Rogue planets roam our galaxy.

We are used to thinking of planetary systems as something orderly and stable, like our own solar system. However, from time to time, astronomers receive indirect evidence that there are a great many “rogue planets” in the Universe, not tied to a star and wandering around the expanses of their galaxies. As a result of a recent study, it turned out that there are actually many more such planets than even the most daring assumptions. Even within the Milky Way alone, there can be billions of such objects!
The census of planets is an incredibly complex process when it comes to a distant system. Modern telescopes allow you to look into deep space and explore it only thanks to light sources. But planets do not emit their own light, unless we are talking about a brown dwarf (which some scientists refuse to recognize as a planet at all). So researchers have to resort to various tricks.
When a planet orbits a star, as it should, there are two ways to detect it. The first is the radial velocity method, where you can calculate a small, but still noticeable effect of the planet’s gravity on the star. The second is the transit method, when the planet passes between the star and the telescope during its rotation, temporarily obscuring the emitted light to some extent.
As you can see, both methods imply that the planet has a star. Accordingly, when it comes to wandering planets, they are completely useless. However, the arsenal of astronomers is not limited to this, and there are still a number of tricks that allow you to find the desired targets. So, two rogue planets were spotted last year due to their gravity bending the light of more distant objects behind them; others were found by IR imaging.
A total of 20 rogue planets have been identified, which is quite a small number compared to the 3,917 known exoplanets of the “normal” type. However, astronomers from Leiden University in the Netherlands decided to test this data and performed complex mathematical simulations over the Orion Trapezoid-a cluster of young stars in the heart of the Orion nebula.
As a result, 500 Sun-like stars were modeled, each with 4-5-6 planets. A total of 2,522 planets participated in the simulation, ranging in mass from three times the mass of Earth to 130 times the mass of Jupiter. Scientists have calculated that for 11 million years, since the formation, 357 planets (16.5% of the total number) should have lost contact with their star systems and go into free drift in the galaxy. Some remained within the cluster, five planets were even captured by other systems, but 282 objects still disappeared in the depths of space. Interestingly, of all the planets that participated in the simulation, 75 crashed into their own star, and 34 rammed other planets.
It turned out that the mass of the planet does not really affect whether it becomes an outcast. Probably, there are many small planets in space, which will not be easy to detect even with the most modern equipment. By extrapolating 16.5% to the entire Milky Way (because the Orion Trapezoid is, by and large, a fairly typical cluster of stars), the scientists got colossal numbers. In total, no less than 100 billion planets should wander through the expanses of our galaxy alone!
Probably, even in the Solar System, there was once another planet that then left it. It is believed that the strange axial tilt of Uranus is just a consequence of the collision with the wandering planet. However, the researchers note that the work is based on only two simulations, so the results should not be considered final. What we can be sure of is that rogue planets are much more common in space than we could have imagined.