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“The search for life is my generation’s Apollo.”
Planet hunting’s grand challenges
To find signs of life on distant worlds, scientists must stare across vast gulfs of space, seeking the firefly light of tiny planets amid the searchlight glare of their parent stars.
In this new era of exploration, the technological challenges are immense. The searchlights must be dimmed so the planets can be seen. Space borne telescopes must maintain a nearly flawless fixation on their targets. Their detectors must be sensitive enough to gather the few sketchy traces of light that make the interstellar journey. And telescope designers must aim big—really big—if they want someday to take a snapshot of another cloudy, watery, living Earth.
Turn the lights down low
Muting the overwhelming brightness of stars to catch glimpses of their planets is the subject of intense focus at NASA’s Jet Propulsion Laboratory. Engineers and astrophysicists are building and testing two very different technologies that essentially do the same thing. The most visually impressive is a gigantic mechanical sunflower the size of a baseball diamond, called the starshade, which would unfold its petals in space to block the light of target stars.
A space telescope would be parked some distance behind it, lining up in near perfect synchrony. The intricate pattern of the starshade’s petals is designed to do much more than simply block the star’s main disk. Each petal, like a manic goalie, also prevents leakage of photons—particles of light—around the edges of the starshade, eliminating as much residual glow as possible. The result: Formerly invisible planets pop out of the background, allowing the telescope to capture direct images as they orbit the star.
The second technology doesn’t have quite the pizazz of the starshade, though it’s at least its equal as an engineering marvel. Called the coronagraph, this comparatively tiny instrument is wholly contained within the body of the telescope. It’s really a suite of three technologies: blunt-force blocking of most of the starlight with a centrally darkened mask, a second light blocker that looks something like the washer in a home plumbing fixture, and a tiny mirror loaded with mechanical pistons.
The mask and the washer do away with so much of the starlight that a display screen showing the telescope’s view looks black—at first. Turn up the light sensitivity, however, and the ugly truth emerges: Blobs of light cluster at the center of the screen like a gaggle of glow-worms. They’re the product of imperfections in the telescope’s optics that allow them to evade the first two starlight suppressors.
That’s where the flexible mirror comes in. The tiny pistons deform the shape of the mirror to precisely match the blobs, canceling their light and causing them to disappear. The light of the planets, meanwhile, enters the telescope at a slight angle compared to that of the star, allowing it to bounce off the mirror, miss the mask, and shoot through the hole in the washer. As the blobs fade, the dim planets slowly come into view. They’re ready for their close-up.
To have any hope of taking reasonably clear portraits of such distant exoplanets, some of them hundreds of light-years away, space telescopes must dampen not only their own jitters and trembles but those of the incoming light as well. This “wavefront” of light is slightly distorted by the telescope’s optics, and must be corrected, again with the help of a deformable mirror. That can reveal hidden exoplanets and make their images much sharper.
The telescopes also must exercise enormous self-control. Vibrations must be held to an absolute minimum, especially in the large multi-mirror arrays contemplated for future missions. That’s even more difficult than it sounds; the whir and grind of mechanical parts, the expansion and contraction of heating and cooling metal, even the soft but relentless buffeting of sunlight has to be counteracted and canceled out. Standard satellite technology like reaction wheels and gyroscopes, which help to point the spacecraft, also cause tiny jitters that must be dampened. Micro thrusters might be used in combination with these more common motion-control technologies to steady future spacecraft.
To sample exoplanet atmospheres for signs of life, space borne observatories will require extremely sensitive light detectors – so sensitive they can measure photons one by one as they straggle in from their long journey through space. Very few photons from the atmosphere of the exoplanet itself actually make the trip. But the ones that do could someday tell a story of plants, animals, and perhaps even of pollutants—a probable sign of a thriving alien civilization.
Light passing through the atmosphere of a planet is really a spectrum – a rainbow of colors, if you like – that can be split up and spread out by earthly instruments. The technique, known as spectroscopy, allows us to identify gases that are present in the atmosphere of a distant planet. The star’s light reflected from its planet is robbed of certain slices of its spectrum by these various gases, which absorb different wavelengths (or colors) of light.
So the missing strips in the light spectrum from the planet reveal which of these gases are present in its atmosphere: methane, carbon dioxide, maybe even oxygen.
To attain that kind of sensitivity, however, engineers must send aloft specialized ultra-low-noise detectors that have never before flown in space. Some now under development are designed to amplify the electronic signal of the faint light from a distant exoplanet, even if it’s only a trickle of solitary photons. The sudden, amplified burst overcomes the subatomic “noise” that might otherwise obscure the planet's signal.
We’re going to need a bigger telescope
When it comes to space telescopes, bigger is definitely better. Astronomers call it angular resolution, which simply means that the bigger the telescope’s mirror, the more separation between a star and its planets the telescope can resolve, making the planets easier to see. And capturing an image of an Earth-like planet, complete with continents, clouds, and oceans, is highly unlikely with ground-based telescopes. The mirage-like dance of air molecules in our atmosphere prevents earthbound observers from seeing a stable-enough image.
But launching a giant, single mirror into space is also an unlikely proposition. Instead, a network of mirror segments would be packed tightly into a rocket payload, designed to unfold into a large disk-shaped array that looks something like a honeycomb. The James Webb Telescope, to be launched as soon as 2018, uses this design.
Another option, perhaps to be considered by astronomers in decades ahead, is to launch multiple telescopes that would link up via remote communication into lockstep formation. One version involves “nulling interferometry”: the telescopes gather light from a star and use the multiple signals to cancel each other out, causing the star to wink out and leaving its hidden planets visible.
The incredible precision needed for such advances make them a daunting technological challenge. Of course, that was once said about NASA’s Kepler Space Telescope, which searches for tiny dips in the light of stars as exoplanets cross in front of them. Critics thought at first that it would never work. Now, seven years after its launch, it has revealed thousands of exoplanets and exoplanet candidates in the galaxy around us.
The coming technological innovations will open a new chapter in the search for life beyond Earth. After taking a galactic census, courtesy of Kepler, we could find ourselves staring at a galaxy full of “pale blue dots”—and closing the book on humanity’s long age of cosmic solitude.