Exoplanets: New missions hunting for alien worlds
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E-ELT’s mirror is three times the diameter of Keck 1 and Keck 2 in Hawaii, so the hope it will spot Earth-like planets up to 20 light years away. (Copyright: Science Photo Library)
Efforts like Kepler to find Earth-like planets outside our galaxy may have ended recently, but astronomers say a wave of bigger and better missions is on its way.

As mountains go, Cerro Armazones may not be much to look at. Standing 3,000 metres (9,800 feet) tall, it is a shapeless reddish dust heap in Chile’s hot and arid Atacama Desert. The only sign of life is a dirt road zig-zagging all the way to the top.

But for astronomers like Joe Liske, this is arguably the world’s most interesting mountain right now, and not just because in the next few months more than 100 tonnes of dynamite will blow off its top to create a flat platform. By the early 2020s, that platform will become home to the biggest-ever eye on the sky, the E-ELT, or European Extremely Large Telescope.

With a mirror that is 39 metres (128ft) in diameter, the E-ELT will dwarf all existing optical telescopes – and those planned to appear in the next decade or two. And it won’t just bring plenty of life to this corner of the Atacama. The hope is that it will also help spot life out in the vastness of space.

In the past decade alone, astronomers have been discovering planets outside our solar system, or exoplanets, with astonishing speed. We now have identified nearly a thousand. Most are much bigger than Earth and almost certainly Jupiter-like gas giants, making making them quite unlikely for hosting life. None has so far been confirmed to bear life – even single-cell organisms – but some of these planets seem to be distinctly rocky and Earth-like: Kepler-62e, Gliese-581g and Kepler-22b, to name but a few.

“The quest for Earth-like exoplanets, and ultimately life on such planets, is one of the great frontiers of science, perhaps the last big piece in the puzzle of how we, humans, fit into the big picture,” says Liske, who works at the European Southern Observatory, an organisation that already operates a number of telescopes in the Chilean desert.

Two of the more high-profile planet hunters have hit rocky ground in recent months, however. The French-led Corot spacecraft, launched in 2006, greatly outlived its original two-and-a-half year mission of spotting terrestrial-sized exoplanets, but in November 2012, it suffered a computer malfunction, which made it impossible to send any data back to Earth. In June 2013, the French Space Agency announced it would switch the satellite off and let it burn up as it re-enters the atmosphere – the usual fate of our mechanical helpers in space.

Nasa’s $600 million space observatory Kepler, launched in 2009, has also been crippled recently. Kepler has helped spot thousands of potential exoplanets – over 130 of which have been confirmed – but two of its four reaction wheels that control the telescope’s direction have failed in recent months, and at least three are necessary to point it in the right direction. Kepler completed its primary mission in November 2012, and there are still thousands of candidates to sift through, but this month Nasa conceded that Kepler will no longer be able to search for exoplanets.

Thankfully, more planet-hunting missions are on the way, which will continue and even extend upon their legacy. Missions like the E-ELT, which Liske says “will completely revolutionise the exoplanet field.”

Size matters

To be considered a “habitable” world, a planet has to be similar to Earth in size, rocky, and located in the so-called Goldilocks zone – an area of space around a parent star that is not too cold or too hot, but just right to support liquid water. Since these planets are expected to be small and faint compared to their sun, spotting them is tricky with existing ground-based optical telescopes.

“It’s like trying to see the light from a feeble little LED 10cm away from a stadium floodlight, from a distance of 200km,” says Liske.

To tell a parent star and a potentially habitable planet apart, astronomers need incredibly sharp, high-resolution pictures. The bigger the mirror, the sharper the image a telescope can capture, and the dimmer the objects it can detect. Hence it takes an extremely large telescope to try to spot any planets that may support alien life many light years away.

Currently, the world’s largest ground-based optical telescope is Gran Telescopio Canarias in the Canary Islands, Spain, with a mirror of 10.4m (34.1ft) in diameter. Then there are Keck 1 and Keck 2 in Hawaii, each sporting a 10m (32.8ft) mirror. “With the E-ELT, we believe that we will be able to directly see exoplanets similar to Earth out to a distance of about 20 light years,” says Liske.

The closest potentially habitable planet is about seven light years away, according to the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. So if aliens there are as eager to spot us as we are them, by the time the E-ELT opens in the early 2020s, they would be receiving light from Earth from today. To them, Armazones would still look intact.

Mirror image

While astronomers wait for the E-ELT to be constructed, other telescopes are busy scanning the skies for far-away worlds. About an hour’s dusty and boulder-strewn drive away from Armazones is another telescope, whose four huge towers resemble some kind of factory rather than a scientific facility. It is the VLT – or Very Large Telescope, composed of four individual 8.2m (26.9ft) mirrors – based at Cerro Paranal, another mountain in the Atacama shorn of its top.

The VLT’s mirrors may not hold any size records but in 2004, it spotted the first exoplanet to be observed directly, 2M1207b, approximately 170 light-years from Earth in the constellation Centaurus. Since then, the VLT has discovered several worlds outside the Solar System, with the help of an instrument called NACO.

NACO is part of the VLT’s adaptive optics (AO) system, and the Keck Observatory in Hawaii uses a similar system. The technology corrects for the blurriness caused by turbulence in the Earth’s atmosphere that makes stars twinkle but gives astronomers headaches. NACO cancels out the turbulence, producing images as sharp as if snapped in space.

But even with these adaptive optics tools, existing ground-based telescopes can only “see” planets bigger than Jupiter – gas giants that orbit their parent stars at a huge distance. The next-generation of AO instruments, Sphere for the VLT and Gemini Planet Imager (GPI) for the Gemini Telescope in Chile “will blow NACO and Keck AO away”, according to Bruce Macintosh, an astronomer at Lawrence Livermore National Laboratory. But while GPI and Sphere will make it possible to spot exoplanets of similar size to Jupiter – even these will still be too big to be considered habitable.

That’s why astronomers put so much hope in the coming generation of optical giants. Besides the E-ELT, two other observatories have recently been given the go-ahead: the Giant Magellan Telescope (GMT) in Las Campanas Observatory in Chile, and the Thirty Meter Telescope (TMT) on Mauna Kea in Hawaii.

The GMT will have seven 8.4m (27.6ft) mirrors, arranged like flower petals. Together they will make up a primary mirror 24.5m (80.4ft) in diameter. The TMT will have a segmented 30m (98.4ft) mirror consisting of 492 small individual hexagonal mirrors, each 1.4m (4.6ft) across. E-ELT, GMT and TMT will all be equipped with tools for exoplanet search, and are expected to be able to peer so deep into the universe so that they could take direct images of relatively small Earth-like worlds some 20-or-so light-years away.

Defying gravity

But they won’t be the only ones on the hunt. Telescopes can produce much clearer images in space, free from our turbulent atmosphere. The Hubble Space Telescope has been circling Earth since 1990 and has spotted a few planets – and also helped determine what some extra-solar worlds are probably made of. But Hubble’s 2.4m (7.9ft) mirror is too small to see planets smaller than Jupiter, says Matt Mountain, the director of the Space Telescope Science Institute at Nasa. Hubble’s planned successor, the James Webb Space Telescope (JWST) – set to go into orbit around 2018 – is expected to do much more.

Equipped with a 6.5-metre (21ft) diameter mirror, JWST will have a variety of tasks, among them the search for planets orbiting nearby stars. It will work in the infrared spectrum, which will allow it to “probe down to smaller planetary sizes than Hubble, to roughly two-to-three times larger than Earth, more Neptune-scale planets,” says Mountain.

The telescope will aim to find out whether these extra-solar worlds are so-called “super-Earths” – rocky planets that could potentially be habitable – or miniature versions of Neptune, unable to support life. Using an instrument called a coronagraph, JWST will try to determine whether a planet has an atmosphere, and – for the first time – analyse it by examining the spectrum of the light coming from the planet.

Elements and molecules in an atmosphere, such as water and oxygen, have specific signatures in the spectrum, giving us an idea about its composition and the likelihood that there is liquid water present – and hence life, says Avi Loeb, Chair of the Astronomy department at Harvard University.

Spotting water and oxygen would be just the start, though, says Liske. “If you found things like water, oxygen, CO, methane, in the right amounts, that would be a strong hint [of life]. Then of course there would be lots of investigations into whether such a combination of elements could be produced by non-biological processes,” he says. “If we are ever in the situation where we've convinced ourselves that we've found life, then the question would be ‘what kind of life’? If we're lucky we may be able to say something about the exoplanet's surface: is it all water? Any signs of vegetation? That sort of thing could come in principle from either ground or space-based telescopes.”

Sic transit gloria

As exciting as the plans for direct imaging are, most exoplanets are still found using indirect techniques – such as detecting a wobble in the position of the star that indicates it is being pulled slightly towards an orbiting planet, or a method called “transiting”, in which planets are identified by the tiny dip in brightness caused when they pass in front of its star. Transiting was used to great success by Kepler, but the method doesn’t allow us to calculate a planet’s mass – a critical factor in determining its density and hence its rockiness. To determine the mass, another instrument enters the stage – a spectrograph.

One such device, the High Accuracy Radial velocity Planet Searcher (Harps), is mounted on a 3.6m- (11.8ft-) mirror telescope at La Silla, another one of ESO’s observatories in the Atacama. It studies space bodies by recording how a planet’s gravity makes its parent star appear to vibrate as it rotates around it. It can use that vibration to detect new planets, but has also been used to learn more about known exoplanets. It determined that an exoplanet discovered by Corot in September 2009 some 500 light-years away and dubbed Corot-7b had a rocky surface and a mass only five times of our Earth. This would make it a likely habitable candidate if it wasn’t for its proximity to its parent star – the planet lies only 2.5 million km (1.6 million miles) away from it, which is just 1/23rd of the distance from the Sun to Mercury.

“By far the most powerful combination right now is the combination of transit detections and Doppler spectroscopy, which gives you both the mass and radius of the planet, so you can start to say what it's made of,” says Macintosh. “Unfortunately, this combination has been hard to do for many of the most interesting cases, because the odds that any single planet discovered by Doppler will also transit are very low, and the huge numbers of transit planets discovered by Kepler are all orbiting relatively faint stars – because Kepler was designed to look at faint stars.” The worlds discovered by Kepler are also thousands of light-years away – totally inaccessible to direct imaging, so it is impossible to determine whether or not any of them host life.

But one more player is set to emerge soon. In April this year, Nasa gave the go-ahead to another space-based mission that will work similar to Kepler, but can discern planets around bright nearby stars: Tess, the Transiting Exoplanet Survey Satellite.

Due to launch in 2017, when Tess finds a potentially habitable planet, just as with Kepler, other tools will come into play to find out more about them, to determine whether they are super-earths or micro-Neptunes. “The big ground-based telescopes can work with it and do the spectroscopy,” says Macintosh. “And if Tess [finds] a habitable planet around a very nearby star, you can then use the James Webb Space Telescope to measure the composition of its atmosphere.”

But to really find a habitable Earth-twin orbiting a star just like our Sun, we will have to go to space, he adds – with a giant telescope fully designed for planet hunting and equipped with a mirror of eight metres or more. That won’t happen any time soon – but Nasa is already thinking about a future JWST replacement, called the Advanced Technology Large Aperture Space Telescope (ATLAST), a space telescope with a 8-to-16m mirror that would be 2,000 times more light-sensitive than Hubble. If all goes according to plan, ATLAST could be launched between 2025 and 2035.

The fact that there have been few good candidates for planets that host life found so far should not discourage the searchers. The data already collected suggests that there are about 100 billion planetary systems in our galaxy alone, says Mountain. So there are plenty of places left to look, as and when we find the tools for the task.

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