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Fresh bid to see exo-Earths

Improved instruments and a telescope windfall could aid the search for extrasolar life.

As NASA’s Kepler spacecraft finds signs that the Galaxy is teeming with Earth-sized planets, astronomers are renewing a stalled quest: to gather light directly from an Earth twin and tease it apart for the chemical signatures of life. That is not possible with current techniques, which find planets indirectly; and in 2006, NASA cancelled a mission that might have had a chance at doing so. The Terrestrial Planet Finder (TPF) would have used either an array of small telescopes or one giant, 8-metre mirror to capture planetary light. Now a new generation of instruments is raising hopes that a smaller, cheaper space tele­scope could do the job.

The TPF cancellation was “a big blow”, says Olivier Guyon, an astronomer at the University of Arizona in Tucson who also works on Japan’s Subaru Telescope in Hawaii, “but it also felt like a challenge to see what we could do with smaller tele­scopes”. Guyon and others are hoping to meet that challenge with advanced corona­graphs: telescope devices that mask starlight like an artificial eclipse, allowing nearby planets that would otherwise be obscured by the star’s glare to be seen (see ‘Blot out the light’). NASA is exploring the idea of putting a coronagraph on one of a pair of 2.4‑metre space telescopes donated to the agency by the National Reconnaissance Office, which operates the US fleet of spy satellites (see Nature 490, 16–17; 2012). Although a telescope that size — the same as the Hubble Space Telescope — would be hard-pressed to gather light from Earth-sized planets, it could image and take chemical spectra from planets the size of Jupiter and possibly even smaller than Neptune.

The impetus for such a mission is growing. This month, at a meeting of the American Astronomical Society in Long Beach, California, astronomers estimated the number of planets in the Milky Way, extrapolated from the number and types of planet that Kepler has already found in one small part of the Galaxy. The result: at least 100 billion extra­solar planets, one for every star. Even more tantalizingly, a potentially habitable Earth-sized orb is likely to reside within 6 parsecs (20 light years) of the Solar System.

Yet NASA, strapped for funds and burdened with the US$8-billion James Webb Space Tele­scope project, is not about to resurrect the TPF. Planet hunters know that a smaller, pathfinder mission is their best chance. “This is the only show in town,” says Alan Boss, an astronomer at the Carnegie Institution for Science in Washington DC.

A NASA-appointed team of astronomers, the Astrophysics Focused Telescope Assets committee, will on 30 April deliver a report on the feasibility of using the donated tele­scopes for several types of observation, including planet imaging. Any planet hunting would have to be squeezed into a jam-packed operations schedule: astrophysicists are also eager to use the telescopes to study the mysterious phenomenon known as dark energy, which is accelerating the expansion of the Universe.

“ It felt like a challenge to see what we could do with smaller telescopes. ”

The structure of the instruments could also be a problem. The 2.4-metre telescopes contain two mirrors; the secondary one is held by six struts that cause light scattering, says Wes Traub, chief scientist for NASA’s exoplanet exploration programme at the Jet Propulsion Laboratory in Pasadena, California. “The struts couldn’t be in a worse place for imaging planets,” he says.

Even so, exoplanet astronomers are enthusiastic. Corona­graphs have improved vastly in recent years: just a decade ago, says Guyon, they were good at blotting out most of the star’s central light, but also threw away about 90% of the light reflected from the planet. One problem was caused by the sharp edges of the telescope mirror, which created a ring-like diffraction pattern in the region of the image where the planet resides.

Guyon’s coronagraph uses two asymmetrically polished mirrors to reshape the distribution of light gathered by the telescope’s main mirror and eliminate the diffraction pattern. The design, which in 2012 won him a ‘genius’ grant from the MacArthur Foundation, allows the starlight to be thrown away without sacrificing light from the planet. Guyon says that his device could enable a 4-metre tele­scope, half the diameter of the TPF, to see an Earth twin — but whether it could enable a 2.4-metre tele­scope to find Earths remains to be seen. “We’re still in the grey zone,” he says.

Dust adds to the uncertainty. Asteroid collisions have filled the inner Solar System with a haze of dust that reflects sunlight, creating what is known as zodiacal light. The worst nightmare of planet hunters is the possibility that the dust in other solar systems could generate so much ‘exo­zodiacal’ light that Earth-sized orbs would be lost within it. “This is the main non-technological risk for an exo-Earth imaging mission,” says Ruslan Belikov, an astrophysicist at the NASA Ames Research Center in Moffett Field, California. Measuring the levels of exo­zodiacal light around Sun-like stars is a key goal for a pathfinder mission, he adds.

Even if NASA decides against putting a corona­graph on one of the donated tele­scopes, says Guyon, he is not about to give up. Finding out if there are other habitable planets in the Galaxy, he says, “is the most interesting question in all of science”.

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Small stars host droves of life-friendly worlds 2013-Jan-09

An image of an exoplanet separated by two diffraction beamwidths from a star 2010-Apr-15

A laboratory demonstration of the capability to image an Earth-like extrasolar planet 2007-Apr-12

A theoretical look at the direct detection of giant planets outside the Solar System 2005-Jan-20

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NASA’s Exoplanet Exploraton Program

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Cowen, R. Fresh bid to see exo-Earths. Nature 493, 464–465 (2013). https://doi.org/10.1038/493464a

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