Twenty-five years ago, we didn’t know whether the planets in our solar system were alone in the galaxy. Now we know planets are everywhere and even outnumber the stars. There are gas giants larger than Jupiter and rocky planets larger than Earth, planets orbiting two stars, made of diamonds, or even ocean worlds. Scientists have found at least nine thousand exoplanets and confirmed at least four thousand. How did we find these planets outside our solar system? One way is with the Kepler telescope, which launched in 2009. Using data from Kepler, and its follow-on mission K2, scientists have broadened our planetary horizons and sparked our imagine for nearly a decade. Listen to Chief Scientist Jim Green and a co-investigator on the Kepler and TESS missions, Jon Jenkins, discuss exoplanet-hunting and all the amazing discoveries Kepler has made.
Jim Green: Our solar system is a wondrous place with a single star, our sun, and everything that orbits around it--planets, moons, asteroids, and comets. What do we know about this beautiful solar system we call home? It’s part of an even larger cosmos with billions of other solar systems.
Hi, I’m Jim Green, NASA’s Chief Scientist, and this is Gravity Assist. With me today is Jon Jenkins, and he’s the co-investigator for data processing on two absolutely fabulous missions, Kepler and TESS. So, John, tell us what that means. What do you really do as a co-investigator for data processing?
Jon Jenkins: Well, Jim, what I do primarily is to design, develop, implement, and then operate the science pipelines for both the Kepler Mission and now for the TESS Mission. And by science pipeline I mean the facility and the software that allows us to take the raw measurements coming down from the spacecraft, the image data. We process it to turn it into planet candidates, essentially.
Jim Green: So, you’re sort of like the first line of defense that sees some of these exciting things that come back from the spacecraft.
Jon Jenkins: Absolutely.
Jim Green: So, what are some of the things that have really popped out in the data that you go, “Whoa, that’s neat,” or “Whoa, here’s something that could be exciting to analyze?”
Jon Jenkins: Well, certainly with Kepler I think some of the most exciting signatures that we saw in the data were ones that we didn’t expect to see. So, for example, when we collected the first science data from Kepler and brought it down, that was our first opportunity, essentially 10 years ago, to see whether or not Kepler would actually work because it was a very challenging experiment, and nobody had done this before.
Jim Green: So, what’s the basic principle behind how Kepler works?
Jon Jenkins: Well, Kepler works by observing a field of stars and measuring the brightness of the stars over time and looking for instances when a planet crosses in front of the face of the star, from our point of view. And when that happens, the star winks at us, and that repeats. So, we see a dimming of the starlight once every orbital period of the planet. And the depth, or the amount of light that drops, tells us what the size of the planet is relative to its star. And the interval between what we call transits or crossings of the star by the planet tell us what the year is for that planet, the orbital period of that planet.
When a distant exoplanet transits, or crosses in front of its star, it causes a slight dip in the star’s brightness. The Kepler space telescope measures this dip in a star’s brightness over time. The U-shaped pattern that emerges is a signal of the presence of a planet as shown by the solid white line. The scatter plot (colored blue) is representative of the recorded data.
Credits: Google
Jim Green: You know, so Kepler has been really successful ‘cause it’s looked at so many stars all at once in its original part of its mission. How many stars did it look at simultaneously, and how fast did it look at each star?
Jon Jenkins: Well, we could observe as many as 170,000 stars at any given time, but typically the largest number of stars that we observed at any given time was 165,000 stars.
Jim Green: Wow.
Jon Jenkins: Now, we measured the brightness of each of those stars every half hour. But, we had a small sample, 512 stars, for which we could make measurements every one minute. And that actually is very important because we need to know about the stars in order to know about the planets. I said before that we could tell what the size of the planet is by looking at what the fractional drop in brightness is when a planet crosses a star.
But, that’s a relative measurement, so we need to know how big the star is in order to infer with precision what the size of the planet is. And so, for the brightest stars that were at least 12th magnitude or brighter we could monitor the stars at one minute and actually measure acoustic oscillations or pulsations of the star caused by star quakes. And this is phenomenal.
Jim Green: Yeah, so it’s not just transits.
Jon Jenkins: Right.
Jim Green: It’s not just planets. Well, even sunspots--you know, and our sun has some enormous sunspots in its cycle.
Jon Jenkins: That’s right.
Jim Green: It’s overall light in certain frequency ranges, like the Kepler frequency range it observes can be dimmed just from the sunspot.
Jon Jenkins: That’s right. In fact, if we could see another Earth-sized planet transit the sun and just measure the brightness, you would see that the drop in brightness from a typical star spot would be as much as five times deeper--.
Jim Green: --Wow--.
Jon Jenkins: --Than the drop in brightness due to an Earth-sized planet crossing the face of the sun.
Jim Green: So, this is really a new and important dimension for other scientists that are using Kepler data to be able to cull through that and look at how active many of these stars are.
Jon Jenkins: Oh, absolutely.
Jim Green: In fact, my understanding is, you know, our star--we call it a typical star in the galaxy.
Jon Jenkins: Right.
Jim Green: You know, it’s classified as a G2 star; there’s something like 10,000 G2 stars that Kepler looked at, you know, in its prime mission. What kind of information, you know, you can get from that is the sunspots and their activity and really kind of understand how active G2 stars can be. And that’s true for almost any star.
Jon Jenkins: That’s correct. And so, one of the unsung stories of Kepler is the fact that we were able to conduct astroseismological investigations of these stars. That’s the study of the songs that the stars sing as--they ring like bells. And, like bells, the more massive and bigger the star is, the lower the tone. Stars like the sun ring with a typical periodicity of about five minutes, so that’s why we needed the one-minute samples.
But, by studying the tones the stars are ringing at, you can measure the size and the mass of a star to a couple percent. And that’s phenomenal because we can’t see the star, we can’t resolve the size of the star by looking at the images. We’re just seeing point sources. So, it’s phenomenal that from, you know, hundreds or thousands of light years away, we can actually measure the mass and the size to a couple percent. I don’t know about you, Jim. I don’t know what my mass is to 2 percent.
Jim Green: Wow. Yeah, okay. You know, that’s really phenomenal. So, let’s listen to what a star song could be like.
Jim Green: So, you know, when Kepler looks out and sees this large a population of stars, I can just imagine that it’s looking at almost every type of star, from what we classify as A all the way to M and others. Is that true?
Jon Jenkins: That is true. For Kepler we had target stars that we observed, and over the lifetime of the mission we observed over 200,000 stars during the Kepler mission itself, ranging from late A stars all the way down to early M stars, for the most part.
Jim Green: Yeah, where the A’s are the really big stars.
Jon Jenkins: That’s right.
Jim Green: And the M’s are the much smaller stars.
Jon Jenkins: That’s correct.
Jim Green: Now, you’ve done a lot of work yourself doing the science analysis. What are some of the fantastic things that get you excited about looking for planets?
Jon Jenkins: Well, I think one of the most exciting things that came out of Kepler in terms of exoplanets that we didn’t expect was the fact that we observed over--well over 400 multiple transited planet systems. This is--these are cases where we don’t just see one planet crossing the face of its star periodically. You see multiple planets. You might see two, three, as many as eight planets transiting a star. Now, Kepler only observed for four years. And so, that means that all of these systems are very compact. The orbital periods are much smaller in general than the orbital periods of the planets in our solar system because we weren’t observing long enough to see the Jupiters in 11-year period orbits. In fact, Kepler 11 was the first really good example of this kind of system, where you had six planets orbiting. And the inner five were within the orbit of Mercury, while the outer one was between the orbit of Venus and Earth. So, these are small, compact planetary systems compared to our own. And that means that nature really likes to make such planetary systems because we saw so many of them. And that means that our general picture for how planetary systems form is a consequence of the collapse of the protoplanetary disk, the inner portion of which collapses to form the star. And the outer part of this disk aggregates into planets.
Jim Green: You know, when I was going to the University of Iowa, and I was getting an undergraduate degree in astronomy, we all recognized as scientists, “Oh, sure, there’s probably planets out there, but we’ll never be able to see them. The stars are so far away.” So, it’s phenomenal to think that in our lifetime some of these techniques have come out that really demonstrate that planets are everywhere in our galaxy. Now, Kepler observed a particular area for, as you said, several years--up to about four years. And then, in its extended mission, it changed that approach. What happened?
Jon Jenkins: Well, the best, worst thing happened to Kepler, and that is that we lost the second of our four reaction wheels. Now, these reaction wheels are kind of like gyroscopes to control the pointing of the spacecraft. It’s a very efficient manner to do so, but you need at least three healthy operating reaction wheels to do that.
And in May of 2013 the second one failed. That meant the end of the Kepler Mission, as we know it, because we could no longer point our spacecraft at the Kepler field of view. But, a clever engineer at Ball, Doug Wiemer, came up with this idea of how we could operate with two wheels using the solar pressure, so the photons bouncing off the spacecraft--.
Jim Green: --Wow--.
Jon Jenkins: --To balance the spacecraft, so that we could control the roll angle. So, with the two reaction wheels we had left, we could control where we pointed on the sky, but we couldn’t control the roll angle.
And we needed to control that. So, we developed and initiated the mission called the K2 Mission, which is the follow-on mission to Kepler, using the Kepler spacecraft, where we were able to observe a field of view in the ecliptic plain, so in the orbital plane of the planets around our own sun. We can point at one of those fields for up to 85 days or so. And that has been a phenomenally successful mission, allowing us to greatly broaden the science-reaching impact of the Kepler spacecraft.
Jim Green: You know, one of the things that came out this last year was a star, TRAPPIST-1, that was close to our sun, 39 light years away, okay.
Jon Jenkins: Right.
Jim Green: Just right around the corner, right? And it had seven, you know, planets, and these are big bodies. You know, these are all Earth-sized. And Kepler has had an opportunity to observe this system, too.
Jon Jenkins: That’s correct.
Jim Green: What is another fantastic discovery Kepler has made, Jon, from your perspective?
Jon Jenkins: One of the most fascinating discoveries that Kepler has made is that planets don’t form just around single stars, but they form around binary star systems.
Jim Green: Wow, Tattooine.
Jon Jenkins: That’s right, exactly like Star Wars, which I remember in 1976, my dad taking me to see Star Wars and seeing Tattooine and seeing the two stars, you know, setting, behind that planet. And, indeed, part of what brought me to the mission was that I was part of a very small team of scientists who were trying to find planets orbiting CM Draconis which is a small M-class pair of stars. And we believe that if there were stars transiting the system, that an Earth-like place, a habitable zone, would be in a 17-day period orbit. And because it’s so small, we could actually from the ground determine whether or not there were planets as small as earth there.
Jim Green: By spending a lot of telescope time but taking--but staring at it.
Jon Jenkins: We--that’s right. We observed for six years. We placed really strong upper limits on the presence of any planets in that system, but we never discovered any.
Jim Green: Okay.
Jon Jenkins: And it took Kepler to fly before we found the first circumbinary planetary system, Kepler-16b. And since then we’ve found a handful of other such systems. And what’s interesting is that within the first several discoveries we discovered a planet in a habitable zone of a circumbinary planetary system.
Jim Green: Wow. You know, this is one of those kind of discoveries that the planetary scientists even are going to be stumped for a while. Right. We have to really figure out what this planet is like. If you look at a star, chances are it’s got a planet--at least one. And so many have, as you have already talked about, quite a solar system, you know, more so than our own. But, what are the kinds of sizes we’re seeing? And what is that kind of distribution?
Jon Jenkins: Well, in terms of the planets that we’ve discovered, we discover more smaller planets than we do larger planets. And that’s very interesting because it’s actually harder to find the smaller planets. It’s kind of like if you were blind and reaching down into a pond fishing, you really want to find the small, cool fish that are down there at the bottom. But, the warmer fish--because the water is warmer at the top, the larger, warmer fish are easier for you to find because they are closer to you, they are easier for you to reach, and they are easier to find and catch. The smaller fish are harder. They dart out of your hands. And that’s the way it seems to happen and what it feels like when you’re looking for very small planets like the Earth in data from Kepler. As exquisite as the data are from Kepler, it’s still very difficult for us to find Earth-sized planets and Earth-sized orbits. We have only a few examples of planets like that. And, in fact, Kepler’s main mission was to determine the frequency and distribution of Earth-sized planets and Earth-like orbits of sun-like stars.
Jim Green: Um-hmm.
Jon Jenkins: And we’re not quite--we have an answer to that question, but it’s not very precise. So, the most current paper we have, and information on that, is that about 10 percent of solar-like stars have a planet about the size of the Earth in a one-year period orbit. But, the error bars on that are very large, so there could be as few as--the frequency could be as few as a percent, or it could be as high as two, meaning that you have as many as two such planets on average. So, that’s a gap in our knowledge that’s large enough for a truck to drive through.
Jim Green: Yeah, but that just means we need to think about more missions to go back up and really close that gap and make that definitive observation.
Jon Jenkins: That’s right.
Jim Green: You know, when we look at our own solar system, we have a set of terrestrial planets and gas giants. And even in between, you know, are Uranus and Neptune, not as big as Jupiter or Saturn. And you would think we’d covered the ground in all the types of planets that must be out there. But, Kepler gave us a number of surprises. What were they?
Of the 1,030 confirmed planets from Kepler, a dozen are less than twice the size of Earth and reside in the habitable zone of their host stars. In this diagram, the sizes of the exoplanets are represented by the size of each sphere. These are arranged by size from left to right, and by the type of star they orbit, from the M stars that are significantly cooler and smaller than the sun, to the K stars that are somewhat cooler and smaller than the sun, to the G stars that include the sun. The sizes of the planets are enlarged by 25 times compared to the stars. The Earth is shown for reference.
Credits: NASA/Ames/JPL-Caltech
Jon Jenkins: Well, one of them is the fact that super-Earth--so, planets that are between the size of Earth and Neptune, which is four times the size of Earth by radius, are the most common planets we find. And yet, we have no examples of these beasts in our own zoo here in orbit around our sun. So, we don’t know what they could be like, right? I mean, we have no examples in our own backyard. So, the question is, are these planets more like super-Earths, you know, like large Earths? Or are they more like mini-Neptunes?
Jim Green: So, terrestrial-based large Earth.
Jon Jenkins: Large rocky.
Jim Green: Yeah, right. Or more like the--a gas giant.
Jon Jenkins: That’s right.
Jim Green: But more like Neptune.
Jon Jenkins: That’s right. So, we do have some hints actually coming out of the work that’s been done to refine the properties of the stars. As I said earlier, we only know the planets as well as we know the stars. And so, it turns out to be very hard when you have a field of view that you’re trying to understand what’s going on on 200,000 stars. It’s really difficult to learn as much as you can about each and every one of those. But, people are doing follow-up observations--.
Jim Green: --Right--.
Jon Jenkins: --To refine the stellar parameters and say, “We see that there’s a break point now between planets that are 1.6 Earth radii or smaller, 60 percent of--bigger than the Earth or smaller--appear to be most likely to be rocky. And planets that are larger than 1.6 Earth radii appear to be more like mini-Neptunes, small, gas, or ice giants.”
So, that’s a big surprise coming out of the Kepler Mission. Again, much like the multi’s, when we were designing this mission, I managed to join this mission back in 1995 while it was still very much on the drawing boards.
Our conception of how it would work and what we had to design this mission to be able to do was predicated entirely on the solar systems that we--we imagined that every star hosted a planetary system just like our own, and that’s what we designed it to be able to do, to find planets like Earth and Venus. And if we were lucky, maybe there would be some big planets like Jupiter closer in. But, it turned out completely different from what we thought 20 years ago.
Jim Green: Well, you know, a lot of people don’t realize that Kepler does a lot of fabulous observations. But, to really be able to confirm planets versus stellar activities or sunspots or some of the other things that we briefly mentioned, you have follow-up observations. And so, what’s the network of follow-up that goes on?
Jon Jenkins: So, it turns out that most of the planets that Kepler has found are simply too far from us, and the stars are too dim for us to do much follow-up work in order to characterize the planets that we’ve discovered. So, there are a few planets that Kepler has discovered that we can measure the masses directly.
Those tend to be the more mass of planets, the Neptunes and Jupiters, and so forth. But, what we can do with the follow-up is first characterize the star, the planetary host, much better than we knew of beforehand. It allows us to refine the planetary properties. And we are talking about planets the size of Earth, typically. Our understanding of the star might end up changing the imputed size of the planet by tens of percent.
Jim Green: So, in other words there’s all kinds of other telescopes that come into play, from ground-based to even space-based to do follow-up. To be able to lend their support or their abilities to the study of that system and confirm, or even improve on, what the observations from Kepler are telling us.
One of the other spectacular things, you know, from a planetary scientist, as I sat here and see the fabulous science coming out of the Kepler Mission, is that there are planets that are on orbits that they couldn’t possibly have been created on, you know, the hot Jupiters and the highly elliptical orbits, you know.
As the initial nebula collapses, and you end up forming planets, one would think it would be in a slightly elliptical or nearly circular orbit but never a highly elliptical orbit. But, we’re finding those.
Jon Jenkins: We are finding some systems like that. So, our best guess as to how this happens is due to late scattering events where you have basically close encounters between large planets. And in many of these instances, a large planet will eject other planets from the system and leave a planetary system behind with highly perturbed orbits, compared to the circular orbits that you would expect from the initial formation process.
Jim Green: Yeah, so those gravitational resonances--when those planets fight each other for position, you know, we actually see that in the solar system because Jupiter really carves up the asteroid belt.
There are certain resonances that occur for which, then, Jupiter pulls asteroids out of the asteroid belt, throws them out of the solar system, or actually interior, you know, so that they end up crossing our orbit and becoming what we call near-Earth objects. So, this is a great connection between astrophysics, observations, and finding planets--and solar system dynamics that planetary scientists do today.
Jon Jenkins: It is. In fact, one of the most recent results out of Kepler has been a study done by Lauren Weiss and her colleagues, who have been following up many of the multiple transient planet systems to look for other planets using radial velocities. But, one of the things that she found was that most planetary systems are not like our own solar system. The planetary systems that Kepler has found typically tend to be very regular in the spacing between the planets, and the planets tend to be of--about the same size. And in our own solar system we find that there are much larger distances between the planets and that they tend to be mixed up.
Jim Green: You know, also when you look at Mercury, you know Mercury has an orbit and a rotational period that are nearly the same. And so, we call that tidally locked. We see one face of the moon all the time, and that’s because of--our pull and tug on the moon has really evened that out, and it’s mass distribution is such that we end up with one face pointing to us. So, many of the stars that you’re finding planets around, actually the smaller stars where the planets are close, may actually be tidally locked.
Jon Jenkins: That’s correct. And that could have great implications for the habitability of such planets because you would expect the sunward-facing side of the planet to be much hotter than the back side of the planet, the dark side of the planet. It’s interesting, though, that Earth’s closest relative is Venus, and because Venus is rotating so slowly, you might expect it to have a hot side and a cold side. And it doesn’t. And that’s because it’s got a thick atmosphere that permits a subsolar to antisolar circulation of the atmosphere at the top levels. And then, it’s got a super-rotated atmosphere.
Jim Green: You know, if we go back and talk a little about TRAPPIST-1, when that was discovered last year, seven planets orbiting an M-dwarf star, smaller star--so, it’s not as big as the sun, doesn’t put out as much light. And so, its habitable zone, as we call it, that area where the light on an Earth-sized planet can allow water to exist in three phases, liquid, vapor, and even solid--in that system it turned out all those planets were tidally locked.
Jon Jenkins: Right.
Jim Green: So, even though that’s a fantastic discovery, and we have three of those planets in the habitable zone, we have a lot of work to do to prove that they’re habitable yet.
Jon Jenkins: We do. And for M stars, in general, they tend to be very active and to have a lot of flares. And we know that planets as close as these are to their star, that flare events--large flare events--could sterilize the surfaces.
Jim Green: Yeah, they’re really getting hammered. Yeah, indeed.
Jon Jenkins: That’s right.
Jim Green: But, here’s what’s really excited and important about the TRAPPIST-1 discovery, and that is that star is only 39 light years away. That’s just right around the block. And it had enough planet-making material that it created seven planets. So, that means when we look around us, our area of the galaxy, of the Milky Way, must have an enormous amount of planetary-making material. And so, we want to be looking for planets closer to home. And that’s where TESS comes in. Can you tell us a little bit about TESS?
This artist's concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets' diameters, masses and distances from the host star, as of February 2018.
Credits: NASA/JPL-Caltech
Jon Jenkins: Absolutely. So, the TESS Mission is a follow-on to Kepler. And the idea is to observe the whole sky, so it’s an all-sky survey for transiting planets. It observes a field of view that’s much, much bigger than Kepler’s field of view. It’s 24 degrees by 96 degrees, and so we are observing a large swath of the sky at any given time. And we re-point the spacecraft to point at a new field of view every month. And after one year we’ve covered one hemisphere, and we’ve flipped our spacecraft upside down and then covered the other hemisphere. And the idea is to observe stars that are much closer, typically 10 times closer and 100 times brighter than the stars that Kepler observed. And we want to eventually learn what their atmospheres are made of and look for biosignatures in the atmospheres of these planets, study their climate, study their weather. And Kepler is--you know, Kepler basically is putting wind in the sails of many missions and investigations to learn more about exoplanets.
Jim Green: Yeah, so TESS is a great follow-up to do exactly that. And as we learn much more about those planetary characteristics, here’s where planetary scientists can come in and help. You know, we can do the modeling. We’ve got models. We’ve got global circulation models of Mars and even Titan. And so, we have all kinds of tools and capability. I think it’s a great opportunity for both astrophysics and planetary scientists to work together. And this helps us understand the origin and evolution of our own solar system and how we got here.
Jon Jenkins: Right. It really provides the context for the Earth and the solar system and the broader tapestry of the galaxy.
Jim Green: I really like to ask each and every one of my guests on this program, what was their Gravity Assist that really got them into the field and propelled them forward to become the scientists they are today? So, Jon, what was your Gravity Assist?
Jon Jenkins: Well, I had several Gravity Assists. I grew up--and was fortunate to grow up in the shadow of the VAB building. Both of my parents were working at Kennedy Space Center. The VAB building is the Vehicle Assembly Building on Kennedy Space Center on Merritt Island, Florida.
Jim Green: It’s huge.
Jon Jenkins: It’s huge. In fact, when they first built it, before they got the air conditioning going, clouds would form, and it would rain inside the building, my dad tells me. So, my dad was an engineer at Kennedy Space Center. He worked on the Mercury, the Gemini--.
Jim Green: --Wow--.
Jon Jenkins: --The Apollo program and then on the shuttle program. And I got to watch the launches. I got--I remember being in my pj’s, being brought out to watch the moon landing. And I have had a lifelong fascination with space. I think the space program and NASA’s endeavors are some of the most important activities that humans can carry out.
Part of it is just exploring our world and exploring the worlds around us, and part of it is just reaching out to understand where we fit in in terms of, how do we get here, and where might we be going, given everything else that’s going on in the universe, so--.
Jim Green: --So, you were really motivated based on those early exposures of what NASA was up to, to study hard and get a degree in astrophysics.
Jon Jenkins: So, I was doing my Ph.D. dissertation on the atmosphere of Venus. I was using data from the Pioneer Venus Orbiter and the Magellan spacecraft. And during that period of time was when I met Bill Borucki, a co-investigator on Pioneer Venus who was studying lightning in the atmosphere of Venus. And he had this really wild idea that we could find Earth-sized planets around sun-like stars. And I don’t know about you, Jim, but, you know, 25 years ago that was really pie in the sky.
Jim Green: Oh, it really was. So, he followed his dream, and you followed him.
Jon Jenkins: That’s right. So, he invited me to join the mission and told me, “Jon, you’re going to figure out how we’re going to be able to find planets like the Earth, even though the stars are highly variable and have star spots.” And so that, indeed, was one of my principle contributions to the Kepler Mission.
Jim Green: I’m Jim Green, and this is your Gravity Assist.
Last Updated: Aug. 1, 2018
Editor: Gary Daines
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