Kepler seems to have detected a bunch of rogue planets drifting through the galaxy

When a star is born, the leftover dust and gas in the cloud from which it formed doesn't just sit there. It clumps together, forming other cosmic objects - asteroids and comets and meteors and, yes, exoplanets. We've detected many of these exoplanets in orbit around alien stars in the Milky Way.

But not all exoplanets stay put. Some get gravitationally kicked away from their parent star, to wander the galaxy, cold and alone. These are less easy to detect - but, after careful combing through data from NASA's planet-hunting Kepler space telescope, astronomers think they've found some.

In data from a two-month observing period, they counted 27 signals hinting that a rogue exoplanet was moving past the telescope's eye. Most of them were known, detected by other instruments - but five were completely new.

And four of these new signals, the researchers said, seem to be from rogue exoplanets around the same mass as Earth.

Kepler, now retired, was not designed to detect exoplanets this way. It relies on something called the transit method: when an exoplanet passes between us and its star, we may observe a faint dip in light. Kepler stared at fields of stars to pick out these dips in starlight, detecting thousands of exoplanets as a result.

Because rogue exoplanets don't orbit a star, they can't be detected in this way. What we can use to find them is a technique called gravitational microlensing - but it's even harder to pick up. When a body with mass moves through space, the gravitational curvature of space-time around it can magnify (very faintly and briefly) the starlight in the background.

"These signals are extremely difficult to find," explained astronomer Iain McDonald, then at the University of Manchester in the UK.

"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."

Nevertheless, Kepler prevailed. During a two-month observing run in 2016, it managed to capture 27 microlensing events, the team found. Along with the 22 known microlensing events, which had been detected by other, ground-based instruments observing at the same time, the researchers identified five previously unknown events.

One of these is the subject of a separate paper, but the remaining four were particularly interesting.

These four events were much shorter than the others, suggesting a population of exoplanets that is on the less massive side (the majority of detected exoplanets to date are large ones, which is at least partially because they are easier to detect).

Now, a star can also create a microlensing event; we know what this looks like, because it happens fairly frequently. The four new events did not have the same signature as a microlensing star, which led the researchers to conclude that these exoplanets were rogue - ejected from their home star systems to soar alone through the cosmos.

These signals are by no means conclusive, but they're an exciting, tantalizing hint of what may be to come with the next generation of instruments, such as the Nancy Grace Roman Space Telescope, due to launch in the mid-2020s.

"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 astronomer Eamonn Kerins of the University of Manchester.

"Now it passes the baton on to other missions that will be designed to find such signals, signals so elusive that Einstein himself thought they were unlikely ever to be observed."

There's never been a more exciting time to be looking at the sky.

The research has been published in the Monthly Notices of the Royal Astronomical Society.

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