How to find a rogue planet

About 80 light years from Earth, a planet wanders alone. Without a star to tie it to a single orbit, the newly discovered PSO J318.5-22 drifts aimlessly across the sky at a slow speed of 20 kilometers per second. This rogue planet is six times the mass of Jupiter and is the closest floating planetary mass object ever to be seen. The international team of astronomers who discovered the PSO J318.5-22 have been following it for two years; their study is published this week in the journal Letters from astrophysical journals.

So how does a planet end up on its own? And how is it possible to find a dark world in the middle of nowhere?

Find a planet without stars

The fact that PSO J318.5-22 does not revolve around a star means that there is little light to obscure it from research telescopes. But it also means that there is little light to illuminate the planet.

The team that found this rogue planet began by looking for brown dwarfs, or failed stars. Using the Pan-STARRS Wide Field Telescope atop the Haleakala volcano in Maui, they studied huge swathes of space and tapped into a database of billions of objects.

“Pan-STARRS monitors the sky using a digital CCD camera with 1.4 billion pixels and takes images using filters that select certain colors of light,” says Katelyn Allers, professor of astronomy at Bucknell University and co-author of the article. “We searched the incredibly large Pan-STARRS database for objects that appeared to have cool, planet-like temperatures, which are indicated by red colors.”

The PSO J318.5-22 appeared in these infrared readings to be pale and red, but much redder than any brown dwarf recorded. Allers and the rest of the team knew they had spotted a good candidate for a floating planet.

“What’s really interesting about our new discovery is that it looks like young planets that have been directly imaged around other stars,” said team leader Michael Liu of the Institute of astronomy from the University of Hawaii.

After astronomers glimpsed PSO J318.5-22’s infrared signature using Pan-STARRS, the team switched to smaller telescopes to image the planet in more detail. “Once we could get a spectrum, we were then able to estimate the temperature and age and measure the distance very accurately,” Liu told PopMech.

How it got there

Researchers estimate that PSO J318.5-22 is 12 million years old. But they don’t know how it ended up floating freely, although there are two possibilities that astronomers think are plausible.

The first idea is that the PSO J318.5-22 was formed in the same way as the Earth and the planets of our solar system: in the circumstellar disk of a star. Then, says Allers, the planet separated from its mother star and flew into space. According to a study in Monthly notices from the Royal Astronomical Society, this could happen when a passing star exerts its gravitational pull and pushes the planet out of orbit. It’s also possible that the parent star, by ending its hydrogen-burning life and developing into a red giant, could push a planet out of its system.

The second possibility is that the planet has been a loner from the start. It could have formed in isolation from a cloud of collapsed matter, a cloud that did not have enough mass to ever reach the hydrogen melting stage, in which case it could have become a star.

“Some recent work by Viki Joergens [of the Max-Planck Institute for Astronomy in Heidelberg, Germany] has shown that some planetary mass objects form like stars and accumulate matter from their own circumstellar disks, ”Allers says. “But we will need to obtain additional data to try to distinguish these two training mechanisms for PSO J318.5-22.”

How many rogue planets are there?

Similar free-floating planetary mass objects have been found in the past, but although their infrared signatures are usually associated with brown dwarfs, PSO J318.5-22 most closely resembles a planet. Brown dwarfs, like stars, form from clouds of gas and dust that collapse, but do not coalesce into objects dense enough to trigger nuclear fusion. They are too big to be planets, but too small to be real stars. Brown dwarfs are believed to be as widespread as stars and much more abundant than floating planets.

Still, there are probably a lot of rogue planets out there. Thousands have been discovered over the past decade, but the majority of them have been too far away to be captured by direct imaging. is 4,600 light years from Earth. These small clouds of matter could have collapsed on themselves, forming planets that broke away from the nebula.

Liu says that although the researchers are not sure the exact number of potential floating planets, now that they have found PSO J318.5-22, they can use its infrared signature as a template to find other candidates among the catalogs amassed. by Pan-ÉTOILES. And there’s a good chance there are more rogue planets closer to home. Judging from the information gathered from regions of star formation, according to Allers, it appears that there is about one planetary mass object for every 10 sun-like stars.

“Very roughly it corresponds to about a dozen floating planets within 100 light years of us,” she says. “We definitely have more work to do to find them all.”

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Arline J. Mercier

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