Facts So Romantic

How We Could Find Alien Life Soon—and Why We Probably Won’t

Not Seeing Stars: Unlike most exoplanets we’ve seen, these four planets orbiting star HR 8799 have been directly spotted, rather than inferred from data about the star. Researchers accomplished this by blocking out the star’s extremely bright light, as seen in the middle of the image.NASA/JPL

Despite their great distance from Earthbound politics, exoplanets were the topic of a joint hearing on May 9th of the House space and research subcommittees. The recent discovery of a trio of temperate super-Earths was the hearing’s impetus, but most of the discussion was devoted to future prospects—chiefly, how and when scientists might learn whether any exoplanet actually harbors an Earth-like environment or even alien life. Leaders from NASA, the National Science Foundation, and the SETI Institute all spoke of steady progress toward those goals, though their sunny forecasts were arguably based more on faith than facts. Right now, speculating about the environment of any potentially habitable exoplanet is rather like guessing an individual’s facial features based only on their height and weight, and without some sea change in policy or funding, this situation seems set to persist for decades to come.

This may be surprising considering that astronomy is in the midst of an exoplanet boom, a golden age of extrasolar discovery, and a host of current and upcoming ground- and space-based projects will ensure the boom continues for years to come. We are learning more than ever before about the formation, distribution, and evolution of exoplanets. The Hubble and Spitzer space telescopes can already crudely study the upper atmospheres of select hot, giant exoplanets that happen to “transit,” or cross the faces of, their stars, as seen from here. The James Webb Space Telescope (JWST), which may launch as early as 2018, could do the same [pdf] for some smaller, cooler exoplanets that orbit and transit nearby stars, while also snapping family portraits of nascent and young planetary systems. Though nearing the end of its life, the Kepler mission has found nearly 3,000 likely exoplanets transiting faraway stars in a single swatch of sky, and a follow-up, all-sky mission called TESS should launch in 2017 to look for more transiting worlds, some of which could then be closely investigated by JWST. These are only a few of the space-based planet-finders; there are too many other new and worthwhile projects to list here.

And yet of all the projects, instruments, and telescopes now operating or soon to debut, not one is capable of delivering what the search for extrasolar life most requires: The atmospheric spectra of any potentially habitable planets around a representative sample of the Sun’s neighboring stars. Whenever starlight shines through or scatters off a planet’s atmosphere, the atoms and molecules in the air absorb some of the light, leaving their faint chemical fingerprints. Astronomers can discern these fingerprints by gathering enough of that light to create a rainbow-like spectrum. A planet’s spectrum can help determine whether that world is habitable or even inhabited. The spectral fingerprints of water vapor or carbon dioxide, two powerful greenhouse gases, would signpost a warm, wet, rocky world, and an abundance of oxygen and methane could be evidence of a biosphere of photosynthetic plants and chemosynthetic bacteria. With such tantalizing clues in hand, more in-depth telescopic observations could eventually reveal even more secrets, as across the light-years astronomers map the distribution of lands, seas, and weather patterns of Earth-like alien planets. 

The exoplanet’s spectrum must be painstakingly sifted from a flood of raw starlight some ten million to ten billion times brighter. Astronomers have already demonstrated a handful of ways to achieve this.

As straightforward as this all sounds, there is an inconvenient underlying truth that few planet-hunters wish to admit—particularly in front of deficit-weary taxpayers: Pursuing the light of other living worlds will almost unavoidably be a multi-billion-dollar venture.

Barring various extremely implausible scenarios, any conceivable Earth-like exoplanet will be a small, dim object very close to its much brighter, larger star. Its spectrum must be gathered photon by photon, with each individual particle of planetary light painstakingly sifted from a flood of raw starlight some ten million to ten billion times brighter. As difficult as this “starlight suppression” sounds, astronomers have already devised and lab-demonstrated a handful of ways to achieve it, using techniques like coronagraphy and interferometry. (See a summary of some of the methods here [pdf].) The bigger, more fundamental problem is that obtaining the spectrum from just one potentially habitable exoplanet is unlikely to be enough; satisfying our search for life, gaining some inkling of our cosmic context, will probably require surveying hundreds or thousands of worlds in the relatively short timespan of one space-telescope mission. To quickly, efficiently perform such a search requires one or more very large, very sophisticated starlight-suppressing space telescopes—telescopes that currently have no funding and very little public awareness.

I would say that in addition to being “very large” and “very sophisticated,” these telescopes are also “very expensive”—except on federal or national scales that’s simply not the case. Assuming it would take some $5 billion to develop, build, launch, and operate one such space observatory, any number of telling comparisons present themselves. That’s less than what the U.S. government spends for a few weeks of its military presence in the Middle East and Central Asia, or for a dozen F-22 fighter jets. As a nation we spend more each year on chocolate candy. The point of course isn’t that we should eliminate defense spending or stop buying candy bars, but simply that the great wealth of our world is more than adequate to bestow a priceless gift to all humankind in perpetuity. What our political leaders and philanthropic billionaires should understand is that the discovery of other Earths and other life beyond our solar system is something that can happen only once in our history, and something that is just now within our grasp. With just a relatively tiny fraction of public or private funding and effort, we could all soon see a day where anyone on Earth can look up to some bright star in the night sky and know a world circles there from which someone may be looking right back.

Wouldn’t that be worthwhile?

Lee Billings is a freelance writer living in New York City. Five Billion Years of Solitude, his book on the search for Earth-like exoplanets, will be published this October by Current/Penguin.

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