“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.
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.