New Planet Discovered Beyond Pluto; Another Has a Moon

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Posted on July 30, 2005 in Solar System

A 10th planet, the biggest since Pluto was found 75 years ago, has been discovered.  Late Friday, a JPL press release announced the find made in January by Dr. Mike Brown of Caltech in research partly funded by NASA.  The planet, temporarily designated 2003 UB313 until a name is approved, is three times farther than Pluto and is estimated to be 1.5 times as big, though its actual size is uncertain.  Brown is confident it is bigger than Pluto.  The object is currently 97 times farther from the sun than Earth.  It has a highly-inclined orbit in a region known as the Kuiper Belt.  See also the,, BBC and Planetary Society reports.
    Another smaller Kuiper Belt object (KBO) announced the same day, though smaller than Pluto, has a moon.  Designated 2003 EL61, it was actually discovered in 2003 but took awhile for its orbit to be determined.  The discovery of both these objects will probably revive the debate over the definition of a planet.  There seems to be a continuum of sizes of KBOs out there; it is even possible larger ones than the new one remain to be discovered.  Alan Boss is not sure tiny Pluto and its kin deserve to be in the same class as Jupiter and Saturn.

Since so many Kuiper Belt objects have been discovered in the last couple of decades, this is not quite the big news it might have been; still, new planets have historically been considered spectacular discoveries.  Uranus launched William Herschel to fame in the late 18th century, and Neptune led to a well-known priority dispute between Adams and Leverrier in the the 19th.  Clyde Tombaugh’s discovery of Pluto in 1930 was a monumental task of searching through photographic plates.  Now, we know there are many other rocky bodies out there beyond Neptune and far past Pluto.  Having a 10th planet will certainly change the textbooks and might turn out to be one of those historical announcements you will tell your grandchildren about.  Just hope they don’t name it Darwin or Huxley or something.  (After Pluto, is another Disney character in the lineup?)
    The intriguing thing about the other object is, why does it still have a moon?  One would think that, after four billion years, a tiny object 1% the size of its parent, in orbit around a small object having low gravity to begin with, would have been swept away long ago.  The shortage of objects to form it by collisions and the unlikelihood of its being captured contribute to the puzzle.  Yet it is one of several (including Pluto) to have a satellite, according to  We’ll have to scrutinize their explanations for this unexpected pairing (see also 10/05/2003 and 05/14/2003 entries).

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