Rogue planets may be more numerous than stars in our galaxy

Planets that go rogue orbit no star. They wander the vacuum of space alone, having been kicked out of their star systems by gravitational interactions with other planets and stars. Nobody really knows how many rogue planets could be out there, but that may change in a few years.

Researchers from NASA’s Goddard Space Flight Center and Osaka University in Japan have used the phenomenon of gravitational microlensing to estimate the number of rogue planets that could be revealed in the heart of the Milky Way. They analyzed data from the Microlensing Observations in Astrophysics (MOA) survey that searched for gravitational microlensing events from 2006 to 2014 to figure out how many more of these events we could expect to find with NASA’s upcoming Nancy Grace Roman Space Telescope.

There are currently only 70 known rogue planets, but there could be hundreds more out there. The researchers now suggest that Roman could discover at least 400 Earth-mass rogues meandering through our galaxy.

Using gravity as a magnifying glass

Anything that has mass bends spacetime since gravity is the curvature of spacetime. When an object passes in front of a distant star, galaxy, or galaxy cluster without being completely aligned with it, the light from that star (or other light-emitting body) will pass through the space that is bent by the object’s mass. This curved space can magnify the object like a lens would, amplifying the brightness of the background star, making it more visible. This phenomenon is known as gravitational lensing.

Most rogue planets tend to be on the smaller side because less mass puts a planet at greater risk of being flung out of its exosolar system. The small size and fact that they’re not associated with a star make them very hard to spot. But gravitational microlensing can give scientists an assist.

Gravitational microlensing events occur the same way other gravitational lensing events do, except microlensing refers to the lensing done by smaller objects. Because of the low mass of many rogue planets, they create a weaker lensing effect that makes events more difficult to see. Still, a number of these microlensing events have been detected, so we know rogue planets are out there.

Rogue planets that range from Mars mass to Earth mass will be future targets for the Roman telescope. As they cross in front of a star and bend spacetime (therefore bending the star’s light), most may not linger longer than a day, but that’s typically just long enough for observations to be made.

“Gravitational microlensing enables us to study a variety of objects with masses ranging from that of exoplanets to black holes,” the researchers said in the first of two studies soon to be published in The Astronomical Journal. The second study can be found here.

Lonely telescope seeks unattached planets

After it launches in 2027, the Roman telescope will search for rogue planets at the heart of the Milky Way. The research team decided to figure out how many planets we might expect it to find. To do so, they used data from MOA and the Optical Gravitational Lensing Experiment (OGLE) to estimate how many of these planets there are. That approximation was then used to make a prediction of how many could be found in the Milky Way’s central galactic bulge based on the Roman telescope’s capabilities.

“We estimate that our galaxy is home to 20 times more rogue planets than stars—trillions of worlds wandering alone,” said a co-author of both papers, senior research scientist David Bennett of NASA’s Goddard Space Flight Center, in a press release. “This is the first measurement of the number of rogue planets in the galaxy that is sensitive to planets less massive than Earth.”

Bennett and his colleagues also estimated that there are at least six times more small rogue planets than planets with wide orbits in our galaxy’s center, which means their orbits are distant from their star. The masses of these predicted rogues are between Mars’ and Earth’s, based on the cutoffs used by the researchers.

If some planets of this mass have wide orbits and their mass is consistent with many rogue planets, it could mean that at least some rogue planets around the mass of Earth were also once in wide orbits around stars but were thrown into space by intense gravitational interactions with objects in their former exosolar systems. They could have also been ejected during the tumultuous formation of their star systems.

Roman’s instruments should be able to pick up on more microlensing events that reveal these planets than the earlier surveys. It already has high expectations to live up to. For now, at least we have an idea of what might be lurking out there.