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.