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For a long time, our understanding of planets has been limited to those in our backyard.
But in the last couple of decades, we’ve found thousands of exoplanets
that reveal that our solar system is kind of weird, actually.
For one thing, there’s one kind of planet we’ve found more often than any other,
and we don’t have one.
They’re called mini-Neptunes,
and some scientists think they’ve figured out why there are just so many of them.
Although we’ve known for sure that exoplanets exist since the 1990s,
it’s only been in the last decade that we’ve built up an inventory
of the different kinds of planets that are out there.
That’s largely been thanks to missions like the Kepler Space Telescope,
which detected more than two and a half thousand exoplanets between 2009 and 2018.
And as these data poured in, researchers began to see an intriguing pattern.
Planets that were about Earth-sized,
up to about twice the diameter of our home planet, had a dense, rocky composition.
But larger planets, between about two and four times the size of Earth,
were noticeably less dense.
Many of them were something like the ice giants, Neptune and Uranus:
basically, a small rocky core surrounded by a thick, puffy atmosphere.
Only, the exoplanet versions tended to be much smaller than our local ice giants,
giving rise to their popular name: mini-Neptunes.
And we’ve found a lot of them.
Like, they made up about three-quarters of all of Kepler’s detections.
As for figuring out why, well, since we don’t have one nearby,
we have to study the planets in our own solar system
to understand how they might form, and how they came to be so numerous.
Overall, the main difference between an ice giant like Neptune or Uranus,
and a straight-up gas giant like Jupiter,
is that ice giants have atmospheres made of more than hydrogen and helium.
They’re also enriched in heavier compounds, like water and methane.
And we think this is true of mini-Neptune exoplanets as well,
since their density seems to match that of the real Neptune.
So, researchers apply similar principles of planetary formation
to explain how these smaller worlds grow.
As the thinking goes, they start out like most planets:
as a small, hot, rocky core orbiting a newborn star.
Then, that core starts to attract
some of the hydrogen and helium from the protoplanetary nebula;
that’s the big cloud of dust and gas that feeds a forming solar system.
But it also attracts icy pebbles and larger rocks.
When these are incorporated,
they add volatile compounds like water and methane to the mini-Neptune.
Ultimately, this gives the planet more mass and more gravity,
so it starts attracting an even larger atmosphere of hydrogen and helium.
So far so good, but that doesn’t explain why most planets that form in this way
never seem to make it to a full-sized Neptune.
In fact, there seems to be a sharp division around three times the diameter of Earth,
where smaller Neptunes are very common, but larger ones are much rarer.
A few hypotheses have been suggested to explain this so-called radius cliff.
Like, a larger atmosphere should be easier to strip off by collisions or the solar wind.
Or maybe the protoplanetary disk runs out of gas before a larger planet can grow.
But both of these depend on very specific conditions,
and can’t explain the range in sizes of mini-Neptunes, or the sharpness of the radius cliff.
In 2019, though, a team published a more thorough explanation.
And they came to it by considering what might be going on inside of a mini-Neptune, rather than outside of it.
They reasoned that a thick atmosphere around the planet would insulate the core, and allow it to stay hotter for longer.
It could even still exist as a ball of molten magma
that’s in direct contact with the bottom of the atmosphere.
Down here, hydrogen in the atmosphere is squeezed so tightly that it can’t compress any further.
So the only way for it to go is down.
And the hydrogen is forced into the magma itself.
Overall, the more the atmosphere grows by accretion from the protoplanetary nebula,
the more hydrogen dissolves into the magma, and so the planet’s overall growth stalls.
Only when the magma is saturated with hydrogen can the mini-Neptune begin to grow again.
But this takes so much more gas,
that there’s a good chance the disk has been completely hoovered up by then.
In the end, this explains why it’s easy enough to make a mini-Neptune,
but way harder to make a bigger one.
Climbing the radius cliff takes an unfeasibly large amount of gas.
Now, it should be said that we don’t have any lab results to support this model,
since we can’t replicate the intense pressures and temperatures
inside a mini-Neptune here on Earth yet.
Instead, this hypothesis is based on gas behaviors we have managed to study,
at lower temperatures and pressures.
But so far, it’s a promising hypothesis.
And as to why our solar system has a big Neptune rather than a mini-one,
well, we’re still not entirely sure.
But there’s a good chance it’s linked to Jupiter.
Early in history, the newborn Jupiter took a trip through the solar system,
pulled around by interactions with gas and other objects.
And theoretically, that could have disrupted an object
that otherwise might have become a mini-Neptune.
If nothing else, though, that’s even more motivation to keep looking into deep space.
By studying types of exoplanets we don’t have here,
we often learn more than just what the rest of the universe is like.
We can uncover more questions and answers about how our solar system came to be.
If you’re sad our solar system doesn’t have a mini-Neptune, I have some good news:
We at SciShow can’t give you th whole planet, but we did make a mini-Neptune pin!
It’s our SciShow Space Pin of the Month for April,
and it’s available all month at DFTBA.com/SciShow.
It’s only available in April, so make sure to order yours soon.
In May, we’ll have a whole new pin for you.
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