Since the first confirmed discovery of planets beyond our solar system in 1993, astronomers have discovered 4,360 extrasolar planets, also known as exoplanets, orbiting 3,223 planetary systems, according to Encyclopedia of extrasolar planets.
However, oddly enough, as the planetary society notes, we’ve actually only seen a relative handful of these distant worlds. At last count, there have been 51 exoplanet discoveries by direct detection, just over 1% of all discoveries, according to POT. All the others have been detected by a variety of indirect means, mainly by measuring the slight dimming of stars as their planets pass in front of them or the small 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 research, circumstantial evidence can be just as powerful as direct detection. In many cases, the careful collection of circumstantial evidence has not only confirmed that a planet exists, but has also provided information about its characteristics, and even about its climate.
Still, astronomers want to capture direct images of these distant worlds, even if the image is just a speck of light. It’s not just the satisfaction of seeing a world orbiting another sun: direct observation will allow us to make measurements that cannot be done reliably in any other way.
A firefly next to a reflector
Compared to stars, planets are small and dim. Detecting one light-years away pushes the limits of even the most powerful telescope. But the real problem is that planets orbit stars. And trying to get images of a planet orbiting close to a star is like trying to capture a firefly sitting on the side of a searchlight.
If you were trying to see something dim that was placed right next to something bright, your first reaction might be to hold something up to shield your eyes from the glare of the bright light, allowing you to focus on the dim object.
And this is exactly what astronomers do. Conveniently, Mother Nature has provided you with an ideal example of this arrangement. The moon, seen from Earth, appears to be almost exactly the same size as the sun. In reality, however, the sun is something 400 times bigger (but also 400 times further).
As a result, during a total solar eclipse, the moon simply blocks out the fiercely bright solar disk, while allowing us a perfect view of the surrounding impressive, but much fainter, solar corona.
conjuring an eclipse
For astronomers, the only limitation of this natural sunshade is that they can’t order a total solar eclipse when they want to. So, they have built devices called coronagraphs, which are equivalent to artificial eclipse generators. They block out the part of the image plane where sunlight would hit so that the rest of the image is not overexposed and can register faint light from the corona.
The same principle can also be used to observe faint objects located near bright stars. This is the technique that astronomers have used to image the few exoplanets that we have directly observed.
Furthermore, the exoplanets we have directly observed so far are all very young and hot, despite the great distance separating them from their parent stars. Therefore, they are much brighter than if we only saw them reflected by starlight.
Enter Star Shadow
To observe Earth-distance planets around numerous Sun-like stars, we need a little more help, and that’s where star shadow technology comes into play.
The concept, as with many space mission concepts, is brilliant in its simplicity: position a spacecraft with the Starshade, a display shaped like flower petals, to outshine a distant star, as seen through an onboard telescope. from another spaceship.
But, as usual in space exploration, the devil is in the details. According to Tiffany Glassman, Ph.D., a Northrop Grumman project engineer, “controlling the shape of Starshade’s edges and positioning them relative to the telescope are the most challenging technologies” for 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 Starshade has to block out almost all starlight in order to see the planets orbiting it.”
Getting the shadow to where it does its job presents the development team with yet another hurdle, Glassman adds. It has to be “a structure that can be deployed, to fit on the rocket, and be as light as possible so that it doesn’t take too much fuel to move it across the sky.”
Fuel (or propellant, to be strictly technical) always looms large in planning a space mission, because getting into space requires a lot of it, not to mention the bigger and heavier the payload, the bigger and more expensive will be the rocket needed to transport it. in the space.
In the case of the Starshade, that meant designing it to fold neatly into the confines of a rocket’s nose cone and then expand seamlessly, without binding or clogging, to provide the necessary deep shadow.
Finally, it must be placed with a surreal level of precision. As Glassman says, “Once it’s deployed the right way, 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 in which either Starshade or the telescope can detect the relative alignment between the two spacecraft and then adjust their position with an accuracy of less than one meter.”
to go boldly
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, hopefully. The reality is that we don’t have to wait for the far future, or even the plausible middle future.
The Starshade doesn’t look much like the USS Enterprise from classic Star Trek. She has no warp drive and no crew in snappy uniforms. But she has the same mission, to search new worlds (more than 4,000 of them) and (as far as we know!) new civilizations among the stars.