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In 2018, the Kepler Space Telescope retired as
the MVP of exoplanet hunters.
When it launched nine years earlier,
we only knew of about 300 planets beyond our solar system.
Kepler found more than two and a half thousand.
Over 60% of the confirmed exoplanets out there today
were discovered by this one telescope!
So at least in volume, the mission was a huge success.
But what about finding Earth’s twin?
That’s where the real excitement’s got to be.
Well in that specific case, not so much.
And that’s partly due to an innocent, little assumption
that its designers made.
They assumed our Sun was a typical star.
But Kepler quickly proved our Sun is even
more unusual than we knew.
[INTRO]
The Sun is the only star we have up-close experience with,
so it’s pretty easy to assume that most of
the other stars out there are just like it.
But scientists have known for more than a century
that stars come in a lot of different sizes and colors.
And it turns out stars like our Sun, known as G stars,
make up only 6% of the ones in the galaxy.
Meanwhile, a whopping 73% of them are M stars,
better known as red dwarf.
And while there’s certainly a chance we could find a habitable,
Earth-like world around one of these, there are some complications.
Red dwarfs are smaller and cooler than our Sun,
so the zone where water can exist as a liquid, called the Habitable Zone,
is both narrower and much closer to the star.
And despite the name Habitable Zone,
a rocky planet that’s the “correct” distance from a red dwarf
for liquid water is also so close its atmosphere
probably got stripped away by all the radiation.
In other words, it’s probably better to go
hunting for Earth 2.0 around Sun 2.0.
Another one of those rare G stars.
Most scientists have agreed on this for decades.
But there’s another assumption that
scientists made when designing Kepler:
That other G stars are just as noisy as the Sun is.
Now, when scientists say a star is noisy,
they’re not talking about needing earplugs.
Noise refers to random fluctuations in a star’s brightness.
The surface of a star is constantly churning, and has lots of
spots, flares, and other features that make the star’s brightness
randomly flicker a little bit.
But this noise can be a problem because of how
the Kepler telescope looked for exoplanets.
It stared at one patch of the sky for years,
watching 150,000 stars all at the same time,
looking for tiny dips in brightness that meant a planet
was crossing in front and blocking a small fraction of its star’s light.
Now this gets easier if the plant is super big, like a gas giant.
Or if the star is smaller, like a red dwarf,
since the planet would block a larger proportion of the light.
But if you were watching the Sun from afar,
the Earth passing in front would cause it to lose
about 0.0085% of its brightness. Or 85 parts per million.
For 10 hours, once per year.
In other words, you’d need to watch the Sun for a long time
with a very sensitive telescope to both pick Earth’s tiny dip up
and know it wasn’t a fluke.
Based on what we know about our Sun, scientists expected that
Kepler would have to handle stellar noise of up to 10 parts per million.
Which is annoying when trying to find an
85 parts per million dip, but not overwhelming.
But after reviewing the first 18 months of data,
astronomers realized that the flickering of G stars
was double their noise limit.
According to a 2011 report, almost no G stars had less than
10 parts per million of noise, and something like 30% of them
had over 50 parts per million!
Our Sun turned out to be the quiet one in the family.
And picking out Earth-sized planets became a whole lot harder.
It was a massive setback, but no one was going to let this telescope
that cost over half a billion dollars go to waste.
And when faced with a problem,
scientists tend to come up with clever solutions.
In this case, it was actually pretty simple: Watch the stars for longer.
By gathering more data, astronomers could average out
the random fluctuations and pick out those tiny, rocky planet dips.
So in late 2012, NASA approved an extension of the Kepler mission.
But in a twist of fate, stabilizing parts on
the telescope had already started to break.
A few months later, Kepler could no longer hold
the position it needed to continue its original mission.
But again, clever solutions.
For the next five years, scientists would use radiation from the Sun
to help stabilize Kepler, and monitor stars for shorter periods of time.
It worked on the same principle as a solar sail.
Light and subatomic particles spat out by the Sun
exert a small amount of pressure, like a kind of solar wind.
So scientists figured out how to brace the telescope against it
and keep it steady, kind of like how you might lean against a wall
to stabilize yourself when taking a picture.
As of 2023, scientists have found over five hundred confirmed exoplanets
in this second set of Kepler data.
And they’ve continued combing through the O.G. data, too.
A study from 2015 used more data than that 2011 report,
combined with different methods of processing,
and was able to both reduce the noise levels
and reconsider the Sun’s noisiness relative to stars more generally.
From a certain point of view, like over longer time-spans,
it might not be as weird.
But what about Kepler’s mission to find Earth 2.0?
Well, in 2011, NASA announced the discovery of Kepler-22b.
It’s a planet just twice the size of Earth,
orbiting a star similar to the Sun in its Habitable Zone.
It was the first exoplanet discovered with this combination of factors!
But we have to hold our space horses.
We still don’t know exactly what the planet is made of,
or if it has an atmosphere, let alone liquid water on the surface.
It might not even be terrestrial!
As of 2023, Kepler has found 361 exoplanet candidates
in their stars’ Habitable Zones.
But so far, only 88 of those have been confirmed to actually exist,
and scientists debate how many are exactly “Earth-like”.
Most of those planets aren’t a perfect Earth 2.0 orbiting a Sun 2.0,
but the telescope has discovered some promising candidates,
including four around G stars.
A definitive second Earth, habitable to life as we know it,
is yet to be confirmed.
We’ll need to follow up on the candidates Kepler found
using other telescopes like the Webb.
But in terms of finding exoplanets, Kepler was just the beginning.
NASA’s planet-hunting telescope TESS launched back in 2018.
And the European Space Agency plans to launch PLATO in 2026.
With its 26 cameras and two billion pixel images,
PLATO will really be able to peer through the noise
and pick out the tiniest planetary dips.
In the end, Kepler may have struggled
to see through that unexpected stellar noise.
But the mission taught us a valuable lesson,
and paved the way for our eventual discovery of Earth’s distant twin.
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[OUTRO]