The brown dwarf WISEA J153429.75-104303.3 — happily nicknamed ‘The Accident’ — is peculiar enough that it may point to a rare population of extremely old brown dwarfs. Dan Caselden, a citizen scientist who built an online program to filter data from the NEOWISE spacecraft, is able to highlight brown dwarfs moving through the NEOWISE field with his software, and while looking at one, he caught a glimpse of another. Call that a lucky catch, because the object didn’t match his program’s profile of a conventional brown dwarf.

We’ve found about 2,000 brown dwarfs thus far, many using data from WISE — Wide Field Infrared Survey Explorer — which was launched in 2009, placed into hibernation in 2011 after its primary mission ended, and then reactivated in 2013 as NEOWISE, a repurposed spacecraft given the new goal of tracking near-Earth objects. WISE 1534?1043 — the shortened name of the object, used by the authors of a new paper on it — stands out from all previously known brown dwarfs because it seems to have very little methane, unlike the brown dwarfs we’re familiar with, among which methane is common at WISE 1534?1043’s temperatures.

What we may be looking at is the signature of an extremely old, and cold, brown dwarf that emerged at a time when the galaxy was low enough in carbon that little methane could form in its atmosphere. That’s the thesis of the paper in Astrophysical Journal Letters, whose lead author is Davy Kirkpatrick (Caltech). The authors believe WISE 1534?1043 may be between 10 and 13 billion years old, making it double the median age of known brown dwarfs.

Adding punch to the hypothesis is the fact that, at about 50 light years from Earth, WISE 1534?1043 is moving much faster — well over 200 kilometers per second — than any other brown dwarf at a comparable distance. This may imply gravitational acceleration from encounters sustained in a long, long lifetime. If one such ancient brown dwarf is out there, we should find others.

Co-author Federico Marocco (Caltech) led the new observations of WISE 1534?1043, which extend earlier studies of the object, using the Keck and Hubble instruments:

“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.”

Image: This video shows data from NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE), launched in 2009 under the moniker WISE. The moving object in the bottom left corner is a brown dwarf officially named WISEA J153429.75-104303.3 and nicknamed “The Accident.” Credit: NASA/JPL-Caltech/Dan Caselden.

This unusual object is too faint at all wavelengths, the paper notes, for spectroscopy to be used at any current facility. Thus the interpretations given in the Kirkpatrick paper rely on trends found among other better known objects extended “into a terra incognita guided by theoretical predictions.” In addition to the idea that it is an extremely old brown dwarf with the low metallicity of its origins in a young galaxy, the authors examine other possibilities.

Are we actually dealing with an extremely low mass young brown dwarf? Evidently not, as the methane deficiency is hard to reconcile with this option. Could WISE 1534?1043 be an ejected exoplanet? Here we need atmospheric data, which we don’t have, to examine the elements therein, knowing that giant planets in our Solar System are more metal rich than the Sun (carbon is enhanced in Jupiter by three times the amount found in the Sun, and more so in Saturn, Uranus and Neptune). Thus we would have a marker, if we could find it. From the paper:

Under this hypothesis, WISE 1534?1043 is photometrically unusual because such elemental differences would profoundly affect its atmospheric composition and emergent spectrum. Unfortunately, forward modeling that incorporates a wide array of elemental abundance differences does not yet exist, so our best method to test this hypothesis is atmospheric retrieval, once a suitable spectrum for WISE 1534?1043 is obtained.

A final possibility is an ultracold stellar remnant, meaning a white dwarf, but this explanation falls short due to models showing a white dwarf could not have cooled to these temperatures within the lifetime of the Milky Way. Thus only one conclusion seems likely:

We conclude that the unique object WISE 1534?1043 is most likely a cold, very metal-poor brown dwarf—perhaps even the first Y-type subdwarf…

But note this:

Verification, refutation, or further befuddlement should be possible via additional photometry and broad-wavelength spectroscopy from the James Webb Space Telescope.

‘Further befuddlement’ indeed! An honest comment about an extremely unusual object.

The paper is Kirkpatrick et al., “The Enigmatic Brown Dwarf WISEA J153429.75-104303.3 (a.k.a. “The Accident”),” Astrophysical Journal Letters Vol. 915, No. 1 (30 June 2021), L6 (abstract / full text).