Rogue planets

Most planets we know circle a star. Rogue planets do not. They drift through the Milky Way on their own, with no sun to light or warm them, and almost nothing to show they are there. Some may outnumber the stars themselves.

Planets without a sun

There are two ways to become a rogue planet. One is to be thrown out. Young planetary systems are violent places, and a close encounter between planets, or with a passing star, can fling a planet out into interstellar space; smaller planets are thrown out most easily. The other is to form alone. A small cloud of gas can collapse into an object of planetary mass instead of a star. In 2025 astronomers using ESO telescopes watched one such young object, Cha 1107-7626, swallowing gas at about six billion tonnes a second, growing the way young stars do.

Finding something that gives no light

A cold, dark planet in interstellar space cannot be seen directly. It can be detected by gravitational microlensing: when it passes almost exactly in front of a more distant star, its gravity bends that star’s light and makes it briefly brighter. For a planet the brightening lasts from a few hours to a couple of days. The shortest event found so far lasted just 41.5 minutes, a sign of an object between the mass of Mars and Earth.

The counts have changed as the surveys improved. In 2011 a study suggested there were almost two Jupiter-mass rogue planets for every star, but a larger survey in 2017 found far fewer giants. A 2023 analysis came out at about 20 rogue planets per star, most of them small, roughly Earth’s size or less. Across the Milky Way that would mean billions to trillions.

Young rogue planets can be seen directly, because they still glow with the heat of their formation. PSO J318.5−22, about 80 light-years away, has about six and a half times Jupiter’s mass. In 2021 a team found between 70 and 170 in the young Upper Scorpius region. In 2023 a Webb survey of the Orion Nebula reported 540 candidates, including dozens of pairs of Jupiter-mass objects orbiting each other, but follow-up studies in 2024 and 2025 found that most of those pairs are distant background objects.

Warm, in the dark?

A rogue planet might not be frozen all the way down. In 1999 David Stevenson suggested that an Earth-sized planet thrown out with a thick hydrogen atmosphere could trap enough of its inner heat to keep liquid water on its surface. In 2011 Dorian Abbot and Eric Switzer proposed a planet a few times Earth’s mass with an ocean under a thick shell of ice, warmed from inside. Both are ideas; no such planet has been found.

Roman

NASA’s Nancy Grace Roman Space Telescope, launched on 30 August 2026, will watch a crowded field of stars towards the centre of the Milky Way again and again, looking for exactly these brief flickers. It is expected to find hundreds of rogue planets, many of them down to the mass of Earth and below, and to settle how common they really are.