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NASA’s Kepler Mission, reflections on planets in the habitable zone
04.16.09
 
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Jesse Carpenter: NASA’s Kepler Mission, reflections on planets in the habitable zone. 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 kinds of planets that Kepler will be looking to find. Joining us in our studio is Kepler Principal Investigator William Borucki from NASA Ames. Also from NASA Ames is Kepler Co-Investigator Jack Lissauer. And from the University of California at Berkeley we have Kepler Science Working Group Member Geoff Marcy. Welcome to everyone, and thank you for joining us. Geoff, what do we already know about planets in the universe?

Geoff Marcy: Well, I can say that astronomers have been lucky since the mid-nineteen nineties in discovering now well over three hundred and fifty planets around other stars. There are two techniques that have been very successful measuring the Doppler shifts of the stars, which allows you to notice the star wobble in response to the gravitational tug of a planet on that star as the planet orbits. And then, the other technique that’s been very successful is indeed detecting the dimming of stars as planets cross in front, transiting the star blocking some of the starlight. And between those two, planets have been discovered like Jupiter in our own solar system, some of then like Saturn others the size of Neptune. They’re in very strange orbits, in many cases some of the planets orbit very close to their host stars. Other planets that have been discovered orbit in elongated, very elliptical orbits like the comets in our own solar system. And so what we’ve learned overall is quite remarkable. That other planetary are in fact different from our own home solar system in some profound ways. And we’re beginning to learn how our own solar system fits into the general family of planetary systems in the cosmos.

Jesse Carpenter: So, how difficult it is to detect and discover planets around other stars?

Geoff Marcy: Discovering planets around other stars is extraordinarily challenging and people have known this for centuries. And the reason is simple: planets by definition don’t generate their own energy, so they’re actually relatively dim in contrast to stars. A star of course by definition has nuclear reactions going on in the center like our sun does converting hydrogen to helium. And so stars are typically about a billion times brighter than any planets that might be orbiting them. So even with the mighty Hubble Space Telescope you can peer up at a nearby star and the planets if any orbiting that star will be lost in its glare. And so we’ve had to find some rather elaborate tricks to find these planets found so far. There are certainly more Saturns than there are Jupiters, Saturn being about a third the mass of our own Jupiter. And so there’s a suggestion that there are more small planets than there are big planets. Just as you approach a beach from afar in your boat, you notice the boulders first, and then as you get closer you might see the rocks on the beach, and then as you’re closer still you’ll see the enormous number of grains of sand. It may be the same with planetary systems, that there are far more smallish planets of Earth size, than the large boulder Saturns and Jupiters that we’ve been able to detect so far.

Jesse Carpenter: Jack, assuming there are many other solar systems like ours, won’t it be difficult to tell which planets are in the habitable zone of those systems?

Jack Lissauer: In essence what we’re trying to do, is a survey of the planetary zoo. And detecting planets of any type is quite challenging around other stars. So, the first planets detected are the easiest ones to see. That doesn’t mean they’re easy, but they’re easier than planets like Earth. So we’re looking essentially at the hippos and rhinos in the zoo. But what’s most important to us, is the primates. And Kepler is sized so that it can detect the primates of the planet population. The planets that are analogous to our Earth. So we want to find planets roughly the size of Earth, so they have nice atmospheres and surfaces, and roughly the temperature of the Earth. So if they orbit a star like our sun, we want them to be roughly Earth’s distance from that star. If they orbit a fainter star, then we want them to be closer to their sun so that fainter star, their sun can heat them to a temperature where water can be liquid, ice can melt but it doesn’t all vaporize making a steam atmosphere and boiling any life as we know it away.

Jesse Carpenter: Bill, how would you define a habitable planet?

William Borucki: When we think about life, and we think about life on Earth, we recognize that liquid water can form in rather strange places. In particular when we look at some of the moons of the planets, these moons are very far from the sun, out at the distance of Saturn and Jupiter. And these moons apparently have oceans. And the oceans are ice covered. And you might ask, “Well, how can the oceans stay unfrozen?” The answer, well those moons often flex because the gravitational field of a major planet moving toward and away from these planets. And that flexing of this moon warms it up, keeps the ocean liquid. We might possibly have life in these oceans. Life that has never seen the sun, that doesn’t depend on sunlight, and it might in some sense be unusual except of course on Earth we have life that’s not that different. We have life in caves, and some of these fish in caves have lost their eyes, so it’s not unreasonable to have life without light. These oceans might be very cold, but of course the Earth’s ocean is very cold, it’s near freezing. So it’s quite possible that planets that we don’t think about, or moons that we don’t think about being in the habitable zone are still habitable. So we don’t want to discard planets that are a little bit too close, or a little bit too far. Life has a way of surprising us in terms of what we might find. Certainly nobody I think expected that when we went to the bottoms of the oceans and we saw water coming up from these vents, way above the boiling point of water at the atmospheric pressure 250 degrees Fahrenheit. Yet there were creatures down there that enjoyed that warmth. There were creatures in fact that were taking in hydrogen sulfide, a poisonous gas to us. There are creatures in various parts of the world that take in methane, and that again is a fuel they us. So, we can certainly imagine planets that have these things, many planets have methane for example, that might survive in ways that might surprise us. So, I don’t think we want to close our minds to the fact that although we’re looking at a narrow zone, we are going to find planets in a wider zone. And some of those planets and some of their moons might very well have life also.

Geoff Marcy: Kepler is designed to answer a question for which we actually don’t know the answer: how common are Earth-like planets? And the reason this is a profound question is that we actually harbor a bias, most of us do. We watched science fiction movies, we read science fiction novels, Earths seemed to be numbered in the billions in our galaxy. But that’s the fiction. The reality is we haven’t found a single Earth-like planet to date. And so the marvel of Kepler is that it is going to answer a question that we don’t have the answer to right now. We might ask, “What do you mean by an Earth-like planet?” We’re using that term. But, what are the attributes of the Earth that render it in our minds Earth-like. The presence of liquid water. But then the question that arises is, “How much liquid water renders a planet Earth-like?” Some planets may have so little water, albeit liquid, that it is buried in the mantles or the crusts and the hydrated minerals of the interior of a planet. Other earths may have so much water that they are literally covered by water. Not a third continents as the Earth has, but no continents at all. Rendering technological life, I think, quite improbable. So we are not just asking a question that we don’t have the answer to, how many earths are out there, but we’re going to learn something about the diversity of and the properties of the Earths when we do find them.

William Borucki: We can always imagine life in very, very strange forms based on silicon, based on all sorts of things. But that’s pure speculation. In reality we know about carbon-based life, we know about life that’s associated with water. And so that’s the first place to look, the most logical place to look. And that’s why we concentrate on Earths even though we recognize in the future people may find life that is quite diverse. First, let’s do Earths, then let’s think about expanding the search.

Geoff Marcy: Kepler answers more than just the anthropocentric question, “How common are planets like our home planet?” We will learn with Kepler how often planets occur closer to their host star than the Earth is from the sun, how often planets are farther. We’ll learn the range of masses of those terrestrial planets, how many planets are twice the mass of the Earth, four times the mass of the Earth. And moreover we will learn how commonly an Earth-like planet orbiting a star is accompanied by a Jupiter sized planet. The Jupiter sized planets presumably playing a very critical role in cleansing the planetary system of its planetary debris, the asteroids and the comets that are the aftermath of planet formation. The Jupiters serve as cosmic vacuum cleaners essentially; cleaning the system out so that the Earth-like planet that remains there doesn’t suffer giant impacts, as the one that wiped out the dinosaurs. So we’ll actually learn an enormous amount about the range of planets, the types of planets and the environments within which those planets exist.

Jack Lissauer: Kepler is key in this because if Kepler finds that planets like Earth, the right distance, the right size from their stars are common, that means some of the nearer stars are likely to have planets like Earth. And it’s worthwhile to build a coronographic telescope, which can look at the atmospheres and find out the compositions of the atmospheres of these planets.

William Borucki: If on the other hand, there are very few such planets, then we’ll need to build a much bigger more expensive instrument called an interferometer that would be space based. And it could look much farther out into space so we would find enough targets.

Geoff Marcy: And we might venture a little more speculatively. What defines this marvelous species of ours, Homo sapiens perhaps more than any other characteristic is our zest for exploration and for traveling to distant places just to find out what’s there. The early hominids did this from east Africa traveling over the globe. The Polynesians did it in little boats that took them eventually to Hawaii and beyond. And we humans are now in a phase of our evolution in which we’re looking outward to the stars, wondering where our next voyage might be. If we find out from Kepler that Earth-like planets are common, it will be obvious to all people of the world that the nearest star, Alpha Centauri, which is in fact a triple star system will be a very logical place to image, to try to detect the planets directly, take spectra of them, learn any possible biological attributes of the planets around the Alpha Centauri stars. And I would go a bit further. A star that’s so close, Alpha Centauri a mere four light years away, will be amenable to small robotic spacecraft someday as our propulsion systems improve. And so Kepler will be the first step in informing us whether indeed Alpha Centauri is the logical next destination for humanity.

William Borucki: The other aspect of this is, when you think about what’s been done, we’ve found these planets, why did we find them? To some extent it’s not only because of our curiosity, because we’ve developed the technology to do so. Our limitations in doing this exploration, whether it’s sending a probe to another star, or building these wonderful instruments we’d like to launch is always technology. And what that means is as we develop that technology we can do more. But, it’s not just more in terms of exploration; it’s more in many ways. We use those principles, those ideas in building better cars, or greener cars. We keep our technologists sharp as we give them these hard problems. So it benefits society not only in exploration, but in developing the technology that helps everyone live better.

Jesse Carpenter: I’d like to thank our guests Bill Borucki, Jack Lissauer and Geoff Marcy 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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