Authors: Marc Kaufman
The announcement was met with some initial skepticism in the exoplanet community, and Swiss planet hunters exploring the same solar system said they had searched their data and had found nothing like Gliese 581G in a potentially habitable zone. Butler replied that the Swiss had substantially less data to make their assessment, though he acknowledged that he and Vogt had published with a slightly lower confidence level than most
of their previous findings. The discovery was at the “raggedy edge” of their planet-finding abilities, he said, and does need to be confirmed by others.
A paper two weeks later in the journal
Science
by Butler's former partner Geoffrey Marcy made clear that regardless of whether Gliese 581G is a habitable Earth-sized planet, there are certain to be many, many other candidates out there. In fact, based on observations at the same Keck Observatory, Marcy and his University of California at Berkeley partner Andrew Howard concluded that Earth-sized planets are absolutely common in the universe. So common, he said, that tens of billions are likely to exist in the Milky Way alone. Not all, and probably not most, Earth-sized planets are actually like Earth. But it is now substantially easier to imagine a galaxy and a universe with rocky planets very similar to ours in all the important ways. “A threshold has been crossed,” Butler declared.
â¢Â   â¢Â   â¢
As coincidence would have it, one of the pioneers in the field of understanding the atmospheres and mantles of faraway exoplanets used to have an office at the Carnegie Institution just down the hall from Butler. She is Sara Seager, now at MIT. By understanding how to collect enough data to seriously analyze those atmospheres, Seager and others maintain, we will someday be able to detect molecules and compounds that are intimately related to life. Oxygen, ozone, and complex carbon compounds like methane would, if found, strongly suggest that some kind of life-form was on the planet and producing those molecules as a by-product of its existence. The techniques being developed now would not provide a 100 percent certain detection of extrasolar life, but would be able to predict to within the 95 percent range that some life-form was responsible for a biologically produced gas if found.
Seager works on the data collected by others, and most centrally she's a theorist. Her theoretical understandings of extrasolar planets, for instance, helped lead a colleague to prove that a previously unsuccessful but important method to detect distant planets could and did work. Using the
Hubble Space Telescope, David Charbonneau was the first to find a planet transiting its sun by measuring the small drop in light that accompanied that passage. Harvard-Smithsonian Center for Astrophysics astronomer David Latham, whom Seager repeatedly urged to explore the “transiting” technique, confirms that Seager did indeed play a “catalytic role” in the subsequent discovery because he passed on the prediction and a transiting candidate to Charbonneau, who was also one of his students. In his 2003 paper reporting on the discovery of sodium in the atmosphere of an exoplanet 150 light-years away, he specifically wrote that another prediction made by Seager was central to his successful effort.
While Seager's work can be highly technical and demanding, what makes her unique is her desire and ability to make the world of planet and atmosphere hunting accessible to a general audience. That and her cut-to-the-chase willingness to say she's in the exoplanet fray because she wants to find the life she's confident is out thereâto be part of science and exploration that will someday significantly change our understanding of the Earth, human life, and the universe. I got an introduction to her effect on people at a most unlikely venue: an outdoor trattoria in one of the old quarters of Rome. She was in town to deliver a talk at a conference on astrobiology called by the Vatican's Pontifical Academy of Sciences, and we met on a day off from the proceedings.
We were seated at the very edge of the covered section of the restaurant, and the tables were set close together and filled. Seager spoke with her customary energy and command about topics such as how to find rocky, Earthlike planets thousands of light-years away, about why learning the chemical makeup of exoplanet atmospheres is a key to finding distant extraterrestrial life, and about the sumptuous but awkward pleasures of being put up on Vatican grounds in what had been the pope's villa. She grew especially animated about the possibilities of launching an “occulter”âa flat, football-field-sized, petal-shaped sunshade that Seager (and others) hope will one day be sent into deep space and aligned with a similarly stationary but space-based telescope. It's essential because an inevitable and unchanging
problem for all exoplanet work is that stars are vastly brighter than the relatively minuscule planets orbiting them. The challenge, Seager explained to what seemed to be a growing audience, is to somehow block the light from potential suns to make their planets and the atmospheres around them detectable. The fantastical occulter arrangement could make this possible. An intricately designed opening in the starshade would theoretically allow a space-based telescope some thirty-five thousand miles away to peek through and see only the tiny planets and, with an attached spectroscope, learn about its component parts. It would be Seager's longtime dream come true, and she spoke faster and faster as she described it.
The skies opened and drenching rain fell on the less-than-sturdy restaurant covering. Rain poured onto our table and onto those of other customers around us, forcing us all to crowd in together even closer where the awning still held. Seager left for a moment and a Dutch diner turned to me and, wide-eyed, asked for what I soon discovered to be a rather large collection of others, “Who is she?” I explained that she was an astrobiologist, part of an international effort to search for life beyond Earth. “I've just never heard someone talk like that,” an American woman said. “Is all that stuff for real?” I assured them that it was.
Real enough that Seager has been on virtually every team put together by NASA or others to dig into and understand the science associated with that next step in planet hunting. She is on the science team of the Kepler mission (launched in 2009 with the goal of determining whether Earthlike planets are common in the more distant galaxy) and was on virtually every incarnation of NASA's Terrestrial Planet Finder effort. That mission, proposed and initially approved in 2004, would have launched two spacecraft designed to find signatures of life on exoplanets. Since 2006, however, it has been on indefinite hold, a victim of agency budget cuts, disputes within the exoplanet community on how to proceed, and extremely challenging and costly engineering and technology. In its place, NASA is considering a plan to design and launch an occulter and connect it with the James Webb Space Telescope, the highly sophisticated and very expensive successor to
the Hubble Space Telescope and scheduled to launch in 2014. Seager is involved with that Webb-related effort, too.
That it might be possible to detect an Earth-sized exoplanet and then learn about the basic chemical makeup of that world illustrates just how far space science and technology have gone. In terms of scale, some have compared the feat of detecting an Earth-sized exoplanet to reading the mint date on a dime at a distance of more than three miles. What's more, the sun of an Earthlike exoplanet can be as much as 10 billion times brighter than the nearby viewing target. So the technological challenge is great and the costs high. Nonetheless, it's also true that the space agencies have made a hash of the effort.
In the early 2000s, as it was becoming clear that extrasolar planets were both plentiful and detectable, both NASA and ESA moved enthusiastically into mission-planning mode. At NASA, first one and then two high-end Terrestrial Planet Finder spacecraft were proposed and to some extent accepted, and ESA put money into a candidate of its own named Darwin. The logic of the missions was that the presence of certain elements and compounds is generally associated with biological activity, and that they could detect them on exoplanets using the most cutting-edge space technology. Oxygen, for instance, bonds quickly with many other elements, and so would be present in atmospheres as pure oxygen only if it were constantly being produced on the planet. The presence of ozone, a form of oxygen, would provide an even stronger sign of possible life. In certain circumstances, so would nitrous oxide and methane if they were present in substantial amounts; they, too, are frequently present when biological activity is going on. And, of course, the TPF missions would be looking for signs of liquid water. But NASA priorities changed a few years into the effort as human space exploration ate up larger amounts of the budget and disputes within the exoplanet community flared while cost estimates grew. By 2006, the exoplanet community learned that NASA had put the dream of taking that next big step regarding exoplanets not only on “hold,” but on “indefinite hold.”
As the big missions were losing support, two teams of exoplanet astronomers came forward with alternate, less ambitious and less costly plans. Both involved the seemingly far-fetched occulters, or sunshades, that Seager would later be describing so enthusiastically. Others had proposed similar techniques decades before, but exoplanets had not yet been discovered and the technology was very experimental, unwieldy, and complex, so it didn't go far. But things had changed by the mid-2000s, in part because a University of Colorado astrophysicist and space visionary, Webster Cash, had found a way to shrink the size of the sunshade from something miles long to something the area of a football field. As proposed, the starshade would be launched wrapped tight and then would be opened like a travel umbrella (using newly declassified technology) at its outer-space destination to a size of 170 or even 250 feet in diameter. The best shape to block the starlight turned out to be something akin to a sunflower or a daisy. The telescope would be 35,000 to 50,000 miles away from the starshade, but its aperture would be shaded by the distant barrier petal. And with the star's light blocked, the much fainter light coming off any exoplanets would be visible and the telescope could take the spectra of atmospheres and consequently find out what elements were present.
It was Cash who contacted NASA about the occulter idea in 2006, reaching out to Ed Weiler, who was then head of the agency's Goddard Space Flight Center, outside Washington. Weiler, who had previously been in charge of science for NASA and would resume that position a few years later, is known as an advocate for astrobiology and someone who believes strongly that life does exist beyond Earth. He's the same Ed Weiler who would later negotiate the NASA-ESA deal that will send methane-sniffing spacecraft and landers to Mars in 2016 and 2018. Weiler invited ten of his research scientists to the meeting with Cash, and he recalls being at first skeptical but then quite enthusiastic. That squares with Cash's clear memory of a moment in his PowerPoint presentation when Weiler realized he was proposing a plan that was much cheaper than the TPF, but capable of collecting much of the same information. “Ed jumped up and
was pretty excited,” is how Cash describes it. Soon after the meeting, Cash's Colorado team and another group at Princeton University won small but useful NASA grants to explore occulters. Northrup Grumman joined the Colorado team and actually built a small portion of an occulter as a limited proof of concept; Lockheed Martin joined with Princeton.
Both teams came up with what were considered to be feasible plans to use an occulter and a dedicated small space telescope together, and both were submitted in 2009 to NASA and to an arm of the National Academy of Sciences, which every ten years reviews all branches of space science and, with input from the scientists, sets priorities for the next decade.
Although both proposals were much less expensive than the original $6 billion TPF plan, they still were in the several-billion-dollar range and, because of NASA's budget problems, they were a long shot. Undeterred, Cash and others called a Hail Mary playâinstead of sending up a telescope solely for the use of exoplanet searching with the sunshade, they would propose using the Webb Telescope, due to launch in 2014, as the second part of a resurrected occulter-telescope mission. The Webb is an enormous, $4.5 billion undertaking and has been in the works since 1996. Its main missions are to see further back than any telescope before to the time of the Big Bang, as well as to explore with greater precision the assembly of galaxies, the origin of stars and of solar systems. Viewing and understanding exoplanets is an important part of its mission, but it will not have the sophisticated equipment needed to analyze atmospheres or planet makeups. The idea of adding a new task at this late date seemed like a long shot, but Cash specializes in long shots. What's more, when he made his big advance in occulter technology in the mid-2000s, Cash first proposed joining forces with the Webb Telescope team and did some preliminary research before being advised to look elsewhere. He still thought the combination could work, and so did an important ally named Matt Mountain, the head of the NASA-funded Space Telescope Science Institute at Johns Hopkins University. The institute is NASA's organizing and data-analyzing center for the Hubble Space Telescope, and it will perform the same role
for the Webb telescope, its successor. It's generally known as the hub of NASA's space-based observatory program, and is home to four hundred space scientists.
Mountain sought out a young, French-born research astronomer and specialist in starlight-blocking technology named Remi Soummer and asked if he was interested in looking into the feasibility of a Webb-sunshade mission. Soummer was eager to take up the challenge, even though his colleagues almost universally thought it was a waste of time for any number of technical, scientific, and bureaucratic reasons.
But a year later, when I met Soummer in his office in Baltimore, the wedding of the Webb to an occulter looked far more promising than anyone could have imagined. The scientific lead for the entire Webb project, Nobel laureate John Mather, supported the idea, as did the ESA principal investigator in charge of the instrument that would be used. (Both ESA and the Canadian Space Agency are partners on the Webb.) Soummer had just finished a white paper the week before (coauthored with Cash and Mountain, among others) outlining how and why the idea would work. There was, however, a big caveat: One minor filter needed upgrading on the Webb to fully shade the aperture and make the project work. It would be quite unusual for such a switch to be approved when a project was as far along as the Webb, but Soummer and others were hopeful. They had to be, because otherwise the characterizing of exoplanets, and consequently any deep knowledge of whether they had molecules and compounds associated with life in their atmospheres, would probably have to wait one or two decades, or more.