The Sun may be collecting rogue planets from as far as 3.8 light-years away, nearly the distance to our closest stellar neighbour, and some of them could already be orbiting out there right now.
Rogue planets are worlds that wander through interstellar space without a star, kicked out of their birth systems by gravitational encounters. Recent estimates suggest there could be trillions of them drifting through the Milky Way. When ‘Oumuamua passed through our solar system in 2017, followed by interstellar comet 2I/Borisov in 2019, it became clear that objects from outside our system regularly pass through. But some of them may not leave.
A study describes how rogue planets and other interstellar objects can become permanently captured by the Sun. The key lies in two gravitational openings in the Sun’s Hill sphere, the region where its gravity dominates over the galaxy’s pull. These openings sit about 3.81 light-years from the Sun, one pointing toward the galactic centre and one away from it. That is remarkably close to the Alpha Centauri system at 4.37 light-years.
When a rogue planet drifts through one of these openings, it enters a state the researchers call “permanent weak capture.” It begins orbiting the Sun very slowly at enormous distances, moving chaotically within the Hill sphere in a fractal-like pattern for roughly 100 million years before gradually spiralling inward over billions of years. Crucially, the object never collides with the Sun and never escapes back into interstellar space.
The Sun could capture objects up to the mass of Jupiter this way. Anything larger would begin to disrupt the gravitational balance of the solar system. If a Jupiter-mass rogue planet ever drifted into the inner solar system, it would dramatically shift planetary orbits, potentially ejecting existing planets and altering conditions for life on Earth. But even if captured rogues remain at extreme distances, their gravitational influence could subtly perturb the orbits of known objects, much like the proposed Planet Nine.
RESEARCH PAPER
Belbruno & Green, “Permanent Capture into the Solar System”, Celestial Mechanics and Dynamical Astronomy (2024)