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鈥檚 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 鈥� and there are probably a lot of them. They could , according to a 2023 analysis by at NASA鈥檚 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. 鈥淗onestly, I was not surprised to find that rogue planets may outnumber stars,鈥� says at Queen Mary University of London.
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Which isn鈥檛 to say astronomers aren鈥檛 awestruck by the prospect. 鈥淚t鈥檚 beautiful to imagine,鈥� says Lisa Kaltenegger at Cornell University in Ithaca, New York. 鈥淏illions of planets that have no home any more, that are just basically travelling through the galaxy.鈥�
We can鈥檛 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 distant stars and galaxies, known as gravitational microlensing. From this, it appears as if most rogue planets are around the size of Earth.
Earth-sized rogue planets
Those that are larger could start life in a similar way to a star, with clouds of gas gathering under gravity and eventually collapsing. But this can鈥檛 account for Earth-sized rogue planets, says Coleman 鈥� those must have formed within a solar system and then been ejected. An outside star might have swept by, tugging on the planet as it went, but this is only likely in star-dense regions of the galaxy, like globular clusters. Another option is that a tussle with another planet in the system flung one closer to the star and one away, out into the wilds.
The most likely explanation for rogue planets in the Milky Way is that they formed around pairs of stars orbiting each other, called binary systems. 鈥淚n binary systems, it is very easy for planets to be placed on orbits that eventually lead to their ejection,鈥� says Coleman.
As for the conditions on rogue planets, we can expect the same rich diversity we see in star-bound planets 鈥� from smaller rocky worlds to massive gas giants like Jupiter and icy exoplanets bigger than Neptune. 鈥淚 imagine that rogue planets will be very similar to a lot of these,鈥� says Coleman. Except for the fact that, with no host star for light, the only source of warmth will have to come from within. As such, it is likely that rogue planets will have frozen surfaces.
One source of energy could come from thermal vents powered by the contraction of the entire planet as it cools down, the mechanism that leads to cryovolcanism on Pluto. But there are ways such a rogue planet could be warmer than this. at the California Institute of Technology, Pasadena, has argued that if they had large amounts of hydrogen in their atmospheres, which is a greenhouse gas at high pressures, rogue planets could even have . 鈥淪uch a planet could be kept warm by the radioactive decays of elements deep in its interior,鈥� says Bennett.
Simulations have even suggested that some rogue planets might have what it takes to be habitable, either in liquid oceans beneath their icy outer crusts or on the surface if they can sustain a thick hydrogen atmosphere to trap enough heat to sustain life.
