Planets that go rogue around no star. They wander alone in the vacuum of space, having been kicked out of their star systems by gravitational interactions with other planets and stars. No one really knows how many rogue planets may be out there, but that could change in a few years.
Researchers from NASA’s Goddard Space Flight Center and Osaka University in Japan have used the phenomenon gravitational microlensing to estimate the number of rogue planets that may be revealed at the heart of the Milky Way. They analyzed data from Microlensing Observations in Astrophysics (MOA) survey that searched for gravitational microlensing events from 2006 to 2014 to find 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 detect at least 400 Earth masses winding through our galaxy.
Using gravity as a magnifying glass
Everything that has mass bends spacetime because gravity is the curvature of spacetime. When an object passes in front of a distant star, galaxy, or galaxy cluster without being perfectly aligned with it, the light from that star (or other light-emitting body) will pass through space bent by the object’s mass. This curved space can magnify the object as a lens would, amplifying the background star’s brightness, making it more visible. This phenomenon is known as gravitational lenses.
Most rogue planets tend to be on the smaller side because less mass makes a planet more likely to be ejected from its exosolar system. The small size and the fact that they are not associated with a star make them very difficult to detect. But gravitational microlensing can give researchers a helping hand.
Gravitational microlensing events occur in the same way as other gravitational lensing events do, except that microlensing refers to the lensing performed by smaller objects. Due to the low mass of many rogue planets, they create a weaker lensing effect that makes events harder to see. Still, a number of these microlensing events have been detected, so we know there are fake planets out there.
Rogue planets ranging from the mass of Mars to the mass of Earth will be future targets for the Roman telescope. When they cross in front of a star and bend spacetime (therefore bending the star’s light), most may not last more than a day, but that’s usually just long enough for observations to be made.
“Gravitational microlensing allows us to study a variety of objects with masses ranging from exoplanets to black holes,” the researchers said in first of two studies soon to be published in The Astronomical Journal. The second study can be found here.
Single telescope searches for loose planets
After it launches in 2027, the Roman telescope will search for rogue planets in the heart of the Milky Way. The research team decided to find out how many planets we can expect 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 central galactic bulge of the Milky Way based on the capabilities of the Roman telescope.
“We estimate that our galaxy is home to 20 times more rogue planets than stars — trillions of worlds wandering alone,” a co-author of both papers, senior scientist David Bennett of NASA’s Goddard Space Flight Center, said in a press release. “This is the first measurement of the number of rogue planets in the galaxy susceptible 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 at the center of our galaxy, meaning their orbits are far from their star. The masses of these predicted rogues are between those of Mars and Earth, based on the limits used by the scientists.
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 Earth-mass rogue planets were also once in wide orbits around stars but were thrown into space by intense gravitational interactions with objects in their former exosolar system. They could also have been ejected during the tumultuous formation of their star system.
Roman’s instrument should be able to pick up more microlensing events revealing these planets than the previous 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.
Elizabeth Rayne is a creature who writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. When she’s not writing, she’s either shape-shifting, drawing, or cosplaying as a character no one has ever heard of. Follow her on Twitter @quothravenrayne.
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