Expert Committee Wants NASA to Build Direct Imaging Space Telescope To Find Life on Exoplanets

A new report from the National Academies recommends that NASA build a space telescope capable of directly imaging planets around other stars — exoplanets — that are within the star’s habitable zone.  It would be in addition to four existing or planned ground- and space-based telescopes already designed to find and study exoplanets, though they are not dedicated to that goal: NASA’s JWST and WFIRST and NSF’s GMT and TMT.   The committee’s co-chairs argued that “now is the time” to build the new space telescope and it could enable answering the question of whether there is life on exoplanets “within 20 years.”

The 2017 NASA Transition Authorization Act directed NASA to contract with the National Academies of Sciences, Engineering, and Medicine to develop a “science strategy for the study and exploration of extrasolar planets.”

The Academies convened a committee of experts co-chaired by Harvard’s David Charbonneau and Ohio State University’s Scott Gaudi.  Charbonneau is a co-investigator on NASA’s Transiting Exoplanet Survey Satellite (TESS), which was launched in April 2018. Gaudi is part of the team working on the Habitable Exoplanet Observatory (HabEx) concept study that will be considered by the next astronomy and astrophysics Decadal Survey.

Their report, Exoplanet Science Strategy, sets two broad goals:

• To understand the formation and evolution of planetary systems as products of star formation and characterize the diversity of their architectures, composition, and environments.
• To learn enough about exoplanets to identify potentially habitable environments and search for scientific evidence of life on worlds orbiting other stars

Accomplishing that will first require launching NASA’s James Webb Space Telescope (JWST) and Wide-Field Infrared Survey Telescope (WFIRST) and investing in NSF’s ground-based Giant Magellan Telescope (GMT) and Thirty-Meter Telescope (TMT) and their exoplanet instrumentation. JWST and WFIRST were recommended as the top priorities for large space telescopes in the 2000 and 2010 Decadal Surveys. The 2010 Decadal Survey recommended a federal investment in “at least one” of the two NSF telescopes, one of which (GMT) will be located in the Southern Hemisphere and the other (TMT) in the Northern Hemisphere.

Decadal Surveys are produced by expert committees at the National Academies every 10 years (a decade) for each of NASA’s science disciplines.  For astronomy and astrophysics, it also prioritizes programs at NSF and the Department of Energy. They determine the top science questions and recommend missions to answer them, creating a consensus-based set of priorities.  The next in this series will get underway soon with publication expected in 2020.

Although JWST, WFIRST, GMT and TMT will provide critical data, the Charbonneau/Gaudi committee concluded that a dedicated large space telescope equipped with a coronagraph or working in tandem with a starshade is needed to provide definitive information on exoplanets in the habitable region of other stars and if there is life.  Habitable zone is defined as the region around a star where a terrestrial planet with an Earth-like atmosphere could support surface liquid water.

A coronagraph or a starshade blocks the light from a star to better enable the detection of planets around it.  A coronagraph is internal to the telescope.  A starshade is a separate spacecraft positioned between the telescope and the star.

NASA is funding four concept studies for consideration by the yet-to-be-appointed 2020 Decadal Survey committee.  Gaudi’s HabEx is one. The others are the Large Ultraviolet Optical Infrared Surveyor (LUVOIR), the Lynx X-ray telescope, and the Origins far-infrared surveyor.

At a press conference releasing the report yesterday, Gaudi and Charbonneau said HabEx and LUVOIR both could contribute to the science goals their report identifies, but neither is a dedicated direct-imaging mission and neither is recommended.   That will be the job of the 2020 Decadal Survey committee.  Charbonneau said this report lays out an architecture, a scientific capability, rather than specifying a design.

NASA has studied concepts for a telescope capable of direct imaging of exoplanets in the past.  The Terrestrial Planet Finder (TPF) and the Space Interferometry Mission (SIM) were never approved for development largely because of their cost and technical complexity.

This report says only that building a direct-imaging space telescope would require “bold investments and a longer time scale to bear fruit…,” but this is the time to get started.

Gaudi offered three reasons why “now is the time, for the first time” to develop a direct-imaging space telescope. First, data from Kepler and other telescopes have shown that exoplanets are common and “one in four stars host a planet” in the habitable zone.  Second, “we are within shouting distance” of having the technologies needed to detect another “pale blue dot” — the poetic phrase used by Carl Sagan to describe Earth.   Third, earlier concerns that dust in the exoplanet systems might obscure the ability to see planets have been determined to be unfounded.

The bottom line, Charbonneau said, is that “if we choose, we could learn the answer to the question [of] whether or not there is life on planets orbiting other stars in the next 20 years.”

The report makes a number of other recommendations, including about the astrophysics workforce.  “We can’t have science without scientists,” Gaudi said.  Among the many issues involved in ensuring an “interdisciplinary, engaged community” is addressing diversity and harassment, particularly because exoplanet research is a such a young field and needs to be attractive to the next generation of astrophysicists.