Brown dwarfs are intriguing objects that scientists study within two fields of astronomy: star formation and extrasolar planets. The reason being that brown dwarfs do not seem to be either one or the other but rather fall in a gray area between these two categories. Intrigued by them, astronomers continue their study, hoping to figure out what these objects really are.
Brown dwarfs can be classified according to their temperature in three types: L, T, or Y. Brown dwarfs type L are the hottest, with temperatures that compare to that of a hot pottery kiln. Type T are in the middle and have temperatures close to that of lava flows. Finally, type Y are the coldest, with temperatures between a hot oven and a cold day on the North Pole.
When talking about their masses, we compare them to the mass of Jupiter. This is because brown dwarfs could have between 13 to 80 times the mass of Jupiter, and this is not enough mass to squeeze the material in their cores to temperatures high enough to eventually ignite the nuclear fusion of atoms of hydrogen or other material - the reason for which we cannot call them stars. The smallest star, where hydrogen fusion is still possible, occurs at about 80 times the mass of Jupiter, or about 8% the mass of our Sun. The mass of brown dwarfs falls below this limit, where only some limited fusion is possible using a rare form of hydrogen called deuterium. Below this point, even deuterium fusion ceases, and we call the object a planet.
We also question the term “planet”, because brown dwarfs are mostly alone in the sky and not associated with another star. However, astronomers have found that some planets do not orbit a star but live in almost complete darkness drifting between stars. They call them rogue planets or free-floating planets and can be found by direct imaging of nearby young star clusters. Some scientists believe these rogue planets might have formed at the same time that the young stellar cluster and hence are still hot enough to be visible in the infrared. With time, however, without a host star, these rogue planets will steadily cool down and fall from detection. Brown dwarfs seem to fit well in this scenario.
Although there are brown dwarfs that have companions, most seem to be alone. Astronomers have several theories to explain why. One theory states that these are rogue planets created around a star in a normal planetary system and later ejected due to some violent gravitational interaction. Another theory tells us that brown dwarfs start forming as other stars but cannot gather enough mass to become one. More complex explanations exist but none can be confirmed, so astronomers continue investigating.
Because brown dwarfs mostly emit infrared light, their study increased dramatically after improved and more powerful infrared observatories started operations. These include the IR channel in the Wide Field Camera on board the Hubble Space Telescope (HST), NASA Spitzer Space Telescope, and the James Webb Space Telescope (JWST). With their ability to observe this new light, these telescopes were able to obtain direct images of many of these objects, allowing us to study them as single entities or as multi-systems that include stellar companions or exoplanets.
Focusing on their individual characteristics, a research group at the American Museum of Natural History in New York City, NY, studied the distribution of sandy clouds in brown dwarfs. Using archival data from the NASA Spitzer Space Telescope, they studied type L brown dwarfs for which they knew the orientation. Analyzing the spectrum of many of these objects, they found that clouds are more concentrated at the equator, making brown dwarfs look different just because of their orientation. For example, when observing brown dwarfs with the equator facing us, we will observe most of the clouds, and the object will look redder. As their orientation tips toward or away from us, these will become bluer because we catch more of their polar regions[2].
In 2023, a team of astronomers used the NIRcam detector in JWST to identify the tiniest brown dwarf ever seen, which is only three to four times the mass of Jupiter. This dwarf is located in IC 348, a young cluster only 5 million years old in the Perseus star-forming region. Because IC 348 is a young cluster, any brown dwarf formed there is still relatively bright[3]. In the image below, we see the three brown dwarfs that the team found in this cluster. Their weight falls between three and eight Jupiter masses. Of the three, the tiniest became the object of interest because it is difficult to explain its size with the current theoretical models of star and planet formation. In this survey, the team also found that two brown dwarfs have an unidentified hydrocarbon signature, similar to a hydrocarbon infrared signature detected previously by NASA’s Cassini mission in the atmospheres of Saturn and its moon Titan. This is the first time this molecule has been detected in the atmosphere of an object outside our solar system[3].
Also in 2023, a team of astronomers discovered the first brown dwarf binary system ever observed by JWST. It is composed of two type Y brown dwarfs[4], both separated by approximately 1 au (au =astronomical unit or ~92 million miles). Using evolutionary models, the team concluded that these brown dwarfs have ages between 1 to 5 Gyrs. The primary has around 7.5-20 Jupiter masses, while the companion is 4-12.5 Jupiter masses. Assuming a Keplerian orbit, the team also derives a period for this extreme binary between 5 and 9 years[4].
The same year, a team used HST to look for low-mass binary and planetary companions of 33 nearby brown dwarfs types T and Y[5]. They found none in their sample, confirming that a binary or planetary companion with a wide separation is rare for the lowest mass and coldest brown dwarfs -- previous studies indicate that only ∼10–20% of L-T brown dwarfs are part of multiple systems [6].
Also with JWST, but this time using the Near Infrared Spectrograph (NIRSpec), a team of astronomers observed several of the coldest brown dwarfs, those type Y. The High-resolution spectra obtained by NIRSpec for two of the objects are almost identical, showing the signatures of water, ammonia, methane, carbon monoxide, and carbon dioxide. In one of them, however, the team also found methane in emission, a feature the research team had not seen in previous observations. After verifying that this feature was real, the team associated the observed emission with similar features previously detected in the auroras of Jupiter. In our solar system, auroras are produced mainly by charged particles transported by the solar wind and trapped in the magnetic fields of planets. However, the observed brown dwarfs are isolated and not close to a star, So how come they have an aurora? A possible explanation for their origin can also be found in the observation of Jupiter and Saturn. These two planets have active moons, Io in Jupiter and Enceladus in Saturn, which astronomers have associated with the production of auroras in some regions of these planets; however, the team cannot say for sure if this is the explanation, so they plan to continue studying these objects looking for a definite answer[7].
Upcoming observations of brown dwarfs taken with JWST should help resolve many mysteries surrounding these objects, like how they form and how these relate to objects we see in our solar system.
References
[4] Calissendorff, p. et al 2023, ApJ, 947:L30
[6] Fontanive, C. et al 2023, MNRAS, 526, 1783