For much longer than the nine years Centauri Dreams has been in existence, I’ve been waiting for the announcement of a planetary discovery around Centauri B. And I’m delighted to turn the first announcement on this site over to Lee Billings, one of the most gifted science writers of our time (and author of a highly regarded piece on the Centauri stars called The Long Shot). Lee puts the find into the broader context of exoplanet research as we turn our gaze to the nearest stars, those that would be the first targets of any future interstellar probes. On Thursday I’ll follow up with specifics, digging into the discovery paper with more on the planet itself and the reasons why Centauri B was a better target than nearby Centauri A. I’ll also be offering my own take on the significance of the find, which I think is considerable.
by Lee Billings
For much of the past century, astronomy has been consumed by a quest to gaze ever deeper out in space and time, in pursuit of the universe’s fundamental origins and ultimate fate. This Old Astronomy has given us a cosmological creation story, one which tells us we live in but one of innumerable galaxies, each populated with hundreds of billions of stars, all in an expanding, accelerating universe that began 13.7 billion years ago and that may endure eternally. It’s an epic, compelling tale, yet something has been missing: us. Lost somewhere in between the universe’s dawn and destiny, passed over and compressed beyond recognition, is the remarkable fact that 4.5 billion years ago our Sun and its worlds were birthed from stardust, and starlight began incubating the planetary ball of rock and iron we call Earth.
Somehow, life emerged and evolved here, eventually producing human beings, creatures with the intellectual capacity to wonder where they came from and the technological capability to determine where they will go. Uniquely among the worlds in our solar system, the Earth has given birth to life that may before the Sun goes dim reach out to touch the stars. Perhaps, on other worlds circling other suns, other curious minds gaze at their night skies and wonder as we do whether they are alone. In this coming century, a New Astronomy is rising, one that focuses not on the edge of space and the beginning of time but on the nearest stars and the uncharted worlds they likely hold. It will be this New Astronomy, rather than the Old, that will at last complete the quest to place our existence on Earth within a cosmic context.
In a major leap forward in this enterprise, today a European planet-hunting team announced their discovery of an alien world about the same mass as Earth. This alone would be noteworthy, for of all the “exoplanets” now known beyond our solar system, only a very few, and very recently, have been shown to at all resemble our own. But there is more to the story. This particular exoplanet resides in a three-day orbit around the dusky orange star Alpha Centauri B, a member of the Sun’s closest neighboring stellar system. There are two other stars in the system as well, the yellow Sun-like star Alpha Centauri A and the red dwarf star Proxima Centauri.
Astronomers began discovering exoplanets about two decades ago, finding at first a few per year. Since then, the pace of discovery has dramatically accelerated. Today there are more than 750 confirmed exoplanets, and a single NASA mission, the Kepler spacecraft, has detected more than 2,300 additional candidates that await confirmation. Most of these exoplanets are far too large, too hot, or too cold to support life as we know it, but a handful appear to be “Goldilocks” worlds the right size and the right distance from their stars where liquid water could flow in streams and pool in seas, worlds where carbon-based organisms could potentially thrive. The discovery of more Goldilocks worlds appears inevitable — statistics from the Kepler mission and other sources suggest that somewhere between ten to thirty percent of stars harbor potentially habitable planets. Among the planet-hunters, the question is no longer whether life exists elsewhere in the universe, but rather how far removed the next-nearest living world might be.
At a distance of just over 4.3 light years, the stars of Alpha Centauri are only a cosmic stone’s throw away. To reach Alpha Centauri B b, as this new world is called, would require a journey of some 25 trillion miles. For comparison, the next-nearest known exoplanet is a gas giant orbiting the orange star Epsilon Eridani, more than twice as far away. But don’t pack your bags quite yet. With a probable surface temperature well above a thousand degrees Fahrenheit, Alpha Centauri B b is no Goldilocks world. Still, its presence is promising: Planets tend to come in packs, and some theorists had believed no planets at all could form in multi-star systems like Alpha Centauri, which are more common than singleton suns throughout our galaxy. It seems increasingly likely that small planets exist around most if not all stars, near and far alike, and that Alpha Centauri B may possess additional worlds further out in clement, habitable orbits, tantalizingly within reach.
Anyone in the Southern Hemisphere can look up on a clear night and easily see Alpha Centauri — to the naked eye, the three suns merge into one of the brightest stars in Earth’s sky, a single golden point piercing the foot of the constellation Centaurus, a few degrees away from the Southern Cross. In galactic terms, the new planet we’ve found there is so very near our own that its night sky shares most of Earth’s constellations. From the planet’s broiling surface, one could see familiar sights such as the Big Dipper and Orion the Hunter, looking just as they do to our eyes here. One of the few major differences would be in the constellation Cassiopeia, which from Earth appears as a 5-starred “W” in the northern sky. Looking out from Alpha Centauri B b and any other planets in that system, Cassiopeia would gain a sixth star, six times brighter than the other five, becoming not a W but a sinuous snake or a winding river. Cassiopeia’s sixth bright point of light would be our Sun and its entire planetary system.
Image: Alpha Centauri as seen by the Cassini orbiter above the limb of Saturn. Credit: NASA/JPL/Space Science Institute.
Despite its close proximity, as with nearly all other known exoplanets, Alpha Centauri B b is as yet unseen. It was detected indirectly, via a periodic 50-centimeter-per-second wobble its orbit raises in the motions of its star, in a painstaking process that took four years of nightly monitoring and careful analysis. The wildly successful Kepler mission finds the bulk of its candidates by a different technique, looking for the minuscule diminution of a star’s light when, by chance, a planet in its orbit transits across the star’s face and casts a shadow toward Earth. Both of these discovery methods can constrain the most basic properties of a planet: its orbit, its mass, and perhaps its size and bulk composition. But neither can readily reveal whether or not any potentially habitable planet is actually a place much like Earth. To do that really requires taking a planet’s picture. Even if that picture amounted to only a meager clump of pixels, astronomers might discern within it a planet’s rotational period — the length of its days — as well as clouds, oceans, and continents. The reflected planetary light would also contain spectroscopic signatures of atmospheric gases. Carbon dioxide would suggest a rocky planet, and water vapor would hint at oceans or seas. Detecting oxygen and methane — gases produced on Earth by living things — would further suggest that the distant planet was not only habitable, but inhabited.
Viewed over interstellar distances in visible light, the Earth is some ten billion times fainter than the Sun, meaning that for every photon bouncing off Earth’s atmosphere or surface, ten billion more are flying out from our star. About the same ratio would apply for any habitable planet around Alpha Centauri’s stars. Distinguishing such faint planetary light from that powerful stellar glare is rather like spotting a firefly hovering a centimeter away from the world’s most powerful spotlight, when the spotlight is in Los Angeles and you are in New York. To see the firefly, that overwhelming ten-billion-to-one background light must be suppressed.
Amazingly, on paper and in laboratory studies, astronomers have already devised multiple ways to do exactly this for potentially Earth-like planets that may exist around nearby stars. Most of these methods require an entirely new multi-billion-dollar space telescope, though a few proposals exist to augment NASA’s upcoming James Webb Space Telescope with starlight-suppressing technology at an estimated cost of $700 million. Considering that three years ago a film about life on Alpha Centauri’s planets — James Cameron’s Avatar — grossed some $2 billion in box-office receipts, it stands to reason there is public appetite to spend at least that much on space telescopes to search for the real thing.
Matt Mountain, the director of the Space Telescope Science Institute in Baltimore, Maryland, likes to quip that the discovery of life beyond our solar system could be to this coming century what Neil Armstrong’s lunar footprints were to the last. Yet today NASA is not seriously funding any life-finding telescopes, and has no real plans to do so in the future. The agency instead is spread thin and lacking any unified direction, strapped for cash and struggling to avoid obsolescence while it maintains the International Space Station, builds a new fleet of rockets to replace the retired Space Shuttles, and completes the James Webb Space Telescope. Yet obsolescence is precisely what NASA will embrace if it fails to invest now in the next giant leap required for this New Astronomy. Of all the scientific institutions and agencies upon this planet, at present NASA alone has the resources to build a telescope capable of directly imaging and characterizing any Earth-like planets around nearby stars. Unless it does so, as the list of potentially habitable planets grows long in years to come, all that shall grow along with it will be our ignorance of what those distant worlds are actually like and what may live upon them.
Meanwhile powerful, influential Old Astronomy, which has revolutionized our understanding of the universe at its largest scales, is wary of the New, and at times has acted quite deliberately against it. Alas, government-funded Big Science is too often a zero-sum game, one in which money that could go toward looking for life on exoplanets around nearby stars would be taken from cosmological efforts to study far-distant galactic clusters and the expansion of the universe. In a perfect world we would fully fund both quests simultaneously. But our world — with its economic instabilities, rising temperatures, growing populations, and plummeting biodiversity — seems to grow more imperfect by the day, in ways that no knowledge of dark energy or dark matter is likely to ever assuage. The New Astronomy is different. We do not yet know whether planets like ours and creatures like us are in fact common or rare in the cosmos, but by trying to find out, we will unavoidably learn just how precious our planet truly is. Perhaps, with luck, discoveries following from today’s announcement will help us finally kick off from our small blue footstool, and find our way among the stars.
Lee Billings is working on a book about the search for other Earth-like planets, forthcoming from Penguin/Current next fall.