The compelling case for giving Pluto back its planetary status
Friday, July 18, 2014, 12:24 PM - For over 75 years, tiny Pluto enjoyed its status as the most distant planet in our solar system, but in 2006, it was demoted down to a 'dwarf planet' and its title was passed on to Neptune. Now, though, the editor of Astronomy magazine is sounding the rallying cry to re-open the debate about Pluto's nature, which could potentially redefine what it means to be a planet.
1) The object must be in orbit around the Sun,
2) The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium, and
3) It must have cleared the neighborhood around its orbit.
Everything in the solar system technically orbits around the Sun, of course. Something like the Moon doesn't qualify, though, even though it's massive enough to be roughly spherical and its 'neighborhood' is as clear as Earth's is, because it only goes around the Sun as a consequence of being in orbit around Earth. Same goes for the moons of the other planets. Asteroids and comets don't qualify because they're not big enough to become spherical by their own gravity. Even Ceres (which is roughly spherical) doesn't make the cut, because it's in the asteroid belt, thus its 'neighborhood' isn't clear.
Pluto suffers the same problem as Ceres. It's definitely in orbit around the Sun (or at least the common gravitational focus it shares with Charon is in orbit around the Sun). It is massive enough to be a sphere. It just isn't considered to have cleared its neighborhood. So, not a planet, at least by the IAU rules.
However, while the first two rules are pretty clear and easy to determine, third isn't. According to Prof. Abel Méndez, of the University of Puerto Rico at Arecibo, "there is no standard 'cleared' metric." It seems that due to the very existence of the Kuiper Belt, Pluto loses its status. However, exactly how cleared does the neighborhood have to be? There are millions of near-Earth asteroids flying around us, and there are even some asteroids that are locked into the same orbit as Earth ('Earth trojans'). There are even more asteroids near Mars' orbit, due to its proximity to the asteroid belt. Jupiter has an extremely large collection of asteroids in its orbit, both preceding it (the Greeks) and following behind (the Trojans).
Even discounting these cases, as it is, when you go further out into the solar system, it gets harder and harder for an object to clear its neighborhood. This is simply because it makes fewer orbits around the Sun compared to objects closer to the Sun, and thus it encounters the other objects in its orbit far less often. Consider Earth, going around the Sun once every year, with Pluto orbiting every 247 years. So, whereas Earth has made roughly 4.5 billion trips around the Sun since it formed, Pluto has only made 18 million similar trips (if it formed at roughly the same time).
As Astronomy magazine editor David Eicher said: "At the Pluto-like distance of 40 astronomical units — 40 times farther away from the Sun then we are now — Earth would not clear its orbit of asteroids, and so would Earth then not be classified as a planet?"
Also, since recent evidence has pointed to the fact that there may be two super-Earth-sized objects out beyond Pluto, both of them would be considered 'dwarf planets' as well, despite one potentially being 10 times the mass of Earth and the other being up to 100 times the mass of Earth.
So, when it comes to Pluto, what's the case for making it a planet again? Based on the facts above and Eicher's own thoughts:
1) the definition of what 'cleared the neighborhood around its orbit' is, itself, unclear
2) it seems unjustifiable that an object even larger than the Earth would not be considered a planet, simply because it orbits far out in our solar system
3) an object's intrinsic characteristics should dictate what kind of object it is, not its location.
Indeed, if you take the IAU's definition and attempt to apply it to all objects we know about, the multitude of worlds that we've discovered outside our solar system aren't technically planets, despite being large enough and even if they've cleared their orbit, because they don't orbit around the Sun.
So, perhaps it's time to revise the IAU's definition, not only to reconsider Pluto for planetary status, but also to make the definition applicable to a wider range of objects. Even if they changed the first rule to have 'a star' instead of 'the Sun' and changed the emphasis of the third rule to be that the object is large enough compared to the rest of the objects in its orbit to be capable of clearing its neighborhood (given enough time), it might be a much better set of conditions to measure everything against.
As Astronomy's editors offer up their time and efforts to host a renewed debate about Pluto, what do you think about its status? Should it be a planet again, remain as a dwarf planet, or perhaps something else? Leave your ideas in the comments below.
(Image credit: Scott Kenyon and Ben Bromley, Harvard-Smithsonian Center for Astrophysics)