In our solar system, objects are close enough that we can check the orbit and see if has been cleared of other objects. This is problematic if we can only infer rough properties of what could be a planet in another solar system. We cannot image the prospective exoplanet but we can see its gravitational effect. If we cannot see the planet then we also cannot know if it is fully round.
A new proposal is to use a lower and upper bound for the mass. The lower bound is below the mass of Mercury but above the mass of Pluto. The upper bound is 13 times more than the mass of Jupiter. As objects get over 6 times the mass of Jupiter then they can start fusion processes and become brown dwarfs.
There have been other planet definition proposals that involve using geological activity. However, observing geological activity in other solar systems would require telescopes a million times better than the ones that our levels where we can barely possible detect exoplanets.
In 2006, the International Astronomical Union (IAU) adopted resolution B5, which contains the following definition:
A planet is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighborhood around its orbit.
The current IAU definition of “planet” is problematic both because it is not quantitative and because it excludes exoplanets. In a previous paper, one of us proposed a possible solution to remedy both problems (Margot 2015). In 2018, IAU Commission F2 “Exoplanets and the Solar System” promulgated the following working definition for exoplanets:
Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4 / L5 instability(M/Mcentral < 2/(25 + √621) ≃ 1/25) are “planets”, no matter how they formed. The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System, which is a mass sufficient both for self-gravity to overcome rigid body forces and for clearing the neighborhood around the object’s orbit.
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This complicated definition will never stick in the public mind. Further, it’s a cheat designed to assign more celestial bodies “planethood” than currently deserve it.
And the claim that we can’t identify geography with today’s instruments is clearly wrong too, since gas giants, waterworlds and other planet compositions have already been identified by the 100s. It’s not all of the 1000s of bodies identified, but that’s how things are now; it’s more incentive to build better telescopes and other instruments and methods.
A planet, in the public mind, will always be:
– In orbit of its star – the sun or whatever is a star in another solar system
– roughly round. A gas giant can bulge in the middle. It can have a large “pimple” volcano like Mars, but we know a planet when we see one.
– Not a star, white/brown dwarf. No nuclear fusion allowed!
– an object that more-or-less clears its orbit. Pluto fits that loose definition. Ceres does not. The objects in the Oort Cloud don’t either. BTW, why does Neptune get a pass on not clearing the orbit of Pluto, while Pluto is penalized for the same result? Because of size? How arbitrary is that? This new definition does nothing to change that.
– Optional: edge cases could be determined by whether a body has moons. If its center of gravity is closer to one body than the other, like Pluto’s is than Charon’s, it’s Pluto that gets to be called a planet, not Charon. Mercury and Venus don’t have a Moon, so that’s why this is optional, but any body that meets the other criteria, but is maybe not a total lane clearer like Pluto, gets a bonus vote for having a moon.
There are other things: rogue planets without a star, dwarf planets like Ceres, asteroids, etc.
Science, like people, loves to classify things, but nature doesn’t care about our silly needs and is free to fling around and form all sorts of celestial bodies, many wondrous and worth exploring, however we can.
The public should get a vote. Maybe then it would support science more too.
That’s really the only reason this is even an issue: “Planet” isn’t a natural category, so there’s no objective definition. They didn’t have objective reasons for demoting Pluto, it was just done to be contrarian.
Planet is just a word right?
Does it matter much what the exact definition is? We just use words to communicate with each other. The real scientific context is encompassed by a bunch of measurable parameters that apply to all celestial bodies. These may or may not be available at a specific time and they also change dynamically.
I would just stop trying to describe all that with a single word and focus on something that matters more.
After the “Pluto incident”, the opinions of people with an itch to redefine something for everyone else have zero weight. Same as star names. Bright stars have silly historical names such as “goat” (Almaaz is “male goat”, Capella is “female goat”), also catalogue numbers (several, each survey makes a new one, which later becomes a useful catalogue reference), also constellation names (Epsilon Aurigae and Alpha Aurigae). So according to some dead people, the sky is full of goats and other clearly irrelevant things, but that is their view of the universe that merely carries on for no reason. Surveys assign codes to stars as an index for catalogue records with star parameters. Also anyone can pay some creative people to name an invisible star the way they like – no one cares. The same thing those people do with planets, and they can be ignored the same way as the goat people are ignored by star surveys, and paid-for names are ignored by everyone. They do not contribute anything to science, only attract attention to themselves and make a few dollars along the way (“How I Killed Pluto and Why It Had It Coming” book sells for $37.25 on Amazon).
Good step in the right direction.
The definition should be concrete and based on measurables.
A minimum mass and a roughly spherical shape due to hydrostatic equilibrium are the bare minimum. Orbiting a star might be another. Rogue planets ought to be another category.
If co-rotating with other similar bodies in the same stellar orbital path, same mass and spherical shape criteria apply, besides of having its rotation barycenter roughly inside the planetary body. Therefore an Earth-like body around a warm Jupiter would be a Moon because the barycenter it’s inside the Jovian.
If all the bodies rotating around a barycenter inside of none of them satisfy the mass requirements, all are planets!
Just come up with another name thats not a planet or a moon. we need another catagory
Neighborhood clearing seems fairly ambiguous about what constitutes “clearing”, as well as excluding a lot of potential cases. Like, if “counter-Earth” actually existed, suddenly neither Earth nor counter-Earth qualify as planets? In the case of Robert Forward’s Rocheworld, neither body is a planet? Clement’s Mesklin isn’t a planet, either?
It’s too solar system centric in terms of that planet clearing criterion.
The lower threshold of being massive enough that gravity dominates the shape is fine. Actual presence of fusion in a body should rule out it being considered a planet, but I see no good reason to exclude particularly massive rocky bodies just because they’d have fusion if they were made of deuterium.
I’d ditch the orbit clearing criterion, and call an object a “planet” so long as it is large enough to be round, doesn’t sustain fusion, and the barycenter between it and any other body otherwise qualifying as a planet that it orbits isn’t within that other body. This allows for twin planets, and planets occupying each others’ L5 positions. Jupiter continues to qualify as a planet because the Sun isn’t a planet. Pluto and Charon qualify as twin planets.
Couldn’t a moon fit that criteria.
I use Star Trek criteria in defining planets. If we can beam down and create a 47 minute episode (minus commercials) around that location, it’s a planet.
So that Dyson sphere was a “planet”?
A one layer Matrioska planet with the star as its core.
They never beamed down to the Dyson sphere, just the crashed ship. So no.
They rescued Scotty, who was in a transporter diagnostic test loop for 70 or so years. So correct, they never stepped foot on the Dyson sphere.