Evidence has been found of four planets with masses similar to Earth's floating in space with no star to warm them. Known as "free-floating", or "rogue" planets, we can't see these planets directly, since nothing lights them up, but their presence has been inferred from the gravitational bending of light from more distant stars.
Towards the end of its life, the planet-hunting Kepler Space Telescope looked towards the center of the galaxy, observing an abundance of stars every 30 minutes in the hope of detecting variations in their brightness. Rather than its usual search for the dimming that occurs when a planet passes across the front of a star, it was seeking out microlensing events, which occur when a massive object's gravitational field bends light from something more distant. Microlensing produces a brief increase in brightness as light is temporarily focused.
Most microlensing events strong enough for detection by existing instruments are caused by stars, or at least very large planets. The University of Manchester's Dr Iain McDonald has reported in Monthly Notices of the Royal Astronomical Society candidate events from two months of Kepler data after illusions caused by variable stars and instrumental error were ruled out. Of these, four are small enough to have been made by Earth-mass objects, though the authors consider none to be confirmed yet and have varying confidence between them, thanks to a lack of ground-based observations.
Small microlensing events have been seen before, but these have been just before, or just after, larger ones. Astronomers believe the main events are caused by a star (usually one we already know about) passing in front of a more distant light source, with the smaller peak produced by one of its planets. The paper reports a previously undetected example of one of these microlensing events in the Kepler data.
"[Free-floating microlensing] signals are extremely difficult to find,” McDonald said in a statement. “Our observations pointed an elderly, ailing telescope with blurred vision at one the most densely crowded parts of the sky, where there are already thousands of bright stars that vary in brightness, and thousands of asteroids that skim across our field. From that cacophony, we try to extract tiny, characteristic brightenings caused by planets, and we only have one chance to see a signal before it's gone. It's about as easy as looking for the single blink of a firefly in the middle of a motorway, using only a handheld phone."
Free-floating planets are thought to have initially been part of star systems but have been booted into the darkness of interstellar space by gravitational encounters with larger objects.
Finding a planet orbiting a star is relatively much easier, since we know any microlensing will take place shortly before or after the much bigger event, and therefore know where to look. Despite, this, we have yet to find all that many Earth-like planets within star systems this way. The fact the team found four free-floating worlds over such a short time period, along with one previous suspected discovery, says something about their relative abundance.
"Kepler has achieved what it was never designed to do, in providing further tentative evidence for the existence of a population of Earth-mass, free-floating planets,” said co-author Dr Eamonn Kerins. “Now it passes the baton on to other missions that will be designed to find such signals, signals so elusive that Einstein himself thought that they were unlikely ever to be observed.”
