NASA should begin developing a space mission that can tell us whether life on nearby planets is abundant, rare or essentially absent, the National Academy of Sciences recommended today.
The call for a new “Grand Observatory” telescope with this ambitious goal represents the first time that the Academy, in its Decadal Survey on Astronomy and Astrophysics, has given top priority to the science of exoplanets and the search for life far beyond Earth.
The long-awaited NAS survey did not select a single mission concept, although several NASA-commissioned studies were extensively researched and presented to the survey panel over the past four years.
Rather, they set the science goal of trying to answer—as completely as possible—the age-old question of whether life exists solely on Earth or whether it might be found on any of the billions of other planets known to be orbiting other suns.
As for what new telescopes might help achieve that goal, the panel combined aspects of two of the NASA studies—chiefly from LUVOIR (the Large UV Optical Infrared Surveyor) and also from the Habitable Exoplanet Observatory (HabEx). While the LUVOIR team had proposed two versions of their telescope, with 15-meter and 8-meter mirrors, the HabEx proposal called for a 4-meter mirror. The Survey steering committee settled on something in between, concluding that a mirror of about six meters, with many of the capabilities of the smaller LUVOIR concept, was all that was scientifically and financially feasible right now.
A space-based telescope of that size, they wrote, should be able to search the solar systems of a hundred or more sun-like stars for small, rocky planets orbiting relatively close to their suns. That survey would be followed by intensive study in several wavelengths of at least 25 especially promising planets within their star’s habitable zones. The goal would be to find well-established signs of a biosphere—particularly the presence of water, oxygen, ozone and methane in the planets’ atmospheres.
The telescope would use direct imaging to observe and analyze stars, solar systems and planets. Thus far, almost all exoplanets have been detected indirectly through their effects on host stars and starlight. The few directly imaged exoplanets have been very large and distant from their star.
Reading the chemicals in the atmospheres of small rocky exoplanets close to their suns is well beyond the technical abilities of any telescope and sun-blocking technology available today, and advancing the state of the art will be extremely challenging and costly.
As part of their recommendations, the Decadal Survey steering committee asked for the establishment of a substantial research and development program for this and other future large space telescope efforts. The proposed Great Observatories Mission and Technology Maturation Program would bring significant changes to how major NASA projects are planned and developed.
Under the timeline envisioned in the report, scientists and engineers would spend an estimated six years determining if the technology exists to allow development of a telescope that could find those 25 potential “exo-Earths,” for a pre-determined price (about $11 billion). If it can in fact be built, the report estimates a launch in the mid-2040s.
The Decadal report as a whole won some quick praise. Keck Observatory Chief Scientist John O’Meara, for instance, said the study is a “very optimistic document” that sets NASA and the nation on an historic pathway.
Even though he had advocated for the largest, 15-meter version of LUVOIR, he said “I’m pragmatic and understand their thinking. And LUVOIR is not a number but a vision, and they adopted that.”
O’Meara and others emphasized that the Decadal report onlyl offers guidelines. It will be up to NASA, scientists and engineers to determine the size and power of the telescope, based on the technology they can develop and the funding available.
Scott Gaudi, professor of astronomy at Ohio State University and community co-chair of the HabEx study group, said he “never expected them to advocate for ‘HabEx’ or ‘LUVOIR,’ but rather I hoped that they would do exactly what they did: prioritize a mission that could accomplish those science goals.”
The HabEx proposal, he said, was judged to not have the power needed to discover enough “exo-Earths” to make a convincing science case for life beyond Earth, while LUVOIR-B as proposed would have taken too long to develop and build.
The question of how big and powerful a telescope is needed to find evidence of extraterrestrial life remains an active, ongoing debate. Unprecedented resolution and precision is required to find distant small rocky exoplanets and to spectrographically analyze their atmospheres. That precision will also determine the uncertainty associated with those readings, since the discovery of an inhabited exoplanet will be based on probability rather than the observation of a living creature.
While the priority given to exoplanets and finding life beyond Earth dominates the new Decadal report, the Academy also offered recommendations on a number of other topics, from how the National Science Foundation uses its funding to the need to invest more in the ground-based Giant Magellan Telescope in Chile and the 30-Meter Telescope in Hawaii, which will play an important role in searching for signatures of life around smaller, red dwarf stars.
The Decadal Study had input from hundreds of scientists in the fields of astronomy and astrophysics, and so is considered to reflect the views of the community as a whole. For that reason, NASA almost always follows its guidelines for setting priorities, which then go to the White House and Congress to approve.
A public briefing on the new Decadal Survey report will be livestreamed here at 2 p.m. Eastern time on November 4.