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A new spin to solving mystery of stellar companions

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Posted December 5, 2017

Taking a picture of a planet in a solar system beyond our sun is no easy task. The light of an exoplanet’s parent star far outshines the light from the planet itself, making the planet difficult to see. While taking a picture of a small rocky planet like Earth is still not feasible, researchers have made strides by snapping images of about 20 giant planet-like bodies (including GU Psc’s companion detected in 2014). These objects, known as planetary-mass companions, are more massive than Jupiter, orbit far from the glare of their stars, and are young enough to still glow with heat from their formation—all traits that make them easier to photograph.

Artistic illustration of an exoplanet. Image credit: H. Knutson.

But one big question remains: Are these planetary-mass companions actually planets, or are they instead small “failed” stars called brown dwarfs? Brown dwarfs form like stars do—out of collapsing clouds of gas—but they lack the mass to ignite and shine with starlight. They can be found floating on their own in space, or they can be found orbiting with other brown dwarfs or stars. The smallest brown dwarfs are similar in size to Jupiter and would look just like a planet when orbiting a star.

An international team of researchers including Björn Benneke, now at the Institute for Research on Exoplanets (iREx), at Université de Montréall, have taken a new approach to the mystery: they have measured the spin rates of three of the photographed planetary-mass companions and compared them to spin rates for small brown dwarfs. The results offer a new set of clues that hint at how the companions may have formed.

“These companions with their high masses and wide separations could have formed either as planets or brown dwarfs,” says graduate student Marta Bryan, lead author of a new study describing the findings in the journal Nature Astronomy. “In this study, we wanted to shed light on their origins.”

The astronomers used the W. M. Keck Observatory in Hawaii—which is managed by Caltech, the University of California, and NASA—to measure the spin rate, or the length of a day, of three planetary-mass companions known as ROXs 42B bGSC 6214-210 b, and VHS 1256-1257 b. They used an instrument at Keck called the Near Infrared Spectrograph (NIRSpec) to dissect the light coming from the companions. As the objects spin on their axes, light from the side that is turning toward us shifts to shorter, bluer wavelengths, while light from the receding side shifts to longer, redder wavelengths. The degree of this shifting indicates the speed of a rotating body. The results showed that the three companions’ spin rates ranged between 6 to 14 kilometers per second, similar to rotation rates of our solar system’s gas giant planets Saturn and Jupiter.

For the study, the researchers also included the two planetary-mass companions for which spin rates had already been measured. One, β Pictoris b, has a rotation rate of 25 kilometers per second—the fastest rotation rate of any planetary-mass body measured so far.

The researchers compared the spin rates for the five companions to those measured previously for small free-floating brown dwarfs. The ranges of rotation rates for the two populations were indistinguishable. In other words, the companions are whirling about their own axes at about the same speeds as their free-floating brown-dwarf counterparts.

“It is very intriguing that these populations of objects appear very similar in their properties like the rotation rate,” says Björn Benneke, also co-author of the study. “This potentially means they have similar formation histories.”

The results suggest two possibilities. One is that the planetary-mass companions are actually brown dwarfs. The second possibility is that the companions looked at in this study are planets that formed, just as planets do, out of disks of material swirling around their stars, but for reasons not yet understood, the objects ended up with spin rates similar to those of brown dwarfs. Some researchers think that both newly forming planets and brown dwarfs are encircled by miniature gas disks that might be helping to slow their spin rates. In other words, similar physical processes may leave planets and brown dwarfs with similar spin rates.

“It’s a question of nature versus nurture,” says Heather Knutson, professor of planetary science at Caltech and a co-author of the paper. “Were the planetary companions born like brown dwarfs, or did they just end up behaving like them with similar spins?”

The team also says that the companions are spinning more slowly than expected. Growing planets tend to be spun up by the material they pull in from a surrounding gas disk, in the same way that spinning ice skaters increase their speed, or angular momentum, when they pull their arms in. The relatively slow rotation rates observed for these objects indicate that they were able to effectively put the brakes on this spin-up process, perhaps by transferring some of this angular momentum back to encircling gas disks. The researchers are planning future studies of spin rates to further investigate the matter.

“The rotation rates of these companions have been little studied so far,” says professor Benneke, “this is the first time we measure it for these objects. We will continue to use this tool to clarify the nature and origin of distant planetary-mass companions.”

Source: University of Montreal

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