Five Billion Years of Solitude (14 page)

In some small fraction, the laws of physics would be identical to or scarcely distinguishable from our own, and those regions would be more likely to generate galaxies, stars, planets, and living creatures. In the
remainder of the regions, natural laws would be so alien that life as we know it would be impossible. The theory of an inflationary “multiverse” is consequently often used in modern cosmology to explain otherwise mysterious fundamental properties of our universe that seem fine-tuned to allow life to arise and persist. In some stillborn universes with physical laws that precluded life’s emergence, there would be no stars. In others there would be no atoms. Some would expand or contract so quickly they shuffled out of existence in an instant; others would contain exactly equal portions of matter and antimatter that would mutually annihilate in a blaze of energy, leaving behind nothing but vacuum and seething radiation fields. In the majority of universes we can conceive of, the existence of observers seems inconceivable. There would be no living creatures within them to gaze out at their surroundings and wonder how it all began. In this telling, the universe we see around us is of course fit for life, for otherwise we would not be here.

No one has yet devised a foolproof way to test most of these ideas—how, exactly, can you detect other universes that by definition are forever inaccessible to us? But if true, an inflationary multiverse holds muddled consequences for Copernican ideas. On the one hand, it would mean that our entire observable universe was only the most minuscule fraction of a much larger cosmos inflated from our Big Bang 13.8 billion years ago. This vastly larger cosmos would itself be only a single member of an infinite ensemble of other universes. Infinity being, well, infinite, it would follow that the multiverse would host infinitudes of living beings on a limitless number of other worlds. On the other hand, the infinitude of bubble universes incapable of supporting life would appear to be very much larger than the infinitude that could. Against the principle of mediocrity, an inflationary multiverse suggests our local universe is a small part of an atypical bubble embedded in a much larger region of inflation, a member of a rather exclusive subset of universes that can harbor life. Whether the physical laws we observe are “average” within this subset, no one can say. A planet, a star, or a galaxy may be only as special and valuable as the cosmos that gave birth to it.

In contemplating eternal inflation, modern cosmology has, in effect, returned to some of the tenets first formulated by the Greek atomists some 2,500 years ago; Democritus would certainly laugh that it took so long. In the far future, as our own universe decays into a dark, cold senescence, life’s last holdouts may find some solace in believing that somewhere, far, far away, over the cosmic horizon, the ceaseless process of creation continues, giving birth to new lives, new worlds, and new universes. Hope springs eternal.

After the Gold Rush

F
rom time to time when Laughlin was deep in thought at his office, he would absentmindedly reach across his desk for a small child’s toy he had purchased in the 1990s, back when he was a postdoc at UC Berkeley. The toy looked much like a hangman’s scaffold. Instead of a noose, the scaffold held a thin steel pendulum, loosely suspended above a steel square by a tiny embedded magnet. He would place magnets of various strengths and shapes strategically upon the square and give the pendulum a gentle bump; it would swing to and fro for long periods, kicking between magnetic fields with sufficient force to overcome the frictional loss of momentum from moving through the air. Its motions followed a chaotic random walk, never exactly repeating any given path.
Laughlin savored the toy for how its complex behavior could unfold solely from the simple initial conditions of each magnet’s position and the strength and trajectory of an initiatory nudge. It reminded him of his struggles to predict the typical outcomes that emerged from the chaotic gravitational interactions of black holes, stars, and planets, and his efforts to squeeze faint signals from backgrounds of meaningless noise.

One night near the end of June in 2006, after coming home late from work, Laughlin sat down at his kitchen table and realized he had brought his work home with him—an idea was effervescing in his brain. Earlier that day he had been pondering the uncertain orbit of Proxima Centauri, a dim red dwarf tenuously bound by gravity to Alpha Centauri’s binary system of two Sun-like stars, Alpha Centauri A and B. Whether it was only a lone star passing in the galactic night or an estranged family member of the Alpha Centauri system, Laughlin was not sure. What mattered was that the trio comprised the closest known stars to our solar system. As he had thought that day about the stars’ celestial motions, the question of whether they had any accompanying planets intermittently tickled the back of his mind. By that night, the tickle had become an irresistible itch. Laughlin scratched it with notes scrawled on scrap paper and calculations keyed into his laptop.

For decades, consensus had held that binary star systems were poor targets for planet searches, because it was thought that gravitational interactions between the two stars would either prevent planet formation or fling planets, once formed, on escape trajectories out of the system. But ever since the exoplanet boom, increasing numbers of binary-star planets had been discovered—the consensus had been wrong. Due to their close proximity to Earth, Alpha Centauri A and B offered plentiful photons for an RV planet search. Alpha Centauri B, a dusky orange star slightly smaller than our Sun, was particularly quiet and stable—an excellent candidate to scour for potentially habitable planets. Earlier searches had already ruled out the presence of any gas-giant planets within a few AUs of each star, but the presence of smaller worlds was still possible. Laughlin thought they might be just within reach.

No matter how many holes Laughlin tried to poke in his thinking, deeper scrutiny deflected each of his criticisms, and the idea of a Centauri-centric RV survey stood unscathed. The more he turned it over in his head, the more ideal and fortuitous the situation seemed. Though most stars appear immobile in the sky on human timescales, the Sun’s 250-million-year orbit about the Milky Way’s center ensures that every few hundred thousand years our solar system has entirely new neighbors. “If we were plopped down at some random point in the galaxy, there’s only a 1 percent chance we’d find ourselves near stars so optimal for detecting small rocky planets like our own,” Laughlin told me during an interview in late 2008. “The hand of fate has dealt us a very interesting situation that has not existed for at least 99.9 percent of Earth’s history. It’s remarkable that Alpha Centauri is right next door just as humans emerge and develop the ability to make these measurements. I’m enamored with that coincidence.”

A survey seemed worth the risk of coming up empty-handed, Laughlin told himself as he sat at his kitchen table on that summer night in 2006. Finding any planets at all around the stars of Alpha Centauri would be a historic discovery. By virtue of their close proximity they would be prime targets for subsequent study, and regardless of their characteristics would likely garner large amounts of in funding for further research.

For a moment he allowed his thoughts to drift far away, into hazy realms of possibility. Detecting potentially habitable worlds around Earth’s nearest neighboring stars would be a truly revolutionary development, one that could stimulate major investments and advances in the quest to learn about our place in the universe. Without actually
seeing
any terrestrial planets that orbited in the stars’ habitable zones, no one could know whether they were places like Venus, Mars, Earth, or something else entirely outside of expectations. In comparison with the prospect of confirming other living worlds right on our galactic doorstep, the cost of building a direct-imaging space telescope to study such planets would seem to shrink. If, by happy chance, those nearby
worlds looked particularly inviting when viewed through new telescopes, they would lure generations of scientists, explorers, and dreamers just as the planets of our own solar system did during astronomy’s earlier, more romantic eras. Alpha Centauri would call across that briefest interstellar gulf, and someone would surely strive to answer. The first emissary would undoubtedly be robotic, maybe something the size of a Coca-Cola can somehow sent voyaging at 10 percent of the speed of light. And if, nearly a half century after its launch, the probe against all odds beamed back to Earth high-resolution images of another clement planet replete with oceans, clouds, continents, and . . .

Laughlin blinked and reined in his mind’s free roaming, which had suddenly slipped beyond the stars. Too much extrapolation was dangerous. He closed his laptop, rose from his kitchen table, and went to bed.

Within months of his kitchen-table reverie, Laughlin had performed numerical simulations of planetary assembly in Alpha Centauri with a graduate student, Javiera Guedes. They began with Moon-size planetary “embryos,” and watched as the embryos gravitationally clustered into small, rocky planets in stable, habitable orbits around each star. Laughlin next approached Debra Fischer, Marcy and Butler’s former collaborator, to propose a search. With funding from the NSF and help from many colleagues, including Laughlin, Butler, and her own students, Fischer began an intensive survey of Alpha Centauri in 2009, using a small 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile. Sixty kilometers to the north, at Cerro Paranal, the Swiss had been monitoring Alpha Centauri B since 2003, but soon after Fischer’s program began, they drastically upped the cadence of their observations. They could not focus as intently on the stars as Fischer and her collaborators—HARPS was simply too valuable a resource to be monopolized by a single star system. In 2011, a third team officially joined the search, obtaining funding to perform a high-cadence survey using a 1-meter telescope at Mount John University Observatory in New Zealand.

Analyzing the RV data proved harder than anticipated, partially due to difficulties with precisely removing the binary orbits—Alpha Centauri A and B orbit each other in a roughly 80-year period, at an average separation somewhat greater than the distance between our Sun and Uranus. The orbit is significantly “eccentric” (non-circular), but its fine details were not known down to the level of centimeters-per-second, making RV signals of small planets in either star’s habitable zone harder to see. That eccentric orbit posed further problems, according to another round of numerical simulations performed by the theoretician Philippe Thébault of Paris Observatory and a few collaborators. In run after run, gravitational perturbations driven by the orbit’s eccentricity disrupted the formation of structure well before any Moon-size building blocks could coalesce. Thébault’s simulations suggested nothing larger than sand grains and pebbles would orbit either star.

Laughlin could find no potential oversights or errors in the newer simulations, save for one: Thébault had assumed that the stars were born at their current distances from each other, with each sporting a protoplanetary disk approximately the same size as the one astronomers think formed our own solar system’s planets long ago. Laughlin believed Alpha Centauri’s stars had begun with very different initial conditions, at wider separations, perhaps with stubbier, smaller protoplanetary disks—any of which could prevent Thébault’s subsequent eccentric disruptions. The presence of the red dwarf Proxima was a potential piece of forensic evidence, Laughlin thought. “Had Alpha Centauri formed in a very dense stellar cluster, like what we see in the Orion Nebula today, then in all likelihood Proxima would have been stripped from its orbit by a passing star,” he explained to me. “Of course Proxima may have been only captured much later on, but I would bet its presence means Alpha Centauri formed in a more open, less cluttered environment, where the stars were further apart. . . . If you start with [Thébault’s] initial conditions, you’ll end with no planets. I just believe the initial conditions were quite different than what he uses.”

In October of 2012, a discovery was finally made. Using more than 450 combined HARPS measurements, the Swiss had detected what looked to be an Earth-mass planet around Alpha Centauri B. It resided in an inhospitable three-day orbit, so close to the star that its surface would be broiled at temperatures exceeding 650 degrees Celsius, yet it was universally acclaimed as a promise of great things to come, of a life-friendly cosmos. Somehow, even in uninviting circumstances, small, rocky planets still found ways to form and persist. Alpha Centauri Bb, as the new world was called, is so light it only creates a wobble of some 50 centimeters per second upon its star—a bit faster than the average speed of a crawling baby. If HARPS could detect that faint signal, the Swiss said, then any undiscovered rocky planets in Alpha Centauri B’s habitable zone were also likely within reach. And more planets were almost certainly there—statistics from the Kepler mission strongly suggested that where one small, close-in planet was detected, several more would lurk farther out, as yet unseen. Astronomers began to murmur that, in all likelihood, all three of Alpha Centauri’s stars possessed planets, and that quiescent, placid B would again be the first to yield additional discoveries. It was only a matter of time.