Planet Nine - also called Planet X - is a massive putative object in an elliptical orbit far beyond Pluto, about such a distance that it would take 10,000 to 20,000 Earth years to complete one revolution around the Sun.
Although there has never been a direct observation of an object that fits the description of Planet Nine, unexpected patterns in the orbits of other, smaller objects in the icy outer solar system can be explained by the gravitational pull of such a body.
In early 2016, Caltech astronomers Mike Brown (known for his role in lowering Pluto to planet 9 in 2005) and Konstantin Batygin announced that the orbit of the dwarf planet 2012 VP113 surprisingly coincides with the orbits of five other extreme trans-Neptunian objects.
According to Brown, the likelihood that such a congestion will occur without some nudging factor is about 1 in 14,000.
Two years later, trans-Neptunian object 2015 BP519 was also calculated to be moving along an unusual trajectory that could be affected by a planet of similar mass.
To date, the best evidence for the existence of an undiscovered planet in the farthest reaches of our solar system remains this cluster, as well as the strange tilt of their orbits and the orbit of 90377 Sedna, which cannot be explained by the presence of Neptune.
What might Planet Nine look like?
If we assume that this planet-like object really exists, then the mass of the Ninth planet will most likely be an order of magnitude (about 10 times) greater than the mass of the Earth, and its girth approaches the size of one of the cosmic ice giants.
According to the likely scenario explaining the distant location of this planet, its origin lies somewhere between the orbits of Jupiter and Neptune, and the birth was the same as that of other gas planets in our solar system.
Despite its large and slow orbit, enough time should have passed since the inception of the solar system for it to have time to clear its orbit of frozen pieces of dust and stones, which would allow this ball of gas to receive the official title of the planet.
Alternatively, Planet Nine could be an object ripped from another star a long time ago.
How can we confirm the existence of planet Nine?
If Planet Nine exists, then its extreme remoteness - about 10 times farther than Neptune - limits the amount of sunlight reflected from its surface. It will be more difficult to notice it, the further away it is. Not only because of its faintness, but also because of the relative decrease in orbital velocity, which makes it difficult to determine the difference in position.
There are telescopes capable of observing an object of this size, but with such a large sky to cover, researchers will first have to cut back on areas of space where the existence of Planet Nine is unlikely.
The article was published by the scientific journal ScienceAlert, today is August 3, 2021.