LWL | Brown Dwarfs and Their Relations to Stars and Planets
By Subin Lee
Abstract
Brown dwarfs are astronomical or substellar objects that underwent a formation failure to become a star. After it was first found at the end of the 1990s, brown dwarfs were categorized as self-luminous objects after discovery. Because brown dwarfs have failed to meet the requirements of both stars and planets, they were distinguished as “failed stars,” leaving an insignificant effect on the subject of astronomy. Nevertheless, brown dwarfs are found to be alike in stars due to their emission of light, hosting of planets, and formation process along with their fundamental foundations. Likewise, brown dwarfs are similar to gas giant planets in our solar system due to the inexistence of nuclear fusion in the core, the resemblance of Jupiter’s outer gas atmosphere, and the presence of weather and atmospheric phenomena. As a result of sharing some properties of stars and planets, brown dwarfs can serve as a paramount intermediate astronomical object that connects and can help to discover more information about both stars and planets.
Introduction
The number of discoveries that can be made about outer space relating to astronomy is endless. Every day, various astronomers, astrophysicists, and space scientists encounter and unveil fascinating facts about our universe. One of the exciting validities that are made is the existence of brown dwarfs in aerospace. Brown dwarfs were initially theoretical objects until the 1980s, but as technology evolved over time, approximately 2800 brown dwarfs have been discovered. Brown dwarfs were first discovered in 1995 (Tillman, 2017), and this was a groundbreaking discovery for many space scientists since it was an essential first step in the search for planetary systems beyond the Solar System (Kulkarni et al., 1995). Figure 1.0 is the first brown dwarf discovered in 1995, called GL229B (Kulkarni et al., 1995). It was found around the Gliese 229, a cold red star that’s 19 light years away from Earth (Kulkarni et al., 1995).
Figure 1.0. Brown Dwarf Gliese 229B.
Despite these findings, after some research, scientists claimed that Brown dwarfs are never true stars because of formation failure but are astronomical objects simply composed mainly of hydrogen gases with no internal energy source and almost no visible light (Tillman, 2017).
However, the implications of the brown dwarf study deserve to be explored further. Although many people believe brown dwarfs have negligible impact on the world of astronomy since they are comprehended as “fallen stars,” in reality, brown dwarfs play a crucial role in helping our understanding of both planets and stars because they serve as a bridge connecting the gap between gas giant planets and small stars.
Analysis
Technically, brown dwarfs are neither stars nor planets. They are not stars because they are not massive enough to influence hydrogen fusion reactions to become a star. Brown dwarfs are not planets either since they are much larger than the most prominent gas giants, such as Jupiter. Brown dwarfs tend to have their own classification as an object above 13 MJ, which is the limiting mass for the thermonuclear fusion of deuterium. Despite having their own classification as “brown dwarfs,” they lie between stars and planets, serving as an essential bridge to the knowledge of both planets and stars, with their temperature and masses being intermediate between the two.
Brown Dwarfs and Stars
First of all, brown dwarfs share some properties with the stars because they both emit light (Allers, 2021). This light is the thermal radiation from the heat within. Brown dwarfs tend to glow in the red because of the heat and the infrared spectrum until they cool down. Then, they emit X-rays and infrared light (Allers, 2021). Figure 1.1 is an X-ray image of an X-ray flare detected from a brown dwarf from Chandra’s observation (“Brown Dwarf LP 944-20,” 2022). The left panel shows there has been no light emitting from the brown dwarf for nine hours and 36 minutes (“Brown Dwarf LP 944-20,” 2022). The right panel is the detected proof of a brown dwarf emitting bright X-ray light and slowly diminishing for the remaining time of the observation (“Brown Dwarf LP 944-20,” 2022).
Figure 1.1. X-ray flare from brown dwarf LP 944-20.
Brown dwarfs are incredibly faint, and most of their light is in infrared wavelengths since their surface temperature is far cooler and dimmer than any star type (Allers, 2021). However, with advanced infrared technology, it is still known for being extremely tough to detect (Allers, 2021). Figure 1.2 demonstrates that brown dwarfs have the lowest temperature and luminosity of all types of stars.
Figure 1.2. H-R diagram.
Also, identical to stars, brown dwarfs can host their own planets (Allers, 2021). Some observations proved that Earth-size planets can form and orbit around brown dwarfs. For now, it is thought that planets are formed when stony worlds form over time as grains orbiting a protostar collide with each other and stick together (Howell, 2012). Scientists previously expected the brown dwarfs to have the potential for this process to happen since they have observed some quickly moving dusty particles encircling brown dwarfs (Howell, 2012). As anticipated, astronomers soon found evidence of tiny solid grains in a disk surrounding a brown dwarf ISO-Oph 102. Soon after, other planetary-mass objects, or exoplanets (2M1207b, MOA-2007-BLG-192 Lb, 2MASSJ044144b and Oph 98 B) orbiting brown dwarfs were discovered (Howell, 2012). Lastly, brown dwarfs and stars share and undergo a similar formation process and basic stellar atmosphere (Allers, 2021). They both form from the same molecular clouds of gas and specks of dust in space. This procedure is done independently and from the gravitational collapse of the same clouds, ending in the formation of a dense core (Allers, 2021). Also, though brown dwarfs cannot initiate maintained hydrogen fusion, they do experience some deuterium and lithium fusion early in their lives, which is analogous to how stars commence their lives. Furthermore, brown dwarfs are composed entirely of hydrogen, just like the sun (Allers, 2021). Most stars are composed of hydrogen and helium, along with a minuscule amount of other elements, and brown dwarfs share a comparable or identical atmospheric composition. These similitudes establish the concept that brown dwarfs and stars have many likenesses.
Brown Dwarfs and Planets
On the other hand, brown dwarfs have some characteristics that match those of planets as well. To begin with, neither planets nor brown dwarfs go through nuclear fusion (hydrogen fusion) in their core for similar reasons (Allers, 2021). Stars we see now are created within dust clouds and scattered throughout most galaxies. Deep within these clouds, turbulence creates knots with enough mass for the gas and dust to start collapsing under its own gravitational pull (Stars - NASA science, n.d.). Then, the pressure from gravity causes the material at the centre to heat up and create a protostar. One day, this core will become hot enough to ignite fusion for the star to be born (Stars - NASA science, n.d.). However, not all clouds of dust become stars. Brown dwarfs are formed along with the stars by the contraction of gases and dust in the interstellar medium (Tillman, 2017). To be a star, the gravitational pull has to push inward to the clouds until hydrogen fusion is started in the core. However, for brown dwarfs, instead of reaching this essential stage for the formation of stars, the closely packed material in the core advances to a stable state before the temperature rises enough for hydrogen fusion to start (Tillman, 2017). Planets cannot go through nuclear fusion either because they lack the necessary conditions for fusion (Allers, 2021). Planets are unable to reach incredibly high temperatures and pressures for fusion. Similarly, brown dwarfs lack temperature, but they also have a considerably low mass that prevents the fusion from taking place, identical to Jupiter.
Secondly, some brown dwarfs tend to resemble Jupiter’s atmosphere (Allers, 2021). Unlike stars, brown dwarfs have clouds surrounding themselves in the atmosphere. Brown dwarfs will have an atmosphere composed of a mix of titanium oxide and carbon monoxide for the first 100 million years. Then, between 100 million and 500 million years, the brown dwarf’s atmosphere cools and forms dusty mineral clouds (Allers, 2021). A billion years later, methane usually tends to dominate the atmosphere. Over long periods of time, brown dwarfs eventually become surrounded by thick, multiple layers of gas, like the outer planets of our solar system (e.g. Jupiter, Saturn, Uranus, and Neptune). As proof, the coldest known brown dwarfs, spectral class Y, exhibit water-ice clouds, water vapour, and methane in the external layer of their atmosphere (Allers, 2021). A significant amount of ammonia gas is expected to be present in class Y brown dwarfs’ atmosphere, equivalent to what can be observed in Jupiter’s outer layer of clouds, as shown below in Figure 1.3 (Allers, 2021).
Figure 1.3. Interior engine and atmosphere of different astronomical objects.
Lastly, weather and atmospheric phenomena have been detected in brown dwarfs, which are common traits that planets tend to have (Kramer, 2013). For instance, Jupiter, which is well-known for being the planet most similar to brown dwarfs, has distinct weather phenomena, such as clouds, storms, and winds (Landau, 2020). Researchers reported the windy and cloud-covered weather of a brown dwarf using NASA’s Spitzer and Hubble space telescopes (Kramer, 2013). Two spacecraft surveyed brown dwarf 2MassJ22282889-431026 (Kramer, 2013). Different light wavelengths revealed that storms the size of Earth were created in the brown dwarf’s atmosphere from an organized cloud system akin to the Great Red Spot in Jupiter (Landau, 2020). Astronomers and scientists believe the cloudy regions on brown dwarfs can produce rainy storms with violent wind and lightning (Clavin, 2014). However, according to the majority of the brown dwarfs studied, their temperature is too hot for water rain (Clavin, 2014). For this reason, astronomers consider the torrential rain and storms in brown dwarfs to be composed of hot sand, molten iron, or salt. In 2014, in “Weather on Other Worlds,” a Spitzer program, scientists anticipated seeing only a few show variations in brightness on brown dwarfs (Clavin, 2014). Still, contrary to most expectations, the findings revealed that approximately half of the brown dwarfs showed variation (Clavin, 2014). Considering that roughly half of the brown dwarfs were oriented in a manner that might conceal or maintain unchanging storms, the outcomes indicate that most, if not all, brown dwarfs experience stormy conditions and weather (Clavin, 2014). Additionally, researchers identified signatures of an aurora from LSR J1835+3259, a brown dwarf that is 20 lightyears away (Landau, 2015). The auroras on this brown dwarf are similar to the ones seen around the magnetic poles on Earth. It is believed that charged electrons would be responsible for the auroras that could be generated by an orbiting planet moving through the brown dwarf’s magnetosphere (Landau, 2015). The auroras on brown dwarfs are generally a million times more potent than the ones on Earth (Landau, 2015). Some of these fundamental overlapping properties establish that brown dwarfs are similar to planets.
Conclusion
In conclusion, brown dwarfs share numerous matching properties with both stars and planets. Though it may seem like brown dwarfs should belong in either the category of stars or planets, they are categorized as intermediate objects between the two. Numerous scientists are still working on searching for more brown dwarfs that may exist in outer space to gain more information about stars and planets because brown dwarfs allow astronomers to explore more about the boundaries between stars and planets (Allers, 2021). They also help us gain valuable insight into various aspects of stellar and planetary science. The fact that brown dwarfs are neither truly stars nor conventional planets allows them to continue emerging as a vital missing link and advance as a treasure trove to many astronomers (Allers, 2021). Their existence has challenged our preconceived notions and provides a fascinating window to the diverse processes that shape our universe (Allers, 2021). In the future, their pivotal role in the cosmos will persist in underscoring the profound impact of understanding these cosmic anomalies, illuminating the fascinating interplay between the stellar and planetary realms, and deepening our appreciation of this intricate web of connections.
