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Astronomers have a photon problem. They need more of these light particles from the right places, like distant stars and exoplanets, and less of them from light pollution and artificial satellites. To do this, they need bigger telescopes 鈥� and now, an astronomer has suggested turning the entire Earth into a kind of lens that could solve this, and potentially help us find the signatures of life on exoplanets.

The 鈥渢errascope鈥� would take advantage of the way light deflects as it passes through Earth鈥檚 atmosphere. 鈥淭his deflection means that distant light sources behind the Earth will have their rays converge towards a focus point, and that鈥檚 where we place our detector,鈥� says David Kipping at Columbia University in New York.

Rays of light traveling at higher altitudes deflect less, so they would meet further away from Earth. All the different meeting points create a focal line, along which detectors can be placed.

The closest point possible would be at about 325,000 kilometres away, or about 85 per cent of the distance between Earth and the moon. 鈥淏ut a better choice is at the Earth鈥檚 Hill sphere,鈥� says Kipping. The Hill sphere is the region where Earth鈥檚 gravity is the dominant force, extending to around 1.5 million kilometres away, one hundredth of the distance to the sun.

He says the light rays that would meet there travel no closer than 14 kilometres to Earth鈥檚 surface, avoiding buildings, mountains, clouds and most of the degrading effects of the lower atmosphere. Kipping estimates a detector 1 metre in diameter placed there would be the equivalent of a 150-metre class telescope on Earth. That is more than 14 times bigger than the biggest telescope on Earth today.

Putting telescopes into space is expensive 鈥� the 6.5-metre James Webb Space Telescope has cost $10 billion to build and still hasn鈥檛 launched 鈥� but Kipping says the terrascope would cost a fraction of that.

鈥淚t鈥檚 exactly the sort of mad, off-the-beaten-track sort of science I like to read,鈥� says Benjamin Pope at New York University. It would be tricky to put in place, he says, and has a lot of challenges.

The terrascope would need a coronagraph to block out light from Earth itself. 鈥淪pace coronagraphs are expensive and fiddly devices,鈥� says Pope. He says getting out past the moon鈥檚 orbit, and the short observation windows while objects are passing behind Earth might be issues, too.

Erika Hamden at the University of Arizona agrees that it is a bold idea but can think of a few practical issues. As a specialist in UV astronomy, Hamden says the telescope would work in the infrared part of the light spectrum, and potentially detect some visible light, but not UV, because of the light from the sun.

鈥淚n practice there are a lot of challenges but it鈥檚 definitely a fun exercise,鈥� she says. 鈥淭hese wild ideas can end up being the seeds to really exciting projects in the future.鈥�

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