Afterword to the Paperback Edition

Since this book first came out in the spring of 2011, the pace of exoplanet discovery has continued unabated. Findings from telescopes in space, especially Kepler, and on the ground make media headlines on a regular basis. As the tally of extrasolar planetary systems grows, so do their astonishing diversity and the prospects of finding worlds that could harbor conditions suitable for life.

Of the first 2,300 planet candidates revealed by Kepler, some two hundred are Earth-size or smaller, while another seven hundred would qualify as “super-Earths” with radii less than twice that of our planet. These numbers confirm that small planets are quite common indeed. Of course, Earth-size does not necessarily mean Earth-like. The vast majority so far are too hot to support life as we know it. Ten of the super-Earth candidates are at about the right distance from their star to have surface temperatures that could sustain liquid water, assuming they are rocky planets with atmospheres. Determining whether a super-Earth is a scaled-up version of our planet or more akin to a mini-Neptune requires a measure of its mass, as well as its size. That’s no easy task for many of the Kepler planet candidates, because Doppler spectroscopy is exceedingly challenging for distant, faint stars. In the case of Kepler-10b, however, scientists have been able to verify that its average density implies a rocky composition. Kepler has also added to the diversity of alien worlds, revealing planets that circle binary stars and what appears to be a slowly disintegrating Mercury-size object. Several research teams have estimated the number of planetary systems in the Galaxy, based on the Kepler findings to date, with one group placing the total at over a hundred billion. Recognizing its spectacular successes, NASA has extended the Kepler mission until 2016, assuming no serious problems arise with the hardware.

Closer to home, a group of European astronomers led by the Geneva team of planet hunters reported a roughly Earth-mass planet around Alpha Centauri B, a member of the star system nearest to the Sun, just over four light-years away (see chapter 8). They used the HARPS spectrograph at the European Southern Observatory to measure the tiny wobble of the star caused by the gravitational tug of a planet circling it every 3.2 days. Since the planet is snuggled so close to the star, it is scorching hot—with an estimated surface temperature of 1,200 degrees Celsius—and thus utterly inhospitable to life. But, as the discoverers wrote in their Nature paper, “This result represents a major step towards the detection of Earth twins in the immediate vicinity of the Sun.” It also raises the possibility of other rocky planets in the same system, perhaps even in the habitable zone. Speaking of which, other teams have reported planets that are several times the mass of the Earth in the habitable zones of nearby stars, including tau Ceti, which is just twelve light-years away.

Meanwhile, new data from the ESO’s Very Large Telescope and the Hubble space telescope favor a water vapor−rich atmosphere for the nearby transiting super-Earth GJ 1214b (see chapter 8). It may have formed far from its star, where water ice was plentiful, and migrated inward. In its current star-hugging orbit, the planet’s surface is too hot to be habitable. The dispute over Gliese 581g, a super-Earth that would fall well within the habitable zone of a nearby red dwarf star, continues. The Geneva team was unable to confirm its existence, but the American team has redoubled on their claim with a new analysis of the radial velocity data. The stakes are high, because, if confirmed, it would be the first known exoplanet that could support life.

Sensitive new infrared observations with the Spitzer space telescope have failed to detect the putative giant planet around the nearby star Fomalhaut. A faint point source inside the star’s dust ring, seen in Hubble pictures, was interpreted as the direct image of a giant planet (see chapter 7). If that were true, the object should shine brighter at infrared wavelengths. But it does not appear in the new Spitzer images at all. In a recent paper in the Astrophysical Journal, Markus Janson of Princeton University and his colleagues wrote: “Hence, we can firmly exclude the hypothesis that any of the observed flux in [the 2008 paper] actually originates from a giant planet.” While re-analyses of earlier Hubble epochs and more recent observations have placed better constraints on its brightness and motion, the true nature of the point source seen in optical images remains a puzzle But, given what we know now, it is unlikely that direct light from a giant planet around Fomalhaut has been detected.

The Atacama Large Millimeter Array in northern Chile, which has begun scientific operations, will provide new insights on planet formation (see chapter 2). Early observations have revealed gas streams flowing across a large gap in a protoplanetary disk surrounding a young star, confirming theoretical predictions.

My collaborators and I continue our studies of exoplanets and brown dwarfs. One recent highlight is the discovery of a free-floating brown dwarf only six times heftier than Jupiter in the young star cluster NGC 1333 (see chapter 6). Another is the discovery of brown dwarf companions around several young stars in the Upper Scorpius association. We are characterizing exoplanet atmospheres through infrared photometry and optical spectroscopy, and following up some interesting Kepler candidates.

There is also some less rosy news to report, however. For one, the launch of the James Webb Space Telescope, the long-awaited successor to Hubble, has been delayed until at least 2018. For another, the SETI Institute in California has struggled to raise funds to extend the search for alien radio signals. After a brief hiatus, the Allen Telescope Array is now back in operation, thanks in part to donations from members of the public. The long-term future of the search for extraterrestrial intelligence remains uncertain.

On a more personal note, I had the privilege of participating in a meteorite-collecting expedition to Antarctica, under Ralph Harvey’s leadership (see chapter 2), during the 2010−11 austral summer. Camping out on a remote ice field halfway between the McMurdo station on the Antarctic coast and the South Pole, eight of us searched for meteorites day after day on snowmobiles and on foot, with the temperature hovering near −20 Celsius. Over thirty-seven days, our total haul came to nine hundred specimens. For me, it was a truly remarkable and rewarding experience, one I shall always cherish. Besides, it was awesome to think that we were scooping up bits of debris left over from the birth of the solar system some four-and-a-half billion years ago. (You can read more about our expedition in the November/December 2011 issue of Muse magazine.) In June 2012, I also had the chance to witness the transit of Venus across the Sun from the Haleakala summit on the island of Maui, Hawaii. That too was a precious spectacle, one that will not be repeated in our lifetime.

I do hope that we will have found definitive signs of life beyond Earth well before the next Venus transit in 2117. The imminent discoveries of Earth twins will surely bring the question of alien life to the forefront, with the obvious impulse to turn our radio telescopes toward candidate habitable worlds. As I wrote in a March 27, 2011, New York Times op-ed,

Some people worry that discovering life elsewhere, especially if it turns out to be in possession of incredible technology, will make us feel small and insignificant. They seem concerned that it will constitute a horrific blow to our collective ego. I happen to be an optimist. It may take decades after the initial indications of alien life for scientists to gather enough evidence to be certain or to decipher a signal of artificial origin. The full ramifications of the discovery may not be felt for generations, giving us plenty of time to get used to the presence of our galactic neighbors. Besides, knowing that we are not alone just might be the kick in the pants we need to grow up as a species.