Roaming the galaxy, alone and untethered, is a mysterious group of free-floating — or rogue — planets. Because of their elusive nature, little is known about their properties or formation.
Now, with the help of some extraordinary luck, an international team of astronomers writing in Science has calculated the mass and distance of one such planet — a Saturn-sized object 10,000 light-years away.
“For the first time, we have a direct measurement of a rogue planet candidate’s mass and not just a rough statistical estimate,” lead author Subo Dong of Peking University explained in a statement. “We know for sure it’s a planet.”
Read More: Hubble Discovers Dracula’s Chivito, the Largest-Known Chaotic Planet Nursery
Celestial Lone Wolves
Not all worlds orbit a star as tightly as our planet orbits the sun. Scientists have known about these celestial lone wolves — which are either entirely unshackled by a central star or on an incredibly wide orbit — for decades.
There are several theories as to how such rogue planets formed. Perhaps they were catapulted out of their planetary system by violent interactions with other planets or by disruptions caused by passing stars. Perhaps they were never part of a planetary system in the first place and were instead forged from celestial clouds and dust, like brown dwarfs.
Whatever the cause, there could be billions traipsing the Milky Way. According to Andrzej Udalski, co-author and astronomer at the Astronomical Observatory of the University of Warsaw, preliminary estimates suggest there is at least one free-floating planet for every star, though some estimates suggest there may be as many as 10 to 20.
Because they emit so little light, they show themselves rarely and only present themselves to us here on Earth through extremely subtle flickers of brightness. Even when this does occur, it has not been possible to calculate the distance between us and them — until now.
A Saturn-Sized Rogue Planet
Astronomers can detect rogue planets via microlensing events. This is when the planet passes a star, and its mass warps the fabric of space-time, causing the star’s light to bend and brighten.
The microlensing event triggered by this particular planet was observed by two separate surveys — one by the Korea Microlensing Telescope Network and the other by the Optical Gravitational Lensing Experiment.
But importantly, the moment was also captured in space by the Gaia satellite. Thanks to some particularly serendipitous timing, Gaia observed the event six times over a 16-hour period.
“We were very lucky to catch the space observations because the Gaia satellite generally observed the same regions of the sky only [approximately every 30 days],” says Udalsk. “In our case, it pointed exactly at our region of the sky — exactly during the maximum of our very short-lived microlensing event.”
By comparing the various observations, astronomers were able to calculate the mass and distance of the planet. Just as the distance between our eyes enables us to perceive depth, the distance between terrestrial telescopes and Gaia can be used to determine the distance between Earth and the roaming planet.
“The difference is that the spacing between the eyes of we humans is a few centimeters, whereas Gaia is about 1.5 million kilometers away from Earth,” said Dong in the press statement.
The event was captured by Gaia approximately two hours after it was recorded on Earth. From this information, the team deduced that the planet is about the size of Saturn, a fifth of Jupiter, 10,000 light-years from Earth, and 3,000 parsecs from the center of the Milky Way. Because of its smaller size, researchers believe it most likely formed in a protoplanetary disc before it was flung out of orbit.
More Rogue Planet Sightings Are Expected
While there are only a few examples of free-floating planets in the scientific literature to date, it is expected that future developments — including the launch of NASA’s Nancy Grace Roman Space Telescope in 2027 — will lead to many more sightings and could help inform our understanding of what might cause a planet to go rogue in the first place.
Gavin Coleman, a post-doctoral research assistant at Queen Mary University of London, U.K., explains in an accompanying perspective in Science that the discovery “highlights the effectiveness of the microlensing technique and how detection of rogue planets can inform the process of planet formation.”
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
This article references information from a recent study published in Science: A free-floating-planet microlensing event caused by a Saturn-mass object
This article references information from a recent study published in Science: Two Views of a Rogue Planet
