Science: Exoplanets

The explosion in planetary knowledge is taking place at such a warp speed that even those most intimately involved are often amazed -- especially since the ultimate goal is nothing less than finding life elsewhere in the universe.

Washington Post staff writer Marc Kaufman will be online at noon on Monday, March 24 to discuss his Science Page story about new discoveries scientists are making about far away planets.

He will be joined by Sara Seager, an exoplanet theorist and professor at the Massachusetts Institute of Technology.

Submit a question or comment now or during the discussion.

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Marc Kaufman: Good morning. We'll be talking today about the exciting field of exosolar planets, and what researchers are finding about the faraway cousins of Earth. Until the mid 1990s, no planet outside our solar system had been identified, but now the number is nearing 300. Joining me on the chat will be Sara Seager, an exoplanet theorist and researcher from MIT who provided comments and context on the NASA teleconference last week that announced the discovery of methane in the atmosphere of a planet 63 light years away.

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Ellicott City, Md.: This is a tremendously exciting field of research, and I think the discovery of an earth sized planet with liquid water WILL happen in the next few decades.

As exciting as that is, the newspaper stories are doing a little too much "cheerleading". For example, calling methane an organic molecule is technically correct, but most people think "organic" means life, and it doesn't - it just means it has carbon and hydrogen in it.

Methane is associated with life on earth because it's not stable in an oxygen bearing atmosphere, and something (life) must be creating it. There's not much oxygen on Jupiter or Saturn, and nobody thinks their methane is biologically generated. The story did make this point fairly late in the text.

I guess what I'm saying is just be careful with the terminology - it's very easy to misinterpret a story like this.

Marc Kaufman: You are absolutely correct that the methane found in the exoplanet atmosphere -- as well as those in the atmospheres of Saturn and Jupiter -- are almost certainly not a sign of life, now or ever on those planets. The great significance of the finding, however, is that science now has a method to detect methane in distant atmospheres, and certainly researchers will find more potentially habitable planets in the future. Using the tools now available, they will then be able to test for that methane presence, as well as the presence of many other molecules. It should be said, however, that the methane finding has to be replicated before it is fully embraced by the astronomy community -- one observation does not a true finding make.

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Washington, D.C.: The math here boggles my mind. How many planets do we think are out there? A trillion? More?

Sara Seager: We think that our galaxy, the Milky Way, has about 100 billion stars.

In the whole Universe, there may be 100 billion galaxies! If each star has

at least one planet, then there are far more than one trillion planets out there

in our Universe.

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Olney, Md.: In our own solar system, the gas giants are thought to have formed further out from the sun because solar wind & radiation pushed the gases away from the sun. Yet many of the new planets being found are gas giants orbiting very close to their parent star. Did they form there or did they get pushed there by some event?

Sara Seager: This is a good question, because it highlights the numerous extrasolar gas giant planets that orbit very close to their stars---about 10 times closer to their stars than Mercury is to our sun.

These close-in giant planets probably did not form close to their parent stars. In the protoplantary disks we see around young stars, there is not nearly enough material (gas and dust) near the star to form a massive planet.

Astronomers think that the planets formed further out in their planetary system (just as Jupiter and Saturn did in the Solar System). The planets then

"migrated" inwards by interactions with the protoplanetary gas disk.

Alternatively, the giant planets may have migrated by collisional events with

smaller planets, just like colliding balls in a billiard balls game on a pool table.

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Reston, Va.: What is the probability of intelligent life in nearby solar systems?

Marc Kaufman: Depends on what you mean by "intelligent" and "life." It also depends on what time frame you're discussing. NASA's Astrobiology Institute is dedicated to answering those kinds of questions, and it is going about its work by helping to develop tools for detecting signs of life elsewhere, and also by studying unusual life forms on Earth that flourish in extremely inhospitable environments. This is because life elsewhere may be quite different from anything we know or can imagine, and it may exist in environments we don't think could sustain life.

This, however, doesn't address the question of "intelligent" life. I don't think anyone would say they know the answer to that one, but increasingly scientists seem to be concluding that Earth-like planets are probably out there, and perhaps many of them will some day be found. Will there be intelligent life? Who knows...

And then there's the question of time. The universe has been around for more than 14 billion years, cosmologists now calculate. While solar systems and planets are relative latecomers, they have still be around for billions of years. So maybe a planet had conditions that could support life a billion years ago, and some life forms developed that could be deemed "intelligent." But now, because of unknown cataclysms, the planet no longer supports life. This is hardly far-fetched since, by all accounts, some time in the distant future our star will begin to burn out and we, too, will disappear (assuming we're still around.)

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Munich, Germany: It's interesting that the search for life on other planets and in other solar systems is the primary focus of the NASA Astrobiology Institute.

What kind of research is being done to figure out how and when mankind can travel to places 100 light years away? Are there people actually working on future propulsion concepts?

Marc Kaufman: While NASA is indeed working on a new generation of spacecraft that can and will leave low earth orbit, nobody that I know of thinks it's possible to build a ship that can ever go those distances. What scientists, and the Astrobiology Institute, are looking for is signs of life, and they do that through a wide range of indirect measurements. Unless someone finds and photographs life found on Mars (if it exists, or ever existed,) then don't expect to see extrasolar life. Instead, expect to learn about what is being found in distant atmospheres that indicate life is, or was, present.

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Auberry, Calif. : Two questions, actually; first, would Kepler be able to directly image an Earth-sized world around another star? And second, what is the status of the proposed Terrestrial Planet Finder mission? I know it was deferred in the previous NASA budget, but has there been any recent news?

Sara Seager: NASA's Kepler mission is a space telescope with the goal to take a census of Earth-size planets orbiting sun-size stars. That is, Kepler aims to find about 50 Earth-size planets and then make a guess at the frequency of Earths.

Kepler does not have the specialized technology needed to directly imagine an Earth-sized world around another star. You are correct that NASA's previously planned Terrestrial Planet Finder mission (to directly image nearby Earths) is deferred. While NASA has made available funds for concept studies, there is no current plan to build the Terrestrial Planet Finder.

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Falls Church, Va.: Exoplanet study is essentially no more than an exercise in philosophy, isn't it?

We cannot travel to planets in other systems because of the distances involved, and realistically we never will be able to do so. Information exchange would take place on such vast time scales as to be useless; whole civilizations might have fallen on Planet X by the time its light reached us.

Thus, the only knowledge that we might even theoretically hope to gain from exoplanet study is perhaps some philosophical exploration of what it might mean to prove that we are not the only intelligent species in the universe, correct?

How much value can this possibly provide? At what point is it fair to ask why we are bothering to expend all this time and effort on the study of exoplanets?

Marc Kaufman: I agree that philosophy (and religion) are central to the quest for exoplanets and life elsewhere, but I wouldn't say, as you do, that it's "no more than an exercise in philosophy." Finding signs of life on other planets would not end war or improve the economy or reduce global warming, but it would add enormously to our understanding of the universe and our place in it. It was Copernicus who, centuries ago, calculated that the Earth revolved around the sun, rather than vice versa. That new knowledge is often cited as an important part of the beginning of the scientific revolution, yet it didn't really change anything on Earth. Some believe that finding life elsewhere would be represent a completion of the circle that Copernicus began to draw when he determined Earth was not the center of the universe.

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Herndon, VA: How are astronomers able to distinguish exoplanets from brown dwarfs?

Sara Seager: We use mass to distinguish between exoplanets and brown dwarfs.

Brown dwarfs are failed stars, but they had nuclear reactions for a short period of time at some earlier stage of their life. For nuclear reactions, the pressure

(and temperature) in the center of the brown dwarf must be high enough. We can convert from this central pressure, to a total mass. Most astronomers accept that below 13 Jupiter masses the astronomical body is an exoplanet.

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Chicago: Our current techniques for finding exoplanets look to gravity wobbles, changes in the spectrum as a planet passes in front of its star -- things that are pretty self-selective for finding really big planets. By amazing coincidence, so far we've found alot of fast-orbiting gas giants.

Will the Webb telescope allow us to look directly for oxygen in an exoplanet's atmosphere? (If not, when can we do that?) How soon before we're trying out telescopes for a direct look at some of these places? How many earth-like planets do you expect we'll know about 10 years from now? Thanks.

Sara Seager: Yes, we have found many fast-orbiting gas giants, and it is amazing that these planets exist at all.

The Webb telescope is able to look for oxygen in an exoplanet's atmosphere.

We just have to find suitable planets for Webb to look at. Webb will

more easily find water vapor in an exoplanet's atmosphere. We expect

to have up to a half dozen planets that Webb can study in this way. These

planets will be big Earths around small stars, more like an Earth cousin than an Earth twin.

By ten years from now, we will know of about 50 Earth-size planets orbiting stars like the Sun. While these planets won't be suitable for observations with Webb, they will help to tell us how common Earth-size planets are.

Beyond Webb, we will need to wait for direct imaging telescopes of the future,

perhaps 15 to 25 years from now.

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McLean, Va.:"Their ultimate goal is nothing less than finding life elsewhere in the universe."

Is there a consensus on how life might have arisen on other planets if we do indeed find living organisms or self-conscious creatures such as ourselves?

-Arie

Marc Kaufman: None whatsoever. It is generally assumed that life needs some form of water to survive, but even that is now being questioned. I was told about micro-organisms found several miles below ground in South African mines that survive by, in effect, using the decay of radioactive compounds to manufacture their own H20 (if I understood correctly.) Nonetheless, the research for organic molecules in distant atmospheres is now central because researchers assume that is most likely where signs of life will be found.

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Playa del Rey, Calif.: Of course, we are only familiar with forms of life that exist on Earth. Thinking out of the box, but within scientifically known possibilities, what forms of life might exist that we have not yet encountered (silicon-based, or crystals, etc.), and how can we begin looking for evidence of such?

Also, any chance of weird life forms living in the atmospheres of Venus or Jupiter?

Thanks...

Sara Seager: This is an excellent question, and one that scientists are only starting to take seriously.

For exoplanets, there is one kind of evidence for life. That is signatures

of gases in the planet atmosphere that do not belong there. On Earth,

we have oxygen, and the related ozone. These gases are so reactive that

they shouldn't be in our atmosphere at all, providing evidence that they

come from life processes. On distant exoplanets, we probably will not be

able to tell what kind of life is producing any suspected biosignatures, just that such life may be present.

People have speculated about atmospheric life on Venus and Jupiter. This life would be floating around in the atmosphere. No veritable signs of such life have yet been found.

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Alexandria, Va.: Do we know if there is any correlation of the 200+ exoplanets found to the stars studied by the SETI project? Thanks.

Sara Seager: Yes, several stars with known exoplanets are SETI targets.

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Mountain View, Calif.: What do the negative results from exoplanet searches tell us? I know they can't exclude smaller planets or planets that are distant from their star, but can they now tell us what percent of stars do NOT have planets above a certain mass and with less than a certain length orbital period?

Sara Seager: This is an excellent question, and one that has not yet been fully answered by astronomers.

I can give you two interesting tidbits. The first is that most of the giant exoplanets have masses below about 10 Jupiter masses. This is significant, because it means that brown dwarfs (or failed stars weighing in above 13 Jupiter masses) are very rare as companions to sun-like stars.

A second interesting negative result is that small stars (called M stars) have many fewer giant planets than larger stars like our sun. This makes sense, because small stars should have smaller protoplanetary disks out of which planets can form.

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Washington, D.C.: Mr. Kaufman, I think it would be useful for you to provide a perspective in articles like this about the enormous research benefits the United States gains from unmanned space flight. Is it not true that the Bush Administration's determination to return to manned exploration of space will cost much more and yield much less, while taking funds away from unmanned flight?

Marc Kaufman: This important question is not directly related to exoplanets, but I wanted to respond because it is such a hot topic in NASA and space circles. Ever since President Bush announced his "Vision for Space Exploration" in 2004, with its goal of building new spacecraft to return to the moon and then fly to Mars, the financial demands on NASA have become enormous. It has to fly the space shuttle --until at least 2010 -- to finish assembly of the international space station, it has to design, test and build a new generation of spacecraft, and it has to maintain what has become one of the wonders of the Earth -- the NASA unmanned science program. The president (and Congress) have generally not increased NASA funding to pay for the added costs, and so programs have been cut and progress has been slower than it might be. The funding shortfall has led to some hard feelings in the space science community, where some programs -- both major and smaller -- have been eliminated or delayed.

While the science program has consistently produced remarkable results for years now -- remember those Hubble images of distant galaxies? -- the manned program also speaks to something important in the culture: the desire to explore. The entire NASA budget is about $17 billion a year. That's a lot of money, but to make a perhaps apples and oranges comparison, it's not much more than the cost to the federal government of one month in Iraq.

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Richmond, Va.: I remember back when the first exoplanets were discovered it was huge news because there was finally confirmation of what previously only been theorized. Now that we've found so many, do you think that stars with planets may be the norm, rather than the exception? Is there any consensus on this in the scientific community, or is there just not enough data to decide yet?

Marc Kaufman: As a reporter, rather than an exo-planet expert with my own research-based opinion, it seems that many stars have planets and solar systems. How many remains unclear, but based on what is being discovered today, there's every reasons to believe planets are common outside our solar systems. And since the scale of the universe is so vast -- a hundred billion stars in our own nothing-special Milky Way, and billions of more galaxies out there -- the number of planets could also be vast. So far, only large gas giants have been discovered because the technology available doesn't allow for identifying small rocky planets like Earth. But that hurdle will probably be overcome in the next decade.

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Rockville, Md.: With current methods, it seems that you need a rather massive planet orbiting fairly close-in to its parent star to detect the wobble in the star's orbit. But I wouldn't think that this represents the norm. Could you detect planetary systems more like our own?

Sara Seager: This is a great question. A planetary system like our own is very difficult to detect. With most planet-hunting methods, we must wait a full orbit. For Jupiter, this is 12 years! The "wobble-method" planet searches have been going on for about 15 years. Astronomers are just beginning to find planetary systems with Jupiters in Jupiter-like orbits. A few are already known. Finding Saturn's would take even longer, about 30 years.

Finding planets like Earth is even more difficult, because Earth is so small and faint compared to the parent star. Nevertheless, astronomers think they can find planets like Earth, with the "wobble" technique, around the brightest stars in the sky.

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Herndon, Va.: Hi. Could you quickly describe how NASA is using its missions and data from other sources to progressively hone its exploration strategy to find "other Earths?"

For example, the LBTI and Keck Interferometer are characterizing protoplanetary disks in order to facilitate target selection for Kepler, SIM PlanetQuest and TPF; the EPOXI mission now is doing the same type of transit photometry as Kepler, but on a lower-resolution scale and on a limited number of stars.

Finding "other Earths" requires space-based efforts for both radial velocity and transit photometry measurements. How do you see all of these missions/efforts being coordinated and realistic rate of discoveries of "other Earths" being made during the coming decade, especially given NASA funding cuts?

Sara Seager: I will start by agreeing with you that NASA and space telescopes are needed to find a decent number of planets just like Earth.

Many astronomers are trying different approaches to find Earths despite the delay or cut of NASA planet-finding missions.

One idea is a "fast-track" to Earths. This fast track

works for a subset of exoplanets: transiting big Earths orbiting small stars, in the so-called habitable zone of the small star. "Transiting" planets are a subset of exoplanets that go in front of the star as seen from Earth, and allow us to measure their densities and atmospheres. We anticipate that several such planets will be found in the next few years.

A second idea is pushing existing technology to find Earth-mass planets orbiting sun-like stars. At least one ground-based "wobble" method effort is trying to find such a planet around one of the brightest stars in the sky.

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Washington, D.C.: No real question, just a comment or two.

It's wonderful to still be able to find science in the Post. I'm sure that many papers have tossed science reporting to the curb, in these times of diminishing sales, but it's just fantastic to still find something that flexs the brain instead of pulling at heart-strings.

Keep up the great work!

Marc Kaufman: Why thank you. Our newsroom is, alas, also shrinking, but I certainly hope our science/health coverage isn't harmed in the process. I've always found a large and well-educated pool of interested readers out there for stories from our pod of science reporters.

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Reston, Va.: You made the statement that no one expects to build spacecraft that can travel 100 light years through space. Yet there is quite an engineering literature on such vehicles especially in the case of unmanned interstellar probes. Such probes can endure long travel times and report back to their source civilizations.

Another possibility is intelligent machines exploring the universe.

Marc Kaufman: Interesting information. Certainly would be wonderful if such fast-moving probes could actually be developed.

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Savoy, Ill.: It might be interesting to some astronomers to find and study the distant tiny little dots near the bright stars, but how do you justify this to the general public who happen to pay for these studies? Many of the people who tune in to this are thinking about aliens and space traveling, but as a scientist you must be fully aware such travel is nearly impossible. The chance of finding evidence of life on these planets are remote and I do not think NASA got their funding by telling Congress that.

Given the scope of astronomy, planet science is only a tiny bit of it. Unless the study of these planets can benefit multiple fields in astronomy, the over emphasis on the search of these planet can in fact harm astronomy as a whole, by diverting resource from studies of blackholes, galaxies and the universe.

Sara Seager: We are trying to answer a hierarchy of ancient questions. "Do other Earths exist?" "Are Earths common?" And, "Do they have signs of life?" Its fair to say that we will answer the first two questions in the next decade, although the third one gets the most attention.

The basic justification of exoplanet astronomy is similar to many pure sciences (education and technology spin offs and pursuit of general knowledge).

New NASA concept studies for a direct imaging space telescope include instrumentation for general astrophysics.

As a scientist, I will not rule out (robotic) space travel to the nearest star system, if an Earth is found there, even if it takes 500 years to reach technology readiness. You have to start somewhere.

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Anonymous: Many of the exoplanets are gas giants orbiting very close to their parent star. In our own solar system, the gas giants are further out. Did these gas giant exoplanets form close to their stars or were they pushed there by other factors?

Sara Seager: Here is a similar answer to a previous question.

These close-in giant planets probably did not form close to their parent stars. In the protoplantary disks we see around young stars, there is not nearly enough material (gas and dust) near the star to form a massive planet.

Astronomers think that the planets formed further out in their planetary system (just as Jupiter and Saturn did in the Solar System). The planets then "migrated" inwards by interactions with the protoplanetary gas disk. Alternatively, the giant planets may have migrated by collisional events with smaller planets, just like colliding balls in a billiard balls game on a pool table.

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Arlington, Va.: Hi, I have a question about the coverage and importance of finding additional planets. I remember seeing the press coverage of the first few exoplanets, and thinking it was really cool that there was evidence for planets beyond the solar system. But then when planets 5, 6, 10, 20, etc. were discovered, there also seemed to be a lot of press coverage. I was wondering the importance of these? Was it because increasingly Earth-like orbits/planetary masses were being discovered?

Marc Kaufman: I think the science of exoplanets is still in its infancy, and so each new step forward is interesting. And yes, the increased excitement (and perhaps coverage) is because researchers are making discoveries that seem closer to home, as it were. For instance, a solar system was recently identified that had two gas giants orbiting in the same relative place as Saturn and Jupiter, suggesting that the planetary order in our solar system may well be common. If true, then the search for Earth-like planets in "habitable zones" becomes that much more exciting. My own view is that once a rocky, Earth-like planet is indeed discovered, interest will grow far greater.

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Marc Kaufman: Sorry, folks, we have to go. Thanks for your questions, and we regret not getting to answer all of them. Next time...

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