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How to define a planet

The International Astronomical UnionI(IAU) is expected to propose wording to delineate planets from other small, round objects at its 12-day General Assembly meeting in Prague this August.

Depending on the outcome of a separate controversial procedural issue — whether IAU members should be allowed to vote on such things — astronomers might then have the chance to weigh in on the definition later in the same meeting.

If approved, the definition would then be announced in September.

The issue surfaced in the late 1990s, when astronomers began discovering Pluto-like objects in the distant reaches of our solar system.

All the newfound worlds—there are several known now—were until recently smaller than Pluto, but they are round and orbit the Sun, two characteristics that had for centuries been sufficient for the implicit definition of ‘planet.’ The hitch: These small objects are typically on wild, elongated orbits that stretch well above and below the main plane of the solar system where eight of the traditional planets travel. Pluto has a wild orbit, too, which is one reason many astronomers do not consider it a planet anymore.

So what to call them? Astronomers have been arguing about it in earnest since 1999.

The controversy came to a head with the July 2005 announcement of 2003 UB313, an object roughly the size of Pluto that orbits the Sun beyond Neptune. The object’s discoverer, Mike Brown of Caltech, has argued it should be called a planet.

But other astronomers say that if planethood is bestowed upon 2003 UB313, then several similar way-out bodies should gain the same status, and the number of planets in our solar system could ultimately climb into the thousands as search technology improves.

posted by: kyawoo at 21:31 | link | comments |
planets

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