Since the first confirmed discovery of planets beyond our solar system in 1993, astronomers have discovered 4,360 extrasolar planets — aka exoplanets — orbiting in 3,223 planetary systems, per the Extrasolar Planets Encyclopedia.
Strangely enough, though, as the Planetary Society notes, we have only actually seen a relative handful of these distant worlds. At latest count, 51 exoplanet discoveries have been made by direct detection — scarcely more than 1% of all discoveries, according to NASA. All the rest have been detected by a variety of indirect means, primarily by measuring the slight dimming of stars as their planets pass in front of them or the tiny wobble effect of the planet’s gravitation on its parent star.
The indirect nature of those 99% of exoplanet detections is no reason to doubt them. In scientific investigation, circumstantial evidence can be as powerful as direct detection. In many cases, the careful amassing of circumstantial evidence has not only confirmed that a planet is out there but also provided insights about its characteristics — and even its weather.
All the same, astronomers want to capture direct images of these distant worlds, even if the image is only a speck of light. It isn’t just the satisfaction of seeing a world orbiting another sun: Direct observation will allow us to make measurements that can be made reliably in no other way.
A Firefly on the Side of a Searchlight
Compared to stars, planets are small and dim. Detecting one from light-years away pushes the limits of even the most powerful telescope. But the real problem is that planets orbit stars. And trying to image a planet orbiting near a star is like trying to snap a firefly sitting on the side of a searchlight.
If you were trying to see something dim that’s positioned right next to something bright, your first reaction might be to hold something up to shade your eyes from the glare of the bright light, allowing you to concentrate on the dim object.
And this is exactly what astronomers do. Conveniently, Mother Nature has provided them with an ideal example of just this arrangement. The moon, as seen from Earth, appears to be almost exactly the same size as the sun. In reality, though, the sun is some 400 times bigger (but also 400 times farther away).
As a result, during a total solar eclipse, the moon just neatly blocks out the fiercely bright solar disk, while allowing us a perfect view of the breathtaking — but much fainter — solar corona that surrounds it.
Conjuring an Eclipse
For astronomers, the only limitation of this natural sunshade is that they can’t order up a total solar eclipse when they want one. So, they have built devices called coronagraphs, which amount to artificial eclipse generators. They block the portion of the image plane where bright sunlight would hit so that the rest of the image won’t be overexposed and can register the faint light of the corona.
The same principle can also be used to observe faint objects located near bright stars. This is the technique astronomers have used to image the few exoplanets that we have observed directly.
In addition, the exoplanets we have directly observed so far are all very young and hot, in spite of their great distance from their parent stars. Thus, they are much brighter than if we only saw them by reflected starlight.
To observe Earth-distance planets around numerous sun-like stars, we need a bit more help, and that’s where Starshade technology comes into play.
The concept, as with many space mission concepts, is brilliant in its simplicity: Position a spacecraft with the Starshade — a screen in the shape of flower petals — so as to eclipse a distant star, as seen through a telescope aboard another spacecraft.
But, as usual in space exploration, the devil is in the details. According to Tiffany Glassman, Ph.D., Northrop Grumman project engineer, “controlling the shape of the Starshade edges and positioning it relative to the telescope are the hardest technologies” for the mission designers.
As Glassman explains, “The edges have to be controlled very precisely because the shape is used to cancel out the light from the star. An Earth-like planet would be about 10 billion times fainter than its star, so the Starshade has to block nearly all the starlight in order to see any planets orbiting it.”
Getting the shade to where it does its work presents the development team with yet another obstacle, Glassman adds. It has to be “a structure that’s deployable, so that it fits into the rocket, and is as light as possible so that it doesn’t take too much fuel to move it around the sky.”
Fuel (or propellant, to be strictly technical) always looms large in space mission planning, because getting into space requires so much of it — not to mention that the larger and heavier the payload, the larger and more expensive the rocket needed to carry it into space.
In the case of the Starshade, that meant designing it to fold neatly into the confines of a rocket’s nose cone, then expand neatly — with no jams or hang-ups — to provide the needed deep shadow.
Finally, it has to be positioned with a surreal level of precision. As Glassman states, “Once it’s deployed to the right shape, it has to be positioned tens of thousands of kilometers from the telescope so that it blocks the star but doesn’t block the planet. This requires an active control system where the Starshade or the telescope can sense the relative alignment between the two spacecraft and then adjust their position to less than one meter accuracy.”
To Boldly Go
Back in the golden age of science fiction, interstellar exploration was thought to be a task for the distant future — the 22nd or 23rd century, perhaps, with luck. The reality is that we don’t have to wait for the far future, or even the plausible mid-future.
The Starshade does not look much like the USS Enterprise of classic Star Trek. It has no warp drive and no crew with snappy uniforms. But it has the same mission, to seek out new worlds (more than 4,000 of them) and (for all we know!) new civilizations out among the stars.