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Guess what? A compelling new mathematical definition makes our humble moon a planet

A hot air ballon floats past a rising moon over Rancho Santa Fe, California.
Reuters/ Mike Blake
Our neighboring planet?
  • Olivia Goldhill
By Olivia Goldhill

Science reporter

Published This article is more than 2 years old.

Deciding what makes a big, round thing a “planet” is surprisingly difficult—just ask Pluto.

UCLA professor Jean-Luc Margot has proposed a new, mathematical definition that would apply to bodies both inside and outside our solar system. By his calculations, detailed in a paper that has been accepted for publication in the Astronomical Journal, our moon could qualify as a planet, as New Scientist reports.

The planet definition debate has been particularly controversial since 2006, when the International Astronomical Union came up with a set of criteria that stripped Pluto of its planet status.

After several bodies were found to be as large as or bigger than Pluto, there were initially plans to increase the number of solar system planets to 12. Ultimately, the IAU voted to define a planet as a nearly round object orbiting the sun and—unfortunately for Pluto—that is large enough to have cleared other objects in its orbit.

Speaking at a meeting of the Division of Planetary Sciences in Maryland this week, Margot said he has calculated the critical mass necessary to meet the IAU’s orbit-clearing criteria. Margot’s formula can be used to identify the many newly discovered planets outside our solar system. It would also make it easier to classify far-off planets, and to do so with a telescope, rather than an interstellar voyage.

“I wanted it to be rigorous, and easy to implement, so we don’t have to wait for interstellar travel to get high resolution images,” he added.

Margot also told New Scientist that he has calculated that Earth’s moon is above the critical mass. According to his formula, that makes the moon a planet in its own right.

And there’s more bad news for Pluto fans: Margot’s mathematical definition still classifies Pluto as a dwarf planet, not a full-grown one.

“Of course it’s just a proposal. I don’t know whether it will stick, whether people will love it, hate it or be indifferent,” said Margot. Judging by the debate over Pluto, astronomers are unlikely to come up with an unequivocal answer any time soon.

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