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!