There could be planets far out in our solar system that come from other stars. Theoretical astrophysicist Amir Siraj from Princeton University has calculated how many strange and undiscovered planets could exist there.
Hypothetically, there could be up to about five alien worlds in the outer solar system. At the edge of our solar system is a disk-shaped field of small, icy rocks that extends beyond Neptune’s orbit: the Kuiper Belt. The dwarf planets Pluto, Eris, Haumea and other celestial bodies such as Ultima Thule are also located there. But this place is so far from the sun that we can hardly see what exactly is going on there.
Even further out is the hypothetical Oort cloud. This is supposed to be a huge spherical field made of small rocks that surround the entire solar system like a bubble. The exact size is unknown. Experts assume that the long-period comets of the solar system come from here.
Siraj calculated mathematically that there could be 5.2 worlds with a mass comparable to Mercury. But there could also be 2.7 planets with the mass of Mars or 1.2 planets with a mass greater than that of the red planet. These are all purely hypothetical considerations – whether there are actually alien worlds from other stars out there is unclear.
Rogue planets in the universe
The idea first came up in 2000 with free-floating planets (also called rogue planets). These are planets that have separated from their star and been thrown out of their home system. These roam the galaxy unbound and can be captured by the gravitational forces of other planetary systems.
Experts believe that gravitational interactions can generate the necessary instability quite easily. Accordingly, there could be a relatively large number of free-floating planets.
For his study, Siraj began estimating the number of rogue planets in the Milky Way. He also estimates the percentage of stars that are likely to capture rogue planets. According to the theoretical astrophysicist, the probability that a rogue planet will be captured by the Sun’s gravity is relatively high.
“We have shown, based on a simple theoretical argument, that there are likely trapped terrestrial planets in the outer solar system”, says his paper. Such a rogue planet on the edge of our solar system should be discovered with the Vera C. Rubin Observatory. This is currently being built in Chile and is expected to begin scientific operation in 2025.
To the study
The paper was published on December 18, 2023 in the journal The Astrophysical Journal Letters: Are There Terrestrial Planets Lurking in the Outer Solar System? (Are there terrestrial planets in the outer solar system?).