Brown dwarfs fill the niche between stars and planets, never quite getting massive enough to start nuclear fusion, the process that causes stars to shine. More than 2,000 have been discovered so far, but a new addition is unlike all the others we've seen yet and so has been given the name “The Accident”. There could be a great many more like it, however.

As brown dwarfs form the gravitational potential energy of the material they draw in is transformed, making them very hot. However, with no source of heat beyond the decay of any radioactive elements they cool down over time, and we determine the age of the brown dwarfs we find from their temperatures, or we did until The Accident.

Properly known as WISEA J153429.75-104303.3, The Accident resists normal methods of temperature measurement, a paper in the Astrophysical Journal Letters reports, and was only found by sheer luck. Compared to other brown dwarfs The Accident is bright at some wavelengths and faint at others.

“This object defied all our expectations,” said lead author Dr Davy Kirkpatrick of Caltech in a statement. 

Kirkpatrick and co-authors' explanation is that The Accident is about 13 billion years old, indicating it formed not long after our galaxy, and is now easily the coolest-known brown dwarf, cold enough to freeze water. That explains why it is so faint at some wavelengths, but the reason for its brightness elsewhere is more complex.

Most brown dwarfs have atmospheres rich in methane. Methane absorbs certain wavelengths of infrared radiation, which is why it is such a powerful greenhouse gas. This makes these brown dwarfs faint at the relevant wavelengths. If The Accident has almost no methane, it would explain its relative brightness in those parts of the infrared spectrum.

The authors think The Accident is so old it dates to before the first stars spread carbon across the galaxy, and without carbon it can have no methane.

“It’s not a surprise to find a brown dwarf this old, but it is a surprise to find one in our backyard,” said co-author Dr Federico Marocco. The Accident is around 50 light-years away, the fourth closest brown dwarf yet found. “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.”

That doesn't mean there are enough brown dwarfs to make up the galaxy's dark matter, as was once proposed, but it does mean they could be important galaxy components.

Other theories have been presented to explain The Accident, such as an expelled exoplanet or even the stripped core of a star, but the paper shows these are very unlikely.

The Accident got its name, however, not from the fact it happens to be close, but from the way it was discovered. Images of it were collected by the Wide-field Infrared Survey Explorer space telescope in its revived (or NEOWISE) mission, but professional astronomers did not notice it because it is so faint.

Citizen Scientist Dan Caselden built his own program to search for brown dwarfs in NEOWISE data. He was studying one known brown dwarf while trying to train the program to ignore other types of astronomical objects when he noticed something very faint moving between NEOWISE images.

Caselden investigated, and found he'd hit on an object so unusual his program would never have recognized its nature.

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