There might be more brown dwarfs lurking in our galaxy than previous thought

An accidental discovery hints at a hidden population of cosmic objects.

NASA scientists recently reported about the discovery of a brown dwarf called WISEA J153429.75-104303.3– nicknamed ‘the Accident.’ It got its name ‘the Accident’ because it was discovered by sheer luck and slipped past normal searches because it doesn’t resemble more than 2,000 brown dwarfs in our galaxy.

Normally, brown dwarfs cool off and change their brightness as they age. The ‘Accident’ is very faint in some key wavelengths- indicating it is very old and cool. Some of its key wavelengths are bright- indicating higher temperature.

Scientists think that the object must be 10 billion to 13 billion years old. It would have formed when our galaxy was much younger and had a different chemical makeup. This discovery suggests that there might be more brown dwarfs in Milky Way galaxy than scientists previously thought.

The Accident was first spotted by NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE). It was discovered by citizen scientist Dan Caselden, who was using an online program he built to find brown dwarfs in NEOWISE data.

Scientists wanted to determine the reason behind some extraordinary properties of this peculiar cosmic object. To do so, they observed it in additional infrared wavelengths with a ground-based telescope at the W. M. Keck Observatory in Hawaii. But the brown dwarf appeared so faint in those wavelengths; they couldn’t detect it at all. Based on this, they confirmed that the object is very cold.

According to precise distance measurements by NASA’s Hubble and Spitzer Space Telescopes, this brown dwarf is moving so fast– about half a million miles per hour (800,000 kph). Considering the speed, scientists concluded that it has probably been careening around the galaxy for a long time, encountering massive objects that accelerate it with their gravity. The data also revealed the object’s distance – about 50 light-years from Earth.

This mosaic shows the entire sky imaged by the Wide-field Infrared Survey Explorer
This mosaic shows the entire sky imaged by the Wide-field Infrared Survey Explorer (WISE). Infrared light refers to wavelengths that are longer than those visible to the human eye. Many cosmic objects radiate infrared, including gas and dust clouds where stars form, and brown dwarfs. Credits: NASA/JPL-Caltech/UCLA

Based on the evidence, scientists suggested that the properties of ‘the Accident’ are not strange at all. Instead, they may be clues to its age.

When it comes to composition, scientists found that the object has very little methane. Like all molecules, methane absorbs specific wavelengths of light, so a methane-rich brown dwarf would be dim in those wavelengths. The Accident, by contrast, is bright in those wavelengths, which could indicate low levels of methane.

Federico Marocco, an astrophysicist at IPAC at Caltech who led the new observations using the Keck and Hubble telescopes, said, “It’s not a surprise to find a brown dwarf this old, but it is a surprise to find one in our backyard. We expected that brown dwarfs this old exist, but we also expected them to be incredibly rare. The chance of finding one so close to the solar system could be a lucky coincidence, or it tells us that they’re more common than we thought.”

Can you see the dark spot moving in the bottom left corner of the screen? It’s a brown dwarf nicknamed “The Accident,” which was discovered by citizen scientist Dan Caselden. It had slipped past typical searches because it doesn’t look like any other known brown dwarfs.
Credits: NASA/JPL-Caltech/Dan Caselden

Davy Kirkpatrick, an astrophysicist at IPAC at Caltech in Pasadena, California, said, “This discovery is telling us that there’s more variety in brown dwarf compositions than we’ve seen so far. There are likely more weird ones out there, and we need to think about how to look for them.”

While highlighting moving objects with similar characteristics to known brown dwarfs, Caselden found one such brown dwarf candidate when he spotted another, a much fainter object moving quickly across the screen. This would turn out to be WISEA J153429.75-104303.3. The object hadn’t been highlighted because it did not match Caselden’s program’s profile of a brown dwarf.

Journal Reference:
  1. J. Davy Kirkpatrick et al. The Enigmatic Brown Dwarf WISEA J153429.75-104303.3 (a.k.a. “The Accident”). DOI: 10.3847/2041-8213/ac0437

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