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[INTRO ♪]
As kids, we’re often taught that the
objects in space can be divided into neat,
orderly categories.
Like, if you have a big, hot, shining thing,
that’s a star.
And if you have a small, cool ball of rock
and gas, that’s a planet.
It all sounds pretty straightforward, and
it’s a great way for a kid to learn
what’s out there.
But in reality, the universe is
a lot more interesting than that.
Like, starting in the late 1990s,
astronomers began to discover objects
that were neither stars nor planets.
Instead, they were somewhere in-between.
These objects are called brown dwarfs,
and they’re more than just cosmic oddities.
As we learn more about them, they could
have a significant influence on our
understanding of the universe.
If you look at a picture of a brown dwarf
— or at least, what a NASA artist imagines
them to look like — you might think
these things are planets.
Maybe even gas giants, like Jupiter.
They’re not, though, because based on
our current understanding of them,
they don’t form the same way.
Planets seem to form when particles of dust,
ice, or rock collide and stick together.
Eventually, some might grow big enough
to attract an envelope of gas, but they
still likely started with a dusty or rocky core.
Brown dwarfs don’t begin like this.
Instead, they form like stars, when a
huge cloud of gas collapses in on itself
due to gravity.
Except... brown dwarfs aren’t stars, either.
See, when a gas cloud collapses to form
a star, the atoms smash together and
release a huge amount of energy — so much
that it starts nuclear fusion.
In this process, hydrogen is turned
to helium, and a bunch of energy is released.
That’s what gives stars their glow.
Brown dwarfs form from smaller gas clouds,
and those aren’t quite big enough to start
real hydrogen fusion.
Early on, these objects can have enough
energy for another type of fusion,
where a form of hydrogen called deuterium
fuses with a proton to make a different
kind of helium.
But deuterium is in comparatively short
supply, and it can get used up within a
few million years.
So, long story short, it doesn’t count.
This all means brown dwarfs aren’t part
of the Planets Club, and not part of the
Stars Club, either.
Instead, they’re oddballs somewhere in between.
At first glance, that might just make them
seem like a curiosity — a kind of non-planet
failed star, thing.
But there’s a lot to learn about them
and a lot they could teach us.
For example, some scientists propose that
brown dwarfs could help us understand the
squishy gray area between stars and planets.
They could help us answer questions like,
“What’s the lightest a star can get?”
That’s actually something that came up
in 2018, when a brown dwarf was discovered
that seemed to be just a little too massive
for its own good.
Right now, most evidence suggests that
brown dwarfs can get up to about 75 times
the mass of Jupiter — because past that point,
gas clouds are big enough to ultimately
cause nuclear fusion.
In 2018, though, one was found that seemed
to be slightly past that cutoff.
And none of our existing models could explain
how it was possible; they predicted you’d get
a star with that kind of mass.
This suggests that there’s something
about tiny star formation that we just don’t
understand, and with more research,
brown dwarfs could help us figure it out.
A few scientists, though, have proposed
that these objects could do even
more than that.
They suggest that, maybe, brown dwarfs
could help us understand the structure
of the universe itself.
See, when researchers look at distant
galaxies, they often notice that the
motion of those objects can’t be explained
just by what they see.
Instead, the galaxies need to have
some extra, invisible mass.
Astronomers believe that mass comes
from what they call dark matter,
which is often thought to be entirely
different, undetectable kind of stuff.
But a handful of scientists have
suggested that this missing mass could,
in fact, be brown dwarfs.
It sounds extreme, but it kind of makes sense.
After all, brown dwarfs only give off
a tiny pinpoint of infrared light, so
they’re really hard to detect, especially
as they get older and dimmer.
Also, they have only slightly less mass
than a star.
So if there were enough of them,
they could play a major role in keeping
galaxies moving the ways they do.
A recent survey even suggests there
could be up to a hundred billion brown
dwarfs in the Milky Way alone.
That’s one for every hydrogen-burning star,
so they could have a significant effect!
Of course, considering that other studies
have found thousands of brown dwarfs —
not millions — this hypothesis might
be a longshot.
But, hey.
There’s not exactly a ton of
conclusive evidence about what dark matter
really is, so it couldn’t hurt
to keep an open mind.
Besides, even if they can’t explain
dark matter, brown dwarfs are already
shaping the way we view the universe.
They’ve proven that the line between
stars and planets is fuzzier than you’d
think, and since we’ve only been discovering
them for a few decades, there’s likely
a lot more to learn.
So for now, they’ll continue to remind
us how varied and surprising space is,
and how things don’t usually exist in
black-and-white categories.
And in the future?
Well, maybe someday,
they could help us answer some of our
biggest astronomical questions.
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