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.