Pluto loses planet status
24 August 2006
Our Solar System seems a little bit smaller today, after the world's astronomers agreed a new definition for planets. Their decision means that Pluto no longer makes the grade - so old textbooks could be heading for the recycling bin.
Antenna finds out more...
Our Solar System now has just eight planets.
Space experts at the International Astronomical Union (IAU) meeting in Prague have voted to settle the score on the difference between planets, and smaller 'solar system bodies' such as asteroids and the many small objects beyond Neptune.
Pluto's status has been fiercely debated in recent years. Thanks to powerful new telescopes we've discovered a whole crop of planet-like bodies. Discoveries like 2003 UB313 and 90377 - dubbed Xena and Sedna by their finders - are similar in size to Pluto. So astronomers had to decide whether to elevate these to planet status, or to demote Pluto.
An artist's impression of 2003 UB313.
Image: NASA, ESA and A Schaller (for STScI)
Last week, an IAU working group suggested a new scheme that elevated the total number of planets to 12. Pluto would have remained a planet, and been joined by Ceres, Charon and 2003 UB313. But this proposal has been rejected by astronomers.
The suggested scheme was controversial because more and more Pluto-like objects are being detected with powerful telescopes. We could have ended up with dozens of new planets over the next few years.
Expanding the list of planets didn't go down well with some astronomers. This artist's impression shows what our Solar System might have looked like. When drawn to scale like this, some of the planets are so small you can't even see them. (Relative distances not correct)
Image: International Astronomical Union
'It's a pity for Pluto fans, but it's a sensible decision, because a lot of astronomers would argue that Pluto should never have been considered a planet in the first place. Clyde Tombaugh's discovery in 1930 was a bit of an accident. At the time astronomers thought there might be another planet beyond Neptune.
'So Tombaugh was scanning the skies and Pluto just happened to be in the right place at the right time. If his telescope had been powerful enough to judge how small Pluto is, he would have realised that it was in a different class from the other planets.'
Alison Boyle, Curator of Astronomy, Science Museum
Alison Boyle, Curator of Astronomy, Science Museum
Alison Boyle, Curator of Astronomy, Science Museum
Image: Terry McCormick / Science Museum
Under the new guidelines, Pluto will be considered a 'dwarf planet', along with 2003 UB313 and Ceres, which was formerly considered an asteroid. But Pluto is still a little bit special - it will be the prototype for the new objects being discovered beyond Neptune.