Some planets float alone through space, with no sun and skies that are always dark. These rogue worlds are incredibly difficult to spot, but two new ones have been discovered. One of them is among the smallest we’ve ever seen, and there may be more small planets like it in the Milky Way than stars.
When astronomers normally spot planets beyond our solar system, they do so by observing a planet passing in front of its star, blocking out some of the starlight. But for planets without stars, this method doesn’t work.
Instead, anyone looking for these wondering worlds must rely on a phenomenon called gravitational microlensing, which occurs when a planet passes in front of a distant background star and the planet’s gravity behaves like a lens, warping and magnifying the star’s light.
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Przemek Mróz at Warsaw University in Poland and his colleagues used this method to find two new starless worlds, using data from the Optical Gravitational Lensing Experiment, a sky survey that looks for gravitational lensing events.
Based on the few Earth-mass worlds like this we have found and how difficult they are to spot, the team calculated that rogue planets might even be more common than stars in the Milky Way, even though only 10 have been discovered so far.
Really big or really close
One of the worlds Mróz and his colleagues spotted behaving like a lens is either about twice or twenty times the mass of Jupiter or 20 times its mass, depending on how distant it is. The other is either about 2.3 times or 23 times Earth’s mass, making it one of the smallest free-floating planets we’ve ever seen.
The team can’t be sure how big these planets are because they have not been able to precisely measure how far they are away — the mass estimates are based on locations either in the main disk of the galaxy or the more distant bulge around the Milky Way’s centre.
“It’s kind of like if you’re looking up at the stars and someone has a bunch of magnifying glasses and is occasionally moving them in front of the star,” says team member David Bennett at NASA’s Goddard Space Flight Center in Maryland. “How do you tell how close the magnifying glass is if all you see is the light from the star? It’s not that easy.”
Some researchers have speculated that rogue worlds like these could be habitable, kept warm by the decay of radioactive elements on their interiors and dense, blanket-like atmospheres. “If there were beings that evolved on that planet they probably would not use optical light to see, because it wouldn’t do them any good,” says Bennett. “Or they would be more sensitive than we are, with giant eyes that can see like a telescope.”
Habitable or not, the detection of a small planet without a star might mean that they are everywhere. “Either we are very lucky, or Earth-mass lenses are quite common in the galaxy,” says Mróz.
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