Interview: exoplanet-hunting with Kepler

The Kepler space telescope may have run out fuel, but for NASA scientists there is alot of important work yet to be done.

We spoke to Kepler scientist Geert Barentsen to find out what the mission has achieved, and what lies ahead.

More planets than stars: the Kepler space telescope has reveal a wealth of diverse planets that have changed what we know about the Universe.
Credit: NASA


On 30 October 2018, NASA announced the retirement of the Kepler space Telescope.

This orbiting observatory spent nearly a decade searching stars in the night sky for signs of exoplanets in orbit around them.

Initial data from the mission has revealed a wealth of information that has transformed our view of the Universe.

We spoke to Geert Barentsen, who is Director of NASA's Kepler Guest Observer Office at NASA Ames in California, to find out what the mission has achieved so far.


What is the feeling amongst the team, now that Kepler has been retired?

A lot of people might expect us to be a sad because after nine and a half years in space, Kepler has run out of fuel.

But even though it is a bit sad that we are no longer collecting new data, we already have so much data on the ground.

It’s a goldmine and it’s really going to take another decade to sift through it and actually get all the science out of it.

Ultimately the science is produced by human beings on Earth, and not by the robotic telescope in space.

I’m excited about all the discoveries that still lie ahead of us.


How does the data get analysed? Can the public help as well as the scientists?

Absolutely. Kepler is a really sensitive digital camera. It takes pictures of stars and is completely uninterrupted in this process.

So for about ten years it has been taking pictures of about half a million stars and it measures their brightness.

We end up with very accurate time series data showing how bright a star is over time. That data reveals a number of things.

If a small exoplanet passes in front of the star as it orbits, it will create a little dip in the light.

But those dips are very small and so it takes a combination of smart algorithms but also human eyes to detect all those small signals in the data.

One approach that has been used very successfully is the use of citizen scientists to look through this data (NB: find out more info here).

They end up finding signals that the algorithms have missed, either because the planet is a bit peculiar or else because there is a strong gravitational shift in the exact timing of when the planets appear.


Do these dips in brightness also allow us to learn anything about what the planet is like?

It’s actually incredible how much information we can get out of those very simple measurements of brightness over time.

The depth of the dip, for example, tells us the size of the planet because if you have a very big planet passing in front of the star, then you get a very deep dip in brightness, and if you have a tiny planet like Earth, which is 100 times smaller in diameter than the Sun, then you get a tiny dip.

And if you measure how frequently the dip occurs, you also find out the rotation period, or the length of one year on that planet.


A Kepler light curve for a hot Jupiter exoplanet, showing dips in the star's brightness as the planet passes in front of it. You can find out how to read these light curves at the dedicated Harvard-Smithsonian Canter for Astrophysics website here.


Was Kepler sensitive enough to pick up dips that could have been caused by exomoons?

There are a few signatures in some of the Kepler light curves that suggest that there might be not only a planet passing a star, but also a tiny extra dip next to the planet.

These always occur at a slightly different time, next to the planet, and right now scientists are debating and using other instruments like the Hubble Space Telescope to find out whether there could be moons around some of the exoplanets.

The jury is still out! Right now people are publishing papers and having the debate, which is how science works.

You see a signal and you study it in more detail to find out whether it’s real or not.

Even now there are some promising signs of exomoons, and that’s crazy because ten years ago we didn’t even know whether planets were common, and now we’re debating the existence of exomoons.

For me personally, Kepler changed the way I look at the night sky.

I’ve always loved looking at the stars at night, but now when I look at the stars, I also see opportunity.

If only our eyes were sensitive enough to see all the exoplanets that Kepler has shown to exist, we would be able to see more planets than stars.

So now I almost feel less lonely when I look at the night sky. I think Kepler has revolutionised how we see the Universe around us.


Has it also introduced new types of planets to us that we didn’t know about before?

Perhaps the biggest surprise to come out of the mission is that a lot of planetary systems and other stars don’t look like our own.

We’ve found an incredible diversity: very compact hot lava planets, fluffy mini Neptune planets, a lot of giant planets, but the most common planet Kepler was able to detect were super Earths.

These are planets that are between the size of Earth and Neptune, so about two or three times the size of Earth.

That’s interesting because there’s no such planet that we know of in our own Solar System, so we’re really struggling to understand what their structure is and what they are made of.

Could our Earth maybe originally have been the core of a one such puffed-up planet?

That’s still very much part of ongoing research that future telescopes like the James Webb Space Telescope might be able to help answer.


Hot Jupiters are another planetary anomaly that we didn’t know about before Kepler, aren’t they?

Hot Jupiters are the first type of exoplanet really that we found back in the 1990s, because a big, Jupiter-sized planet orbiting close to its star is going to block a lot of light.

One of the things Kepler found is that they’re not actually that common, but the fact that they exist is very curious, because you wouldn’t expect such a big planet to be so close to its star.

When a star forms, there’s material left around the star in a sort of pre-planetary disc, but there’s not that much material for big planets to form so close to the star.

Also, there aren’t a lot of volatiles like water, due to the hot temperatures near the star. So the fact that hot Jupiters exist is very surprising.

People are even today coming up with new models to explain how these planets might exist, and perhaps it’s because of gravitational interactions over time, big Jupiters might circle inwards and migrate close to their stars. 

That’s really interesting from a scientific perspective.


A NASA graphic showing the number of confirmed exoplanets discovered by Kepler
Credit: NASA/Ames Research Center/Jessie Dotson and Wendy Stenzel


Are we able to use the knowledge gained from the Kepler mission to learn more about our own Solar System?

Absolutely. Kepler studied stars very intensively and it has been studying the structures of stars using their oscillations and pulsations.

That has given us new insights into the evolution of stars, which ultimately applies to our Sun.

In the last few years, Kepler has been finding planets around stars in clusters. This is interesting because when a star is in a cluster we can measure its age very precisely.

Now we are starting to find a sequence of different planets for which we know precise ages, and that is helping us create new models that show us how planets evolve over time.

Ultimately this helps us understand how the Solar System might have evolved. 


What has Kepler told us about other Earth-like rocky planets?

Before Kepler launched, we didn’t know whether Earth-like rocky planets were common around other stars. What Kepler has shown is that they are very common.

We found numerous Earth-sized planets in the habitable zones of their stars - the orbital distance where the temperature on the exoplanet is just right, so that liquid water might be found on its surface.

This is especially true for smaller stars. If you have a smaller star then a smaller planet like Earth is easier to detect because by contrast the size of the dip, the amount of light blocked, is much greater.

Kepler even found a small number of Earth-sized planets in or near the habitable zone of more Sun-like stars.

This is an incredibly different measurement to make because you need to measure the light with a precision of about ten parts per million.

It’s a bit like looking at a mosquito flying in front of the distant headlight of a car. 

The most recent estimates suggest that perhaps between 20 and 50 per cent of Sun-like stars have an Earth-like planet in the habitable zone.

These analyses are still continuing, but for the first time we know that Earth is definitely not alone, in terms of a planet of that size existing in the habitable zone around its star.


What do you think are the chances that there exists an Earth-like planet that humans could live on, if we could get to it?

We have not found any planet that is exactly like Earth, with continents and oceans: so far the only planet like this that we know of is our own planet.

But if you consider the factors you might need for such a planet to exist, the first question you would ask is ‘how common are planets to begin with?’

Now we know that Earth-like planets in the habitable zone are common. The beauty of Kepler is that it’s not just a standalone mission.

NASA has a strategic objective to answer the question ‘are we alone?’

Kepler was the first step in this endeavour, and now future missions such as the James Webb Space Telescope will look more closely at the planets we’ve found.

James Webb has really sensitive spectrographs that split the light from stars into different colours, and this might reveal the chemical fingerprints, the different atoms and molecules in the atmospheres of some of the exoplanets. 

It might be able to detect molecules such as water, or CO2 or methane. 

On Earth we know that those molecules are linked to life but on another planet they could also be a sign of volcanic activity, so it’s never going to be easy to know in the near future whether or not there is life on an exoplanet.

But we are slowly, in a very strategic way closing in on understanding whether such planets might exist.


A NASA diagram showing the light curve of a planet passing in front of a star
Credit: NASA Ames


How did the K2 mission come about, and why was the original Kepler mission halted?

It’s one of those great stories at NASA. Just because we encounter a technical challenge, doesn’t mean the mission is over.

In fact, one of the best parts about my job here as an astronomer is that I get to work with insanely amazing engineers who can make anything work.

Kepler was originally designed to work for about four years, but at the end of that time two of the reaction wheels -  spinning wheels that keep the telescope very stable - malfunctioned. 

For a while people thought that might be the end of the mission, but the engineers came up with a really clever solution.

They used the two remaining reaction wheels to keep two axes of the telescope stable, and the third axis was kept stable by pointing the solar panels of the telescope towards the Sun so that the radiation pressure, the photons hitting the sun-shield of the telescope, enabled Kepler to sail on the solar wind.

By balancing the spacecraft with respect to the radiation from the Sun, they were able to keep the third axis stable swell.

This is important; we need to keep our telescope really stable to make precise measurements.

This meant that we could no longer look at the same field in the sky for four years, but we started surveying different parts of the Galaxy.

For an astronomer, that was a blessing because those different parts of the sky had different star clusters in them. They had supernovae explosions.

So all sorts of new discoveries and new physics were enabled. Even though it wasn’t planned, its’ been an incredible success.


What did Kepler tell us about supernovae?

In the last few months and years, Kepler has been looking at a sample of galaxies, about 100,000.

We know that if we look at enough galaxies for long enough, statistically a few stars in those galaxies will explode.

Very massive stars end their lives in dramatic explosions called supernovae and for a few days they increase in brightness a billion-fold.

It’s very difficult to observe the first hours and minutes of these explosions because we never know where those explosions might happen.

With Kepler, we were able to use its big telescope and camera to look at tens of thousands of galaxies and just sit and wait.

We caught a number of supernova explosions that right now are being analysed by astronomers, and for the first time they’ve given us an incredible view of the physics of the first minutes and hours of these explosions. 

I know for a fact there are some really interesting papers that are going to come out in the next few months about discoveries Kepler has been making in this field.


What do you think is Kepler’s lasting legacy?

I think its ultimate legacy is that we now know there are more planets than stars. Aside from that, we now know that planets are diverse, we know that they occur in all sorts of sizes, densities and orbital configurations around their star.

But we have also learned so much about the physics of stars: the interior structure of stars has been revealed.

So while I would say that the discovery that planets are ubiquitous is the number one result, there’s a huge wealth of smaller yet equally intriguing discoveries Kepler is making and is going to continue to make over the coming decades because the scientists are still working on the data.

There’s going to be the opportunity to do even more science with Kepler.

We believe there are a large number of planets that have yet to be found because astronomers on Earth have been overwhelmed with the data.

But we’ll not only get discoveries from Kepler, we’ve also launched a brand new mission called TESS, which is the Transiting Exoplanet Survey Satellite.

It was successfully launched in April 2018 and it builds on the Kepler legacy and some of the things we have learned from Kepler, and is now surveying the entire sky.

Kepler originally only studied a small patch of sky and it found thousands of planets, which is incredible, but now TESS is an array of smaller telescopes designed to look at all the bright stars in the sky.

This will give us a really good catalogue of all the nearby exoplanets.

These are really cool planets because if a planet is nearby around a bright star it’s much easier to study using the James Webb Space Telescope or other future telescopes.



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