Pluto - Planet 9 again?


Should Pluto be allowed back in the planet club? ~New Atlas


Pluto is one among the many icy Trans-Neptunian Objects(TNO) in the Kuiper belt. Discovered by Clyde Tombaugh in 1930, Pluto was considered a planet for more than 75 years. In 2006, the International Astronomical Union (IAU), the body in charge of naming planets, updated its criterion for which all astronomical bodies constitute a planet. According to the IAU, 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 neighbourhood around its orbit." Following this, Pluto was demoted to the status of a dwarf planet as it failed to meet the third condition. The controversial decision to drop the ninth planet from the solar system was, and is, much debated.


A new study led by planetary scientists Philip Metzger and Alan Stern analysing not only the qualities that uniquely and unambiguously allow for a celestial body to be named a planet, but also more than two centuries worth of astronomical literature, claim that Pluto is a planet, and should never have been demoted in the first place. Their contention is that the criterion that doomed Pluto's fate as a planet, namely, that it has to "clear its orbit" is ill-defined and is a "sloppy definition".

Metzger, a scientist at ther the University of Central Florida and a key figure behind the New Horizons mission which had a Pluto fly-by in 2015, lead the new study. It found that in over two centuries of astronomical literature, only in one instance has "clearing its orbit" been a criterion for classification as a planet. Further, the paper in question was published in 1802, and was based on long since disproven schools of thought. If taken literally, the condition that an object should "clear its orbit" to become a planet would disqualify all current planets too as all of their orbits are frequently overlapped with those of asteroids and other such bodies. Many major Moons of the existing 8 planets would also satisfy the condition and would have to be designated as planets by the IAU definition.

Metzger states that Pluto is the second most interesting active planet in the solar system after Earth. Even organic molecules have been detected on the surface of its moons, and quite possibly hides an icy ocean beneath its surface. According to Metzger, the criteria to differentiate asteroids and planets should be whether the body's gravity is strong enough to pull it into a roughly spherical shape. This is a watershed moment in the evolution of a planet as major geological activity starts once the planet has achieved its shape. By revoking the third (and arbitrary) condition, the nine-planet solar system would be back, with Pluto regaining its place as the Ninth Planet.


Pluto, at one third the size of Earth's Moon, is the largest TNO in the known solar system, but, at one-sixth the mass of the Moon, is not the most massive. The ninth most massive object in the discovered Solar System is another TNO called Eris(UB313) which is 27% more massive than Pluto. It was the discovery of Eris in January 2005 that precipitated the events leading to the eventual removal of Pluto's status as a planet. Pluto, Eris, and a host of other objects in the Kuiper belt are now considered as dwarf-planets.

Pluto is a sore thumb if considered as a planet along with the other 8 planets of the solar system. Its orbit is out of axis with the rest of the nine, thus making it occasionally closer the sun than Neptune. Pluto's orbit is also affected by Neptune's gravity, creating a resonance that makes it complete its trajectory around the sun exactly twice in the same time Neptune completes its orbit thrice. Pluto's biggest "moon" Charon has a diameter more than half of Pluto, and the duo is often considered as a binary system rather than as a planet-moon system as the point around which they orbit each other lies outside either body.

Planets being demoted on basis of fresh evidence is not new -  Ceres, Pallas, Juno and Vesta are asteroids in the asteroid belt, but were initially, up to the early 19th century, considered as planets before the asteroid belt was discovered. Similarly, the Kuiper belt and various TNOs in it were discovered post-1992, and they all were a call to action to remove Pluto's status as a planet.  Pluto's designation as a planet was a historical mistake made due to lack of information, but with the facts before us, it was the duty of the posterity that was us to correct that error. That correction happened in 2006. Any move to overrule it would be a regressive step backwards which would serve no purpose other than to create needless controversy.

Athul Krishna A is an ardent fan of everything DC, everything fantasy and everything sci-fi. You can read short stories written by him at YourStoryClub. Currently pursing a B.Tech degree from College of Engineering Trivandrum. He can be reached at

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