If you want to know what scientists are thinking about, you don’t need to ask—all you need to do is count. That’s what I did as I was touring the Long Beach Convention and Entertainment Center in Long Beach, California, where the American Astronomical Society is currently holding its semi-annual meeting. I tallied meeting abstracts, scientific sessions, press conferences, and (more impressionistically) casual conversations in the pannini-and-pizza lunch area. So what is on the mind of the world’s greatest explorers of the cosmos? By any measure, the answer comes down to one word: exoplanets.
An illustration depicts the compact solar system KOI-961, home to several small exoplanets. Photo courtesy of NASA. “Exoplanet” is the technical term for any planet that does not circle the sun—or, to put it another way, any planet that lies beyond the bound of our solar system. Two decades ago, nobody was sure such worlds even existed. Twenty one years ago, almost to the day, astronomers reported the first planets around another star. Yesterday, scientists working with NASA’s space-based Kepler Observatory announced 461 new likely exoplanets, bringing the number of worlds discovered just by that one mission to 2,740. Including all the ones identified by other searches, the number is over 3,000. Stop and think about that number. From 5000 BC to AD 1992, the number of planets known to science grew just from 5 to 9. We live in the middle of an explosion of planetary knowledge, and Long Beach sits at ground zero. One by one, the visiting astronomers stepped forward to sketch in more details about these new lands. Christopher Burke of the SETI Institute highlighted one of the latest batch of Kepler planets, currently burdened with the catalog name KOI 172.02. Based on the latest analysis, this is a world just slightly larger than Earth (20,000 miles wide, perhaps) circling a very sunlike yellow star every 242 days, at a distance that is just right for liquid water. It is, so far as we can tell, a near-twin of our own planet—certainly the closest that anyone has found so far. Earth-Size Planets But that is just a single example. Francois Fressin at the Harvard-Smithsonian Center for Astrophysics has done a clever analysis to find out how many Earth-size worlds are out there in total. After sorting through the Kepler dataset he has good news. “Almost all sunlike stars have planets,” he announced, “and at least one in six stars has an Earth-size planet.” The true fraction is very likely even better than that. John Johnson at Caltech pushed the numbers further and came up with an estimated tally of allthe planets in our galaxy. He made headlines recently when he announced his result: 100 billion planets in our galaxy alone. (There are also 100 billion other galaxies in the universe. Do the math, if you dare.) Again, that’s a lower limit, held back by the fact that Kepler has been operating for less than four years.
A NASA chart compares the smallest known exoplanets to planets in our solar system. Photo courtesy of NASA. What’s frustrating is that, for all of these amazing statistics, these planets themselves are little more than numbers. The smartest astronomers in the room know only the most basic details about the new worlds: diameter, orbital period, and decidedly approximate estimates of their temperatures and ages. Do these planets have air, oceans, continents, lakes, clouds, oxygen, methane, any of the plausible chemical signatures of life? Shrugs all around. What’s even more frustrating is that NASA, the world’s leading astronomical agency and the one with the best resources to find out, has no successor to the groundbreaking Kepler mission. Scientists are scrambling to reconfigure the upcoming James Webb Space Telescope—a kind of super-Hubble—so it can study the atmospheres of exoplanets. One cunning proposal would fly a second spaceraft, called New Worlds, in front of the Webb telescope to blot out starlight and come much more clearly into view. Another would build a new Transiting Exoplanet Survey Satellite, or TESS, that would scan the whole sky for planets around relatively bright, nearby stars that the Webb telescope could study in detail. One of the biggest challenges right now is simply not knowing where to look. Neither of these proposals is funded, however. The most exciting trend in astronomy today—and, really, the most exciting development in human exploration today—is looking at a future budget allocation of zero. Now there is a grim number. --
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