Solving the mystery of stellar companions

The study of planet-like bodies has revealed they are very similar to brown dwarfs. Are these objects in fact failed stars?

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An image of the planetary-mass companion VHS 1256-1257 b (bottom right) and its host star (centre). Might these planet-like companions actually be brown dwarf stars?
Credit: Gauza, B. et al 2015, MNRAS, 452, 1677-1683

 

Astronomers have been attempting to solve the mystery of planet-like bodies orbiting stars beyond the Solar System. Are these bodies actually planets, or are they a type of failed star known as a brown dwarf?

The uncertainty exists because, if they are planets, they appear to have properties very similar to brown dwarfs.

A team of astronomers at Caltech have been taking images of 20 planet-like bodies.

These objects are more massive than Jupiter, orbit far from their stars, and are young enough that they are still glowing with the heat produced during their formation.

But the smallest brown dwarfs that astronomers know of are similar in size to Jupiter and could appear like a planet when orbiting a star, raising questions as to the identity of these objects.

 


Read more about brown dwarfs from BBC Sky at Night Magazine:


 

The team measured the spin rates of three of the objects, known as ‘planetary-mass companions’, and compared them to the spin rates for small brown dwarfs.

"These new spin measurements suggest that if these bodies are massive planets located far away from their stars, they have properties that are very similar to those of the smallest brown dwarfs," says Heather Knutson, professor of planetary science at Caltech and a co-author of the paper.

The team were able to get a good look at the companions by using the W.M. Keck Observatory in Hawaii. The three objects are known as ROXs 42B b, GSC 6214-210 b, and VHS 1256-1257 b.

As these companions spin, light from the side turning towards Earth shifts to shorter wavelengths, while light from the other side shifts to longer wavelengths. This information can be used to calculate the rate of spin.

They then compared these spin rates to the spin rates previously deduced for small, free-floating brown dwarfs. They found the companions are spinning at about the same speed as the dwarfs.

 


The W.M. Keck Observatory and its two 10-meter telescopes, on Mauna Kea in Hawaii. The observatory has enabled astronomers to get a good look at planetary-mass companions to learn more about their properties.
Image credit: NASA/JPL-Caltech

 

The results suggest that either planetary-mass companions are actually brown dwarfs, or that they are planets that ended up with the same spin rate as brown dwarfs.

Perhaps planets and brown dwarfs go through formation processes that leave them with similar spin rates.

"It's a question of nature versus nurture," says Knutson. "Were the planetary companions born like brown dwarfs, or did they just end up behaving like them with similar spins?"

"Spin rates of planetary-mass bodies outside our solar system have not been fully explored," says Marta Bryan, lead author of the study.

"We are just now beginning to use this as a tool for understanding formation histories of planetary-mass objects."

 


Carousel image: An artist’s impression of WISEA J114724.10−204021.3, a free-floating object thought to be a brown dwarf.
Credit: NASA/JPL-Caltech
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