« dnia: Sierpień 15, 2018, 12:00 »
Gravity Assist: Exoplanet Hunting with Jon Jenkins (1)
Aug. 1, 2018
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: GoogleJim 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. NASA heliophysicist Alex Young explains how sound connects us with the Sun and all other stars. This piece features low frequency sounds of the Sun. Credits: NASA's Goddard Space Flight CenterJim 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.Kepler-11 is a sun-like star around which six planets orbit. At times, two or more planets pass in front of the star at once, as shown in this artist's conception of a simultaneous transit of three planets observed by NASA's Kepler spacecraft on Aug. 26, 2010. Credits: NASA/Tim PyleJim 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?
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