Why are there so many rogue planets and what do they look like?

Imagine a world where it is always night, no matter the time of day or year. There are no days or years, in fact, because there is no sun, meaning no cycle of daylight to mark time’s passing. And if there are moons, they are barely visible. For this is a lonely world, drifting through interstellar space.

Rogue planets, as they are known, do exist – and there are probably a lot of them. They could outnumber stars by up to 20 times, according to a 2023 analysis by David Bennett at NASA’s Goddard Space Flight Center in Maryland and his colleagues, which would mean there are possibly trillions of them in our galaxy alone.

That might sound like an outlandishly large number, given that we tend to think of planets orbiting stars. But the existence of free-floating planets is perfectly compatible with planetary formation theory. “Honestly, I was not surprised to find that rogue planets may outnumber stars,” says Gavin Coleman at Queen Mary University of London.

Which isn’t to say astronomers aren’t awestruck by the prospect. “It’s beautiful to imagine,” says Lisa Kaltenegger at Cornell University in Ithaca, New York. “Billions of planets that have no home any more, that are just basically travelling through the galaxy.”

We can’t see rogue planets directly. Since the first candidate was discovered in 2012, we have been inferring their presence by the way they bend the light coming from more…