Shading stars to see exoplanets

A flower-like starshade could give astronomers a direct look at planets orbiting distant stars. By CATHAL O’CONNELL.

Forty billion. That’s the number of habitable, Earth-like planets in our galaxy. But do any of them host life? NASA has a farout plan to find out: put a giant sunflower light-shield in space.

The 40 billion number is based on the thousands of exoplanets discovered so far by planet-hunters such as the Kepler Space telescope, which watches for the periodic dimming of stars as a planet passes in front of them. Researchers use the dimming effect to calculate the planet’s size and orbital distance – but the technique gives no inkling as to what the planet is made of.

If only we could see one of these potentially second Earths directly. By analysing the light glinting off the planet, we’d be able to tell if it had liquid water. As the planet spun on its axis, the light would change, offering clues about how much of the surface was covered in ocean. We could look for oxygen in the planet’s atmosphere, clouds in its sky, and possibly detect other molecules associated with life.

The problem is, we can’t see these planets, as our telescopes are blinded by the parent star, which typically blazes 10 billion times brighter than the dim light reflected by Earth-like planets.

The starshade is a sunflower-shaped disc, half the size of a soccer field, designed to block that dazzling starlight. Flying in tandem with a space telescope, the starshade would use its own thrusters to position itself 50,000 kilometres in front of the telescope, where its 50-metre diameter disc would cover the target star, while leaving any planets visible.

We’re not talking close-up pictures here, explains Jonti Horner, an astrobiologist at the University of Southern Queensland. “The planets would still just be a single pixel, a tiny speck of light in the inky blackness of space. But if we could see that speck, then we could begin to analyse the light from it,” he says.

The starshade idea harks back to Lyman Spitzer, the father of spacebased astronomy. Back in the 1960s, he envisioned a simple disc to block the starlight. But the idea was shelved when reseachers hit the diffraction problem.

Any kind of waves, including sound, water and light waves, tend to diffract around an obstacle, their paths bending to follow its contours. That’s why you can hear someone speaking from around a corner. And it’s why a disc-shaped starshade could never work. Too much of the parent star’s light would diffract around its edges, obscuring any planets within a bright halo.

The sunflower design solves this problem by directing the diffracted starlight onto special paths that cause neighbouring waves to overlap and cancel each other out, an effect known as destructive interference. So the brightness of the star could be reduced by 10 billion times – enough to reveal planets as close to their stars as Venus is to our Sun.

Astronomers at the University of