What Really Is A Planet? New Definition Solves Dilemma

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The official definition of a planet remains controversial. The demotion of Pluto to dwarf planet status in 2006 caused an uproar when the International Astronomical Union (IAU) changed their official definition, saying a planet must be a spherical body in orbit around a star that had cleared its orbit of other debris, the latter being something Pluto apparently has not done.

But while this is all well and good for the Solar System, where bodies are close enough to study their orbits, it is essentially useless for exoplanets, most of which are too far away to see if they have cleared their orbit. So one scientist, Jean-Luc Margot from the University of California, Los Angeles, has devised a formula that can work this out just by knowing a body's mass, its orbital period, and the mass of the star it orbits – readily available data for most exoplanets.

“One should not need a teleportation device to decide whether a newly discovered object is a planet,” Margot said in a statement. His paper is due to appear in the Astronomical Journal, but a preprint is available on Arxiv. The formula can be seen in section three. Margot presented his research this week at the 47th American Astronomical Society’s annual meeting of the Division for Planetary Sciences in Maryland.

Plugging in the orbital and mass data to the formula, it can be worked out if a body will have cleared its orbit in a specific time frame – such as the lifetime of its star. Only a body that has cleared all debris within up to five times its gravitational sphere of influence, known as its "feeding zone" or Hill radius, is considered a planet by Margot.

“In the spirit of the IAU resolution, we suggest that a body that is capable of clearing its orbit within a well-defined time interval is a planet,” Margot wrote in his paper.

For our own Solar System this creates a clear boundary between the eight major planets and the dwarf planets, while Margot says the method of classification also works for 99 percent of all known exoplanets. He notes that a planet must also be below 13 Jupiter masses in size, as this is the limiting mass for thermonuclear fusion of deuterium, when a body begins to become a star. 

Shown is the formula applied to our Solar System, clearly defining the planets. Margot 2015.

“The disparity between planets and non-planets is striking,” Margot added in the statement. “The sharp distinction suggests that there is a fundamental difference in how these bodies formed, and the mere act of classifying them reveals something profound about nature.”

Thus, his new definition is as follows: A body is a planet when it is in orbit around one or more stars, it dominates its orbit as per the formula, and has a mass below 13 Jupiters. There's no need to require an object to be spherical, like the IAU definition, because bodies that can clear their orbits will almost certainly be round.

There’s no word yet on whether the IAU will consider using this new classification, although IFLScience has asked them to comment on it. Considering the next IAU general assembly isn’t to be held until 2018 though, when things like this are typically decided, don’t go holding your breath any time soon. It's an interesting idea, at least.


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