Science

Another Planet in our Solar System?

Dom Smith explores research on the ninth planet

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Image Credit: NASA's Marshall Space Flight Centre

Scientists believe a ninth planet almost certainly exists at the very depths of our Solar System. A celestial body currently nicknamed ‘Planet Nine’ is believed to lie beyond the Kuiper belt — a layer of asteroid-type ice rocks that starts near the orbit of Neptune and continues outwards past the orbit of dwarf planet Pluto, and into the furthest reaches of the Solar System.

It is after this layer that Planet Nine is expected to sit, if indeed it exists at all. Planet Nine’s existence could provide explanations for a series of unlikely clusters of objects beyond Neptune. As Neptune is the final known planet in the Solar System, objects that lie beyond it (but which are still in our Solar System) are known as trans-Neptunian objects (TNOs). Objects which lie much further even than these — and which are deemed to be of considerable size — are known as extreme TNOs (or, ETNOs). Planet Nine could explain the otherwise highly unlikely clustering of some nearby ETNOs.

One of the researchers working closest on this project is Michael E. Brown, professor of planetary astronomy at Caltech. Brown was the man responsible for demoting Pluto from a planet to a dwarf planet, by virtue of its size. Brown and Caltech colleague Konstantin Batygin have suggested that Planet Nine may have formed from the core of a previous planet which collided with the Solar System’s biggest, Jupiter, in the very early stages of the Solar System’s formation — the nebula hypothesis.

A number of fear mongering articles have been written on this topic, rumouring that Planet Nine might send asteroids and comets flying towards Earth that could spark the end of life on our planet. There is no evidence for this. The only way in which Planet Nine has been interacting with other objects is by influencing small icy bodies much, much closer to Planet Nine itself. It is false to say that Earth would be in danger if indeed Planet Nine exists.

Quite apart from any such fake news, Planet Nine is an exciting proposition for scientists. It is estimated as having a radius of between 2–4 times the size of Earth’s, and a mass of 4–8 times the Earth’s. That would make Planet Nine one of the largest planets in the Solar System, and make its history — whether aligned with Batygin’s and Brown’s hypothesis for its formation or not — a thrilling and eventful one.

Predictably, one of the most contentious topics surrounding Planet Nine is its name. Only once there are images showing the planet does exist will it be given a formal name. Hence, its placeholder remains ‘Planet Nine’ at the moment. While Greek and Roman mythology tend to be used for Planetary names, there is not much of a precedent for the naming of planets. Only two planets in the Solar System have been discovered during recorded history: the two most distant, Uranus and Neptune.

However, some are not even happy with the use of ‘Planet Nine’ in the meantime. Between Clyde Tombaugh’s discovery of Pluto in 1930 and its removal of planet status in 2006, it was the ninth planet from The Sun. This led to American planetary scientist Alan Stern commenting in 2018 that repurposing the name ‘Planet Nine’ is “an effort to erase Clyde Tombaugh’s legacy, and it’s frankly insulting.”



Some have suggested that the name ‘Persephone’ could be used for Planet Nine if its existence is ever confirmed. The name is often used as a name for fictional planets further out from the sun than Neptune. This is due to the fact that in Greek mythology, the deity Pluto’s wife is named Persephone. However, it is unlikely that ‘Persephone’ will ever become Planet Nine’s official name, as there already exists an asteroid called ‘399 Persephone,’ which was discovered in Heidelberg, Germany by Max Wolf in 1895.

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