ANTOINE LABEYRIE has a dream. He wants to photograph tropical forests, mountain ranges, oceans and deserts, but the Amazon, the Himalayas, the Pacific and the Sahara leave him cold. Instead, he has his sights firmly set on the clouds and continents of worlds many light years away from Earth. Sitting on Labeyrie’s drawing board are plans for a hypertelescope, a new breed of space telescope that is capable of mapping distant cousins of Earth in exquisite detail.
Measuring hundreds of kilometres across, these hypertelescopes will be large enough to resolve a green patch the size of the Amazon basin on an alien world 10 light years away. Such images could tell us about the sway of the seasons – clinching evidence for life on planets outside our solar system.
It is a remarkable prospect. Twenty years ago, nobody even knew about the existence of “exoplanets”. The first one, orbiting a sun-like star known as 51 Pegasi, wasn’t discovered until 1995. It bears little resemblance to Earth: the planet (nicknamed Bellerophon by its Swiss discoverers, Didier Queloz and Michel Mayor of the Geneva Observatory) is a bloated ball of hot gas whirling around its parent star every four days. Since then, over 160 massive exoplanets have been found in more than 100 planetary systems.
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Even though none of them look like our own solar system, most astronomers are convinced that Earth-like planets must be fairly common – just incredibly hard to find using today’s techniques. The smallest found so far appears to weigh 5.5 times as much as Earth (Â鶹´«Ă˝, 28 January, p 12), but now two missions are aiming to change that. Later this year, the French space agency CNES hopes to launch a spacecraft called Corot. It will simultaneously monitor 12,000 stars, looking for minute dips in a star’s brightness caused by a planet passing in front of it. And NASA aims to go one better in June 2008 with the launch of Kepler, a 95-centimetre space telescope that will monitor 100,000 stars at once. Both missions could discover several hundred planets as small as our own.
Even so, planet hunters agree they need to do more than simply count up the number of exoplanets, and so they have ambitious plans: both NASA and the European Space Agency (ESA) are planning multibillion-dollar missions to image the exoplanets as distant specks of light and study the chemical make-up of the planets’ atmospheres for possible signs of life. However, the two missions, the Terrestrial Planet Finder and Darwin, are unlikely to get off the ground before the end of the next decade, if at all: this month NASA announced that it has delayed its mission indefinitely. Even if the missions go ahead, they will be unable to fulfil planet hunters’ ultimate dream of observing weather and vegetation patterns.
Little wonder then that inventive scientists like Labeyrie are thinking up alternatives that might deliver results much earlier. “It could happen in 2011,” says Webster Cash, who also works on futuristic telescope designs at the University of Colorado in Boulder.
Imaging exoplanets will be no easy task, though. Even from the outer parts of our own solar system, Earth is little more than a pale blue dot. So how can we ever hope to resolve details on a planet orbiting another star? In astronomy, size matters: the bigger the telescope, the more you can see. As well as collecting more light, a bigger telescope will also give you better resolution. Trouble is, to spot a 1000-kilometre chunk of forest on a planet 30 light years away, you would need a telescope with a mirror 150 kilometres wide, which is impossible to build. Across the world, there are only about a dozen telescopes with mirrors measuring a mere 8 to 10 metres across, and even though astronomers are planning much larger telescopes of between 30 and 100 metres in diameter, this is still a fraction of what is needed to map exoplanets.
Instead Labeyrie, who is an astronomer at the Observatory of Haute-Provence in southern France, is working on a concept known as diluted optics, where the light from many small telescopes is pooled to give the same results as a much larger telescope.
Extreme optics
To see how this might work, think of the Keck telescope on the summit of Mauna Kea in Hawaii. At 10 metres across, Keck’s mirror is among the largest in the world. Yet it is not a single mirror; instead it is made up of 36 hexagonal segments. If 30 of the segments were removed at random, the sensitivity of Keck would drop. This is because the remaining mirrors would collect much less light, so it would take far longer to image faint objects. But it would still be able to pick out fine detail because its resolution depends on the widest separation between two segments. So Keck would still have the same resolution as a 10-metre telescope, as long as two segments at opposite edges of the mirror remain intact.
Labeyrie’s design for a hypertelescope takes dilute optics to the extreme. Ultimately his Exo-Earth Imager will consist of at least 150 mirror elements, each measuring 3 metres across, and spread out over an area of about 8000 square kilometres. Together, they would fly in formation around the sun to make a hypertelescope with a diameter of 100 kilometres – large enough to pick out clouds and continents on a distant relative of our home planet.
Each of the mirrors will be carefully positioned so that it reflects its light onto a common focal point, where a camera will collect the images and relay them back to Earth. In effect it will act as one giant mirror, albeit one with a small light-collecting area. Interferometry expert Andreas Quirrenbach of Leiden Observatory in the Netherlands calls it “a very clever solution”, but Labeyrie is kicking himself for not coming up with the concept until 1996. “I should have thought about it 20 years earlier,” he says.
Labeyrie is currently working out the details of the Exo-Earth Imager, and is building prototypes of the various parts for a forerunner of the final design. He has already obtained results from a much smaller version at the Observatory of Haute-Provence. The device comprises three small mirrors spaced a couple of metres apart on the ground and pointing at the sky. Light from the mirrors reflects onto a detector suspended from a tethered balloon where it combines to form a single image. Over the past year, Labeyrie’s set-up has captured its first high-resolution pictures of stars, which show them as extended discs rather than just points of light.
Formation flying
It is a promising start and, crucially, this performance comes with a modest price tag of less than ¬20,000 – something that has caught the eye of the French Space Agency, CNES. The agency has started an in-house study of the concept, and within a few months Labeyrie expects it to decide whether to fund a demonstration flight to test the technology in space. If the project gets the go-ahead, a miniature version of the hypertelescope could be off the ground in a few years.
Before then, a much larger hypertelescope could be deployed in craters at the Plateau de Calern in France, says Labeyrie, or maybe even in the Taburiente caldera on La Palma in the Canary Islands. Dozens of small mirrors placed inside these craters would mimic a giant curved mirror. If all goes well, Labeyrie has plans for a precursor mission called Luciola. It would consist of up to two dozen lightweight mirrors, each about 20 centimetres across and spread out over an area of about 1 square kilometre. A satellite flying at the mirrors’ focal point would carry a CCD camera that would beam images back to Earth. According to Labeyrie, “it could find an exo-Earth out to a distance of 10 light years or so, and take direct images.”
Labeyrie is not the only person with a yen to image exoplanets directly. Cash recently received NASA funding to continue developing the design of a space telescope called the New Worlds Observer (NWO). Building on earlier ideas by engineer Gordon Woodcock of Boeing and astronomer Glenn Starkman at Case Western Reserve University in Cleveland, Ohio, NWO will use a giant dark sheet to block the light from an exoplanet’s star before it reaches the telescope. Getting rid of a star’s glare is essential given that a typical exo-Earth is 10 billion times fainter than its parent star. “It’s a bit like putting your hand in front of the sun, so you can see something next to it,” says Cash.
According to Cash’s design, the NWO “star shade” will be 50 metres across and float some 50,000 kilometres in front of a large space telescope – a distance about four times the diameter of the Earth. If aligned precisely enough, it will just cover the very inner regions of a planetary system some 30 light years away. That means it will hide any exoplanets orbiting close to their parent star, but the most interesting planets, in Earth-like orbits, would still be visible. Cash believes the main challenge will be to control the position of the star shade relative to the telescope for long enough to study one planetary system in detail. “You have to hold the star shade aligned for about one week,” he says.
Formation flying in orbit around the sun is not the only issue that Cash and his colleagues have to deal with. No matter how good the star shade is at blocking light, it can never get rid of a star’s glare completely – there will always be some starlight seeping round the edge of the shade and into the telescope, like ocean waves bending round a promontory. This is known as diffraction, and it threatens to drown out the faint light from a small exoplanet.
However, last year Cash and his colleague Bob Vanderbey of Princeton University found a way to reduce diffraction effects. By looking at the mathematical equations that describe diffraction around objects, they discovered that star shade in the shape of a flower with a few dozen petals worked best. It turns out that this weird shape cancels most of the diffraction effects. “Last April, we finally found the mathematical solution that would work,” says Cash. “You have to closely look at the equations to understand it – it’s not intuitive at all.”
Right now, the New Worlds Observer team is trying to demonstrate the petal technology in the lab, as part of a two-year feasibility study sponsored by a $400,000 grant from NASA’s Institute for Advanced Concepts.
There is still a long way to go before we can swap photographs of alien continents and cloud patterns, however. Both Cash’s New Worlds Observer and Labeyrie’s Luciola are too small to resolve any features on extrasolar planets, but their successors should be up to the task. Cash is spending part of the money he received from NASA on a preliminary design for a follow-up mission called the New Worlds Imager that would actually be able to see details on exo-Earths.
“The New Worlds Imager is basically two New Worlds Observers, flying 1500 kilometres apart and looking at the same star,” says Cash. It would use star shades measuring 100 metres across and mirrors as large as the 10-metre Keck telescope to observe the faint glow from exoplanets. By combining their light, the two observatories could slowly build up a high-resolution image of an exo-Earth.
Not everyone shares Cash’s enthusiasm. “There’s nothing wrong with the principle,” says Quirrenbach, but he worries that the star shades could cause difficulties. “It seems to be quite complicated to reorient the whole thing if you want to observe another star.”
If it is already a problem to reorient the two star shades and telescopes of Cash’s New Worlds Imager, how do you go about slewing a 100-kilometre hypertelescope made of 150 individual mirrors? Labeyrie believes the answer is simple: you just launch hundreds more mirrors and create a spherical diluted hypertelescope 400 kilometres across. A smaller flotilla of satellites moving around within the sphere would image the focused light. This way you create a telescope that looks in every direction simultaneously.
Cash has reservations about Labeyrie’s Exo-Earth Imager, which would be a dramatically scaled-up version of Luciola. “It’s an absolutely fascinating idea, and in principle it must work,” he says, “but I don’t think Labeyrie is on the verge of actually doing it because of the daunting technological hurdles, and that’s a shame because I really want to see it fly.”
One challenge is the mass production of ultra-lightweight mirrors; another is precisely measuring the relative positions of all the mirrors and controlling the overall geometry. Even so, Labeyrie remains undeterred. “It’s of course a very complex and costly machine,” he says, “but I guess it will be done sooner or later.”
Malcolm Fridlund, project scientist for ESA’s Darwin mission in Noordwijk, the Netherlands, is pragmatic. “The costs would be really prohibitive,” he points out. Already, NASA’s Terrestrial Planet Finder has been put on hold indefinitely because of budget problems, and ESA is considering a less ambitious version of its proposed exoplanet observatory, called Darwin-light. “But if we ever find a nearby Earth-like planet that might harbour life, I personally don’t think the problems will be so big any longer,” adds Fridlund. “The whole world will understand the scientific questions we want to answer.”
“The case is so compelling that sooner or later it will happen. I expect to see a resolved image of an exo-Earth in my lifetime”
Quirrenbach agrees. “The case is so compelling,” he says, “that sooner or later it will happen. I hope I will still be alive when it does.” Fridlund is confident about it. “I’m only 53,” he says. “So if I stop smoking and lose a few kilograms more, I expect to see a resolved image of an exo-Earth in my lifetime.”
