Neither Star nor Planet: A Strange Brown Dwarf Puzzles Astronomers

Dan Caselden was up late on November 3, 2018, playing the video game Counter-Strike, when he made astronomy history. Every time he died, he would jump on his laptop to check in on an automated search he was running of NASA space telescope images.

Suddenly, in the early hours of the morning, something bizarre popped into view. “It was very confusing,” said Caselden. “It was moving faster than anything I’ve discovered. It was faint and fast, which made it very weird.”

Caselden emailed the astronomers he was working with as part of the Backyard Worlds: Planet 9 project. Once they ruled out the possibility that it was an image artifact, they realized they were looking at something wholly unusual, an exceedingly faint object 50 light-years away blazing through the galaxy at 200 kilometers per second. It was given the name WISE 1534-1043, but by virtue of its singular characteristics and chance discovery, it soon earned the nickname “The Accident.”

Astronomers now think Caselden found a brown dwarf — a failed star that lacks the necessary bulk to begin nuclear fusion in its core. “It forms like a star,” said Sarah Casewell, an astronomer at the University of Leicester in the U.K. “However, it never gains enough mass to fuse hydrogen into helium and start burning anything.”

The discovery of the Accident highlighted how we still have much to learn about brown dwarfs. These objects range in mass from an estimated 13 times the mass of Jupiter to 75 times or more, but exactly where those two boundaries lie is an ongoing dilemma. “People argue about that in conferences all the time,” said Beth Biller, an astronomer at the University of Edinburgh in the U.K., particularly the lower limit. While 13 Jupiter masses is roughly the mass at which deuterium fusion can take place — the characteristic that differentiates brown dwarfs from gas giant planets — the boundary can vary. “There’s nothing special about 13 Jupiter masses,” said Biller. “It’s completely ad hoc.”

Brown dwarfs also vary greatly in temperature. The hottest ones have surface temperatures of around 2,000 degrees Celsius — “about that of a candle flame,” said Biller. The coldest are below 200 degrees. As they do not have their own source of heat, brown dwarfs will gradually cool over billions of years to these lower temperatures. (Subdwarfs, which blur the boundary further between planets and brown dwarfs, can be cooler still. An object called WISE 0855-0714 is below freezing. “It’s the coldest object we know of outside of our solar system,” said Biller.)

What a brown dwarf might look like up close is also unclear. Despite their name — proposed by astronomer Jill Tarter in 1975 — they are likely not brown. They’re more orange or red. “For better or worse it’s stuck as a name,” said Davy Kirkpatrick, an astronomer at the California Institute of Technology.

They also have atmospheres, and those atmospheres may show some kind of banding and spotlike storms, like on Jupiter. Last year, Biller and her colleagues used these storms to measure the wind speed on a brown dwarf about 34 light-years away. They first watched features in its atmosphere come into and out of view as they rotated, and then compared this speed to a measurement of the object’s interior rotation speed gleaned from its magnetic field. Comparing the two values, the researchers calculated a wind speed of over 2,300 kilometers per hour — more than five times that of Jupiter’s winds.

Because brown dwarfs bridge the gap between stars and planets, they can help us understand both. At the upper end of the mass scale, the boundary between the largest brown dwarfs and the smallest stars can give us insights into how nuclear fusion begins. An object needs to reach temperatures of around 3 million degrees Celsius in its core to kick-start nuclear fusion, said Nolan Grieves of the University of Geneva in Switzerland; this ignites a chain reaction that turns hydrogen into helium. But no one is exactly sure how much mass is needed for that to happen, and at what point a brown dwarf becomes a star. “There’s a lot of aspects of stellar evolution that our knowledge is still pretty uncertain on,” said Biller. “Where that fusion limit is exactly is one of those questions.”

Recent work led by Grieves identified five high-mass brown dwarfs with masses between 77 and 98 times that of Jupiter. “They’re right on the border where hydrogen fusion starts to take place,” said Grieves. It’s unclear at the moment, however, which side of the boundary these five objects actually sit on. “We don’t know the true nature of these objects,” said Grieves, “because they’re so close to this limit.”

Some brown dwarfs may even be so starlike that they could host their own planets. “We know of some brown dwarf systems that look like they have protoplanetary disks around them,” said Kirkpatrick. “And there’s every indication that there are probably brown dwarfs that have their own exoplanets in orbit around them as well. A holy grail is to find a brown dwarf with a transiting exoplanet.”

On the opposite end of the brown dwarf mass scale lies the Accident. It’s an extremely small, cold and faint object — “just barely at the level where we could detect it,” said Kirkpatrick. Astronomers are eager to work out what the difference is between a low-mass brown dwarf and a high-mass gas giant planet. This makes small and faint brown dwarfs like the Accident useful targets.

The Accident also appears to be made of some strange stuff. As the universe ages, supernovas spit out lots of heavier elements such as carbon and oxygen — what astronomers call “metals.” Because of this, old objects that formed early in the universe’s history tend to have few metals, while new objects have more. Yet despite being found in our local solar neighborhood — home mostly to young, metal-rich stars — the Accident appears to be metal-poor. “We think this is probably an older brown dwarf, probably one that was created before the Milky Way had all the metal enrichment it does now,” said Kirkpatrick. Casewell added it was likely “one of the first brown dwarfs formed” in our galaxy, originating in the outer galactic halo surrounding the Milky Way and then migrating inward.

As with so many other phenomena in our universe, the finding highlights that these puzzling yet mysterious objects come in all manner of flavors, and fitting them into rigidly defined categories is no simple task. Caselden, meanwhile, hopes he can contribute more to the field in future, perhaps homing in on similar objects now he knows what to look for. “I want to find another Accident,” he said. “And I want to have it not be an accident.”

Correction: August 5, 2021
An earlier version of this article stated that stellar cores need to reach temperatures of 100 million degrees Celsius to sustain nuclear fusion. Extremely low-mass stars can burn hydrogen at temperatures as low as 3 million degrees Celsius.