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
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
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):
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.
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.
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
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 .
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 –\hthat 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.