Rogue worlds two ways
How does a planet find itself drifting alone through space? There are two main ways that we know of.
Some rogue worlds start off the same way the planets in our Solar System did, coalescing from the swirling disk of gas and dust that surrounds a young star. Then, through gravitational interactions with other large bodies, a planet can be flung out of its star’s orbit, left to wander interstellar space. Astronomers think this tends to happen more to Earth-sized planets, but can also happen to giant ones.
Other rogue worlds are thought to form in much the same way as stars: When material in a cloud of dust and gas becomes dense enough, gravity causes it to collapse into a single object. If the resulting object is massive enough, it begins to fuse hydrogen into helium in its core, becoming a star. If it’s not quite massive enough to fuse hydrogen like a star, it’s considered a brown dwarf, or sometimes a rogue planet — the distinction is fuzzy.
Some scientists say that it comes down to how the object formed: If it started out in orbit around a star and then got ejected, it’s a rogue planet, but if it formed through cloud collapse, it’s a brown dwarf. Even if two free-floating objects are identical, their formation could determine their categorization. But this definition isn’t universally accepted in the astronomy community.
Finding a needle in a great cosmic void
The confusion and disagreement around what counts as a rogue planet could potentially be resolved by studying more of them. But finding them can be tricky.
The usual techniques we use for spotting exoplanets don’t always translate well to free-floating worlds. The transit photometry method, for example, looks for the dip in a star’s brightness caused when an orbiting exoplanet passes in front of it, from our perspective. This method only works when the planet orbits a star, though — not the case with rogue planets.
Other techniques have proved more useful for finding free-floating worlds. Large sky surveys using infrared light were the first to detect very faint, cool objects drifting alone in space. Rogue planets that are just a few million years old (babies, in cosmic terms) can also glow strongly from the heat created by their formation, emitting dim light that can be directly detectable by sensitive cameras on large telescopes.
One technique for spotting smaller, more unambiguous rogue planets is gravitational microlensing.
This effect was actually predicted by Einstein’s General Theory of Relativity long before it was observed. Einstein recognized that mass warps the fabric of spacetime. (You don’t need to get your head around why — just know that a large object can noticeably bend light as it passes through space.) When a massive object aligns between us and a more distant light source like a star or galaxy, the effect is that the background light appears to brighten. By looking at dense stellar fields and watching for brief moments of increased brightness, astronomers have been able to find possibly hundreds of rogue planets — and some candidates are as small as Earth, or even smaller.