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Looking nearly due north through binoculars on a clear day at CTIO, you just may catch the glint from one of the telescopes at the European Southern Observatory’s facilities on the peak of La Silla, another mountain some 60 kilometers away. Here, a Swiss team led by Michel Mayor and his protégé Stéphane Udry operates the High Accuracy Radial velocity Planet Searcher (HARPS). Thanks to the high precision of its spectroscopic equipment, running on a telescope more than twice the size of Fischer’s, HARPS is the world’s premier RV experiment. Since 2003, Mayor and his team have used HARPS to search for planets around Alpha Centauri B. Last August, they began observing the star every available night in a strategy similar to Fischer’s.
Mayor’s team can’t devote the same amount of time to Alpha Centauri as Fischer; HARPS is simply too valuable to the astronomical community to be committed to a single star system. But even if they could, they don’t want to. In a recent survey, the team showed that one-third of about 200 nearby Sun-like stars harbor rocky, terrestrial worlds several times more massive than Earth in short-period orbits. They’re called “super-Earths.”
“These objects aren’t exactly like our Earth, but they may very well be habitable,” Mayor said. “We’re discovering them everywhere we look. So what lies below? What is the frequency of planets twice the size of Earth, or truly Earth-size? [Fischer’s team] is focusing solely on one star system. Personally, I prefer to explore a larger sample.”
Like several other rival teams, the Swiss are playing a numbers game, hoping that surveying a larger number of stars with a smaller number of observations from their superior instrument will net the first clearly habitable world outside our solar system. They’re simply hedging their bets by observing Alpha Centauri B. If a super-Earth with three-to-five Earth-masses is there, the Swiss will probably detect it before Fischer.
“The borderline between where they’ll get it and where we will would be a planet about 2.5 Earth-masses in the habitable zone,” Laughlin said. “But even with hundreds of observations on HARPS, they won’t be able to probe to Earth-mass planets in B’s habitable zone like Debra will.”
There are already firm constraints on what resides in Alpha Centauri. Previous searches have ruled out anything larger than Jupiter in close orbits around either star, and dynamical simulations clearly show that, for both stars, orbits beyond two to three astronomical units (one astronomical unit is the distance between the Earth and the Sun) are unstable. Past that boundary, the perturbative effect of the second star either ejects planets from the system or makes them fall inward, where they are engulfed and destroyed by their host star. The same perturbations could also have proved disastrous for the formation of planets, leaving nothing orbiting either star but sand.
This is the bleakest scenario for both Fischer’s team and the Swiss, and precisely what Philippe Thébault, a young theoretician at the Paris Observatory, seems to have proved in a pair of papers he co-authored in 2007 and 2008. Using rigorous numerical simulations of the “standard model” of planet formation, he showed that planets could not form around Alpha Centauri A or B, refuting the work of Laughlin, Guedes, and several other researchers.
When we discussed his work, Thébault spoke impeccable English with a slight French accent, yet still apologized profusely for his “meager” grasp of the language. He was also contrite about his Alpha Centauri results.
“If you ask a doctor about a fatal diagnosis he makes for a cancer patient, of course he wishes he is wrong. It is the same for me. It appears to be very difficult to form planets around close binary stars. I don’t wish for such a universe—I wish for another universe where planet formation is always very easy. I hope that Greg is right and that I’m wrong.”
No one yet knows for certain precisely how planets form, but the process seems to be a complex chain reaction that is highly dependent upon initial conditions. It begins with the creation of a star, which forms from a gravitationally collapsing cloud of gas and dust. The leftovers flatten out, due to the conservation of angular momentum, forming a spinning disk of material. To create a rocky world like Earth, dust must condense in the disk to form grains, grains must settle to form pebbles and rocks, and rocks must collide to form planetesimals, kilometer-sized objects that can gravitationally attract each other. These planetesimals must collide to form embryos, Moon-sized objects that collide in turn to finally form a planet.
Laughlin and Guedes only accounted for this last stage in their simulation—investigating previous steps was too computationally intensive. Thébault, in contrast, used several simplifying assumptions to probe a step deeper, to the planetesimal-to-embryo stage within a stellar binary system. Embryo formation requires planetesimals colliding gently, so that they aren’t shattered. Thébault’s results showed that, in the Alpha Centauri system, the influence of one star would stir up the other star’s disk of debris, increasing the relative velocities of planetesimals to such high speeds that they couldn’t stick together to form embryos. The causal chain was broken; planet formation seemed impossible.
Fischer and Laughlin were so impressed with Thébault’s work, they offered him a co-investigator position on their project, to help interpret any emerging signals. And though Thébault accepted, they still are skeptical of his conclusions.
“I take no issue with Philippe’s calculations,” Laughlin said. “If you start with his initial conditions, you’ll end with no planets. I just believe the initial conditions were quite different than what he uses.” Laughlin reeled off several possible ways planets could still emerge in Alpha Centauri, stretching back to the early history of the system, when the stars’ orbital separations could have been greater, when their debris disks could have looked very different than the one that formed our own solar system. The list of planet-friendly possibilities is impressively long, but so filled with uncertainty that believing them remains largely a matter of faith.
Fischer preferred to assault Thébault’s claims with an inconvenient truth. “Here’s my problem with those papers: HD196885 is another binary star system, one almost exactly like Alpha Centauri, and we know it has a gas giant planet, orbiting at 3.5 astronomical units. I asked Philippe, ‘Can you explain this with your models?’ He said, ‘I can’t…. It shouldn’t be there.’”
One can whimsically show that essentially any occurrence, any circumstance, is a virtual impossibility. Simply trace its origins back through time until the accumulated serendipities within that chain of chance overwhelm logic and reason. This game applies to everything, from the formation of the Earth and life upon it, to your birth or your first love. All that occurs and exists is but the thinnest sliver of light, bookended by the infinite, muted depths of what will never be. Call it luck, call it destiny, but this intangible something seems the only arbiter between being and nothingness.
Thus, the Alpha Centauri project should not exist—numerous unlikely midwives have coaxed it into being. It should not succeed, since it relies on scarce funding for outdated equipment to provide unprecedentedly precise observations of uncertain stellar behavior, seeking planets that by standard estimations should not be there. But of course, the project does exist, and it may somehow succeed. It’s a long shot, cast outward into a distant, hazy region where our understanding dwindles, where probabilities coalesce to create reality.
For Laughlin, the potential rewards outweigh the risks; betting on planets in Alpha Centauri is a gamble he can’t help but make. “From a strictly monetary point of view, I think the discovery of habitable planets around Alpha Centauri would be worth a billion dollars, and ultimately perhaps far more,” he speculated. “Whereas Debra’s project, the price tag is running in the hundreds of thousands, maybe a million. Given a strict probabilistic analysis, that means if there is a one-in-a-thousand chance there’s something there, it’s worthwhile to spend a million dollars because of that expectation of coming out ahead, and I think the odds are much better than that. The payoff could be enormous.”
The discovery of habitable worlds around any star would be front-page news, but finding them around our next-door neighbors would catalyze a scientific and cultural revolution, an immense rising wave of effort to learn whether our sister stars’ habitable planets were in fact inhabited. The ripples would spread beyond science to touch and change our literature and art, our politics and religion, perhaps even aiding our struggles to unite, survive, and expand as a species. The chain of chance that brought us into existence would swing to point outward to the stars, strengthening our resolve to someday reach them.
“If planets are found around Alpha Centauri, it’s very clear to me what will happen,” Marcy said. “NASA will immediately convene a committee of its most thoughtful space propulsion experts, and they’ll attempt to ascertain whether they can get a probe there, something scarcely more than a digital camera, at let’s say a tenth the speed of light. They’ll plan the first-ever mission to the stars.”
In fact, planning is already underway. In their scant spare time, Fischer and Laughlin are examining the propulsion requirements and orbital mechanics for a robotic interstellar mission and determining how to reliably transmit any collected information back to Earth. Traveling at 10 percent light-speed, a probe would take more than 40 years to reach Alpha Centauri, and its data wouldn’t come back for more than another four. Fischer and Laughlin are both middle-aged. Even if such a mission launched within the next decade, it seems unlikely either would live to see its returns; for them, any planets found there will probably be, at best, flickering dots in an image from a space telescope. Their work is sacrificial, sustained by dreams of future generations escaping Earth’s cradle, of a universe made less lonesome and senseless.
Dusk had given way to darkness at CTIO, and though the stars blazed above the windswept summit, no one was there to see them except for Fischer and me; Walp and the other astronomers had retreated to their domes and control rooms to watch the skies through telescopes and computer screens. The gravel crunched beneath our feet as we walked to the edge of the abyss, and Fischer gestured to the spot on the sky where, in a few hours, Alpha Centauri would rise above the mountains. “I wonder every time I point it out, whether there’s someone there, pointing back at me,” she said. It was getting late, and there was much to do. Waving goodbye, Fischer turned and walked back to the telescope to prepare for that night’s many hours of observations. My visit was at an end, but her long shot had only just begun.
Originally published May 19, 2009
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