Feature

Space is suspected to be filled with worlds wandering alone without a star, but could such loners ever shelter life?

Reported by Giles Sparrow

Rogue planet

© NASA

In the past few decades, we’ve grown used to the idea of exoplanets – distant worlds orbiting stars in alien solar systems. Some I of these planets are very different from our own – scorched gas giants skimming the surfaces of their stars or frozen balls of ice larger than Earth. But perhaps the strangest exoplanets are those that drift alone through the darkness of interstellar space, far from the heat and light of any star. Only a handful of these objects are known, but many billions of them could be scattered across our galaxy – estimates vary from at least a couple for each of the Milky Way’s 200-billionplus stars, up to an astonishing 100,000.

Astronomers call these mysterious worlds interstellar or ‘rogue’ planets, but in order to understand them, we first need to find them and learn more about their characteristics – and that’s a challenge in itself. The vast majority of exoplanets discovered so far have been detected through their influence on their parent stars – either the tug of their gravity on the star’s path through space or slight dips in the star’s brightness as a planet transits in front of its star as seen from Earth. But for interstellar planets, neither of these techniques work. The other obvious route of observing them through their own light and other radiation is also caught in a catch-22 situation: while the light of exoplanets is often drowned by that of the stars they orbit, most are only visible at all thanks to reflected starlight. Earth-like interstellar planets – relatively tiny balls of rock floating far from any stars – are so small, cold and faint as to be almost impossible to detect. The only exception might be if they wander by chance in front of a more distant star and create a microlensing event, where the starlight is bent.

Fortunately, there are occasional lone planets, still hot from the gravitational collapse in which they formed, that give out light of their own, shining dimly but distinctly if viewed through powerful telescopes and emitting radiation that can reveal important clues to their other properties. One intriguing example, catalogued as PSO J318.5-22, was discovered in 2013 by a team including Dr Niall Deacon, now at the Max Planck Institute for Astronomy. Deacon specialises in studying wide-orbit planets. But it was during a search for brown dwarfs that he and his colleagues stumbled across the object he calls PSO J318.

“We were doing a large survey with the Pan-STARRS [Panoramic Survey Telescope and Rapid Response System] observatory on Maui, Hawaii. We looked for very red objects that drift across the sky a bit – lots of distant galaxies are red, but if an object is moving quickly across the sky, it must be nearby. We detected a bunch of candidates and did follow-up measurements with the United Kingdom InfraRed Telescope on Mauna Kea, and Mike Liu noticed that this object was really, really red. We followed it up with a spectrum from NASA’s Infrared Telescope Facility and found that its light was very similar to that of the planets around HR 8799. Based on its colours and spectrum, we knew that it might be a free-floating planet.”