Engineering the Search for Earth-like Exoplanets
SARA SEAGER
Massachusetts Institute of Technology
MITCHELL WALKER
Georgia Institute of Technology
Every star in the sky is a sun, and if Earth’s Sun has planets then it seems logical that other stars should have planets also. And they do: in the past two decades, astronomers have found thousands of “exoplanets” orbiting stars other than the Sun. Surprisingly, no solar system copies have yet been found; instead, an incredible diversity of exoplanets and planetary systems has been uncovered. Extrapolating from these discoveries, every star in the Milky Way galaxy should have at least one planet. With hundreds of billions of stars in the Milky Way and upwards of hundreds of billions of galaxies in the universe, the chance for one of those planets to be Earth-like should be a near certainty.
But currently available techniques to find and study small rocky planets have measurement capabilities that limit the detectable planet size (or mass) and orbit. In other words, similar-sized planets such as Venus (with a scorching surface hot enough to melt lead) and Earth (with a clement surface and temperatures supportive of a liquid water ocean and suitable for life) would look the same to current observational capabilities. The ability to observe rocky exoplanet atmospheres and detect biosignature gases—gases produced by life that accumulate in the atmosphere to detectable levels—is a prime goal of the search for other Earths.
A new generation of space-based telescope is needed to find and identify an Earth-like exoplanet (even though such a telescope will have only the nearest stars within reach). It must operate above the blurring effects of Earth’s atmosphere. Moreover, the signal of an Earth-like planet orbiting a nearby sun-like star is so dim that less than one visible-light photon would strike the telescope’s primary mirror each second for a mirror 10 m in diameter (the larger the aperture, the more stars are accessible and the higher the chance of finding an exo-Earth). But many billions of photons from that planet’s host star will flood the telescope in
that same second, requiring precise separation, suppression, and/or shadowing of the star if the Earth-like exoplanet is to be detected and studied.
Ultraprecise starlight suppression and the deployment or construction of large optical telescopes in space depend on advanced engineering to enable the search for Earth-like exoplanets. The presenters in this session described aspects of this advanced engineering. Amy Lo (Northrop Grumman) set the stage for large space telescopes by describing the James Webb Space Telescope (JWST), an international, NASA-led mission to be launched in 2018. JWST has nearly four times the collecting area of the Hubble Space Telescope and is cryogenically cooled to detect infrared wavelengths. She provided the industry perspective for large civilian space missions. Next, Dmitry Savransky (Cornell University) explained the two main techniques for starlight suppression: the internal occulter, or coronagraph, that blocks light inside the telescope and works with wavefront sensing and control to create a stable optical system; and the external occulter, or starshade, a specially shaped screen tens of meters in diameter that formation flies tens of thousands of kilometers from the telescope, blocking out the star light so that only planet light enters the telescope. Jeremy Banik then addressed the construction of large structures in space, from large deployables to space-based assembly and construction. He leads the Large Deployable Structures Technology Thrust Area at the Air Force Research Laboratory Space Vehicles Directorate at Kirtland Air Force Base. The final speaker, Jonathan Black (Virginia Tech), presented the cutting edge in sensing controls for formation flying and satellite proximity operations (primarily to enable autonomy for small satellites) and discussed challenges in the advances needed to apply those technologies to space-based planet-finding missions.
