Stellar surprise – astronomers uncover methane emission on a cold brown dwarf for the first time

Posted on: 17 April 2024

Published today in leading international journal Nature, the findings suggest this brown dwarf might generate aurorae similar to those seen on our own planet as well as on Jupiter and Saturn. 

More massive than planets but lighter than stars, brown dwarfs are ubiquitous in our solar neighborhood, with thousands identified. 

Last year, Jackie Faherty, a senior research scientist and senior education manager at the American Museum of Natural History, led a team of researchers awarded time on JWST to investigate 12 brown dwarfs. Among those was CWISEP J193518.59–154620.3 (or W1935 for short). This cold brown dwarf, 47 light years away, was co-discovered by Backyard Worlds: Planet 9 citizen science volunteer Dan Caselden and the NASA CatWISE team. 

W1935 is a cold brown dwarf with a surface temperature of about 200° Celsius, which is a similar temperature to that at which you’d bake chocolate chip cookies. The mass for W1935 isn’t well known but it likely ranges between 6–35 times that of Jupiter. 

After looking at a number of brown dwarfs observed with JWST, Faherty’s team noticed that W1935 looked similar but with one striking exception: it was emitting methane, something that’s never been seen before on a brown dwarf.  

This artist concept portrays the brown dwarf W1935. The team speculates that the methane emission may be due to processes generating aurorae, shown here in red. Credit: NASA, ESA, CSA, Leah Hustak (Space Telescope Science Institute).

“Methane gas is expected in giant planets and brown dwarfs but we usually see it absorbing light, not glowing,” said Faherty, the lead author of the study. “We were confused about what we were seeing at first but ultimately that transformed into pure excitement at the discovery.”

Computer modelling yielded another surprise: the brown dwarf likely has a temperature inversion, a phenomenon in which the atmosphere gets warmer with increasing altitude. Temperature inversions can easily happen to planets orbiting stars, but W1935 is isolated, with no obvious external heat source.  

“We were pleasantly shocked when the model clearly predicted a temperature inversion,” said co-author Ben Burningham from the University of Hertfordshire. “But we also had to figure out where that extra upper atmosphere heat was coming from.”

To investigate, the researchers turned to our solar system, looking at studies of Jupiter and Saturn, which both show methane emission and have temperature inversions. The likely cause for this feature on solar system giants is aurorae. Therefore, the research team surmised that they had uncovered that same phenomenon on W1935. 

Planetary scientists know that one of the major drivers of aurorae on Jupiter and Saturn is high-energy particles from the Sun that interact with the planets’ magnetic fields and atmospheres, heating the upper layers. This is also the reason for the aurorae that we see on Earth, commonly referred to as the Northern or Southern Lights since they are most extraordinary near the poles.  

But with no host star for W1935, a solar wind cannot contribute to the explanation. 

There is an enticing additional reason for the aurora in our solar system. Both Jupiter and Saturn have active moons that occasionally eject material into space, interact with the planets, and enhance the auroral footprint on those worlds.  

Jupiter’s moon Io is the most volcanically active world in the solar system, spewing lava fountains dozens of miles high, and Saturn’s moon Enceleadus ejects water vapor from its geysers that simultaneously freezes and boils when it hits space. 

More observations are needed, but the researchers speculate that one explanation for the aurora on W1935 might be an active, yet-to-be discovered moon.  

Speaking to the implications of the work, Johanna Vos, assistant professor in Trinity College Dublin’s School of Physics, said: “Our findings are really exciting because they offer the first glimpse at how aurorae might affect the atmospheres of worlds beyond our solar system. 

“While this was a completely unexpected discovery, we can now search for similar hints of aurorae in even more extrasolar atmospheres with JWST. We have never had a telescope with comparable sensitivity or wavelength range as JWST, so it’s not surprising that a whole range of exciting new discoveries are popping up across all fields of astrophysics. It really is an excellent time to be an astrophysicist!

“This has been a great project to work on from start to finish. It has been so satisfying to go from first noticing the mysterious little bump in our spectrum to eventually converging on the idea that aurorae are heating the upper atmosphere. And it’s not every day that you get to publish a Nature paper with its roots in citizen science – this work underlines the impact that citizen scientists can make and I’m very proud to be part of this fantastic team.”

This work was supported in part by NASA and the Space Telescope Science Institute.

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