On the Hybrid Nature of Brown Dwarfs

Brown dwarfs, objects within the mass range of 13–80 MJ, exhibit similarities with both low-mass stars and giant planets. Despite great progress in both detection and theory in the past few years, the main formation mechanism of brown dwarfs remains unclear. To understand their origin, I tested whether lowmass (≤42.5 MJ) and high-mass (≥42.5 MJ) brown dwarfs came from distinct populations, as has been claimed in the literature. I collected all the available data on brown dwarfs from the NASA Exoplanet Archive and the Extrasolar Planets Encyclopaedia. Then, using SPSS Software, I performed nonparametric tests to analyze their statistical properties. Finally, I compared the results to the theories of gravitational collapse, core accretion, and disk instability. Results showed that the two mass groups did not come from distinct populations. In fact, inside the brown-dwarf desert, stellar and planetary processes met and produced almost indistinguishable objects. Despite the hybrid origins of brown dwarfs, disk instability seems to be their main formation mechanism. Brown dwarfs, starlike objects, are common yet mysterious. The Milky Way contains about 25–100 billion brown dwarfs (Mužić et al., 2017) and approximately 60 billion stars (McMillan, 2011), suggesting that there might be one brown dwarf for every star in the Milky Way—perhaps even more. Despite the frequency of brown dwarfs, their origin is still unknown. Kumar theorized their existence in 1963, but not until 1995 did Oppenheimer and colleagues observe the first brown dwarf. Unlike stars, brown dwarfs are unable to fuse hydrogen (Nakajima et al., 1995), but they do share several characteristics with stars (Luhman, 2012), such as radial velocity dispersions, spatial distributions in young clusters, and outflows. Because of this overlap, Chabrier et al. (2014) argued that stars and brown dwarfs must have a common origin; still, a clear consensus is lacking in the literature. For instance, Mollière and Mordasini (2012) reasoned that brown dwarfs and planets could form correspondingly. Since 1995, at least seven theories for formation have been proposed, ranging from turbulent fragmentation (Padoan & BUTLER JOURNAL OF UNDERGRADUATE RESEARCH, VOLUME 7 171 Nordlund, 2002) to accretion-ejection (Reipurth & Clarke, 2001); however, because the formation process in each of these theories can be classified as a variation of either a stellar or planetary process, I simplify the scenarios by discussing only the major theory of star formation—gravitational collapse—and the two main theories of planetary formation—core accretion and disk instability—the latter two operating at different distances. Debate is still underway to determine which of the three is the main mechanism responsible for brown-dwarf formation. The theory of gravitational collapse argues that a molecular cloud, which consists of gas and dust atoms bound together, collapses under its gravity to create a star (Bodenheimer, 2011). On the one hand, in the theory of core accretion, small particles collide to form a solid core that then gathers or accretes mass and thus eventually grows into a planet (Armitage, 2010). On the other hand, in the theory of disk instability, the gaseous disk surrounding a star gravitationally fragments into a planet (Armitage, 2010). While core accretion is associated with close companions, disk instability can effectively produce wide companions (Luhman, 2012). The answer might also be closely related to the brown-dwarf desert—the theorized absence of brown dwarfs that have low orbital periods (≤100 days) within short separations (≤3 AU) of their main-sequence stars (Carmichael et al., 2019)—as the desert is seen as evidence not only of different mechanisms in operation (Maldonado & Villaver, 2017) but also of the boundary between them. If we succeed in understanding the nature of brown dwarfs, we will improve our knowledge of galaxies and their composition, determine the minimum stellar mass (Pinochet, 2019), and reassess the boundary between stars and planets. To serve this aim, this paper collects all the available data about brown-dwarf binaries (i.e., systems in which a brown dwarf orbits a host star or another brown dwarf) in the NASA Exoplanet Archive and the Extrasolar Planets Encyclopaedia. Next, it divides the sample into low-mass and high-mass groups to test the hypothesis that these, particularly inside the brown-dwarf desert, come from different populations. Then, the results are compared with the theories of gravitational collapse, core accretion, and disk instability. Finally, based on these results, this paper suggests the main formation mechanism of brown dwarfs.