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[♪ INTRO]
Astronomers have detected thousands of exoplanets over the last few decades,
but for most of them, we don’t really get to see them,
we just infer what they’re like indirectly.
So whenever a new exoplanet discovery comes
along with some amazing direct images, they’re always worth a look.
But a discovery published in 2022 by a large joint
US-Japanese team is special even by those standards:
they’ve directly imaged one of the youngest exoplanets ever found.
It’s so young, in fact, that it’s still forming, and may even have
some things to teach us about how planets in our own Solar System came to be.
The team looked at a star about 530 light years away called AB Aurigae.
It’s similar in size to the Sun, but it’s practically a baby at only 2 million years
old,
compared to our Sun’s 4.5 billion years!
Thanks to previous work, we’d seen images of the swirling disk of gas
around the young star, and we think disks like those give birth to new planets,
so the team was hoping to find a young protoplanet there.
But they still had to work for it.
The search took two telescopes: the Hubble Space Telescope
and Japan’s Subaru telescope in Hawaii.
It also took years of observation time,
as well as a trawl through years of archival telescope data.
It even took a tool on Subaru called the Visible Aperture Masking Polarimetric
Imager for Resolved Exoplanetary Structures, or VAMPIRES for short.
That’s not super important right now, but it is extremely fun.
And then the team did a bunch of statistical analyses on their data to make sure
their signal wasn’t just noise, or a background source, or something else.
But the result was worth it: a clear-as-day bright spot in the disk
showing a new planet being formed right now, give or take 500 years.
It may be the youngest gas giant ever discovered.
And it’s kind of a weird planet, too.
AB Aur b, as it’s called, is around nine times the mass
and three times the radius of Jupiter.
It’s very hot at around 2,000 degrees, and it’s located way,
way far out from its host star.
Specifically, it’s over ninety astronomical units away,
which in our Solar System would put it three times further out than Neptune.
These numbers aren’t just curiosities, either.
For a planet that big to exist that far out from such a young star
actually raises serious questions about our understanding of planet formation.
See, there’s been a long-standing debate in the astronomy community
over how gas giants like Jupiter form.
One of the leading ideas is an intuitive one called core accretion.
Remember that disk of stuff around our young star?
Basically, you start with a small lump of matter in the disk, and then
gradually amass material from the rest of the disk until you get a gas giant.
But there are some problems with that theory.
Mainly, researchers now think that those disks of gas and material
usually don't last long enough for Jupiter-sized planets to grow from them.
And discoveries like this one put serious holes in the theory, too.
It’s kind of hard to imagine a planet nine times bigger than Jupiter, that far away
from the core of the system, getting that big just by accretion in such a short time.
Because yes, in astronomy, a star being just two million years old
means that not a lot of time has passed!
In fact, this discovery might be good evidence
for an alternative theory of planet formation called disk instability.
In that theory, if a disk of gas surrounding a star becomes big enough,
then gravitational instabilities can make it fragment into clumps.
Those clumps on their own can even be Jupiter-sized,
so they can go on to form the cores of even bigger gas giants.
It’s more sudden and jerky than core accretion,
which is predicted to be gradual and smooth.
And the advantage of the disk instability model is that because the cores start big,
they can accrete material a lot quicker,
on timescales of thousands of years, not millions of years.
The team took the parameters of the exoplanet they’d seen, plugged them
into a computer simulation of the disk instability model,
and found that it matched the predictions of that theory quite well.
In fact, these observations provide a perfect test for the model,
because the planet is at such an early stage of formation, the model can be
checked against reality at the earliest points in a planet’s birth.
So the fact that the data and simulations agree
so well has the researchers pretty excited.
That said, not everyone is convinced by the discovery.
Because hey, direct imaging is hard, and despite their robust analysis,
sometimes shiny dots turn out to be false alarms.
And of course, even if it is a real detection, like with all things in science,
the formation method question is a bit more complicated than a simple ‘either-or’.
It’s likely that some gas giants form by one method
and others form by the other method.
It’s working out which planets formed which way that’s the tricky part.
Or as one of the researchers put it, “There's more than one way to cook an egg,
and apparently there may be more than one way to form a Jupiter-like planet.”
The data from this new protoplanet is a great clue that planets like Jupiter can be
formed by the instability method, not the accretion one, but it’s not a slam dunk.
As always, we’ll need to gradually accrete more data to know for sure.
Thanks for watching this episode of SciShow Space,
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