Space starshade to better see exoplanets
A flower-like starshade could give astronomers a direct look at planets orbiting distant stars. Cathal O’Connell explains how it would work.
Forty billion. That’s the number of habitable, Earth-like planets in our galaxy. But do any of them host life? NASA has a far-out 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 space-based astronomy. Back in the 1960s, he envisioned a simple disc to block the starlight. But the idea was shelved when researchers 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 Colorado pioneered the sunflower design using mathematical models to figure out the optimum shape. They tested it in the Nevada desert at night by setting up a powerful light as the “star” and a feeble LED, a billion times less bright, as the planet. A telescope two kilometres away was initially blinded by the “star”, but when the starshade was positioned in front, the “planet” was revealed.
The team then equipped the McMath Solar Observatory in Arizona with their starshade and pointed the telescope at the star Vega. The starshade blocked Vega’s glare so effectively that several stars previously invisible to the telescope popped into view.
These early successes got the attention of NASA, which put its Jet Propulsion Laboratory on the case – figuring out how to make the starshade’s delicate petals, then fold it up inside a rocket and unfurl it in space. It’s a challenge, Horner admits – but not an insurmountable one.
The starshade is still in its early stages of development, but it could see a mission sooner rather than later. The Wide-Field Infrared Survey Telescope (WFIRST), scheduled to launch in 2025, could be equipped with one if NASA can find the extra billion dollars it would cost. WFIRST would be a perfect partner for a starshade because the telescope will be highly manoeuvrable in its stable solar orbit. After examining one planetary system, starshade and WFIRST could reorient towards another system and continue the search for life.
And with up to 40 billion chances, who’d bet against them?
StarshadeThe starshade blocks the dazzling light from an exoplanet’s star. The shade’s “hypergaussian” petal arrangement causes waves of starlight spilling around the starshade to cancel each other out, making the exoplanet easier to spot.
Unfurling mechanismWhen launched from Earth, the starshade structure is folded. To unfurl, thin poles push the petals into position. Starshade blooms 50 metres tip-to-tip and must deploy within 0.1 millimetre precision, approximately the width of a human hair.
Alignment50,000 kilometres away from the telescope, the starshade must sit exactly along the telescope’s line of sight towards a distant star, with just a two-metre margin for error.
Space telescopeTo spot a second Earth, the telescope’s optical power would have to at least match that of the Hubble Space Telescope.
See the starshield unfurl here.