NASA Podcasts

Kepler: A Search for Habitable Planets
02.27.09
 
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Jesse Carpenter: NASA’s Kepler Mission to look for Earth-like planets. Hi, I'm Jesse Carpenter and you're listening to a podcast about NASA’s Kepler Mission from the NASA Ames Research Center. Planned for launch in March of 2009, the Kepler space telescope will observe one area of the universe continuously for a period of four years in its mission to identify the presence of planets that are similar to Earth. Today we have three Kepler project team members who are here to discuss the Kepler mission and its goals. Joining us in our studio are Kepler Principal Investigator William Borucki from NASA Ames, Kepler Deputy Principal Investigator David Koch also from NASA Ames and Kepler Science Council Member Alan Boss from the Carnegie Institute of Washington. Welcome to everyone, and thank you for joining us. Bill, why don’t you start by giving us an overview of what Kepler is.

William Borucki: Well, Kepler basically is a space mission, where we have a large telescope, a photometer really, that measures the brightness of over a hundred thousand stars, searching planets, around other stars particularly planets that are Earth size. And in fact, we’d like to know not only how many Earths are out there, but how many are in a habitable zone of their star. That’s the area where there could be water on the surface, where life might be possible. And so, basically the spacecraft looks at these hundreds of thousands of stars looking for transits, when a planet moves across the star, the light dims, and the amount of dimming tells us about the size of the planet and how often it repeats tells us the orbital period. And we use Kepler’s third law, which relates how often a planet orbits, what the orbital period is, and the distance from the star. And since we know the temperature of the star from other measurements, we can find out whether it’s at the right temperature, not too close to the star, so it’s too hot, the oceans boil, not too far away where everything is ice. So, basically that allows us to find out whether these planets are in the habitable zone, and find out how frequent they are. Clearly if we see that, many, many of these stars do have planets; life might be ubiquitous throughout the universe. They may be just waiting for us to say hello. On the other hand if we find none at all, well, that might very well mean that things are quite a bit different than expected, and we might be alone in the universe. And I think Alan can address what we’ve learned so far from the several hundred planets we’ve found, giant planets around other stars.

Alan Boss: Well as Bill said, Kepler is going to be able to determine how frequently earths occur, and the expectation from theorists in particular is that earths should be quite frequent. And, that’s based on several lines of evidence. The first is that astronomers, who study young stars, see that essentially all young stars are surrounded by disks of gas and dust, formed as part of the same process that formed the star itself. And planets are thought to form out of these disks of gas and dust. And so those disks are quite common and we expect therefore planet formation to be quite common just from a very basic observational point of view. Second of all, from a theoretical point of view, theorists who study what happens inside those so-called proto-planetary disks find that planet formation is pretty much an inevitable process, at least certainly the formation of planets like the Earth. Because the dust that’s inside these disks tends to settle down toward a thin plane and become so dense that the dust grains start hitting each other and sticking together, and growing larger and larger. And over a time period of perhaps a few hundred thousand years, bodies the size of the moon are thought to form, rocky bodies. And then those moon-like bodies take another several hundred million years perhaps to manage to find each other, hit and form Earth-like planets. So there’s a good chance that essentially every star has an Earth-like planet or something similar to it and so, expectations are that Kepler is going to be finding just oodles of these planets.

Jesse Carpenter: So Alan, what are the techniques that are used in the search for planets?

Alan Boss: The most common method for detecting planets is the so-called Doppler survey where you look for the radio velocity wobble of a star around its center of mass of a system implying that you have a planet there, which is then sensed indirectly. Most of these tend to be gas giants like Jupiter and Saturn, rather large planets full of gas and a bit of a rocky, icy core perhaps, but in addition other techniques have found even lower and lower mass planets. In particular, the micro-lensing technique has found objects as low in mass as perhaps Neptune or less, or maybe as low as five to ten times the mass of the Earth. And these objects are out orbiting at distances that they’re quite cold and icy and they’re probably very good analogs to Uranus and Neptune in our solar system that is that they’re sort of like ice giant planets. In addition, the radio velocity folks have been able to improve their precision quite a bit and they now can get even lower and lower mass objects orbiting in close to their star, and they’ve been able to find a third class of planets which might be called the hot super-earths or the warm super-earths, these are objects with masses as low as again maybe five or ten times the mass of the Earth, they’re orbiting closer to their star so they’re sort of analogous to Mercury and Venus in our solar system, although a bit more massive. So we have now evidence for all three classes of planets within out solar system basically. Jovian planets, gas giants that is, ice giants like Uranus and Neptune, and probably the tip of the iceberg of the terrestrial planets the so-called hot, warm super-earths. So that makes us feel like maybe the solar system is not such an oddball place at all. We’ve not yet found an Earth-like planet, that’s what Kepler is going to do, and the indications are from what we’ve seen so far is that Kepler is going to find lots of them.

William Borucki: One of the things of course is that Kepler is a different kind of mission. It’s a transit method, where we use photometry and Dave Koch is here and he can tell us about the optical systems that are required to use this new method that is especially good at finding small planets.

David Koch: Well as Bill mentioned, we’re looking for transits and we’re looking for sequence of transits not just a single one. The method then requires us to look at lots of stars because if you’re looking for transits you don’t see them very often. So to look at lots of stars, we have to have a different kind of telescope than your normal astronomical telescope. Hubble, Keck and all the large ground based telescopes have a very tiny field of view. They’re designed to look very deep in our universe at things like galaxies. We need to look at lots of stars, and that requires a special kind of telescope that’s called a Schmidt telescope. Our telescope has a hundred square degree square field of view, that’s about equal to taking two dips from the Big Dipper on the sky. It’s enormous for an astronomical telescope. What we do with data then is we image it onto an array of CCD’s, charged coupled devices. We look at the stars continuously; we add the data up from the stars every thirty minutes and we save that information, and we pick out then only the pixels where we have stars that are appropriate for our studies, about a hundred thousand stars is what we need. That means we use only about three percent of the pixels. Our focal plane has ninety-five mega pixels, ninety-five million pixels as compared to your commercial, consumer camera that has a few, maybe up to eight or ten mega pixels. Once we have the data on the ground, we then need to do our data processing to convert that digital information into individual light curves from each of the stars and then we can start looking for the tiny dips in brightness caused by an earth. Now the tiny dip is something like one-hundredth of one percent, so when we see that dip that tells us directly the size of the planet we’re looking at relative to the star. If we know the size of the star, we then know the size of the planet. The other thing we get directly from the data is the orbital period from the sequence of transits. Using Kepler’s third law, we know then the relationship of the orbital period to the distance from the star. If you now know the distance and the luminosity or temperature of the star, you can now calculate the characteristic temperature of the planet, once you that, you can determine whether there’s liquid water on board.

Jesse Carpenter: Dave, tell us why the mission was named after Kepler, and give us little background as to who he was.

David Koch: Kepler was a very prominent scientist. Kepler lived four hundred years ago and in fact, the year that we’re launching this in 2009 is the four hundredth anniversary year of Kepler publishing his first two laws of planetary motion. So it’s a wonderful coincidence that the mission name that we picked is going to be launched on the four hundredth anniversary of his spectacular work.

William Borucki: We might also mention however, that at first when we were developing the mission, it had a different acronym that basically everybody disliked. It was called FRESIP, the FRequency of Earth-Sized Inner Planets and when Carl Sagan was a member of the team and Jill Tartar from the SETI Institute and Dave Koch finally convinced me that it was a terrible acronym, and Kepler was so much better. And I had to admit that they were right, so we changed the name of the mission to Kepler from what it used to be when we first started out.

Jesse Carpenter: Bill, give us a sense of perspective, tell us how important it is that we conduct this mission.

William Borucki: Basically when you look through the Earth’s atmosphere, the atmosphere is full of dust and clouds, there’s a day night cycle, so you can’t keep looking continuously and you don’t have anywhere enough precision to find small planets, so you must go out into space to find these planets. You have to look uninterruptedly for years to make the measurements that we want, and the measurements are important because when we recognize that Kepler will find how often there are earths, how often there are earths in the habitable zone, that’s not the end of our search. It’s just one step. The next step is to build more capable missions that can look at the atmospheres of these planets, and find out are biological or gases or gases like methane and oxygen, the things that we might associate with life. So Kepler is one necessary step, in a series of steps that we will use to find life in our universe.

Alan Boss: It’s important to emphasize that Kepler is really going to be looking for something, Earth-like planets in the habitable zone that really cannot be done by any other astronomical technique that we have available to us right now. Ground based studies can do wonderful things, and other space telescopes like Hubble and Spitzer can do wonderful things, but there’s no facility we have available to us that now that can do what Kepler can do which is to find Earth mass planets orbiting at Earth-like distances. So Kepler is unique and important and crucial, especially for what NASA will be planning to do after Kepler finds the earths it’s going to find.

William Borucki: This mission does not have the capability of telling you whether the planet has life. All we can do is tell you the size of the planet. We cannot even tell you if the planet has an atmosphere. So one has to be very careful to distinguish what Kepler can do and what future missions might do. This is a mission of basically… it’s very quantitative. What is the fraction of stars that have Earth-sized planets and what is their distribution of orbits and planets sizes. It does not really look for life. It cannot find life.

David Koch: What we’re really looking for is ET’s home. When you’re looking at habitable planets there are really two principles that are important. One is the size of the planet, and that tells you whether you can have a life-sustaining atmosphere, if the planet is too small it can’t hold onto an atmosphere. If it’s too large it turns into a gas giant, so there’s a right size and the planet has to be at the right distance from its star. Too close it’s too hot, you lose an atmosphere, you would lose any water that you could have. Too far away it’s too cold, water freezes. So there’s a right size, a just right size and at just the right distance from the star that we would consider habitable, that is a place for ET to have a home.

Alan Boss: Basically, Kepler is going to try to count the number of rocky planets around some sun-like stars and figure out how many of those rocky planets are conducive to perhaps having water be liquid on their surfaces. That’s not the same thing as finding life on them, but we would expect that such planets would eventually evolve some sort of life, but Kepler will just tell us how frequently the abodes for life occur.

William Borucki: The summary of what we’re doing really is that we’re going to be looking for several years to find these planetary orbits where we have to find sets of transits. But, certainly in a year or so we’ll find some planets, Earth-size planets hopefully around the coolest, smallest stars and in the coming years, certainly by the end of three years we would expect to find earths in the habitable zone. So basically that’s what Kepler is all about. Are there earths out there, how frequent are they, what kind of stars are they around? We will have an idea of whether life is likely to be possible throughout the galaxy, or if we don’t find any, that life is going to be very rare in our galaxy. And that’s what Kepler is about.

Jesse Carpenter: I’d like to thank our guests Bill Borucki, David Koch and Alan Boss for joining us. I’m Jesse Carpenter, and you’ve been listening to a podcast from the NASA Ames Research Center.

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