First rock from the sun

THREE centuries have passed since the polymath Sir Christopher Wren predicted that “a time will come when men will stretch out their eyes—they should see planets like our Earth.” By most astronomers' accounts, that time is just about nigh. Indeed, detecting big planets orbiting other stars is no longer tricky—nearly 450 such exoplanets have been catalogued. Smaller, rocky planets orbiting at a comfortable distance from their stars—as the Earth does—remain more elusive.

Most exoplanets have been discovered by inferring their presence from the rhythmic wobble their gravity imparts on their home star—like a waltz between two dancers of markedly different weights. The problem is that this method favours the discovery of large planets close to their stars. As a result, the catalogue of planets is filled with “hot Jupiters”, huge bodies baking brightly in the light of their sun.

To find places that might support life it is necessary to look for planets a little farther away from their stars, but not so far away that they are frozen lumps of ice and rock, like the dwarf planet Pluto. But it isn't easy. Spotting the light from a tiny planet across astronomical distances is akin to discerning a red-hot pinhead right next to a floodlight from many kilometres away.

Astronomers have partially solved the problem of looking at objects near to a star's bright glare by inventing the coronagraph, which focuses all the incoming light around a dark spot in the telescope's innards, neatly clipping off most of the central starlight and passing on much of the planetary glow. For exoplanet hunting that is not quite good enough. Some of the starlight still gets through, easily obscuring planets that are millions of times fainter than their parent stars.

A novel approach plays with the peaks and troughs of the light waves to do the job more effectively. This week, in Nature, Eugene Serabyn of the Jet Propulsion Laboratory in California and his colleagues describe a stunning implementation of what is known as an optical vortex coronagraph. In place of a dark spot, this uses a disk of glassy material, etched with a carefully designed pattern which changes the phase of the incoming light, in effect twisting it back onto itself and creating a dark hole in the centre of the image. This blots out the starlight more effectively, making it easier to see any nearby planets.

In order for this technique to work, the distortions imposed on the incoming light during its passage through the Earth's atmosphere must be removed using a trick called “wavefront correction”. The researchers did this using a small part of the giant Hale telescope in California, with which they examined a star called HR 8799 in the constellation of Pegasus. When images showing three planets orbiting this star were obtained in 2008, it was the first time exoplanets had been directly observed. The researchers could also see the planets—and their telescope was in effect five times smaller than the telescopes used in 2008.

With a two-metre telescope—small by modern astronomical standards—Dr Serabyn and his team say they could spot a planet 33 light-years away orbiting its host star at a similar distance to that at which the Earth orbits the Sun. Such a planet would fall into the so-called “Goldilocks zone” (neither too hot nor too cold) where the interesting chemistry should happen. As well as bringing planet-hunting within the reach of many smallish telescopes on Earth, the new technique also means that smaller, cheaper, or perhaps more numerous space telescopes—entirely free of deleterious atmospheric effects—could be employed to stretch out the eyes of men and finally fulfil Wren's prediction.