Astronomers have discovered four faraway Earth-mass exoplanets that appear to be floating in space all by themselves without a parent star to orbit.
The strange bodies were unearthed by a team of astroboffins led by the University of Manchester in the UK after digging through data collected by NASA’s now-decommissioned Kepler space telescope. It's the first time Earth-sized planets have been spotted using Kepler's little-known microlensing capability.
Planetary systems exist in all sorts of configurations. Some may contain a lone star, like our Sun, or twin binary stars, or even three sets of them. These fiery balls of gas can be surrounded by all types of exoplanets, such as hot Jupiters or super-Earths. No matter how much they differ by, however, they pretty much always contain at least one host star in the mix.
But one group of planets with similar masses to Earth previously spotted by Kepler don’t seem to be gravitationally bound to any stars at all. Instead, it’s a “free-floating planet population,” according to results published in a paper in the Monthly Notices of the Royal Astronomical Society.
These objects were either ejected from their original orbits around a star or they could have been birthed from a protoplanetary disk that a star never grew out of, Eamonn Kerins, a senior astronomy lecturer at the University of Manchester and co-author of the study, explained to El Reg.
“Theories suggest that in the early phases of planet formation planets can migrate, typically moving from orbits further out to closer in," he said. "This could result in close encounters between larger and smaller planets and the tug of the larger planet would typically kick the smaller one out of the system. But it's also possible that planet sized objects may just form by themselves from the same gas that forms stars. So maybe they never orbited a star.”
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Star-less planets are rare; only a few dozen candidates have been spotted so far, Kerins said. The lack of discoveries are down to the techniques space telescopes typically use to find exoplanets, and there are probably more free-floating planet populations out there.
Kepler, for example, was designed to hunt for alien worlds using the transit method, which involves monitoring the characteristic dip in brightness of a star when a planet crosses in front of the sun during its orbit.
Since the majority of exoplanets are found using the transit method, it’s no wonder that they’re mostly found along with their parent stars. The most recent four free-floating candidates, however, were detected using microlensing.
“With microlensing we don't need to be able to see the planet at all," Kerins told us. "We're looking instead for its gravitational influence on the light from background stars that is observable as a temporary and very characteristic brightening.
"But it's a very low-probability effect so we have to monitor millions of background stars to find any such signals. For these signals we see no evidence of a similar effect from a host star. So either these planets are alone or they orbit at huge distances from their hosts.”
For two months in 2016 Kepler took readings from specific sectors in the universe for 30 minutes at a time, to see if microlensing would work. It did, and the academics found 27 apparently star-less exoplanets, four of which were comparable to Earth.
Lead author of the paper, Iain McDonald, a research fellow at Manchester and an astronomy lecturer at the Open University, said finding these microlensing signals is “about as easy as looking for the single blink of a firefly in the middle of a motorway, using only a handheld phone.”
Microlensing is well-suited to finding objects that are very distant. It’s unclear how far away the unmoored planets are. Kerins said they were “well in excess” of being 3,000 light-years away and are probably closer to 10,000 light-years away. The team hopes that they will be able to confirm the planets are free floaters in the future with telescopes designed to better probe the effects of gravity, dark matter, and energy.
“In the next few years there'll be two missions, NASA Roman and possibly ESA Euclid, that will both be perfect for this kind of work,” he suggested.
"Not only should they find hundreds of these events (if they are as common as suggested), we'll be able to measure their masses and distances. We'll be able to use this to decide if they really are alone and if they are little guys getting kicked out from their homes, or perhaps bigger planets that never had a home star." ®