Brrr. JWST Looks at the Coldest Brown Dwarf

What are the atmospheric compositions of cold brown dwarf stars? This is what a recent study published in The Astronomical Journal hopes to address as an international team of researchers used NASA’s James Webb Space Telescope (JWST) to investigate the coldest known brown dwarf star, WISE J085510.83?071442.5 (WISE 0855). This study holds the potential to help astronomers better understand the compositions of brown dwarf stars, which are also known as “failed stars” since while they form like other stars, they fail to reach the necessary mass to produce nuclear fusion. So, what was the motivation behind using JWST to examine the coldest known brown dwarf star?

“The coldest brown dwarfs are brightest at infrared wavelengths and extremely faint and difficult to observe at visible wavelengths, so they are very well suited for JWST,” Dr. Kevin Luhman, who is a professor in the Department of Astronomy and Astrophysics at Penn State University and lead author of the study, tells Universe Today. “The target of our paper, WISE 0855, is one of the most appealing targets of any kind for JWST because it is the coldest brown dwarf and is very close to our solar system (the fourth closest system). It is such an obvious object to observe with JWST that it was selected (by multiple teams) for guaranteed time observations with all of the instruments on JWST.”

Dr. Luhman was responsible for discovering WISE 0855, which is located approximately 7.43 light-years from Earth, announcing his findings in a 2014 paper published in The Astrophysical Journal Letters. He concluded that WISE 0855 exhibited a surface temperature of approximately 250 Kelvin (K), henceforth dubbing WISE 0855 as the coldest known brown dwarf star. For context, our Sun’s surface temperature is just under 5800 K, making WISE 0855’s surface temperature less than 5 percent of our Sun. Additionally, Dr. Luhman is responsible for discovering the third closest system, Luhman 16, which is a binary brown dwarf system located approximately 6.5 light-years from Earth.

For this study, the researchers used JWST’s Near Infrared Spectrograph (NIRSpec) instrument to examine the atmospheric composition of WISE 0855, to include making new measurements of the surface temperature, which the team concluded is 285 K using several computer models for their calculations. They also attempted to detect phosphine (PH₃), which they note has been identified in Y-class brown dwarf stars, along with searching for evidence of water ice clouds based on previous ground-based research. Therefore, what are the most significant results from this study?

“As discussed in our paper, the spectrum produced by NIRSpec is far superior to previous spectroscopy of WISE 0855, which allows much better characterization of its atmosphere, and better testing of theoretical models for cool, planet-like atmospheres,” Dr. Luhman tells Universe Today. “For instance, the NIRSpec data show that WISE 0855 does not have phosphine (PH₃) in its atmosphere, unlike Jupiter’s atmosphere, which is difficult to explain. In addition, there has been a debate in previous studies about whether WISE 0855 shows evidence of water ice clouds (it should be just cold enough that it could have water ice in its atmosphere). We find that the data can be reproduced reasonably well with models that do not have clouds, so it remains unclear whether water ice clouds are present.”

The study mentions how better models and unpublished spectroscopy data from JWST’s Mid-Infrared Instrument (MIRI) could help identify the presence of water ice clouds, with Dr. Luhman telling Universe Today how another team of researchers used NIRSpec in November 2023 to identify spectroscopy variances over time that could contribute to this, as well. As noted, brown dwarf stars are considered “failed stars” since they do not become large enough to produce nuclear fusion like our Sun. Therefore, what is the importance of studying brown dwarf stars?

Dr. Luhman tells Universe Today, “Brown dwarfs are important because they allow us to study the process of star formation in an extreme range of masses (below 10 Jupiter masses), and they allow us to study cool atmospheres that may be similar to those of gas giant planets.”

WISE 0855 does not currently possess any known exoplanets, with exoplanets orbiting brown dwarf stars being incredibly rare finds. One example includes a 2004 study published in Astronomy & Astrophysics identified exoplanet, 2M1207b, orbiting at approximately 55 astronomical units (AU) from its brown dwarf parent star, and is located approximately 170 light-years from Earth. A few years later, a 2008 study published in The Astrophysical Journal identified MOA-2007-BLG-192Lb, which was the first exoplanet discovered orbiting a brown dwarf star at a much smaller distance, only 0.62 astronomical units (AU), and is located approximately 3,000 light-years from Earth. But with so few exoplanets being discovered around brown dwarf stars, what can brown dwarf stars teach us about finding life beyond Earth?

“Brown dwarfs are primarily relevant to studies of gas giant planets, and such planets are unlikely to harbor life since they lack solid surfaces, so brown dwarfs may not tell us much about the prospects of life beyond Earth,” Dr. Luhman tells Universe Today. “But astronomers do speculate about whether life might be possible on planets that orbit brown dwarfs. The main complication of that scenario is that brown dwarfs steadily fade and cool over time, so the temperature of an orbiting planet also would change over time, which might make it difficult for life to survive for billions of years.”

What new discoveries will astronomers make about brown dwarf stars in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!