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 鈥減lanet鈥� has been a sore point since 2006, when, after a hurried argument, the (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.
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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鈥檚 dwarf-planet status have misgivings about the definition. Its criteria are vague, and it refers only to our solar system 鈥� so it excludes the thousands of worlds detected around other stars since the definition was written.
鈥淚 want a classification that applies both to the solar system and to exoplanets,鈥� says 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. 鈥淚 wanted it to be rigorous, and easy to implement, so we don鈥檛 have to wait for interstellar travel to get high resolution images.鈥�
He has worked out how massive a body must be to conform to the IAU鈥檚 orbit-clearing criterion. Above this critical mass, a planet鈥檚 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鈥檚 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 杏吧原创 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鈥檚 a planet too.
鈥淏ut we should be careful here,鈥� he adds. 鈥淭he IAU has not defined the term 鈥榮atellite鈥�. When they do, that will affect what they might decide about double planets versus satellites.鈥� 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鈥檚 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鈥檛 yet good enough to tell. And conveniently, the proposal makes the iffy matter of 鈥渞oundness鈥� redundant 鈥� anything above orbit-clearing mass is so big that its gravity must pull it into a round shape.
鈥淥f course it鈥檚 just a proposal,鈥� says Margot. 鈥淚 don鈥檛 know whether it will stick, whether people will love it, hate it or be indifferent.鈥�
His suggestion certainly won鈥檛 satisfy those who think the IAU was wrong to require orbit clearance in the first place. 鈥淚鈥檓 sympathetic with what he鈥檚 trying to do,鈥� says of the Massachusetts Institute of Technology. 鈥淏ut to me, it鈥檚 about the body itself, not its location.鈥� Binzel prefers an earlier suggestion that roundness be the main criterion 鈥� which would planetise many more objects.
Reference: ArXiv,
Image credit: NASA
