. Like Pluto, the new planet is a member of the Kuiper belt, a swarm of icy
bodies beyond Neptune in orbit around the sun. Until this discovery Pluto was
frequently described as "the largest Kuiper belt object" in addition to being called a
planet. Pluto is now the second largest Kuiper belt object, while this is the largest
currently known.

Where is it?
The new planet is the most distant object ever seen in orbit around the sun, even
more distant than Sedna, the planetoid discovered almost 2 years ago. It is almost
10 billion miles from the sun and more than 3 times more distant than the next
closest planet, Pluto and takes more than twice as long to orbit the sun as Pluto.
At the time of the announcement (late July 2005), the new planet can be seen high
in the morning sky a few hours before the sun comes up in the constellation Cetus.
The planet can be seen using very high-end amateur equipment, but you need to
know where to look. The best way to find precise coordinates (of this planet, or any
other body in the solar system) is with JPL's horizons system. Click on "select
target" and  then enter "2003 UB313" under small (!) bodies.
The orbit of the new planet is even more eccentric than that of Pluto. Pluto moves
from 30 to 50 times the sun-earth distance over its 250 year orbit, while the new
planet moves from 38 to 97 times the sun-earth distance over its 560 year orbit.

How big is it?
Usually when we first discover distant objects in the outer solar system we don't
know for sure how large they are. Why not? Because all we see is a dot of light, like
the top of the page. This dot of light is sunlight reflected off the surface of the planet
(interestingly the sunlight takes almost a day to get out to the planet, reflect off of it,
and get back to the earth!), but we don't know if the object is bright because it is
large or if it is bright because it is highly reflective or both. In the case of the new
planet, however, we know that even if it is extremely reflective (like fresh snow, for
example) it still cannot be as bright as it is unless it is bigger than Pluto.
This image of the newfound planet in our solar
system, called 2003UB313, was taken on Oct. 21,
2003, using the Samuel Oschin Telescope at the
Palomar Observatory near San Diego, Calif. The
planet, circled in white, is seen moving across a field
of stars. The three images were taken about 90
minutes apart.

Scientists did not discover that the object in these
pictures was a planet until Jan. 8, 2005. Image credit:
Samuel Oschin Telescope, Palomar Observatory
Click for larger image
© 2008 David A. Trapani
All Rights Reserved