The outer solar system resembles the wild reaches of nature. It is so far from the Sun that our telescopes can hardly penetrate the secrets of its far corners. One can only guess what mysterious objects are hidden in this planetary space.

We know that beyond Neptune’s orbit lies a field of small icy rocks known as the Kuiper Belt. This belt is home to dwarf planets such as Pluto, Eris and Haumea. Further, even further, is the hypothetical Oort Cloud – a huge spherical cloud of small rocks, covering the entire solar system. Its true size remains a mystery.

It is believed that this is where the long-period comets of the Solar System are formed, but what else could these distant expanses hide?

One exciting possibility is the existence of planets—not just planets, but planets from other star systems.

Now a theoretical astrophysicist Amir Siraj from Princeton University calculated how many alien worlds may be hidden from our view. According to his estimates, on a mathematical level, there could be 1.2 planets in our Galaxy with a mass greater than that of Mars, 2.7 with a mass comparable to Mars, and 5.2 with a mass approximately equal to the mass of Mercury. These results were published in The Astrophysical Journal Letters.

These are just educated guesses based on other assumptions, but the idea of ​​hidden worlds coming from distant stars is incredibly exciting.

It all started with the discovery of so-called free-roaming planets, or rogue planets, first recorded back in 2000. These are planets that have become detached from their stars and have been expelled from their home systems to wander the Galaxy. It is speculated that gravitational influences could easily create the necessary instability, causing these planets to wander in space.

How common this phenomenon is remains unclear, but we are becoming more and more adept at detecting these solitary individuals, improving the accuracy of population estimates.

Even more exciting is the idea that these planets may not necessarily remain free. If they pass too close to a star, their gravity may be captured. This phenomenon, although to a lesser extent, has been observed in our solar system: Jupiter seems to be a real gravitational tycoon, attracting space rocks to itself.

Siraj wondered what the likelihood was of something like this happening to the Sun. He began by estimating the number of free-roaming planets in the Milky Way and the percentage of stars that could capture those planets. Using this data, he calculated the probability that such planets would pass close enough to the solar system to be captured by the sun’s gravity.

His calculations showed that there was a fairly high probability that somewhere between the masses of Mercury and Earth there was a planet wandering alone in the cold and darkness.

“Based on direct theoretical assumptions, we show that terrestrial planets captured by rogues likely exist in the outer solar system,” he writes. “Future studies should include more detailed modeling of the capture and retention of free-roaming planets, as well as planets associated with other stars. In addition, modeling may shed light on the probability distribution for orbital parameters and the location of captured stars on the celestial sphere.”

Future work should also explore other methods for observing captured planets.

He adds that if someone happens to be in a lucky enough position in the sky, we might be able to detect them with the help of the Vera K. Rubin Observatory, which is currently under construction in Chile and is expected to begin its scientific work in 2025.