An exoplanet too far
Like many exoplanets, Proxima b was observed indirectly, but convincingly, through its gravitational sway. As the planet orbits, it tugs the star very slightly toward it. This wobble is detectable as a Doppler shift, a minor but regular change in the color of the star’s light—slightly bluer as it moves toward an observer on Earth, then slightly redder as it moves away. The scientific team, led by Guillem Anglada-Escudé, of Queen Mary University of London, began to suspect the existence of Proxima b after analyzing sixteen years’ worth of Doppler measurements from the European Southern Observatory network. A follow-up series of observations, conducted nearly every night from January 19th to March 31st of this year, confirmed it. “Statistically, there’s no doubt,” Anglada-Escudé said yesterday in a press conference.
Proxima Centauri belongs to a trio of stars in the Alpha Centauri system. The two largest stars, Alpha Centauri A and Alpha Centauri B, are slightly smaller than our sun, while Proxima Centauri is considerably smaller—an M dwarf, faint and red, only about one-eighth the size and mass of the sun and one-seventh as bright. Anglada-Escudé and his colleagues estimate that Proxima b is at least 1.3 times as big as Earth and is most likely a terrestrial planet, with a surface. It orbits its star at a dizzying pace, once every eleven days, at a distance twenty times closer than Earth is to the sun. But, because the star is much dimmer than the sun, the average temperature on Proxima b could potentially be temperate and the orbit “is within the range where water could be liquid on its surface,” the researchers write in their Nature paper.
Whether Proxima b actually has water or an atmosphere is unknown. “It depends entirely on the history of the planet,” Ansgar Reiners, an astrophysicist at the University of Göttingen and a co-author on the paper, said yesterday. “The main question is what happened during its formation.” Radiation from the star could have entirely stripped away the atmosphere from the planet long ago; then again, the planet could have formed farther from the star than it is now, with a thick enough atmosphere that some of it remains today. “From what we know, there is a non-zero probability that there is an atmosphere,” Reiners said, which seemed to be his way of saying that he was excited. He added that M dwarves, which are among the most abundant stars in our galaxy, are relatively easy places to look for exoplanets: because they’re so much less massive than stars like the sun, their Doppler wobble is larger and more detectable. And, as new planet-hunting telescopes come online, the early stages of these stars’ lives will become better understood.
Alongside a commentary that accompanies the new paper, Nature published an artist’s rendition of the surface of Proxima b. It looks like the Snake River Canyon might have appeared during an ice age, or perhaps like the view north across the frozen wastes from atop the Wall in “Game of Thrones.” It’s an optimistic impression. Although scientists will say that the study of Earth-like planets offers to shed light on how our own planet formed, surely part of the thrill is imagining what it would be like to live in such places. Proxima b has little to offer humans, however. Its star is turbulent, flooding the planet with X rays and ultraviolet radiation in amounts at least a hundred times higher than what Earth receives. The planet is also likely tidally locked, like our moon, with one side permanently in light; good luck to anyone doomed to live on the frigid dark side.
Of course, the main problem with exoplanets is the “exo” part. Proxima b is 4.3 light-years away—the distance that light, moving at a hundred and eighty-six thousand miles per second, travels in 4.3 years. That means the Proxima b detectable today is the exoplanet as it existed 4.3 years ago, when those light rays left its surface. Such is the weirdness of space-time that residents of Proxima b could say the same about Earth. If they aimed their radio telescopes our way, they’d find Taylor Swift’s “We Are Never Ever Getting Back Together” climbing up the Billboard charts and Gotye’s “Somebody That I Used to Know” fading away. Temporally, Proxima b is our next-door neighbor, a contemporary compared with the crotchety galaxies several billion light-years away.
But in physical terms the distance between us is effectively unbridgeable. Late last year, astronomers detected the presence of what they believe is a massive ninth planet in the far reaches of our solar system, at least two hundred and fifty—and sometimes as far as twelve hundred—astronomical units from the sun. (An astronomical unit is the average distance from the sun to Earth, or about ninety-three million miles.) Reaching Planet Nine with current propulsion technology would take fifty years; the Voyager 1 spacecraft, which was launched in 1977, is barely halfway there. Proxima b is a thousand times farther away still—more than two hundred and sixty-seven thousand astronomical units distant. In April, Stephen Hawking and Yuri Milner announced an engineering initiative, Breakthrough Starshot, to design spacecraft that could travel at a fifth the speed of light and reach the Alpha Centauri system in a couple of decades—ships an inch or two in size, propelled through the cosmos by lasers, like interstellar spitballs. Until such dreams come to pass, we’re looking at a journey of more than eighty thousand years.
Sit with that for a moment. In the coming decades, we will discover exoplanets by the tens of thousands and will come to know them, from afar, in intimate detail. Yet the nearest one is an eighty-thousand-year drive away. Whatever Proxima b is like today—that is, 4.3 years ago—it’s likely to be vastly different eighty thousand years from now. In fact, it wouldn’t even be there by the time we arrived. Stars wander ever so slightly, and the cosmos as a whole is expanding; in the next eighty thousand years, Proxima Centauri and its planet will have moved two light-years farther from Earth, adding another forty thousand years to the trip. “The universe is moving,” Pedro Amado, of the Instituto de Astrofísica de Andalucía, said yesterday.
Moving, yes, and stirring to behold. The hunt for exoplanets embodies all that is wondrous about the human endeavor—our innovation, our spirit, our thirst to know how we came to be, how we fit in, and what our cosmic destiny might hold. But there’s a whiff of the desperate about it, too. We seem just a little too eager to escape our history and envisage our species out among the stars, with a red sun rising above icy peaks or river-carved canyons. As if our one planet were not enough and its man-made troubles too much for our ingenuity. But we’re looking for salvation in exactly the wrong place. The cosmos is a hall of mirrors. There is no future out there, only an ancient and unfathomable past, its horizon receding ever faster.
Alan Burdick