New research suggests that there might be a ninth planet in the Solar System. Probably a twin of Earth buried in the depths of the icy debris of the Kuiper Belt beyond Neptune. These speculations originate from a difficulty. Astronomers struggle to explain the properties of “transneptunian” objects (TNOs). These are small icy bodies orbiting the Sun beyond Neptune’s orbit. Their orbits have distances greater than 30 astronomical units (AU).

Strange Transneptunians

TNOs represent the remaining fragments of the formation of the Solar System. These objects were cast out to the outer edges during the chaotic early years of planetary creation. Extremely difficult to discover and map due to their small size, distance, and faintness, known TNOs have strange properties that defy logical explanations.

Firstly, about 10% of all TNOs are “detached” having no orbital relationship with Neptune. Despite Neptune being the smallest of the gas giants, its gravitational force dominates in the outer Solar System. It is strange that so many TNOs do not have motion influenced by the eighth planet. Current models of Solar System formation predict far fewer detached TNOs.

Secondly, there is a significant population of TNOs with very high orbital inclinations. They orbit the Sun with angles greater than 45 degrees. Once again, models of Solar System formation struggle to place so many objects on such extremely tilted orbits.

Next, there are some TNOs with very large orbits known as “extreme” ones. The most famous example is Sedna, a minor planet with a Perihelion of about 76 AU and an Aphelion of over 900 AU. Orbits like these require gravitational pushes not explained by the gas giants of the Solar System.

The Ninth Planet of the Solar System?

A coherent theory of the outer Solar System must be able to explain all these discordant observations. A pair of researchers, reporting their results in an article in The Astronomical Journal, have found an intriguing answer (ref.). In the past, the presence of an extremely distant massive planet hundreds of AUs away was proposed.

However, these hypotheses were mainly based on the behavior of a handful of TNOs. The problem is that the presence of such a massive planet, rivaling Neptune, further complicates the ideas and possible answers. In the new study, researchers conducted hundreds of simulations of Solar System formation.

The primordial system likely produced 10 to 100 planets with at least Earth’s mass beyond Neptune’s orbit. Most of these planets were completely ejected from the Solar System due to the gravitational influences of the remaining planets.

Hunting for the Mysterious Planet

The researchers found that a single Earth-sized planet, with a weight ranging from 1.5 to three times Earth’s mass, could hide in the outer Solar System. With an orbit ranging from 250 to 500 AU, it would be firmly within the Kuiper Belt.

This hypothetical large planet, if placed on an orbit with an inclination of 45 degrees, could explain the known collective behavior of TNOs. However, there is no evidence of the existence of this Earth-sized planet or other newly proposed planets in the Kuiper Belt. So far, all we know is the strange orbital dance of TNOs. Nevertheless, this is an intriguing hypothesis and one of the best ways to explain all available evidence.

The next step for researchers is to develop more detailed models so they can begin to predict where this Earth-sized planet might be in its orbit and devise plans for an observation campaign to hunt it down.

Stefano Gallotta
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