The accidental discovery of a brown dwarf that moves on the border between star and planet, and which they called "The Accident", breaks the astronomers' data so far for these objects.
Dan Caselden woke up late at night, on November 3, 2018, and started playing the video game Counter-Strike, when he wrote an astronomy story. Each time he died, he flew to his laptop to check his automatic search of NASA space telescope images.
Suddenly, in the early hours of the morning, something strange happened. As Kasselden said: "It was very strange. It was moving faster than anything I have discovered. It was pale and fast, which made it very strange. "
Castleden emailed the astronomers he was working with on the Backyard Worlds: Planet 9 project . Once they had ruled out an artifact in the image, they realized they were looking at something completely unusual: an extremely faint object, 50 light-years away, blazing through the galaxy at 200 kilometers per second. It was given the name WISE 1534-1043, but because of its unique characteristics and chance discovery, it soon earned the nickname “The Accident . ”
Astronomers now believe that Casselden has found a brown dwarf, a failed star that lacks the mass needed to start nuclear fusion at its core. “It forms like a star,” said Sarah Casewell , an astronomer at the University of Leicester in the United Kingdom, “However, it never gains enough mass to fuse hydrogen into helium and start burning anything.”
The discovery of the "Accident" showed that we still have a lot to learn about brown dwarfs. These objects range in mass from about 13 times the mass of Jupiter to 75 times or more, but exactly where these two limits are is a constant dilemma.
“Astronomers argue about this lower limit all the time,” said Beth Biller , an astronomer at the University of Edinburgh in the United Kingdom. While 13 Jupiter masses is roughly the mass at which deuterium fusion can occur (the characteristic that differentiates brown dwarfs from gas giant planets), the limit can vary. “There’s nothing special about 13 Jupiter masses,” Biller said.
According to Biller, “Brown dwarfs also vary greatly in temperature. The hottest ones have surface temperatures of about 2.000 degrees Celsius. The coldest ones are below 200 degrees. Because they don’t have their own heat source, brown dwarfs will gradually cool down over billions of years to these lower temperatures. Dwarfs, which blur the line between planets and brown dwarfs, can be even colder. An object called WISE 0855-0714 lies below this freezing point. It is the coldest object we know of outside our solar system.”
What a brown dwarf might look like up close is also unclear. Despite their name, proposed by astronomer Jill Tarter in 1975 , they probably aren't brown. They're more likely 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 can produce some kind of storm, like 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 monitored the elements that appeared in its atmosphere as it rotated, and then compared that speed with a measurement of the object's internal rotation speed, gleaned from its magnetic field. By comparing the two values, the researchers calculated a wind speed of more than 2.300 kilometers per hour, five times faster than Jupiter's winds.
Because brown dwarfs bridge the gap between stars and planets, they can help us understand both. At the top of the mass scale, the boundary between the largest brown dwarfs and the smallest stars can give us information about how nuclear fusion begins.
An object must reach temperatures of about 3 million degrees Celsius at its core to start nuclear fusion, said Nolan Grieves of the University of Geneva in Switzerland. This triggers a chain reaction that converts hydrogen into helium. But no one is exactly sure how much mass is needed for this to happen and at what point a brown dwarf becomes a star. “There are many aspects of stellar evolution that our knowledge is still relatively uncertain about,” said Biller. “Where exactly that fusion limit lies is one of those questions.”
Recent work led by Nolan Grieves has identified five high-mass brown dwarfs , with masses between 77 and 98 times that of Jupiter. “They’re right on the boundary where hydrogen fusion starts to happen,” Grieves said. However, it’s not clear at this time which side of the boundary these five objects are on. “We don’t know the true nature of these objects,” Grieves said, “because they’re so close to that boundary.”
Some brown dwarfs may even be so magnetically strong that they could host their own planets . "We know of some brown dwarf systems that look like they have protoplanetary disks around them," Kirkpatrick said. "And there's every indication that there are probably brown dwarfs that have their own exoplanets orbiting them."
At the opposite end of the mass scale of a brown dwarf is "The Accident". It is an extremely small, cold and dim object, "just at the level we could detect it," Kirkpatrick said. Astronomers are eager to find out the difference between a low-mass brown dwarf and a giant massive gas planet. This makes small and dim brown dwarfs, like Accident, useful targets.
The Accident also seems to be made of some strange things. As the universe ages, supernovae spit out heavier elements, such as carbon and oxygen, what astronomers call "metals." Because of this, old objects formed early in the history of the universe tend to have fewer metals, while new objects tend to have more.
However, despite being found in our local solar neighborhood, which hosts mostly young, metal-rich stars, Accident seems to be poor in minerals. "We think this is probably an old brown dwarf, probably created before our Galaxy," Kirkpatrick said. Casewell added that it was probably "one of the first brown dwarfs to form" in our galaxy, coming from the outer galactic halo surrounding our galaxy and then migrating inward.
As with so many other phenomena in our universe, the "Accident" finding emphasizes that these enigmatic and mysterious objects come in all sorts of forms and that placing them in strictly defined categories is not a simple matter.
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