Milky Way has trillions of wandering worlds – 08/22/2023 – Science

Rogue planets — dark, isolated orbs that wander the universe without being attached to any host star — don’t just appear out of nowhere, in the middle of cosmic nowhere. They are likely to form in the same way as other planets: within the rotating disk of gas and dust that surrounds a “baby” star.

But unlike their planetary brethren, these worlds are violently expelled from their celestial neighborhoods.

Astronomers once calculated that there would be billions of wandering or orphan planets in the Milky Way. Now, scientists at NASA and Osaka University in Japan have raised the estimate to trillions. In conclusions detailed in two scientific papers accepted for publication in The Astronomical Journal, the researchers deduced that these planets are six times more abundant than the worlds orbiting their own suns, and identified the second Earth-sized rogue planet ever detected.

The existence of wandering worlds that have been orphaned from their star systems has long been known but poorly understood. Previous discoveries have suggested that most of these planets would be roughly the size of Jupiter, the largest planet in our solar system. But this conclusion met with much resistance; there were even scientists who declared that they found it surprising.

To better study these wandering worlds, David Bennett, an astronomer at NASA’s Goddard Space Flight Center, and his team used nine years of data from the Microlensing Observations in Astrophysics (MOA) telescope at Canterbury Mount John Observatory in New Zealand. Exoplanets were detected indirectly by measuring how their gravity distorted and magnified light arriving from distant stars behind them, in an effect known as gravitational microlensing.

With the help of empirical models, the researchers calculated the mass spread of more than 3,500 gravitational microlensing events (data from one of these candidates was convincing enough to prompt the team to announce the discovery of a new rogue planet). From that analysis, they estimate that there are about 20 times more wandering worlds in our Milky Way than there are stars, and that Earth-mass planets are 180 times more common than wandering Jupiters.

The conclusion that most rogue worlds are small makes more sense than the idea that they are Jupiter-sized, Bennett said. That’s because planets are thought to become errant when two protoplanets collide. The impact force is so great that one of them ejects the other completely from the emerging star system.

But planets can only be kicked out of their star systems by objects larger than themselves. If most of these orphan stars were comparable in size to Jupiter, many so-called super-Jupiters would have to be orbiting their host stars — but there are few of them. On the other hand, these results suggest that the lower mass planets are at risk of being ejected.

“So the situation is dangerous for the lands,” Bennett said.

He also said that the abundance of orphan stars in the Milky Way suggests that instances of planet-sized objects colliding during the formation process “are perhaps more common than theorists would have imagined”.

Przemek Mróz, an astronomer at the University of Warsaw who was not involved in the work, said the results obtained by the group reinforce previous indications about wandering worlds coming from observations made within the Gravitational Microlensing Optical Experiment and the Korean Microlensing Telescope Network. “So we now have three independent studies and three lines of evidence indicating that low-mass rogue planets are very common in the Milky Way,” he wrote in an email.

It’s still unclear whether these planets are truly rogue or just orbiting far enough away that scientists can’t link them to a host star. Mróz thinks the observed population probably includes a mix of both, but it will be difficult to deduce the relative numbers of each from gravitational microlensing measurements alone.

Astronomers responsible for the new studies are anticipating even better data on loose planets obtained by the Nancy Grace Roman Space Telescope, a NASA mission scheduled to be launched in 2027. well positioned on land, scientists will be able to measure mass more directly, with less use of models.

Is there a possibility that some of these planets are habitable? Perhaps, Bennett theorized, explaining that without a host star they must be dim, but not necessarily icy. The hydrogen in a planet’s atmosphere can act as a greenhouse and trap heat emanating from the planet’s interior. That’s what sustains microbial life in deep marine crevices on Earth.

For now, however, we have no way of looking for signs of life on these isolated worlds. “In a hundred years, maybe there will be a method to do it,” Bennett said. “But for now, scientists are looking for things we can actually do.”

The team looked no further than the Milky Way. “But we imagine that the situation is similar in other galaxies,” Bennett said — namely, that these evicted planets could be scattered throughout our entire universe.

^{Translated by Clara Allain}