March 21, 2014
By suppressing starlight with a starshade, scientists hope to make it possible for space telescopes to capture direct images of rocky, Earth-like exoplanets. This could give us important clues in discovering habitable worlds outside our solar system.
Starshade blocking the starlight on its way to a telescope. Credit: Credit: NASA/JPL-Caltech
Taking a picture of an exoplanet seems like an impossible task at the moment. Compared to the brightness of stars, distant planets are pitch black. The fact that they, from our perspective, closely orbit the star that outshines them, makes them invisible to us. Majority of the almost 1800 exoplanets discovered are found by indirect means. Those few that we imaged directly are gas giants larger than Jupiter with thermal emission of their own, which is possible to pick up in infrared if the conditions are right. Taking a picture of a rocky Earth-sized planet is impossible.
But don't despair! There's a project by Jet Propulsion Laboratory (JPL) in Pasadena, USA, with the goal to design a starshade which is to be launched into orbit, position itself between any star and a space telescope and block the light from that star, so that the telescope could observe planets around that star. This starlight suppression is actually something that the Moon does during the solar eclipse, when it blocks the Sun, making it possible to see stars that the Sun outshines.
"A starshade mission would allow us to directly image Earth-size, rocky exoplanets, which is something we can't do from the ground," says Jeremy Kasdin of Princeton University, Princeton, USA. He is Principal Investigator of the starshade project. "We'll be able to show people a picture of a dot and explain that that's another Earth."
Images of rocky exoplanets, similar to Earth and at the right distances to keep water liquid, could tell us if the life on those planets is possible. Whether we image it in visible light or at any other wavelength, such information would be valuable in finding if the atmosphere is rich in oxygen or if the planet is covered with liquid water.
Starshade is a deployable structure that could launch with a telescope, and once in space, unfurl its petals and position itself between the telescope and the star. Although the starshade's flower-shaped petals may look like an overkill, they are actually crucial to the effectiveness of the project and it is important that they are positioned accurately.
"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. He is JPL's lead engineer on the 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."
Also, the starshade will be independent of any telescope and able to move around, which gives it a much-needed flexibility. "We can use a pre-existing space telescope to take the pictures," says Shaklan. "The starshade has thrusters that will allow it to move around in order to block the light from different stars."
If the project is successful, the starshade could be used in combination with telescopes in other fields that need starlight suppression in order to improve their observations.
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