The International Astronomical Union defines
Brown Dwarfs as balls of gas in space that are too small to be bona-fide hydrogen-burning
stars, but large enough to burn deuterium, which anything bigger than about 13 times
the mass of Jupiter can do. Because of this, brown dwarfs are often called
“failed stars” or “super Jupiters.” However, there’s a major problem with this
deuterium-burning-based definition: it doesn’t make any scientific sense. First, unlike how hydrogen fusion is huge
since it means you can shine brightly for millions or billions of years, burning deuterium
doesn’t appear to affect an astronomical object in any particularly meaningful way,
which is probably why you haven’t heard much about it. I mean, on this density vs mass plot, hydrogen
burning is a cutoff that clearly distinguishes stars from non-hydrogen-burning things, while
deuterium burning doesn’t appear distinguishing at all. So it may seem like there is no distinction
between things that are slightly-too-small to be stars (which we call brown dwarfs) and
giant gas planets, and that they’re all really the same kind of object. However, just because deuterium isn’t a
good cutoff doesn’t mean there aren’t other options. So, let’s briefly list the features that
DO scientifically distinguish brown dwarf-like objects from gas-giant-like objects (And a
caveat here: some of these statements are still being debated within the astronomical
community, but for each one there are at least some researchers arguing in favor of it): 1. Movement: Brown dwarfs (whether above or below
the deuterium limit) and stars appear to be located and move in similar ways: in loose
clusters with other similar objects moving with roughly the same relative speeds. Planets, on the other hand, move around stars
in orbits, and are much closer to the nearest star – which can even be a brown dwarf. 2. Formation: Brown dwarfs (whether above or
below the deuterium limit) and stars appear to follow the same distribution of masses,
suggesting they form the same way: the gravitational collapse of a cloud of gas and dust. Planets appear to follow a different distribution
of masses, suggesting they form in their own way: by accreting from the protoplanetary
disk of gas and dust leftover around a star (or brown dwarf) after it forms. 3. Metallicity: The dust and gas leftover from
star formation has higher concentrations of metal, so the atmospheres of gas giant planets
have elevated levels of metal. Brown dwarfs (whether above or below the deuterium
limit) have around the same amount of metal as stars. 4. Size of Orbits: Protoplanetary disks around
stars typically don’t extend much farther than a few hundred times the distance between
the earth and the sun, so that’s about as far out as you find planets. However, brown dwarfs (whether above or below
the deuterium limit) often orbit stars or other brown dwarfs in binary pairs at significantly
greater distances . 5. Mass Ratio: Protoplanetary disks are pretty
much never more than 10% of the mass of their parent star (or brown dwarf), so a planet-to-star
mass ratio is almost always more extreme than 1 to 10. However, brown dwarfs (whether above or below
the deuterium limit) and stars regularly orbit in pairs with mass ratios much closer to 1
to 1, suggesting they formed from their own clouds of gas and dust. Basically, a lot of evidence points to two
separate populations of objects: things that form from gravitationally collapsing clouds
of gas, and things that form from the leftovers. It appears an unfortunate coincidence that
the overlap in these two populations is roughly at the mass where deuterium-burning becomes
possible. I mean, IF we didn’t have any other good
ways to distinguish between brown dwarfs and planets, sure, deuterium burning might be
a reasonable rule of thumb. It’s also possible, as some researchers
contend, that there IS no real, clear way of distinguishing between brown dwarfs and
giant planets, and that they really do just exist on a spectrum. But either way, deuterium is more or less
a distraction. So, among those who think that the evidence
suggests brown dwarfs are different from giant planets, what supposedly distinguishes them
is how they formed, their consequent behavior, and their composition. The claim is this: planets, no matter how
big, appear to be the leftovers of star formation. And brown dwarfs, no matter how small, appear
to be failed stars: they started off the same ways stars do by gravitationally collapsing
from a cloud of dust, but failed to capture enough mass to burn hydrogen. Perhaps in the end it doesn’t matter how
badly they failed – that is, it doesn’t matter if they also can’t burn deuterium
– what matters is that they aspired to be stars, and fell short. Thanks to NASA’s James Webb Space Telescope
Project at the Space Telescope Science Institute for supporting this video. JWST is literally the perfect telescope for
studying brown dwarfs: it sees best in infrared light, and guess what brown dwarfs mostly
emit: yup, infrared! Unlike stars which are super hot, the smallest
brown dwarfs are about the same temperature as you and me, and they give off infrared
light right in the middle of JWST’s spectrum. This also means that if you and I were in
space, we’d shine out like a beacon to JWST even with no stars nearby. So JWST will be able to find and study human-temperature
brown dwarfs and compare them with super Jupiters to help settle the brown dwarf debate. Video source: https://www.youtube.com/watch?v=FDjeKuAu-8E
