Planetary Sciences News

Starlike Brown Dwarf? Not Anymore

Because of a new, surprisingly smaller mass estimate for a much-studied, nearby brown dwarf, astronomers now regard the familiar object as merely planetlike.

By

When is a star really a star? For a brown dwarf, this line can blur. These lightweight objects begin their lives, as do stars like our Sun, in a nebula as masses of dust and gas collapsing under the weight of their own gravity. But a brown dwarf never gets massive enough to kick-start the nuclear fusion engine that transforms hydrogen into helium. In the cores of stars, such engines allow them to burn hot and bright for billions of years.

For brown dwarf SIMP J013656.5+093347, another line has now blurred—that between star and gas giant planet. In a paper published 9 May in Astrophysical Journal Letters, scientists reanalyzed old data and added a pinch of new data to reveal that the object, better known as SIMP0136, is much less weighty than originally thought, which means it might more closely resemble a gas giant planetlike object floating alone in space.

Scientists can’t officially call the downsized SIMP0136 a planet, however. That label implies that it orbits something or used to orbit something. That’s impossible for scientists to know, said Jonathan Gagné, an astrophysics Ph.D. candidate at the Carnegie Institution for Science in Washington, D. C., and lead author of the new paper.

SIMP0136 lies only 18 light-years from Earth, and it already interests scientists because it shows strange weather variations. The patterns even remind him of the storms on Jupiter, Gagné said. Having a gas giant so close, without the glare of a parent star, offers a tantalizing prospect for scientists eager to explore the atmospheres of the thousands of giant, gassy exoplanets discovered in recent years. It could “give us a lot of valuable information about giant planets,” he said.

A Cosmic Accident

Gagné came up with this new view of SIMP0136 entirely by accident while hunting for other planetlike objects whose atmospheres aren’t outshined by a parent star. To find these, he searches for distant pinpricks of infrared light. Although brown dwarfs and slightly smaller, planetlike objects don’t power themselves with nuclear fusion, they still generate enough heat to glow in infrared light.

Gagné conducted his search using ground-based and space-based surveys of the cosmos and a statistical model he created. The model calculates the likelihood that any given infrared point in the sky could be a free-floating planetary mass, allowing him to narrow millions of candidates to mere hundreds. One of the objects his model picked out was SIMP0136, which had been considered more massive and starlike since its discovery in 2006.

This result left Gagné scratching his head. SIMP0136 was a widely known brown dwarf, the third closest to Earth and relatively well studied by the scientific community. Was his code not working properly? Why would it tag this brown dwarf, which was estimated to be 20 to 80 times the size of Jupiter, as a much smaller object? Not only that, but the model also placed SIMP0136 within a star system called the Carina-Near Moving Group, a 200-million-year-old group of 20 or so stars that all formed at the same time.

“I was extremely surprised, and then I didn’t believe it,” Gagné said. “It seemed surprising that such a well-studied brown dwarf had not been found already to be a member of the young [Carina-Near] group.”

Gathering New Data

Gagné then set out to prove his code wrong. Specifically, he wanted to find out if SIMP0136’s velocity moving away from Earth matched Carina-Near’s velocity. If SIMP0136 was moving away from Earth at the same speed as the stars in Carina-Near, then he could be certain it belonged to the same group and was therefore the same age as its stellar companions.

For brown dwarfs, age and mass are inextricably linked. Without knowing a brown dwarf’s age, scientists can’t constrain the object’s mass. That’s because, unlike stars, brown dwarfs immediately start cooling once they form. Without knowing the age, scientists can’t be sure whether the brown dwarf is lightweight but too young to have cooled very much or massive but so old that it had been cooling for a long time, Gagné said.

He and his colleagues turned to the Keck II telescope in Hawaii to observe SIMP0136. Once they determined SIMP0136’s velocity, the answer was clear: This object was indeed part of Carina-Near, which meant it matched Carina-Near’s age of 200 million years. Having figured out SIMP0136’s age, the team realized that everything else about the object once thought to be a starlike brown dwarf needed to be reevaluated.

Astronomers had previously measured SIMP0136’s temperature as 1100 kelvins (about 827°C), which placed the object directly on the line between planetlike and starlike, Gagné said. Researchers consequently assumed that the object’s mass also fell on that line. However, new calculations based on SIMP0136’s age and brown dwarf dynamics told the team that the object was only about 13 times the mass of Jupiter and therefore much more likely to be a large, gassy, planetlike object.

Atmospheric Observations

SIMP0136 was already interesting because of its weird weather, but this new finding “hints that the dramatic weather variations may be a sign of youth,” said John Gizis, an astronomer at the University of Delaware in Newark who wasn’t involved in the research. Studying SIMP1036 will allow scientists to develop better models of massive gas giant atmospheres, he continued.

Even though the Kepler telescope has discovered thousands of giant, gassy exoplanets for scientists to gaze at, “if you try to see only the planet, you get completely blinded by the star” around which it orbits, Gagné said. However, SIMP0136’s proximity will now allow scientists to see the atmosphere of a gassy, planetlike object without the glare from a star.

Correction, 18 May 2017: This article was updated to reflect that brown dwarfs are not stars.

—JoAnna Wendel (@JoAnnaScience), Staff Writer

Citation: Wendel, J. (2017), Brown dwarf star? Not anymore, Eos, 98, https://doi.org/10.1029/2017EO073625. Published on 17 May 2017.
© 2017. The authors. CC BY-NC-ND 3.0