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Coronagraph, starshade could assist imaging of Earth-like planets


An artist’s rendering of a starshade unfurling after separating from its host telescope. By traveling thousands of kilometers ahead of the telescope, the shade can block out much of the light from a particular star, enabling the detection of an exoplanet’s atmosphere. Technologies like this, as well as a coronagraph, could enable direct imaging of Earth-sized planets in the next decade. Image Credit: JPL

Two new technologies in development could yield detailed images of Earth-like exoplanets within the next decade—a pinhead-sized coronagraph and baseball-diamond-sized starshade.

Nearly 2,000 exoplanets—planets orbiting stars other than the Sun—have been discovered to date with almost twice that number pending confirmation. Scientists estimate every star may host at least one planet while one in five could harbor rocky planets in their habitable zones, the regions neither too hot nor too cold for liquid water to exist on their surfaces.

The coronagraph and the starshade, when paired with space telescopes, could offer unprecedented opportunities to study alien worlds.


A coronagraph on an exoplanet-hunting space telescope must be the size of a pinhead. In this simulated image, the mirrors of the telescope focused all the light of the target star onto this point (the black circle in the center) to block out the glare from the host star to see any orbiting bodies. It is the same concept as the starshade, only much smaller. Image Credit: JPL

Coronagraphs are tiny instruments fitted inside telescopes to block the light of a star, enabling the imaging of orbiting planets. Using the dim light reflected off planets once a star’s light is blocked, scientists can then search for atmospheres using spectrometers. A planets’ atmosphere can provide important clues as to whether life is present.

“Coronagraphs are like visors in your car—you use them to block the light of the Sun, so you can see the road,” explained Nick Siegler, program chief technologist at NASA’s Exoplanet Exploration Program office at California’s Jet Propulsion Laboratory (JPL).

JPL scientists are developing coronagraphs for the space agency’s Wide-Field Infrared Survey Telescope (WFIRST), a space observatory scheduled for launch in 2025.

WFIRST will have a 2.4-meter mirror and be capable of imaging panoramic views about a hundred times larger than those Hubble can photograph today. Two different types of coronagraphs will be placed on the new telescope to address challenges caused by the diffraction of starlight.

“The starlight likes to walk all over the place and into the area where you want to image the planet,” said WFIRST project scientist Wes Traub. “The goal now is to get more practical with the kind of telescope we will use for WFIRST.”

Coronagraphs must be able to adjust for the “jitter” or tiny vibrations of space telescopes, a challenge JPL scientists are addressing by working with a mock WFIRST coronagraph under simulated space-like conditions.

Unlike coronagraphs, starshades are separate structures about the size of a baseball diamond that would be placed in front of space telescopes like WFIRST.

Shaped like a large sunflower, these paper-thin structures could be either be folded inside a rocket and launched together with WFIRST before separating and unfurling or be launched separately and then unfurled at a location tens of thousands of kilometers ahead of the telescope.

Starshades do not need to be custom-made for specific telescopes. Their feathered shapes prevent waves of light from flowing around them. Both JPL and aerospace company Northrop Grumman are researching these concepts.

John Grunsfeld, the outgoing associate administrator of NASA’s Science Mission Directorate, acknowledged starshades would enable more detailed research than coronagraphs but qualified the statement by noting starshade technology is “still in its infancy”.

WFIRST will be placed 930,000 miles (1.5 million kilometers) from Earth, a location where a starshade could function without being disturbed. Because it will be observing in the direction of the galactic bulge, the telescope will likely discover tens of thousands of exoplanets.

WFIRST’s coronagraph is expected to enable imaging of “super-Earths” and “mini-Neptunes” with a minimum size twice that of Earth. By contrast, starshades could enable a telescope like WFIRST to photograph Earth-sized planets by the end of the 2020s.

The two technologies can be used to complement one another. Coronagraphs are best at discovering planets because they can easily switch targets from one star to another. Starshades, on the other hand, can take up to several weeks to change targets, but have better broadband sensitivity, making them capable of studying individual planets in greater detail.

Aki Roberge, an astrophysicist at NASA’s Goddard Space Flight Center, studies future telescopes for the agency. She believes a combination of both technologies offers the best chance of finding another Earth.

Video courtesy of JPL


Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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