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You can get a $100 60-day credit on a new Linode account at linode.com/scishow. 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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