It is estimated that up to 60 billion brown dwarfs make their home in the Milky Way. Because these elusive celestial objects do not fuse hydrogen in their core, they spend their lives cooling as they lose that gravitational energy from their formation, morphing as they age from looking like a low-mass star to looking like Jupiter. Every brown dwarf that was ever created still exists because they can’t fuse hydrogen, giving them a calm, sustained existence on the vast timeframe of the cosmos.
Historically, brown dwarfs have been defined as objects with 13 – 80 Jupiter masses that are unable to fuse hydrogen but still massive enough to fuse deuterium, which is an isotope of hydrogen with a single neutron paired with its proton in the nucleus. Recently, however, astronomers suggest a different definition should be applied that encapsulates their formation process or other physical attributes. For example, Jovian planets likely formed from accretion of small planetesimals into a solid core followed by accretion of the surrounding gas, whereas binary stars formed via fragmentation of molecular clouds or their gaseous primordial accretion disks.
This past week, an international team characterized five companions that were originally identified with the Transiting Exoplanet Survey Satellite (TESS) as TESS objects of interest (TOIs) – TOI-148, TOI-587, TOI-681, TOI-746 and TOI-1213. These are called “companions” because they orbit their respective host stars with periods of 5 to 27 days, but with radii between 0.81 and 1.66 times that of Jupiter and between 77 and 98 Jupiter masses. Hence, these five objects are right on the border of the deuterium vs. hydrogen fusion limits.
The artist’s illustration below represents the five brown dwarfs discovered with the satellite TESS. These objects are all in close orbits of 5-27 days (at least 3 times closer than Mercury is to the sun) around their much larger host stars. © 2021 Creatives Commons Attribution 4.0 International (CC BY-NC-SA 4.0) – Thibaut Roger – UNIGE
Brown dwarfs cool from 3,000 K to only 500 K during the 13.8 billion year age of the Universe. The five newly discovered TOIs are on the hotter side, roughly 2,500 K, due to their close proximity to their host stars. This is still a factor of two lower than the temperature of our sun.
These five new objects contain valuable information about the nature of brown dwarfs. “Each new discovery reveals additional clues about the nature of brown dwarfs and gives us a better understanding of how they form and why they are so rare,” says Monika Lendl, a researcher in the Department of Astronomy at the UNIGE and a member of the NCCR PlanetS.
“It is still unclear what the pathway of formation is for brown dwarfs. Likely they do not form exclusively via one method,” says astrophysicist and dailygalaxy.com editor, Jackie Faherty. “Rather, they may form through the collapse of a molecular cloud which makes stars and alternatively they may form through accretion around a higher mass host which makes planets. These five new objects are bridge sources toward a better understanding of the formation pathways available for substellar mass objects.”
One of the clues the scientists found to show these objects are brown dwarfs is the relationship between their size and age. “Brown dwarfs are supposed to shrink over time as they burn up their deuterium reserves and cool down,” explains François Bouchy, professor at UNIGE and member of the NCCR PlanetS. “Here we found that the two oldest objects, TOI 148 and 746, have a smaller radius, while the two younger companions have larger radii.”
“Even with these additional objects, we still lack the numbers to draw definitive conclusions about the differences between brown dwarfs and low-mass stars. Further studies are needed to find out more,” concludes Grieves. These objects are so close to the limit that they could just as easily be very low-mass stars, and astronomers are still unsure whether they are brown dwarfs.
Source: Nolan Grieves et al, Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit, Astronomy & Astrophysics (2021). DOI: 10.1051/0004-6361/202141145
Image at the top of the page: the faster a brown dwarf rotates, the narrower the different-colored atmospheric bands on it likely become, as shown in this illustration. Some brown dwarfs glow in visible light, but they are typically brightest in infrared wavelengths. (NASA / JPL-Caltech).