Can Intelligent Life Thrive in Close Quarters?
An Interview with Christopher Chyba
Parts 1 * 2 * 3
|SETI's Jill Tarter (left), Peter Backus (center), and Rocco Mancinelli (right).
Image Credit: SETI Institute
Christopher Chyba is the principal investigator for the SETI Institute lead team of the NASA Astrobiology Institute. Chyba formerly headed the SETI Institute's Center for the Study of Life in the Universe. His NAI team is pursuing a wide range of research activities, looking at both life's beginnings on Earth and the possibility of life on other worlds. One of his team's research projects will explore a question critical to the search for extraterrestrial intelligence: Can planets orbiting red dwarf M-type stars support life - perhaps even intelligent life? Astrobiology Magazine's managing editor Henry Bortman recently spoke with Chyba about Tarter's, Mancinelli's, and Backus's research.
Astrobiology Magazine: Jill Tarter and Peter Backus, and microbiologist Rocco Mancinelli, all of whom are with the SETI Institute, are involved in the search for extraterrestrial intelligence. Their contribution to your NAI team's research will be to consider the habitability of M-class red dwarf stars, their potential as SETI targets.
At a forum last winter, Frank Drake explained his famous Drake Equation, which calculates the likelihood that there are other intelligent species in our galaxy. One of the equation's factors, R*, has to do with the rate at which the galaxy produces stars that provide environments capable of supporting intelligent life.
He said that a major question in determining this factor had to do with the habitability of M-class stars. Of the 20 or so stars produced in the galaxy each year, 15 of them are dim M dwarfs. And so, if it turns out that planets around M dwarfs could support intelligent life, there would be many more stars on the SETI target list than if M dwarfs were excluded. Is that the question that Jill, Rocco and Peter are hoping to answer?
|Frank Drake's famous Drake Equation calculates the likelihood that there are other intelligent species in our galaxy.
Image Credit: SETI Institute
Christopher Chyba: You're right on target. We're going to have a series of workshops that involve project co-investigators and other people who are experts in stellar evolution - and also biologists, microbiologists, and it's likely we'll bring others in - to look at this question of the habitability of worlds around M stars.
There are two reasons why that question is tough. One's mostly a red herring, and the other is more significant. For a planet to be in the so-called habitable zone of an M star, it has to be close enough to have liquid water. For a red dwarf, that means it has to be real close, because the star is dim. But that also means that it's going to be close enough that on a geologically short time span it's going to be spin-locked, with the same side of the planet always facing the star, the way the moon is with the Earth.
There was concern for a while - this is featured in Rare Earth - that that would mean that all of the planet's atmosphere would freeze out on its dark side, and that would be that. But, in fact, if you do the greenhouse simulations Joshi et al. published these results in 1997-- for those kinds of worlds, you find that you need only around a tenth of a bar of carbon dioxide to give you a thick enough atmosphere, enough greenhouse effect, that you redistribute the heat so that your atmosphere doesn't freeze out. That's a lot more carbon dioxide than we have on Earth, but well within the range of what's plausible.
If you get up into the range of a bar or so of carbon dioxide, not only does your atmosphere not freeze out, but you stay warm on both sides of the planet, so you have liquid water potentially all over the planet. You have no guarantees that a planet around an M dwarf will have that much carbon dioxide. But you don't have any guarantee of having the atmosphere you want on a planet around a G-class (sun-like) star, either. So I don't think that's a decisive problem, although we will certainly revisit that.
The other issue, though, has to do with flaring from those stars. Its radiation environment might be too harsh for life on planets around M stars, although papers published as early as 1991 called this into question. And that's something that we need a much better understanding of, astrophysically and atmospherically. We also want to have biologists in the picture so that we can get a better handle on just how challenging the radiation and ultraviolet environments would be either for microscopic life or for more sophisticated forms of life. And that will ultimately lead to an operational decision about whether or not we expand the list of target stars for our SETI search to include M stars. Remember, as Frank said, these are 75 percent of the nearby stars. So answering this question will have a huge impact on our search strategy.
|Of the 20 or so stars produced in the galaxy each year, 15 of them are dim M dwarfs.
Image Credit: NASA
AM: I'm surprised that NASA is funding a research project whose focus is the search for extraterrestrial intelligence. I was under the impression that NASA was forbidden by Congress from funding SETI.
CC: No, that's no longer the case. Let me clarify, and then explain what's changed, because this is something that has often caused confusion.
There was never any prohibition against NASA funding research projects out of the SETI Institute. What the prohibition was understood to be - really, misunderstood to be - was a prohibition against funding SETI science, whether it was the SETI Institute, or the Harvard SETI project, or the Berkeley SETI project, or any other SETI project. The interpretation was that they were not permitted to fund that entire area of science.
What was really the case was that Congress zeroed SETI research out of the NASA budget in '93 - but that was a one-time elimination from the budget. There wasn't any language saying, "You shall never fund SETI research ever again." But it was interpreted that way, and I understand that. A government agency thinks it gets a directive from Congress and it wants to be faithful to that.
|Christopher Chyba, principal investigator for NASA Astrobiology Institute's SETI lead team.
Image Credit: SETI Institute
What's changed is that the sense of Congress is different now. And you can see that back to at least 2001, when the Aeronautics and Space Subcommittee of the House Science Committee held a hearing on life in the universe, and they had four or five people testifying, including myself. I testified mostly about astrobiology and how SETI is a natural component of the continuum of questions asked in astrobiology. And there was a clear sense of the committee at that time, at the hearing, that SETI was part of astrobiology. In fact, this led to a statement that NASA would consider SETI science projects equally under peer review with other projects that fit within astrobiology. That is to say, NASA could make these decisions on the basis of peer review, rather than on the basis of some extra-scientific reason.
That was, after all, reflected in the new Astrobiology Roadmap, which for the first time includes SETI as a component of astrobiology. And the funding for a SETI project within this proposal is just a kind of natural outcome of that process.
But you're right, in the sense that that's a shift in at least perception in the last few years. And I think a few things have happened. One is that, while it's still entirely possible that there's no life anywhere else in the known universe other than Earth - we don't have definite evidence that there's life anywhere else - there has been a whole series of discoveries that make it seem more plausible that there's extraterrestrial life. So I think that's the intellectual context. And there's a lot of enthusiasm in Congress now.
The other thing to mention is that there is a standing committee of the National Academy of Sciences National Research Council, the Committee on the Origin and Evolution of Life, which was asked by Congress to do a report on astrobiology, and in particular to assess the role of SETI in astrobiology. And that committee's report is effusive in its praise for SETI and, to be frank, the SETI Institute, so I think that played a role, too, because that represented outside support.
So I think a lot of things came together in the space of the last few years.
Related Web Pages
Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs:
Conditions for Atmospheric Collapse and the Implications for Habitability (M. M. Joshi, R. M. Haberle, and R. T. Reynolds)
The Drake Equation Revisited (Astrobiology Magazine)
The Search for Life in the Universe (Astrobiology Magazine)
Scientists Hunt for Light Flashes from Extraterrestrial Civilizations (NASA Astrobiology Institute)
SETI and the Search for Life (NASA Astrobiology Institute)