A group of astronomers want to change the definition of a planet. Their new proposed definition wouldn’t bring Pluto back into the planetary fold, but it could reclassify thousands of celestial bodies across the universe. From a report:
The International Astronomical Union’s (IAU) current definition of a planet, established in 2006, includes only celestial bodies that are nearly round, are gravitationally dominant and orbit our Sun. This Sun-centric definition excludes all of the bodies we’ve discovered outside our solar system, even if they may fit all other parameters. They are instead considered exoplanets. Those behind the new proposal critiqued the IAU’s definition in an upcoming paper in the Planetary Science Journal, arguing it’s vague, not quantitative and unnecessarily exclusionary.
Their new proposal would instead classify planets based on their mass, considering a planet to be any celestial body that:
- orbits one or more stars, brown dwarfs or stellar remnants and,
- is more massive than 10ÂÂ kilograms (kg) and,
- is less massive than 13 Jupiter masses (2.5 X 10^28Âkg).
Better definition: big enough to be round, too small to fuse hydrogen.
Wouldn’t that get rid of moons?
Planets orbit stars
Moons orbit planetsOne day we’re gonna find something orbiting a moon and then you’re gonna feel really silly.
A subsatellite, also known as a submoon or a moonmoon, is a “moon of a moon” or a hypothetical natural satellite that orbits the moon of a planet.
While that may be possible, I highly doubt such a system can be stable? Maybe it can be stable for a while though (which could still be thousands of years).
What about Pluto and Charon? Their barycenter is outside Pluto.
I propose binary (and more generally n-ary) planets.
And Ceres, you want it to be a planet?
Yes, it’s already a planet. Just like dwarf stars are already stars, dwarf planets are already planets. It’s right there in the name.
Ok, then, Pluto is also a (dwarf) planet, what is the big fuss?
PS: sorry, I thought you were the
OPdude at the top of this thread, we are in agreement, then.
Since I think that the “definition game” is mostly a waste of time, I’m all for more objective measures, like mass. It’s still useful to distinguish between planets in our system vs. elsewhere, so perhaps keep planet vs. exoplanet? The later is still shorter than “planet not orbiting Sol”.
Definitions aren’t a waste of time when they’re based on meaningful distinctions and natural classifications. As Plato once said, “you should cleave nature at the joints.”
In the case of the IAU definition of “planet”, they picked a very good joint to cleave the population of objects at. For sound physically-based dynamic reasons you tend to get objects that are either really good at clearing their orbital neighborhood of other objects or not at all good at doing that, you never expect to see objects that are somewhere in a fuzzy middle ground.
The Wikipedia article on clearing the neighbourhood has a table of values for the planets and also for some of the prominent dwarf planets, and there’s a very clear multiple-orders-of-magnitude gap between the two populations under all of the various mechanisms by which neighbourhood-clearing can be measured or calculated.
Frankly, this is a way better approach than an arbitrary “at least this many kilograms” cutoff. With a cutoff like that you can easily get objects that straddle the line and are impossible to classify. It’s not based on any meaningful dynamic orbital properties of the object. I don’t like this proposal for exoplanets, they should use the same one that we use for solar system planets.
