Exoplanet report recommends development of large space telescope

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WASHINGTON — A National Academies committee recommends that NASA pursue development of a large space telescope to search for potentially habitable exoplanets, but declined to choose a specific concept for such a mission.

The “Exoplanet Science Strategy” report, requested by Congress in a 2017 NASA authorization act and released by the National Academies of Science, Engineering and Medicine Sept. 5, also recommends continued development of the Wide Field Infrared Survey Telescope (WFIRST) and funding by the National Science Foundation of two large ground-based observatories.

The recommendations of the report are based on an underlying conclusion that the science of exoplanet research has advanced to the point where it will soon be possible identify planets that are potentially habitable and even discern so-called “biosignatures,” or evidence of life, on them.

“If we choose, we could learn the answer to that question — we could figure out whether or not there’s life on planets orbiting other stars — in the next 20 years,” said David Charbonneau, a Harvard University professor and one of the co-chairs of the study, in a briefing about the report here.

Achieving that goal, the committee concluded, will ultimately require development of a “large strategic,” or flagship, direct imaging mission that can measure the reflected light spectra of Earth-sized planets orbiting in the habitable zones of sun-like stars. Such spectra can reveal the abundance of molecular oxygen, water and methane that could help determine the habitability of those planets.

“We did not have the knowledge necessary even just 10 years ago” to support such a mission, said the report’s other co-chair, Scott Gaudi, an Ohio State University professor. NASA’s Kepler mission and other research, he said, has demonstrated Earth-sized planets should be common around other stars, including many in their stars’ habitable zones. “None of that was known 10 years ago. So that means that we now have targets for which we can aim for to try and develop these missions.”

While the report endorsed the concept of a flagship-class space telescope to study exoplanets, it did not recommend a specific mission concept. The report does mention two concepts NASA is funding studies of, one called the Habitable Exoplanet Imaging Mission, or HabEx, and the other the Large Ultraviolet/Optical/Infrared Surveyor, or LUVOIR. HabEx and LUVOIR are among the four missions being studied for consideration by the next astrophysics decadal survey, due to be completed in 2020.

While HabEx and LUVOIR would both be able to carry out the direct imaging mission recommended by the report, the two are very different spacecraft. HabEx will be devoted primarily to exoplanet studies, and in its current baseline design features a mirror four meters in diameter. LUVOIR is intended for more general astrophysics work, but proposals call for a larger, segmented mirror up to 15 meters across. The James Webb Space Telescope, by comparison, has a segmented mirror 6.5 meters in diameter.

The report did not offer a preference for either HabEx or LUVOIR. “They can be thought of as points on a continuum of missions, with the ultimate trade between cost, complexity, exoplanet characterization capability, and other astrophysical capabilities to be made by the 2020 Decadal Survey,” the report stated.

Gaudi emphasized that at the briefing, noting that the mission concepts remain under study, with final reports not expected until the middle of 2019. “We do not feel it is up to us to decide where in that buffet of options the mission architecture should be,” he said. “That is the province of the decadal survey. We are simply providing them with the recommendation that a mission that is capable of achieving these science goals should be launched, and HabEx and LUVOIR both satisfy that.”

The report also backed development of WFIRST because of both its ability to carry out a search for exoplanets known as a microlensing survey as well as test a coronagraph that can be used to block light from individual stars, enabling direct observations of planets or dust dusks orbiting them. The coronagraph was downgraded from a full-fledged science instrument to a technology demonstration as part of efforts last year to reduce the mission’s cost.

The administration’s fiscal year 2019 budget proposal called for cancelling WFIRST, but both House and Senate versions of spending bills provide at least partial funding to continue the mission.

“WFIRST will address two very important science questions,” Gaudi said, hence the committee’s recommendation “that NASA should launch WFIRST to do the microlensing survey and to demonstrate the technique of coronagraphy on exoplanets.”

For ground-based telescopes, the report recommended the NSF financially support development of the Giant Magellan Telescope and Thirty Meter Telescope, two large observatories in development. Both telescopes, the committee concluded in the report, could make “profound advances in imaging and spectroscopy” of exoplanet systems.

Both, though, have struggled to raise all of the money needed to fund their development, and the Thirty Meter Telescope has also faced legal challenges to its plans to build the observatory atop Mauna Kea in Hawaii. “We believe that those facilities, once completed, will enable transformative science across a wide range of exoplanetary investigations,” Charbonneau said.

Other recommendations in the report involved use of JWST for studies of exoplanet atmospheres and cooperation between NASA and NSF on approaches for measuring the masses of Earth-sized planets.

The committee looked beyond specific missions and technologies to emphasize supporting the scientists involved in exoplanet research. That included recommendations on developing a “cross-divisional exoplanet research coordination network” and increased funding opportunities for research. It also included findings on the importance of a diverse workforce and addressing discrimination and harassment issues that may deter some from pursuing careers in the field.

“We need the very best minds to go take on what is arguably one of the greatest questions in all of science: are we alone in the universe? Unfortunately, right now in the field of exoplanets we’re not drawing from the widest possible pool,” Charbonneau said, acknowledging that the problem is true for much of science, not just exoplanets.

“You might wonder why this is part of our statement of task,” Gaudi said. “The answer is actually quite simple: you can’t do science without scientists.”