The other night I trained my telescope on a few stars that resemble the Sun and are now known to have planets—inconspicuous and previously unheralded stars such as 61 Virginis and 47 Ursae Majoris, each found to be orbited by at least three planets, and HD 81040, home to a gas giant six times as massive as mighty Jupiter.
I could see none of the actual planets—lost in the glare of their stars, exoplanets can only rarely be discerned through even the largest telescopes—but just knowing they were there enhanced the experience. Watching those yellow stars dancing in the eyepiece, I found myself grinning widely in the dark, like an interstellar Peeping Tom.
When I was a boy, the prospect of finding exoplanets was as dim and distant as the planets themselves. Theorists had their theories, but nobody knew whether planets were commonplace or cosmically rare. My 1959 edition of the opulent Larousse Encyclopedia of Astronomy noted that no planets of other stars had yet been identified, but predicted that “future instrumental and technical improvements may confidently be expected to reveal many things that are now hidden.”
And so they did. Thanks to space telescopes, digital cameras, high-speed computers and other innovations scarcely dreamt of a half century ago, astronomers today have located hundreds of exoplanets. Thousands more are awaiting confirmation. New worlds are being discovered on an almost daily basis.
These revelations advance the quest to find extraterrestrial life, help scientists better understand how our solar system evolved and provide a more accurate picture of how the universe—which is to say, the system that created us—actually works.
Two techniques are responsible for most of the planet-finding boom.
The transit method discerns the slight dimming in a star’s light that occurs when a planet passes in front of it. Some transits can be observed from Earth’s surface—even a few amateur astronomers have verified the presence of transiting exoplanets—but the technique came into its own with the launch in March 2009 of NASA’s Kepler satellite, a one-ton space telescope with a 95-megapixel camera that repeatedly photographs 150,000 stars in a single swath of sky off the left wing of Cygnus the Swan. Computers comb the images to find evidence of transits. The degree to which a star’s light is reduced (typically by less than one-thousandths of 1 percent) suggests each planet’s diameter, while the time the transit lasts reveals the size of the planet’s orbit. As I write this, the Kepler mission has discovered 74 planets; hundreds more are expected to be confirmed soon.
Doppler spectroscopy measures the subtle wobbling of stars—really surface distortions, like those of a tossed water balloon—caused by the gravitational tug of orbiting planets. When a star is tugged toward or away from us its light is shifted to slightly shorter or longer wavelengths, respectively, much as an ambulance siren sounds higher and then lower in pitch as the ambulance speeds past. The technique has revealed nearly 500 exoplanets.
Both approaches are better at finding massive planets orbiting close to their stars—the so-called “hot Jupiters”—than earthlike planets in earthlike orbits around sunlike stars. So it may be some time before planets that closely resemble Earth are identified, and even longer before astronomers can capture their meager, reflected light and interrogate it for the chemical signatures of life as we know it.
But it is the nature of exploration to find things different from what one expected to find, and the exoplanet hunters have unveiled planets quite unlike any previously envisioned. One is GJ 1214b, a “water world” more than twice Earth’s diameter that whips around a red dwarf star 40 light-years from Earth every 38 hours, its steamy surface boiling at an oven-hot 446 degrees Fahrenheit. The sunlike star Kepler-20, some 950 light-years away, has five planets, two of them comparable in mass to Earth, all packed into orbits smaller than Mercury’s around the Sun. WASP-17b is a big wisp of a world, about twice the size of Jupiter but only a tenth as dense, orbiting a star a thousand light-years from us.
Back in 1959, astronomers were still debating whether a planet could orbit a binary star, a pair of stars that orbit each other. The answer turns out to be yes: Three such planetary systems have been found. The planets have twin suns in their skies, like Tatooine in Star Wars.
Such exotica aside, the planet-hunting enterprise calls attention to what the stellar population of our galaxy is really like, as opposed to the initial impressions one acquires through casual stargazing. The constellations we learn as children—Orion the Hunter, Canis Major the Big Dog, Lyra the Lyre—are made memorable by their brightest stars, giants like Rigel, Sirius and Vega. But such big, bright stars, though conspicuous, are rare: For every giant like Sirius there are a dozen or so sunlike stars and an astounding 100 million dim dwarf stars. The disparity arises because dwarf stars form much more abundantly than do giants, and last a lot longer. Giants burn so furiously that they can run out of fuel within millions of years. Middleweight stars like the Sun last around ten billion years. Dwarf stars burn their fuel frugally enough to make them effectively immortal: So far as astronomers can tell, no M-class dwarf star that ever formed has yet stopped shining.
In all, roughly 80 percent of the stars in our galaxy are dwarfs. So isn’t it more likely that life would be found on a dwarf-star planet than on a planet orbiting a much more rare sunlike star?
Perhaps, but dwarfs are so dim that their habitable zones—the “Goldilocks” region, cool enough that water won’t boil off yet warm enough that water isn’t permanently frozen—are necessarily quite close to the star, for the same reason that campers must huddle closer to a small fire than to a roaring blaze. The habitable zones of dwarf stars can be so cramped that planets orbiting there are virtually skimming the star’s surface, whirling through “years” lasting only days or hours. If you grew up on, say, Kepler-42c, which orbits in the habitable zone of a dwarf star only 13 percent as massive as the Sun, your birthday would roll around every ten hours and 53 minutes.
Life on such a world could be chancy. Even small dwarf stars, with surface temperatures not much hotter than a cup of coffee, can produce sterilizing X-ray flares as powerful as the Sun’s. If you were vacationing on KOI-961c and its star flared, the radiation might well kill you before you could reach shelter. Planets so close to their stars may also become gravitationally locked, so that one side is baked dry while the other freezes.
And even if you were content with your planet’s orbit, what are the chances of its remaining there? We terrestrials live in the habitable zone of a rather orderly system whose planets evidently have plodded along in pretty much the same old orbits for a very long time. But many exoplanetary systems are proving to be more chaotic. There, astronomers are finding planets that must somehow have migrated to their present locations from quite different original orbits.
Stars and planets form together, congealing gravitationally into a rotating disk of gas and dust with the proto-star sitting at the center like the yolk of an egg. Once the star ignites, a constant blast of particles blown off its surface sweeps light gasses like hydrogen and helium out of the inner part of the disk. Hence the Sun’s inner planets (like Earth) are rocky while the outer planets (like Jupiter) contain light gasses aplenty. That means the hot Jupiters orbiting close to many stars could not have formed there, but must instead have originated farther out and subsequently migrated to their present locations. Such migrations could have been caused by interactions among planets or by the tidal pull of passing stars and nebulae.
Planets not only change orbits much more often than had been thought, but can even be hurled out of their systems altogether, to wander ever after in the cold and darkness of interstellar space. A recent NASA study estimates that our galaxy contains more “free floating” planets than stars. That would put the number of benighted, exiled worlds into the hundreds of billions.
All known phenomena reside somewhere between total orderliness, which would make their behavior predictable in every detail, and utter chaos, which would make them utterly unpredictable. Prior to the rise of science, nature seemed to be mostly chaotic. Unable to predict most natural phenomena, people relegated even the appearances of comets and thunderstorms to what legal documents still refer to as “acts of God.” Once science got going, philosophers, impressed by its predictive power, went to the opposite extreme and began imagining that everything was completely orderly. Science came to be haunted by the specter of “strict determinism”—the notion that if the precise locations and motions of every atom in a system were known, one could reliably calculate its future in every detail. Since human beings are made of atoms, strict determinism implied that humans are but living robots, their every thought and action predetermined at the beginning of time.
The behavior of the solar system seemed to support strict determinism. The picture of an orderly “clockwork universe,” as predictable as a mechanical orrery, dates back to Isaac Newton’s working out the dynamical laws governing the motions of the Sun’s planets. When the mathematician Pierre-Simon de Laplace refined Newton’s clockwork and ran it backward to accurately “predict” a conjunction of Saturn with the star Gamma Virginis that Babylonian stargazers had observed in 228 B.C., it began to seem reasonable to conclude that every single event, even one’s own thoughts, were part of a strictly deterministic cosmic clockwork.
Yet Newton himself had doubts about that. He appreciated that although the Sun dominates its gravitational environment, the planets exert small but persistent gravitational forces on one another. He suspected that these interactions might sooner or later upset the solar system’s clockwork predictability, but he was unable to calculate their effects. “To define these motions by exact laws admitting of easy calculation exceeds, if I am not mistaken, the force of any human mind,” he wrote.
He was right. It took the power of modern computation to reveal that all planetary systems, even those as seemingly serene as the Sun’s, are infected by potential chaos. Computer simulations indicate, for instance, that Jupiter’s gravity has repeatedly altered the polar axis of Mars and may one day pull Mercury into an orbit so elliptical that it might collide with Venus or Earth. (Even a near miss between Mercury and Earth would generate enough tidal friction to transform both planets into balls of lava.) Troubled by his intimations of chaos, Newton wondered aloud whether God might have had to intervene from time to time to keep the solar system running so smoothly. Today it might be said that only an act of God could save strict determinism.
Bidding good night to planet-bearing stars that increasingly looked like ports of call, I closed up the observatory and paused to scratch a few numbers on a scrap of paper. NASA estimates that the Milky Way galaxy contains at least 100 billion planets, not counting the lonely free-floaters. If the “instrumental and technical improvements” I read about back in 1959 eventually attain such a state of excellence that astronomers are finding new planets every minute, day and night, they would be at it for 100,000 years before they’d mapped half the planets in our galaxy. And ours is one among more than 100 billion galaxies.
In short, we stand at the onset of a great age of adventure—and always shall, so long as we keep doing science.