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Needless to say, the observation opened a whole new field for cosmic inquiry, and added force to the question that has intrigued scientists, philosophers, poets, religious believers, science-fiction devotees and every child who has ever gazed upward on a clear, moonless night: Are we alone?
As if on cue, NASA then announced that its Kepler spacecraft, two years into a three-and-a-half year mission to find Earth-size planets around nearby stars, had found a totally unexpected profusion of candidates. Of the 1,235 suspected planets spotted so far, moreover, about a third were in multiplanet solar systems like ours. Judging from these discoveries, it would appear that planets out there are as numerous as grains of sand. Twenty-five years ago, when I was a student in high school, only nine planets were known, all in our solar system. We learned their names and sequence from the sun, from the fleet-footed Mercury to icy Pluto. We learned of the runaway greenhouse effect that had stoked Venus to blistering temperatures and read about the giant storm that is Jupiter's red spot, and we gazed at pictures of the rings of Saturn that the Voyager spacecraft had sent back.
The rest we could only imagine, fed by a rich body of science fiction for which outer space provided an endless source of content. Already in the second century A.D., Lucian of Samosta wrote (in Greek) of a war between the kings of the Moon and the Sun over the Morning Star. In From the Earth to the Moon, Jules Verne fantasized about two Americans and a Frenchman shooting themselves to the Moon with a huge gun, while H.G. Wells wrote of Martians invading London in The War of the Worlds. The universe became more civilized with Star Trek and its United Federation of Planets - reachable, of course, with Warp Drive - while Star Wars added a broad new range of cosmic sound effects and weapons to the repertory of children, and E.T., the Extra-Terrestrial established that distant worlds were populated by brown creatures with outsize heads.
The heavens were also the subject of the most fundamental questions raised by the great thinkers of recorded history. Epicurus, an ancient Greek philosopher, wondered about the plurality of worlds like ours. The Dominican friar Albertus Magnus, who was deeply interested in the physical sciences, pondered whether there is just one world or more, declaring that "this is one of the most noble and exalted questions in the study of Nature." About 300 years later, another Dominican friar, Giordano Bruno, proclaimed his belief that there were "innumerable" suns, with each one having multiple Earths - a belief that, combined with other ideas deemed heretical at the time, led to his being burned at the stake in 1600. The idea persisted. Writing about formation of the solar system in 1755, the German philosopher Immanuel Kant hypothesized that other stars have solar systems like ours.
So it may come as a surprise that it was only in 1995 that a planet beyond our solar system was first sighted. The discovery by the Swiss astronomers Michel Mayor and Didier Queloz was confirmed soon after by an independent team in the United States. I was a graduate student then and remember the great excitement this stirred among astronomers. Like Kant, many had believed that the processes that gave rise to our solar system were not unique, and that there were other planets in the universe. Now, observations had finally caught up with belief.
Finding "exoplanets" (for extrasolar planet, as planets outside our solar system are now referred to) is no easy matter. Planets emit no light of their own, and only reflect the light of their stars. Given the interstellar distances involved, even the stars nearest to us appear only as pinpoints, so it's a technological challenge to identify a planet thousands of times dimmer.
Mayor and Queloz met the challenge by using a spectrograph at the Haute-Provence Observatory in southeastern France to observe the rhythmic wobble of a sun-like star known as 51 Pegasi, a wobble created by the gravitational tug of an orbiting planet. This "radial velocity" technique has been used since to find many planets, but its reliance on spotting the wobble of a star tends to pick out larger planets close to their parent star - like the Jupiter-sized one Mayor and Queloz reported - which most scientists think could not be capable of supporting life.
There are ways to detect smaller planets, and the Kepler spacecraft launched on March 7, 2009, was specifically designed, according to NASA, "to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets." Kepler continuously monitors 145,000 stars in the Milky Way for the brief dimming of light that would indicate a "planetary transit" - a planet crossing the face of the star.
The team that discovered the wandering orphan planets, led by Takahiro Sumi and including David Bennett from the University of Notre Dame, used an even more arcane technique - gravitational microlensing - to spot these otherwise totally invisible bodies. Based on Einstein's premise that gravity bends light, the technique can see dark objects in the sky by measuring the light they bend from stars behind them. The astrophysicists thus saw 10 drifters, and estimated that there may be one or two of them for each of the approximately 200 billion stars in the Milky Way.
That's a quantum leap from the nine I knew in high school (reduced to eight after Pluto was demoted to a "dwarf planet" in 2006 by the International Astronomical Union), and even from the 500 or so exoplanets confirmed as of early this year. And if Jupiter-size planets, which are easier to spot, are numbered in the billions, surely there must be many Earth-size planets out there, spinning around their stars at just the right distance to support life? It is time to rewrite the texts.
You may wonder at this point why something so Earth shattering as the discovery of innumerable planets has not caused more excitement in the broad public. One reason, I would suggest, is that scientific discoveries take a while to register in the popular consciousness. In 1925, the astronomer Edwin Hubble first confirmed the existence of galaxies other than our own; four years later, he showed that they were moving apart from one another, supplying astonishing evidence of an expanding universe. Time magazine put him on its cover only in February 1948.
Besides, the confirmation that planets are a dime a dozen is really the culmination of the scientific revolution first started by Copernicus and Galileo and Kepler more than four centuries ago, a revolution in which our home planet lost its special place at the center of the universe. The prevailing cosmology before Copernicus - codified by the astronomer Claudius Ptolemy in the first century A.D. and, though dead wrong, accepted for the next 1,500 years - held that the Sun, Moon and planets (the six known ones) all revolved around Mother Earth, under a canopy of stars. It was a rational and well organized universe, in which the Roman Catholic Church could point with authority to heaven above and hell below.
Then Nicolaus Copernicus, a timid Polish canon, put forward an alternate, heliocentric system in which the Sun replaced Earth at the center. In 1543, just before he died, Copernicus finally summoned the courage to publish his treatise, De Revolutionibus Orbium Coelestium ("On the Revolutions of Heavenly Spheres"), which would inspire Galileo Galilei and Johannes Kepler to pursue the studies that became modern astronomy. In an age when science was inextricably linked to religion, the Catholic Church did not surrender lightly its geocentric universe. To challenge it was "false and contrary to Scripture," Galileo was told by the Inquisition, and even though he disowned his ideas, he spent his last years under house arrest. (In 2000, Pope John Paul II formally apologized for Galileo's trial).
But there was no turning back. Within a few decades, Isaac Newton confirmed Kepler's ideas on planetary motion and described the natural laws that have shaped our view of the cosmos ever since. Once the Earth had been displaced from the center of the universe, it was only a matter of time before the Sun was reduced to a garden variety star in a remote spiral arm of the Milky Way galaxy; the Milky Way itself to one of a hundred billion galaxies; and our planet to a speck of cosmic dust.
We have been humbled. Or have we? We have confirmed Bruno's belief that "innumerable suns exist, and innumerable Earths revolve about these suns," but we have not yet answered the "noble and exalted" question Magnus posed: "Do there exist many worlds, or is there but a single world?" For all that we have learned, it still remains possible that only on this minute blue speck are there intelligent creatures, shaped by an improbable confluence of events and forces, or, as some would argue, by a great supranatural power; creatures who are capable of gazing out at the heavens and wondering, are we alone?
Saswato R. Das first learned about the stars at his grandfather's knee in a village in rural West Bengal in India. He was captivated by the Milky Way, and his fascination led him to pursue graduate studies in astronomy and observe stars in Arizona and Hawaii. These days, he lives in Manhattan, where, on most nights, the stars are elusive - although he does occasionally see one or two over Central Park. He has written about science and technology for many publications, including Scientific American, New Scientist and the Times Literary Supplement.
"You may wonder at this point why something so Earth shattering as the discovery of innumerable planets has not caused more excitement in the broad public. One reason, I would suggest, is that scientific discoveries take a while to register in the popular consciousness."
I think people have known (or at least assumed the existence) of these planets for a very long time. The recent find of exoplanets is less of a discovery than a confirmation.