Brown dwarfs are defined as the border between yuuge planets like Jupiter and a hydrogen-burning star, the lowest-mass of which are red dwarfs. Traditionally that’s between 13 Jupiters and 80. Why there? Because those bodies actually burn fuel, where something like Jupiter just steadily emits infrared from, basically, rain. And maybe some heavy metal fission.
The brown dwarf’s fuel is deuterium – hey, just like a fusion rocket! Problem: the brown dwarf does not have deuterium to spare, as a main-sequence star like our Sun has hydrogen. So it starts not very hot, and will just get colder. I suppose at the end, it’s just another dead planet.
It turns out that we don’t know a lot about the higher-mass dwarfs, because they are rare. I didn’t actually know they were rare: Luhman 16 is only a light-year past the other side of Alpha Centauri. But, again, these 30-jupiter objects aren’t easy to see on their own – it took until a decade ago just to see the Luhman set. If we don’t see them we can hardly count them.
In particular the University of Geneva wanted some samples of dwarfs at that upper edge, to fine tune that difference between a brown dwarf and a red one. So, never mind Luhman 16. Nolan Grieves has gone through TESS’s “objects of interest” from which his team found five of interest to us.
TESS means Transit. You're imaging the shadow, not the dwarf. On the other hand you do get the radius (as % of star), and maybe even a look in the atmosphere. Grieves warns that TOI-681 and -1213 do not transit very much (check out -1213’s error-bar in Fig. 10!). But all five present a valuable addition to our sample-size.
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