The Secret to Planet Formation Might Be Hiding in the ‘Fulton Gap’


Starting in 2009, the spacecraft Kepler scanned
the night sky hunting for distant worlds. Over the course of its nearly decade long
mission, it identified over 2,300 confirmed exoplanets, making us realize the galaxy was
much fuller than we previously thought. But poring over the data, something jumps
out at you: we seem to be missing planets of a particular size. It appears there are a lot of rocky planets
in the ballpark of Earth’s size, up to 1.5 earth radii. And there seem to be plenty of gas planets
that are like mini-Neptunes, 2 to 3 times as big as Earth across. But a 2017 paper on the data noted that between
those two peaks is a noticeable dip. There are way fewer planets 1.5 to 2 times
Earth’s size than we’d expect. For some reason, instead of a smooth distribution
curve, there’s a big gap. It looks like my teeth in middle school. The anomaly has been dubbed the Fulton Gap. The Fulton Gap calls into question what we
think we know about planetary formation, and we’ve had to come up with explanations to
make the data fit. Some of them focus on atmospheric loss. The mini-Neptunes we see could have cores
massive enough to hold onto their gassy atmospheres. Gas can add to a planet’s size easily since
it takes up so much space. But if the cores aren’t big enough, astronomers
suggest the atmosphere could get blown away, leaving the much smaller and denser rocky
mass behind. It’s like a reverse Goldilocks scenario,
and any planets we do see in the gap may not be there for long. Atmospheric loss is one of three general ideas. It’s possible that planets just naturally
fall on one side of this gap or the other, maybe due to how dust and gas is distributed
after a star forms. Or perhaps planets of a certain size just
cook their atmosphere away as they cool, an effect called “core-powered mass-loss.” Core-powered mass-loss is also what I’m
calling my ab-focused fitness program. This is assuming the larger planets are gassy
layers over a rocky core, but truth be told we aren’t sure about their makeup just yet. What some astronomers think is hydrogen gas,
others argue could be water. Really what we need is more data. Kepler told us a lot, but it only looked at
one tiny patch of sky near the Cygnus constellation for most of its lifespan. It also studied distant dim stars that are
harder to follow-up on using additional telescopes on the ground. The Transient Exoplanet Survey Satellite,
or TESS, should remedy a lot of those shortcomings. Launched in April of 2018, TESS will monitor
an area of the sky 400 times larger than Kepler did, observing stars 30 to 100 times brighter. And it will be able to detect planets farther
away from their stars that take longer to pass in front of it. So far TESS has only had enough time to confirm
20 more exoplanets. Once it’s had enough time to document a
statistically significant number, we can revisit the Fulton gap. Maybe it’ll end up going away, or maybe
it’ll persist and the cosmic mystery will remain unsolved. Thanks for watching be sure to subscribe because
we love talking about exoplanets, and the craft that found most of them was a testament
to human ingenuity and can-do spirit. For the full story check out Trace’s video
here. That’s all for now, see you next time on
Seeker!

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