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The definition of a planet February 28, 2006

Posted by dr. gonzo in Space.

Plutonian System.Nine planets, twenty planets, how many planets are there in our Solar System anyway?

This is a question that the astronomical community has been discussing with zeal since the discovery of 2003UB313 (temporarily designated Xena, though this won’t be its official name). Xena is about 3000 km in diameter according to observations, which makes its about 25 percent larger than Pluto, the last discovered object to be designated a planet.

The International Astronomical Union, the organization charged with naming and classifying celestial objects, never really set down criteria for new planets in our Solar System. Because they, like most of the astronomical community, assumed there were no other planets in our star system.

The definition of a planet, astronomically speaking, will be set by the IAU at their General Assembly in August 14-25, 2006, in Prague. They say they will announce and publish a definition of a planet at the beginning of September.

Xena is a part of a broader area of the outer solar system called the Kupier Belt. This collection of mostly small, icy bodies orbits the Sun well past the orbit of Pluto and in Xena’s case have a year equal to 557 Earth years.

This debate really isn’t new. When the first asteroids were discovered they remained classified as planets until a host of smaller objects in the same region began to be discovered. Objects like Ceres and Vesta are now solidly not planets in the annals of astronomy.

IAU action thus far

The IAU hasn’t said a lot on this debate so far, they are unlikely to until their General Assembly in August.

In 2001, however, the IAU began to approach this issue in response to the growing number of bodies being discovered orbiting extrasolar stars.

They developed a preliminary definition which they called “evolving.” It was revised in 2003.

The thing about this definition is that it walks us around in circles.

From the IAU’s definition (emphasis added):

“Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are “planets” (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

Guess what? There is no criteria on minimum size requirements for planets in our Solar System.

What the IAU has done, it would appear, (according to persistent rumors throughout the scientific community) is to have narrowly passed a preliminary measure at the end of 2005 in favor of the “Pluto and anything larger” definition for planet. Then they wanted a recommendation from the American Astronomical Society, the AAS wanted a committee to look at it and the committee formed a special committee. Now the rumor is that during the General Assembly they will decide on how to make a final, binding decision.

What will the IAU do?

The IAU will not change Pluto’s status as a planet, despite the rash of Kupier Belt Object Discoveries in recent years.

What is likely to happen instead is that the IAU will ascribe a planet classification system. They talk of it a bit on the link above when the mention Trans-Neptunian Planets, which would include objects like Pluto, UB313, and Sedna.

Other likely categories include Terrestrial planets (Mercury, Venus, Earth and Mars), Jovian Planets (Jupiter, Saturn, Uranus, Neptune) and then we will likely see additional categories to encompass extrasolar planets. Categories such as the Hot Jovian or Hot Neptunian. Other proposals include a cis-Jovian planets designation for objects closer to the Sun than Jupiter.

Of course, depending on your viewpoint this may all be a moot debate.

“Planet” doesn’t belong to scientists

The co-discoverer of Xena, Michael Brown, made a valid point on his Web site when he proposed that the term planet is more a cultural term than it is a scientific one.

He compared it to the term continent, which has a scientific and cultural meaning. In science a continent is divided by plate boundaries but humans have attached their own definition of continent, as India is considered part of the “continent” of Asia.

It is a valid point too. It’s likely that 70+ years of nine planets being implanted into brains has made it near impossible that any other object will be considered a planet unless it is comparable to the gas giants.

So even if Xena is declared a planet it is unlikely to gain widespread cultural acceptance as such.

Regardless, this won’t diminish the scientific value of discoveries such as Xena.


Since I am, again, on the topic of astronomy here is an update on the Near Earth Object 2004VD17.

The half kilometer wide object still sits at a Torino Scale maximum of 2. However, since the other day the chance of impact increased slightly to 1 in 1560.

The headline grabbing asteroid Apophis has had no change in its status.

Related Posts:
The battle over science and money
Asteroid reaches two on Torino Scale
Close encounters of the asteroid kind
Apophis collision more likely

Photos: Photo 1: The Pluto system, note the three moons. (Credit: NASA)


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