Free-floating planets — dark, isolated orbs wandering the universe unconstrained by any host star — don’t just pop up in the middle of cosmic nowhere. They likely form the same way other planets do: within a rotating disk of gas and dust surrounding an infant star.
But unlike their planetary brethren, these worlds have been violently removed from their celestial neighborhoods.
Astronomers once calculated that billions of planets have gone rogue in the Milky Way. Now, scientists at NASA and Osaka University in Japan are raising the estimate to trillions. Detailed in two papers accepted for publication in The Astronomical Journal, the researchers concluded that these planets are six times more abundant than the worlds orbiting their own suns, and they identified the second Earth-sized free floater ever seen.
The existence of wandering worlds orphaned from their star systems has long been known, but poorly understood. Previous findings It has been suggested that most of these planets are about the size of Jupiter, the most massive planet of our solar system. But that conclusion got a lot of pushback; even the scientists who revealed it were surprising.
To better study these rogue worlds, David Bennett, an astronomer at 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 have been detected indirectly by measuring how their gravity warps and magnifies the light coming from distant stars behind them, an effect known as microlensing.
With help from empirical models, the researchers worked out the mass distribution for more than 3,500 microlensing events, which include stars, stellar remnants, brown dwarfs and planet candidates. (The data from one of those candidates is 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 as many free-floating worlds in our Milky Way than the stars, with Earth. -mass planet 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. That’s because planets are thought to go rogue when two protoplanets collide with each other. The force of the impact was so strong that it knocked out one of the developing star systems.
But planets can be knocked out of their star systems only by massive objects. If most of these stellar orphans were Jupiter-sized, many so-called super-Jupiters should be orbiting host stars — but they are few and far between. On the other hand, these results suggest that the lower mass planets are the ones at risk of ejection.
“So things are dangerous for Earth,” said Dr. Bennett.
He also said that the abundance of free floaters in the Milky Way suggests that planet-sized objects slamming into each other during the formation process “may be more common than scientists might have predicted.” theorist.”
Przemek Mróz, an astronomer at the University of Warsaw who was not involved in the work, said the group’s results strengthened a while ago 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 common in the Milky Way,” he wrote in an email.
There is still some ambiguity about whether these planets were truly ejected, or just thrown into wide orbits that scientists can’t pin down to a host star. Dr. thinks Mróz said the observed population likely included a mix of both, but it would be difficult to determine the relative numbers of each with microlensing measurements alone.
Astronomers of the new study hope better free-floating planet data taken with the Nancy Grace Roman Space Telescope, a NASA mission scheduled to launch in 2027, could detect hundreds of rogue Earths. Combined with data from the European Space Agency’s Euclid Telescope, or well-positioned observatories on the ground, scientists can measure mass more directly, with less reliance on models.
Are any of these planets habitable? Possibly, predicts Dr. Bennett, explains that they would be dark without a host star, but not too cold. Hydrogen in a planet’s atmosphere can act like a greenhouse and trap the heat emanating from its interior — which sustains microbial life in Earth’s deep sea tunnels.
But for now, finding life on these lonely worlds is out of reach. “Maybe they’ll find a way to do it in a hundred years,” said Dr. Bennett. “But scientists today are looking for things we can do.”
The team did not look beyond the borders of the Milky Way. “But we expect that other galaxies are quite similar,” said Dr. Bennett – which means these outcasts can be sprinkled throughout our universe.
