WISEA J153429.75-104303.3 (WISE 1534-1043 for short) — nicknamed ‘The Accident’ by its discoverer, citizen scientist Dan Caselden — is a cold, very metal-poor brown dwarf located approximately 50 light-years away in the constellation of Libra.
This is an artist’s impression of a free-floating Y-dwarf. Image credit: NASA / JPL-Caltech.
As brown dwarfs age, they cool off, and their brightness in different wavelengths of light changes. It’s not unlike how some metals, when heated, go from bright white to deep red as they cool.
WISE 1534-1043 confused astronomers because it was faint in some key wavelengths, suggesting it was very cold (and old), but bright in others, indicating a higher temperature.
“This object defied all our expectations,” said Dr. Davy Kirkpatrick, an astrophysicist at IPAC/Caltech.
In their new study, Dr. Kirkpatrick and colleagues posit that WISE 1534-1043 might be between 10 and 13 billion years old — at least double the median age of other known brown dwarfs.
That means the brown dwarf would have formed when our Milky Way Galaxy was much younger and had a different chemical makeup.
The astronomers observed WISE 1534-1043 in infrared wavelengths with a ground-based telescope at the W.M. Keck Observatory.
But the object appeared so faint in those wavelengths, they couldn’t detect it at all, apparently confirming their suggestion that it was very cold.
They also determined the object’s distance — about 50 light-years from Earth — and realized that it is moving fast — about 800,000 kph (500,000 mph).
That’s much faster than all other brown dwarfs known to be at this distance from Earth, which means it has probably been careening around the Galaxy for a long time, encountering massive objects that accelerate it with their gravity.
WISE 1534-1043 was discovered by citizen scientist Dan Caselden. Image credit: NASA / JPL-Caltech / Dan Caselden.
With a mound of evidence suggesting WISE 1534-1043 is extremely old, the researchers propose that its strange properties aren’t strange at all and that they may be a clue to its age.
When the Milky Way formed about 13.6 billion years ago, it was composed almost entirely of hydrogen and helium. Other elements, like carbon, formed inside stars; when the most massive stars exploded as supernovae, they scattered the elements throughout the Galaxy.
Methane, composed of hydrogen and carbon, is common in most brown dwarfs that have a temperature similar to WISE 1534-1043. But the object’s light profile suggests it contains very little methane.
Like all molecules, methane absorbs specific wavelengths of light, so a methane-rich brown dwarf would be dim in those wavelengths. WISE 1534-1043, by contrast, is bright in those wavelengths, which could indicate low levels of methane.
Thus, the light profile of WISE 1534-1043 could match that of a very old brown dwarf that formed when the galaxy was still carbon poor; very little carbon at formation means very little methane in its atmosphere today.
“It’s not a surprise to find a brown dwarf this old, but it is a surprise to find one in our backyard,” said Dr. Federico Marocco, an astrophysicist at IPAC/Caltech.
“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.”
The study was published in the Astrophysical Journal Letters.
J. Davy Kirkpatrick et al. 2021. The Enigmatic Brown Dwarf WISEA J153429.75-104303.3 (a.k.a. ‘The Accident’). ApJL 915, L6; doi: 10.3847/2041-8213/ac0437