行星越大越好

What's so super about super-Earths? This artist's impression shows Earth alongside the super-Earth known as 55 Cancri C, which is thought to be a little more than twice as wide as our planet and 7.8 times as massive. By Alan Boyle Two years ago, Harvard astronomer Dimitar Sasselov stunned the world when he claimed there might well be 100 million Earth-size planets in the Milky Way. To some, the number sounded shockingly high. But the torrents of data that have come in from planet-hunters since then suggest that, if anything, the estimate was almost laughably low. Just this month, researchers reported that there are probably more planets than stars in our galaxy, which would bring the total count well past the 100 billion mark. What's more, astronomers say the planets toward the lower end of the scale — "super-Earths" that are up to 10 times as massive as our own planet — are likely to be more common than Jupiter-scale planets. Planet-hunters have already identified more than two dozen super-Earths beyond our solar system, including a batch of 16 announced on a single day last September. A couple of weeks ago, scientists spread the news about three planets smaller than Earth, and last week the science team for NASA's Kepler space telescope mission added still more super-Earths to the list. That kind of planetary plenitude has even had an impact on the funny pages: "I don't know why this isn't the only thing people are talking about!" one character told another last week in the Arlo & Janis comic strip. It's the main thing that Sasselov is talking about, for more than one reason. He's a co-investigator for the $600 million Kepler mission, the director of the Harvard Origins of Life Initiative, and the author of a new book titled "The Life of Super-Earths." In the book, he makes the case that super-Earths could be as hospitable to life as our own planet, and perhaps even more so. Super-Earths that lie in the habitable zones around their parent stars — that is, the zones where water can exist in liquid form — would be prime candidates in the search for signs of extraterrestrial life. "Life is not rare, it seems," the Bulgarian-born astronomer says. Sasselov talked about the Kepler mission, the plenitude of planets and its implications for the search for alien life during our wide-ranging interview. Here's an edited transcript of last week's Q&A: Cosmic Log: Do you look back at your estimate from two years ago and just shake your head at the idea that you were guessing so low? Were people making a fuss over something that now seems obvious? Dimitar Sasselov:I feel that I was on the right track. Basically, yes, we have on one hand an even larger number of planetary candidates than I anticipated two years ago. The numbers went up. However, there is also a result which cancels those large numbers. There is a fly in the ointment. The caveat is that as it happens, most of our planetary candidates and confirmed planets are in relatively short orbits. That means two things. First of all, they don’t directly tell you what the exact prediction about planets in the habitable zone should be. Second, a lot of our small-planet candidates are in compact, multi-planet systems. Planets are closely packed next to each other, and these planets usually are within the orbit of Mercury around a star which is not that different from the sun. So there must be something extraordinary about the way they formed. It's quite possible that the formation and evolution required to create such architectures in planetary orbits is different in some fundamental way from planetary formation and early evolution in our solar system. So it is still a question mark as to what these planets are telling us, and what they are made of. For the Kepler-11 system, we have the mean density of the planets. Those little planets are very low-density planets. They’re nothing like a bigger version of Earth. They have envelopes of hydrogen, or probably hydrogen and helium. They're like mini-versions of Neptune and Uranus. There are no planets like that in our solar system, so we don't know much about them. It’s a cautionary tale there. Yes, there may be plenty of planets that are just two to three times more massive than our own Earth. But their mean density may be very low, because they formed farther out and migrated inward, and ended up in the moderate temperature regions of their planetary systems. What would happen if we have a very large number, maybe billions, of super-Earth-size planets in the habitable zones — but half of them, or even nine out of 10 of them, are these mini-Neptunes? Would I consider them Earthlike? Definitely not, because they don't have the same geochemistry. So while on one hand, the numbers have gone beyond my expectations, the diversity has gone beyond my expectations, too. And that means we might have a lot of planets with something different from an Earthlike geochemistry. Looking at the physics and the geochemistry is the only way we can go to the next step — and that is the search for signatures of life. advertisement Q: What is the next step? How do you go from Kepler and planet detection to getting at the more fundamental questions? A: To me, the next big step is to go from discovery and detection of planets like our Earth, to understanding their geochemistry. We have to do that to be effective in searching for biosignatures. The way we would do the first step — that is, understanding geochemistry — is by finding enough planets that are close to us. Kepler's candidates are a little bit too far for a good follow-up on characterization. So in terms of a practical approach, we should be gearing up for surveys of the nearby population of stars, and discovering those nearby planets. There, the news from Kepler is good, because the statistics are high. If the statistics were low, then it would take more of an effort. Once we make that survey, and we can practically accomplish that in the next 10 years, we can jump onto those planetary candidates, and do atmospheric analysis, and try to understand the diversity of their atmospheres. This is a necessary step to talk about the signatures of life. Otherwise, we'd be looking blindly. Q: Some people might say, well, let's just look for oxygen or methane, or something we associate with life on Earth.  A: That wouldn't be prudent at all. If we just look at biosignatures as we understand them on our own Earth today, they correspond to a particular moment in time in which the microbial communities on this planet have managed to change the atmosphere in a particular way. For about half of the history of life on Earth, the atmosphere wasn't anything like what it is today. It would be foolish to just assume that all life shares the same biochemistry and the same history. Theoretically speaking, we should not assume that all planets that otherwise resemble Earth have the same geochemical cycle. There are alternatives. Q: What sort of mission would work for this next step? A: There are two approaches that need to be taken. The first one, when it comes to discovery, is a combination of space- and ground-based surveys. The space surveys would use smaller arrays of telescopes in orbit, and would scan the entire sky by observing the brightest stars, nearest to us, in a selective manner. But as opposed to concentrating in one direction, which was necessary due to the design of Kepler, we can select the nearest stars over the entire sky. This can also be done from the ground for a particular subset of stars, which are the M stars. These stars are so much smaller than a sunlike star that the transits for Earth-size planets are much more prominent. You can see them using ground-based telescopes. You don't need to go to space. The trick is to do the whole sky and catch all those M dwarfs, and catch the transits. Q: One of themes in your book is that we shouldn't limit the planet search to Earth-size planets, because the planets that are bigger than Earth — the super-Earths — might be more conducive to life than even our own planet. How can that be? A: What we're finding out about super-Earths places them front and center as the most suitable places for life to emerge. These are planets that are only slightly bigger than Earth. In terms of size, we're talking about an average of 50 percent larger. In terms of mass, we're talking about two, three, five times as massive — maybe 10 in some cases, but overall, made of the same stuff. Then you just compare the whole range of planets, from Mars to Earth to the largest super-Earths. In all different levels of comparison, the super-Earths end up being equal or slightly better when compared with Earth. For example, one of the problems a planet could encounter is the ability to keep water liquid on the surface, and to have the good chemical exchange between the interior and the surface. That’s very difficult to do if you don’t have an atmosphere. An atmosphere in the habitable zone is difficult to keep, because it evaporates over the course of billions of years. If you have a small planet, made of rock but still low mass, like Mars is, eventually you lose more of your atmosphere than if you have a bigger planet. There is no negative factor, it is just more of a good thing. Here's another example. A lot of people would say we have it good here on Earth because the moon keeps the axis of Earth's rotation more stable than it otherwise would be. It's the kind of momentum effect you get when you're on a bicycle — you can let the handlebars go and you still go straight. In a similar way, the existence of the moon out there cancels out the additional push and pull from the other planets, which could from time to time turn the axis of Earth dramatically and change the climate. This is what we think happened a few times on Mars. The more massive a planet is, the less vulnerable it would be to these effects. Q: Is it always "the bigger, the better," until you get into a Neptune-class ice giant? A: It's always the bigger the better. There's either no difference, or it's better. I didn’t find anything which was actually detrimental about having a big planet. Larger g-force, having more gravity on the surface, has a small effect when it comes to building biological structures, such as the membranes of cells. The list goes on and on. Everything gets better when you're slightly bigger. Q: How long do you expect this book to stand up? I suppose that's an occupational hazard when you're writing about planet-hunting. A: I would say it should stand up until we discover life out there on another planet, or in the lab when we manage to put it together as an artificial minimal cell. Then, of course, we'll open a whole new chapter in the history of science — and it will be so exciting that I wouldn't care. If a new book needs to be written, I will be happy to do so.