Free-floating planets — dark, isolated orbs roaming the universe unfettered by any host star — don’t just pop into existence in the middle of cosmic nowhere. They probably form the same way other planets do: within the swirling disk of gas and dust surrounding an infant star.
But unlike their planetary siblings, these worlds get violently chucked out of their celestial neighborhoods.
Astronomers had once calculated that billions of planets had gone rogue in the Milky Way. Now, scientists at NASA and Osaka University in Japan are upping the estimate to trillions. Detailed in two papers accepted for publication in The Astronomical Journal, the researchers have deduced that these planets are six times more abundant than worlds orbiting their own suns, and they identified the second Earth-size free floater ever detected.
The existence of wandering worlds orphaned from their star systems has long been known, but poorly understood. Previous findings suggested that most of these planets were about the size of Jupiter, our solar system’s most massive planet. But that conclusion garnered a lot of pushback; even scientists who announced it found it surprising.
To better study these rogue worlds, David Bennett, an astronomer at the NASA Goddard Space Flight Center, and his team used nine years of data from the Microlensing Observations in Astrophysics telescope at the University of Canterbury Mount John Observatory in New Zealand. Exoplanets were indirectly detected by measuring how their gravity warped and magnified the light arriving from faraway stars behind them, an effect known as microlensing.
With help from empirical models, the researchers worked out the spread of the masses for more than 3,500 microlensing events, which included stars, stellar remnants, brown dwarfs and planet candidates. (Data from one of those candidates was compelling enough for the team to claim the discovery of a new rogue Earth.) From this analysis, they estimate that there are about 20 times more free-floating worlds in our Milky Way than stars, with Earth-mass planets 180 times more common than rogue Jupiters.
The conclusion that most rogue worlds are small makes more sense than the idea that they are Jupiter-size, Dr. Bennett said. That’s because planets are thought to go rogue when two protoplanets slam into each other. The force of the impact is so strong that it knocks one out of the emerging star system altogether.
But planets can be kicked out of their star systems only by larger objects. If most of these stellar orphans were Jupiter-size, a lot of so-called super-Jupiters must be orbiting host stars — but those are scarce. On the other hand, these results suggest that lower mass planets are the ones at risk of ejection.
“So things are dangerous for the Earths,” Dr. Bennett said.
He also said that the abundance of free floaters in the Milky Way suggests that planet-size objects slamming into each other during the formation process “are maybe more common than theorists might have guessed.”
Przemek Mróz, an astronomer at the University of Warsaw who was not involved in the work, said that the group’s results strengthened earlier hints about rogue worlds from observations made with the Optical Gravitational Lensing Experiment and the Korean Microlensing Telescope Network. “So now we have three independent studies and three independent lines of evidence that low mass free-floating planets are very common in the Milky Way,” he wrote in an email.
There’s still some ambiguity about whether these planets are truly unleashed, or just cast out to wide enough orbits that scientists can’t link them to a host star. Dr. Mróz thinks the observed population probably includes a mix of both, but it’ll be difficult to deduce the relative numbers of each with microlensing measurements alone.
The new studies’ astronomers are looking forward to even better free-floating planet data taken with the Nancy Grace Roman Space Telescope, a NASA mission set to launch in 2027, which could spot hundreds of rogue Earths. Combined with data from the European Space Agency’s Euclid Telescope, or well-positioned observatories on the ground, scientists will be able to measure the mass more directly, with less reliance on models.
Could any of these planets be habitable? Possibly, Dr. Bennett surmised, explaining that they’d be dark without a host star, but not necessarily frigid. Hydrogen in a planet’s atmosphere could act like a greenhouse and trap heat emanating from its interior — which is what sustains microbial life in deep sea vents on Earth.
But for now, searching for life on these lone worlds is out of reach. “Maybe they’ll have a method to do it in a hundred years,” Dr. Bennett said. “But scientists now are looking for things that we can actually do.”
The team didn’t look beyond the bounds of the Milky Way. “But we expect that other galaxies are pretty similar,” Dr. Bennett said — meaning that these outcasts might be sprinkled across our entire universe.
