Astronomers have been indirectly detecting exoplanets for more than 15 years, but actually taking a picture of one has proven an immensely difficult task. Picking out the dim light of a planet from a star billions of times brighter is akin to finding a needle in a cosmic haystack, especially when the planet in question is a small, rocky world similar to Earth. In order to achieve this feat, researchers are developing techniques to block out the starlight while preserving the light emitted by the planet. This is called starlight suppression.
It’s a task that NASA’s flower-shaped starshade is designed to make easier. Working in conjunction with a space-based telescope, the starshade is able to position itself precisely between the telescope and the star that’s being observed, and can block the starlight before it even reaches the telescope’s mirrors..
With the starlight suppressed, light coming from exoplanets orbiting the star would be visible. Using this technology, astronomers would be able to take actual pictures of exoplanets – images that could provide clues as to whether such worlds could support life as we know it.
The flower-shaped petals are part of what makes the starshade so effective. “The shape of the petals, when seen from far away, creates a softer edge that causes less bending of light waves,” said Dr. Stuart Shaklan, JPL’s lead engineer on the starshade project. “Less light bending means that the starshade shadow is very dark, so the telescope can take images of the planets without being overwhelmed by starlight .”
The starshade is also unique in that, unlike most space-based instruments, it’s one part of a two-spacecraft observation system. “We can use a pre-existing space telescope to take the pictures,” explains Shaklan. “The starshade has thrusters that will allow it to move around in order to block the light from different stars.”
This process presents a number of engineering challenges that Shaklan and his team are working hard to unravel, from positioning the starshade precisely in space, to ensuring that it can be deployed accurately. “Our current task is figuring out how to unfurl the starshade in space so that all the petals end up in the right place, with millimeter accuracy,” said Professor Jeremy Kasdin, a Princeton researcher who is the Principal Investigator of the starshade project. Kasdin’s group will create a smaller scale starshade at Princeton to verify that the design blocks the light as predicted by the computer simulations. Concurrently, the JPL team will test the deployment of a near-full scale starshade system in the lab to measure its accuracy.
Despite these challenges, the starshade approach could offer planet-hunters many advantages. “One of the starshade’s strengths is simplicity,” said Kasdin. “Light from the star never reaches the telescope because it’s blocked by the starshade, which allows the telescope system to be simpler.” Another advantage of the starshade approach is that it can be used with a multi-purpose space telescope designed to make observations that could be useful to astronomers working in fields other than exoplanets.
NASA’s starshade engineers are optimistic that refining their technology could be the key to major exoplanet discoveries in the future. “A starshade mission would allow us to directly image Earth-size, rocky exoplanets, which is something we can’t do from the ground,” says Kasdin. “We’ll be able to show people a picture of a dot and explain that that’s another Earth.”