The Exoplanet Pipeline

Looking into Astro2020’s recommendations for ground-based astronomy, I was heartened with the emphasis on ELTs (Extremely Large Telescopes), as found within the US-ELT project to develop the Thirty Meter Telescope and the Giant Magellan Telescope, both now under construction. Such instruments represent our best chance for studying exoplanets from the ground, even rocky worlds that could hold life. An Astro2020 with different priorities could have spelled the end of both these ELT efforts in the US even as the European Extremely Large Telescope, with its 40-meter mirror, moves ahead, with first light at Cerro Armazones (Chile) projected for 2027.

So the ELTs persist in both US and European plans for the future, a context within which to consider how planet detection continues to evolve. So much of what we know about exoplanets has come from radial velocity methods. These in turn rely critically on spectrographs like HARPS (High Accuracy Radial Velocity Planet Searcher), which is installed at the European Southern Observatory’s 3.6m telescope at La Silla in Chile, and its successor ESPRESSO (Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations). We can add the NEID spectrometer on the WIYN 3.5m telescope at Kitt Peak to the mix, now operational and in the hunt for ever tinier Doppler shifts in the light of host stars.

We’re measuring the tug a planet puts on its star by looking radially — how is the star pulled toward us, then away, as the planet moves along its orbit? Given that the Earth produces a movement of a mere 9 centimeters per second on the Sun, it’s heartening to see that astronomers are closing on that range right now. NEID has demonstrated a precision of better than 25 centimeters per second in the tests that led up to its commissioning, giving us another tool for exoplanet detection and confirmation.

But this is a story that also reminds us of the vast amount of data being generated in such observations, and the methods needed to get this information distributed and analyzed. On an average night, NEID will collect about 150 gigabytes of data that is sent to Caltech, and from there via a data management network called Globus to the Texas Advanced Computing Center (TACC) for analysis and processing. TACC, in turn, extracts metadata and returns the data to Caltech for further analysis. The results are made available by the NASA Exoplanet Science Institute via its NEID Archive.

Image: The NEID instrument is shown mounted on the 3.5-meter WIYN telescope at the Kitt Peak National Observatory. Credit: NSF’s National Optical-Infrared Astronomy Research Laboratory/KPNO/NSF/AURA.

What a contrast with the now ancient image of the astronomer on a mountaintop coming away with photographic plates that would be analyzed with instruments like the blink comparator Clyde Tombaugh used to discover Pluto in 1930. The data now come in avalanche form, with breakthrough work occurring not only on mountaintops but in the building of data pipelines like these that can be generalized for analysis on supercomputers. The vast caches of data contain the seeds of future discovery.

Joe Stubbs leads the Cloud & Interactive Computing group at TACC:

“NEID is the first of hopefully many collaborations with the NASA Jet Propulsion Laboratory (JPL) and other institutions where automated data analysis pipelines run with no human-in-the-loop. Tapis Pipelines, a new project that has grown out of this collaboration, generalizes the concepts developed for NEID so that other projects can automate distributed data analysis on TACC’s supercomputers in a secure and reliable way with minimal human supervision.”

NEID also makes a unique contribution to exoplanet detection by being given over to the analysis of activity on our own star. Radial velocity is vulnerable to confusion over starspots — created by convection on the surface of exoplanet host stars and mistaken for planetary signatures. The plan is to use NEID during daylight hours with a smaller solar telescope developed for the purpose to track this activity. Eric Ford (Penn State) is an astrophysicist at the university where NEID was designed and built:

“Thanks to the NEID solar telescope, funded by the Heising-Simons Foundation, NEID won’t sit idle during the day. Instead, it will carry out a second mission, collecting a unique dataset that will enhance the ability of machine learning algorithms to recognize the signals of low-mass planets during the nighttime.”

Image: A new instrument called NEID is helping astronomers scan the skies for alien planets. TACC supports NEID with supercomputers and expertise to automate the data analysis of distant starlight, which holds evidence of new planets waiting to be discovered. WIYN telescope at the Kitt Peak National Observatory. Credit: Mark Hanna/NOAO/AURA/NSF.

Modern astronomy in a nutshell. We’re talking about data pipelines operational without human intervention, and machine-learning algorithms that are being tuned to pull exoplanet signals out of the noise of starlight. In such ways does a just commissioned spectrograph contribute to exoplanetary science through an ever-flowing data network now indispensable to such work. Supercomputing expertise is part of the package that will one day extract potential biosignatures from newly discovered rocky worlds. Bring on the ELTs.


Going After Sagittarius A*

Only time will tell whether humanity has a future beyond the Solar System, but if we do have prospects among the stars — and I fervently hope that we do — it’s interesting to speculate on what future historians will consider the beginning of the interstellar era. Teasing out origins is tricky. You could label the first crossing of the heliopause by a functioning probe (Voyager 1) as a beginning, but neither the Voyagers nor the Pioneers (nor, for that matter, New Horizons) were built as interstellar missions.

I’m going to play the ‘future history’ game by offering my own candidate. I think the image of the black hole in the galaxy M87 marks the beginning of an era, one in which our culture begins to look more and more at the universe beyond the Solar System. I say that not because of what we found at M87, remarkable as it was, but because of the instrument used. The creation of a telescope that, through interferometry, can create an aperture the size of our planet speaks volumes about what a small species can accomplish. An entire planet is looking into the cosmos.

So will some future historian look back on the M87 detection as the beginning of the ‘interstellar era’? No one can know, but from the standpoint of symbolism — and that’s what this defining of eras is all about — the creation of a telescope like this is a civilizational accomplishment. I think its cultural significance will only grow with time.

Image: Composite image showing how the M87 system looked, across the entire electromagnetic spectrum, during the Event Horizon Telescope’s April 2017 campaign to take the iconic first image of a black hole. Requiring 19 different facilities on the Earth and in space, this image reveals the enormous scales spanned by the black hole and its forward-pointing jet, launched just outside the event horizon and spanning the entire galaxy. Credit: the EHT Multi-Wavelength Science Working Group; the EHT Collaboration; ALMA (ESO/NAOJ/NRAO); the EVN; the EAVN Collaboration; VLBA (NRAO); the GMVA; the Hubble Space Telescope, the Neil Gehrels Swift Observatory; the Chandra X-ray Observatory; the Nuclear Spectroscopic Telescope Array; the Fermi-LAT Collaboration; the H.E.S.S. collaboration; the MAGIC collaboration; the VERITAS collaboration; NASA and ESA. Composition by J.C. Algaba.

Into the Milky Way’s Heart

The Event Horizon Telescope (EHT) is not a single physical installation but a collection of telescopes around the world that use Very Long Baseline Interferometry to produce a virtual observatory with, as mentioned above, an aperture the size of our planet. Heino Falcke’s book Light in the Darkness (HarperOne, 2021) tells this story from the inside, and it’s as exhilarating an account of scientific research as any I’ve read.

M87 seemed in some ways an ideal target, with a black hole thought to mass well over 6 billion times more than the Sun. In terms of sheer size, M87 dwarfed estimates of the Milky Way’s supermassive black hole (Sgr A*), which weighs in at 4.3 million solar masses, but it’s also 2,000 times farther away. Even so, it was the better target, for M87 was well off the galactic plane, whereas astronomers hoping to study the Milky Way’s black hole have to contend with shrouds of gas and dust and the fact that, while average quasars consume one sun per year, Sgr A* pulls in 106 times less.

But the investigation of Sgr A* continues as new technologies come into play, with the James Webb Space Telescope now awaiting launch in December and already on the scene in French Guiana. Early in JWST’s observing regime, Sgr A* is to be probed at infrared wavelengths, adding the new space-based observatory to the existing Event Horizon Telescope. Farhad Yusef-Zadeh, principal investigator on the Webb Sgr A* program, points out that JWST will allow data capture at two different wavelengths simultaneously and continuously, further enhancing the EHT’s powers.

Among other reasons, a compelling driver for looking hard at Sgr A* is the fact that it produces flares in the dust and gas surrounding it. Yusef-Zadeh (Northwestern University) notes that the Milky Way’s supermassive black hole is the only one yet observed with this kind of flare activity, which makes it more difficult to image the black hole but also adds considerably to the scientific interest of the investigation. The flares are thought to be the result of particles accelerating around the object, but details of the mechanism of light emission here are not well understood.

Image: An enormous swirling vortex of hot gas glows with infrared light, marking the approximate location of the supermassive black hole at the heart of our Milky Way galaxy. This multiwavelength composite image includes near-infrared light captured by NASA’s Hubble Space Telescope, and was the sharpest infrared image ever made of the galactic center region when it was released in 2009. While the black hole itself does not emit light and so cannot be detected by a telescope, the EHT team is working to capture it by getting a clear image of the hot glowing gas and dust directly surrounding it. Credit: NASA, ESA, SSC, CXC, STS.

Thus we combine radio data from the Event Horizon Telescope with JWST’s infrared data. How different wavelengths can tease out more information is evident in the image above. Here we have a composite showing Hubble near-infrared observations in yellow, and deeper infrared observations from the Spitzer Space Telescope in red, while light detected by the Chandra X-Ray Observatory appears in blue and violet. Flare detection and better imagery of the region as enabled by adding JWST to the EHT mix, which will include X-ray and other observatories, should make for the most detailed look at Sgr A* that has ever been attempted.

What light we detect associated with a black hole is from the accreting material surrounding it, with the event horizon being its inner edge — this is what we saw in the famous M87 image. The early JWST observations, expected in its first year of operation, are to be supplemented by further work to build up our knowledge of the flare activity and enhance our understanding of how Sgr A* differs from other supermassive black holes.

Image: Heated gas swirls around the region of the Milky Way galaxy’s supermassive black hole, illuminated in near-infrared light captured by NASA’s Hubble Space Telescope. Released in 2009 to celebrate the International Year of Astronomy, this was the sharpest infrared image ever made of the galactic center region. NASA’s upcoming James Webb Space Telescope, scheduled to launch in December 2021, will continue this research, pairing Hubble-strength resolution with even more infrared-detecting capability. Of particular interest for astronomers will be Webb’s observations of flares in the area, which have not been observed around any other supermassive black hole and the cause of which is unknown. The flares have complicated the Event Horizon Telescope (EHT) collaboration’s quest to capture an image of the area immediately surrounding the black hole, and Webb’s infrared data is expected to help greatly in producing a clean image. Credit: NASA, ESA, STScI, Q. Daniel Wang (UMass).

Whether we’re entering an interstellar era or not, we’re going to be learning a lot more about the heart of the Milky Way, assuming we can get JWST aloft. How many hopes and plans ride on that Ariane 5!


Interesting Transient: A New Class of Object toward Galactic Center?

The 36 dish antennae at ASKAP — the Australian Square Kilometre Array Pathfinder in outback Western Australia — comprise an interferometer with a total collecting area of about 4,000 square meters. ASKAP has commanded attention as a technology demonstrator for the planned Square Kilometer Array, but today we’re looking at the discovery of a highly polarized, highly variable radio source labeled ASKAP J173608.2?321635, about 4 degrees from galactic center in the galactic plane.

According to Ziteng Wang, who is lead author of the study on this signal and a University of Sydney PhD student, the observations are strikingly different from other variable radio sources:

“The strangest property of this new signal is that it has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time. The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We’ve never seen anything like it.”

Variable celestial objects are common enough, from supernovae to pulsars, not to mention interesting sources like Fast Radio Bursts and, of course, the Cepheid variable stars that have played such a large role in astronomical history in helping us determine the scale of the universe. Any new variable source might be looked upon in light of such objects, perhaps as a type of flare star intermittently spewing out bursts of radiation. But none of these match the odd behavior of the new source. While J173608.2?321635 was found at ASKAP, Wang and team performed follow-up observations with the MeerKET telescope in South Africa.

So we have a source toward galactic center that is at first unseen, then brightens, fades, and reappears. Having detected six such signals from the source over nine months in 2020, the astronomers searched in vain for it in visible light, even as a search with the Parkes radio telescope turned up nothing. That’s when the team turned to MeerKAT, where it was once again detected. Tara Murphy, who is Wang’s PhD supervisor at Sydney, notes what happened next:

“Because the signal was intermittent, we observed it for 15 minutes every few weeks, hoping that we would see it again. Luckily, the signal returned, but we found that the behaviour of the source was dramatically different — the source disappeared in a single day, even though it had lasted for weeks in our previous ASKAP observations.”

Image: The ASKAP telescope array. Credit: CSIRO.

Other low frequency transients from galactic center have been detected in recent years, including GCRT J1745-3009, which was quickly labeled a ‘burper’ by its discoverers due to its intermittent bursts after detection in 1998. Five bursts of equal brightness were noted, each about ten minutes in duration, and occurring every 77 minutes. No explanation has been agreed upon for that one either, although a pulsar, a neutron star pair, or a radio-emitting white dwarf have all been discussed in the literature.

For the ASKAP transient, the authors have considered pulsar scenarios, a transient magnetar, and “a low-mass star/substellar object with extremely low infrared luminosity,” with none of these providing a satisfactory answer. The suspicion grows that this is a new class of objects that future radio imaging surveys will observe as our capabilities improve. With the Square Kilometer Array coming online in the next decade, we are probably looking at a phenomenon that will generate a great deal of study and, doubtless, many more examples.

The paper is Wang et al., “Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder,” The Astrophysical Journal Vol. 920, No. 1 (12 October 2021), 45. Abstract.


Adjusting the Clock: Hydrogen Burning in White Dwarfs

White dwarfs have turned out to be more interesting than I had imagined. We know how they form: A star like the Sun exhausts the hydrogen in its core and swells into a red giant, a scenario that is a trope in science fiction, as it posits an Earth of the far-future incinerated by its star. Losing its outer layers near the end of nuclear burning, a red giant ultimately leaves behind an object with much of the mass of the Sun now crammed into a white dwarf that is about the size of the Earth. For years I assumed white dwarfs were dead ends, a terminus for life whose only function seemed to be in binary systems, where they could be the locus, through accretion from the other star, of a stellar explosion in the form of a nova.

Lately we’ve been learning, though, that through analysis of their atmospheres, white dwarfs can yield information about objects that have fallen into them, such as remnants of the original stellar system. Some white dwarfs may have habitable zones lasting several billion years, an interesting thought if surviving planets in the system migrate inward. And now, thanks to Hubble data, we are learning that white dwarfs may have a longer lifetime than previously thought. Continuing to burn hydrogen, some of these stars may be older than they look.

The work was coordinated by Francesco Ferraro (Alma Mater Studiorum Università di Bologna / Italian National Institute for Astrophysics), who points to the useful role of white dwarfs in determining the ages of stars. The white dwarf cooling rate has been used as a natural clock as a way of calibrating the age of star clusters, as their cooling seemed easy to model by following the relationship between age and temperature. But the new study finds that white dwarf aging is nowhere near as simple as this, so an accepted method of measuring cluster ages may have to be re-examined.

To investigate white dwarf aging, the scientists looked at the globular clusters M3 in the constellation Canes Venatici and M13 in Hercules, in which stars in aggregate show common values of metallicity and age. As always in astronomy, the word ‘metals’ refers to elements higher than hydrogen and helium, with the bulk of a star like the Sun being made up of the latter two, while a scant 1.3 percent is given over to metals.

Ferraro’s team compared cooling white dwarfs in the two clusters and found that among the stars that will eventually become white dwarfs, the color of stars in the Horizontal Branch is bluer in M13 than in M3. The Horizontal Branch is a stage in the evolution of a star in which it has begun helium burning in the core, a stage that is flagged by a reduction in luminosity and increases in surface temperature. So the enhanced blue color in M13 is indicative of hotter stars on their way toward white dwarf status.

Image: To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity, but the populations of stars which will eventually give rise to white dwarfs are different. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool. Credit: ESA/Hubble & NASA, G. Piotto et al.

The researchers consider the M3 and M13 clusters to be “a classical horizontal branch (HB) morphology pair” because they share many physical properties including metallicity, making the color difference the salient feature. Homing in on the white dwarfs among this population, the team finds the outer envelope of hydrogen in these bluer white dwarfs allows them to burn for longer and cool more slowly than the standard white dwarf model. Using near-ultraviolet data from Hubble’s Wide Field Camera 3, the team compared more than 700 white dwarfs in the two clusters.

The result: M3’s white dwarfs follow the expected model, consisting of predictably cooling stellar cores and no stable thermonuclear activity. But in M13, two populations of white dwarfs can be found, the second being those that have retained an outer hydrogen envelope, continue thermonuclear burning and therefore cool at a slower pace. In fact, 70 percent of the white dwarfs in M13 appear to be burning hydrogen on their surface. This appears to be the only viable explanation for the ‘blue tail’ found in the Hubble data that distinguishes the two clusters.

From the paper:

At the moment, this [hydrogen burning on the surface] appears to be the most viable and natural explanation, while alternative scenarios should invoke ad hoc and unknown mechanisms able to increase the production or slow down the cooling process of the WDs in M13, and not in M3. The discovery reported in this paper represents the first direct evidence for the occurrence of stable nuclear burning in the residual hydrogen envelope of cooling WDs and offers an empirical measure of the delay in the flow of time marked by the WD clock in the presence of slowly cooling WDs.

The authors believe the road ahead should involve studying other clusters that show differences when at the Horizontal Branch of stellar evolution, while also examining clusters with different degrees of metallicity to determine the role it plays. Clarifying how white dwarfs evolve will force us to adjust the use of these stellar remnants in calibrating age, where current uncertainties can be as large as a billion years.

Adds Ferraro:

“Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old. We are now investigating other clusters similar to M13 to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly.”

The paper is Chen et al., “Slowly cooling white dwarfs in M13 from stable hydrogen Burning,” Nature Astronomy 6 September 2021 (abstract).


Predicting a Supernova in 2037

Here’s a story that’s both mind-bending and light-bending. It involves a supernova that, on the one hand, happened 10 billion years ago, and on the other hand, has appeared in our skies not once but three times, with a fourth in the works. In play here is gravitational lensing, in which light from a background galaxy bends around a foreground galactic cluster known as MACS J0138.0-2155. Out of this we get multiple mirror images, and researchers predict another supernova appearance in the year 2037.

Three of the appearances of the supernova, labeled AT 2016jka and nicknamed ‘Requiem,’ are in the image below, a Hubble view from 2016, all three circled for ease of identification. The light of the supernova has been split into different images by the lensing effect. Using archival data, researchers led by Steve Rodney (University of South Carolina) have analyzed differences in brightness and color that reflect different phases of the event as the supernova faded.

“This new discovery is the third example of a multiply imaged supernova for which we can actually measure the delay in arrival times,” says Rodney. “It is the most distant of the three, and the predicted delay is extraordinarily long. We will be able to come back and see the final arrival, which we predict will be in 2037, plus or minus a couple of years.”

Image: Three views of the same supernova appear in the 2016 image on the left, taken by the Hubble Space Telescope. But they’re gone in the 2019 image. The distant supernova, named Requiem, is embedded in the giant galaxy cluster MACS J0138. The cluster is so massive that its powerful gravity bends and magnifies the light from the supernova, located in a galaxy far behind it. Called gravitational lensing, this phenomenon also splits the supernova’s light into multiple mirror images, highlighted by the white circles in the 2016 image. The multiply imaged supernova disappears in the 2019 image of the same cluster, at right. The snapshot, taken in 2019, helped astronomers confirm the object’s pedigree. Supernovae explode and fade away over time. Researchers predict that a rerun of the same supernova will make an appearance in 2037. The predicted location of that fourth image is highlighted by the yellow circle at top left. The images were taken in near-infrared light by Hubble’s Wide Field Camera 3. Image processing credit: Joseph DePasquale (STScI).

Cluster and supernova are at vastly different distances from us, with the light from the lensing cluster MACS J0138.0-2155 taking about four billion years to reach us, while the light from the supernova has traveled an estimated 10 billion years. Computer modeling makes the call on the supernova’s return appearance as researchers untangle the complex path followed by the light.

In fact, says Rodney, the longer delay in the predicted 2037 light is the result of its traveling through the middle of the cluster and thus encountering the densest amount of dark matter. While dark matter remains controversial in many ways, it’s telling that the assumption of dark matter in the cluster explains the current three images and makes the call on the upcoming fourth. There appears to be a likelihood for a fifth appearance some time after the 2037 event, although the prediction is that it will be extremely hard to detect.

The lensing supernova images were discovered by Gabe Brammer (Niels Bohr Institute, University of Copenhagen), who found the three mirrored images while analyzing lensing magnification effects for the REQUIEM (REsolved QUIEscent Magnified Galaxies ) program, which uses Hubble data. Comparing the 2019 data with data from three years earlier showed that what he thought was a single image of a lensed galaxy had disappeared. Says Brammer:

“But then, on further inspection of the 2016 data, I noticed there were actually three magnified objects, two red and a purple. Each of the three objects was paired with a lensed image of a distant massive galaxy. Immediately it suggested to me that it was not a distant galaxy but actually a transient source in this system that had faded from view in the 2019 images like a light bulb that had been flicked off.”

Co-author Johan Richard (University of Lyon) developed a map of the amount of dark matter in the foreground cluster that drew on inferences from the lensing effects found in Brammer’s data. The map fits with the locations of the lensed objects based on Richard’s assumptions. Analyzing the fourth image in 2037, assuming all happens as expected, will allow astronomers to more accurately measure the time delays between the four images, which in turn will yield further data on the distortions to spacetime through which the light transited. Adds Rodney:

“These long time delays are particularly valuable because you can get a good, precise measurement of that time delay if you are just patient and wait years, in this case more than a decade, for the final image to return. It is a completely independent path to calculate the universe’s expansion rate. The real value in the future will be using a larger sample of these to improve the precision.”

In the excerpt from the paper below, MRG-M0138 refers to the background galaxy containing the supernova, which is being lensed by the foreground galaxy cluster MACS J0138.0-2155:

We model the mass distribution in the cluster core as the combination of a cluster-scale and galaxy-scale potentials… From this model we derive estimates for the lensing magnification and time delay of each of the SN images, including two predicted future images… The lens model predicts that the SN should appear in the fourth MRG-M0138 image in the year 2037±2, demagnified with µ = 0.4 ± 0.2. A fifth image will also appear at a still later date, located near the center of the cluster and much more significantly demagnified, so it will not be easily observable. We anticipate that future lens modelling of the cluster will improve on these predictions primarily by exploring a wider range of mass models and incorporating more observational constraints.

The building of the model that makes the 2037 prediction is fascinating, fully explicated in the ‘Methods’ section of the paper, as the researchers use a software program called LENSTOOL to develop five lens models that would yield the lensing effects seen in the data. The best fit model was then used to predict the magnification and time delays for the three images observed by Hubble, as well as the location of the fourth and fifth images, which have yet to appear. An education on lens modeling is available here for those interested in digging into the details.

The name Requiem comes into use for a reason beyond the reference to the REQUIEM program. I like the note at the end of the paper that explains it:

HST observations enabled us to find this SN. We anticipate that HST may be deorbited and make its final plummet to Earth around the time of the reappearance of AT 2016jka, so we coin the name SN Requiem as an ode to the vast new discovery space that HST continues to unveil.

Nicely put.

The paper is Rodney et al., “A gravitationally lensed supernova with an observable two-decade time delay,” Nature Astronomy 13 September 2021 (abstract / preprint).


Odd Find: An Ancient Brown Dwarf?

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).