Seager went on to summarize a recent announcement about scientists finding a planet in the Goldilocks zone around the red dwarf star Gliese 581.1 She reminded the audience that it was a “three sigma result” or has a “confidence level of 99.7 percent,” meaning that there are three chances in a thousand that it is wrong. This level of uncertainty is acceptable in science, “but it could be wrong,” she stressed.
She then discussed what she considered the less-than-optimal method by which the finding was announced. The National Science Foundation (NSF) held a “hush, hush” press conference that could only be attended by a select group, she said, and NSF embargoed all the data until after the press conference. Consequently, a competing scientific team from Switzerland announced 2 weeks later at a meeting in Italy that it had different data from the U.S. scientists, and they saw no evidence of a planet. The U.S. team’s data were published in a scientific journal, but the Swiss team did not release their data, so it is difficult for scientists like herself to determine what is going on without documentation. Further analysis must wait for the Swiss team’s data. She added on a positive note that the episode did stimulate interest in exoplanets in the media.
Seager talked about the large number of exoplanets that have been found to date using a variety of methods. “Any kind of planet you can imagine exists somewhere,” she said.
The search for Earth-size planets is the task of NASA’s new Kepler space telescope. It will tell us “how common are other Earths.” It will find Earth-size planets, but she emphasized again they are not necessarily Earth-like planets. Although Kepler is finding Earth-size planets, direct imaging 2 is necessary to find Earth-like planets (“Earth twins”). Scientists will not be able to directly image a planet the size of Earth with good spatial resolution for the next 100 years, she said, comparing it to trying to see a firefly next to a search light at the distance of the west coast to the east coast of the United States. What is needed, she said, is the Terrestrial Planet Finder mission that has been proposed but not yet approved by NASA. “People are really working on this,” Seager said, and “if we had the money, we could do it.”
Absent that mission, scientists are looking for gases in the atmospheres of exoplanets that could indicate if life exists or could exist there. We won’t find life, she said, but may find signs of life, even though “we’ll never be 100 percent sure.” Seager joked that Jill Tarter, a prominent scientist at the Search for Extraterrestrial Intelligence (SETI) Institute, laughs when Seager says that because SETI scientists will be 100 percent sure when their search succeeds. SETI uses radio telescopes to search for signals from other intelligent civilizations.
Seager ended her talk by saying that people who work in exoplanet research have an idealistic vision that they can change the way we see ourselves in the universe—”we really think we can change the world.”
Steven Benner, distinguished fellow at the Foundation for Applied Molecular Evolution, broadened the discussion to talk about life forms that might be quite different from the liquid-water-based, carbon-based life we know. It is especially difficult to search for life that could be completely different from anything the searchers know, he stressed.
Delving into some detail about the chemistry of life, Benner explained that despite the millions of examples of life on Earth (“terran” life), the closer one looks the more they are the same at the molecular level. Four nucleobases, or “letters,” in the deoxyribonucleic acid (DNA) “alphabet” “are all shared by all terran life,” he said.
1 Seager explained that at least six planets have been observed about Gliese 581. The one in the “Goldilocks zone” is Gliese 581G. Gliese 581 is 20.3 light years from Earth in the constellation Libra.
2 Kepler stares at stars watching for objects that move across (“transit”) their faces at regular intervals, which suggests they are planets. It does not actually see the planet with any fine degree of resolution like taking a picture, as direct imaging would do.