A planet that is about the same mass as Earth appears to be floating on its own through space. It is the smallest such rogue planet we have ever spotted, but there are probably many more in the galaxy.
Usually, we find planets beyond our solar system using observations of the light from the planet鈥檚 host star. Rogue, or free-floating, planets have no host star, making them difficult to spot. Astronomers have only found a handful, and most are聽far more massive聽than Jupiter.
These were mainly spotted using gravitational microlensing, which occurs when a planet passes in front of a brighter object and the planet鈥檚 gravitational pull bends that object鈥檚 light. The size of the planet can be measured based on how long the lensing persists.
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Przemek Mr贸z at the California Institute of Technology and his colleagues found the shortest microlensing event ever seen, lasting just 41.5 minutes, using the Optical Gravitational Lensing Experiment in Poland and part of the Korea Microlensing Telescope Network. The shortness of the event suggests the object has a similar mass to Mars or Earth, depending on how far away it is, which the team couldn鈥檛 determine.
The group ruled out the planet having a star within about eight times the distance between Earth and the sun. 鈥淚f there is a more distant star, we can鈥檛 detect it in the聽light curve of the microlensing event鈥�, so we can鈥檛 be entirely certain that this small world is really聽free-floating, says Mr贸z.
Astronomers expect that while systems of planets are forming, it should be fairly easy to kick small worlds out of orbit and turn them into rogue planets. 鈥淲e estimate that Earth-mass free-floating planets may be as common as stars in the Milky Way,鈥� says Mr贸z.
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