When is the moon not a moon? If a new proposal for defining planethood is adopted, the moon could be considered a planet in its own right.

The meaning of the word “planet” has been a sore point since 2006, when, after a hurried argument, the International Astronomical Union (IAU) adopted a definition demanding that a planet be nearly round and be massive enough to have accreted or flung away other objects in its orbital neighbourhood.

Famously, this excludes Pluto, recently revealed by the New Horizons mission to be a stunningly complex world.


Even astronomers who are quite happy with Pluto’s dwarf-planet status have misgivings about the definition. Its criteria are vague, and it refers only to our solar system – so it excludes the thousands of worlds detected around other stars since the definition was written.

“I want a classification that applies both to the solar system and to exoplanets,” says Jean-Luc Margot at the University of California, Los Angeles.

Margot has now proposed a mathematical definition, which he outlined at a meeting of the Division of Planetary Sciences in National Harbor, Maryland, this week. “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 has worked out how massive a body must be to conform to the IAU’s orbit-clearing criterion. Above this critical mass, a planet’s gravity should be powerful enough to sling away or pull in any smaller bodies within a precisely-defined territory called the feeding zone. Given the mass of the star and the size of the planet’s orbit, you can work out the critical mass using a fairly simple formula.

Double trouble

That formula yields one peculiar consequence. Margot defines a pair of orbiting objects that are both above the critical mass as a double planet. So, New Scientist asked him at the meeting, what about Earth and the moon? With a quick glance at a graph, Margot confirmed that the moon is above the critical mass. So by his proposed definition, it’s a planet too.

“But we should be careful here,” he adds. “The IAU has not defined the term ‘satellite’. When they do, that will affect what they might decide about double planets versus satellites.” The next opportunity for the IAU to reopen the case would be their general assembly in Vienna in 2018.

As for the rest of the solar system, Margot’s criterion leaves a gulf between planets and dwarf planets. Pluto would retain its dwarf status, because it still has so much company in the Kuiper belt. The least planet-like planet, Mars, has more than 50 times the orbit-clearing mass; whereas the most dominant dwarf, Ceres, has only a few per cent of the mass required. It also means that all known exoplanets are indeed planets, except in the few cases where measurements aren’t yet good enough to tell. And conveniently, the proposal makes the iffy matter of “roundness” redundant – anything above orbit-clearing mass is so big that its gravity must pull it into a round shape.

“Of course it’s just a proposal,” says Margot. “I don’t know whether it will stick, whether people will love it, hate it or be indifferent.”

His suggestion certainly won’t satisfy those who think the IAU was wrong to require orbit clearance in the first place. “I’m sympathetic with what he’s trying to do,” says Richard Binzel of the Massachusetts Institute of Technology. “But to me, it’s about the body itself, not its location.” Binzel prefers an earlier suggestion that roundness be the main criterion – which would planetise many more objects.

Reference: ArXiv, arxiv.org/abs/1507.06300

Image credit: NASA