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The Exo-C mission concept under study would fly a space telescope equipped with a coronagraph to attempt to directly observe exoplanets. A study nearing completion suggests the mission could be done for no more than $1 billion. (credit: NASA)

Debating the future of exoplanet missions concepts and community

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The last several years have made it clear we are living in a golden era of extrasolar planet studies. Less than 20 years ago, astronomers discovered the first planets orbiting Sun-like stars. Now, the number of exoplanets is in the thousands, including a growing number of “Earth-like” planets, a designation based on some combination of the planets’ mass, radius, and orbit around their stars. Just this past Friday, for example, astronomers reported detecting three planets between 1.5 and 2.4 times the radius of the Earth orbiting a single star, one of which lies in the star’s habitable zone, where liquid water could exist on its surface.

“Everybody wanted a piece of the pie, a pie that had not yet been made or baked,” Beichman said of the exoplanet community in 2010.

We don’t know, however, if these or other Earth-like planets are really like the Earth in the characteristics that really count: whether they have atmospheres like the Earth, oceans of liquid water like the Earth, and life like the Earth. Those determinations are largely beyond the capabilities of ground- and space-based observatories in operation today. A new generation—arguably, generations—of instruments and telescopes will be needed to determine just how Earth-like these Earth-like worlds really are. And the ability to develop those instruments will depend, at least in part, on the ability of exoplanet scientists to come into agreement on what’s needed to enable that next round of discoveries.

Avoiding the circular firing squad

That agreement is necessary since exoplanet scientists will be competing with researchers in other fields of astrophysics for funding from NASA and the National Science Foundation (NSF) that’s unlikely to increase significantly for the foreseeable future, even as missions and instruments become increasingly sophisticated and expensive. Yet, in recent years, the exoplanet research community has gained a reputation—at least among other astronomers—as a fractious one, prone to infighting and disagreement about scientific priorities to pursue and how.

In an effort to overcome that reputation, and get as much as the community as possible on the same page regarding future research priorities, the Exoplanet Exploration Program Analysis Group (ExoPAG), a scientific group that advises NASA, discussed development of an “exoplanet community plan” during a meeting in Seattle January 4, just before the start of the American Astronomical Society (AAS) meeting there.

“We all want to see this science happen, and want to maximize the science we can achieve,” Scott Gaudi, an Ohio State University astronomer who is chairman of the ExoPAG, said in remarks kicking off the discussion.

Exoplanet astronomers also want to avoid repeating what happened in the last astrophysics decadal survey, published in 2010. That study, prepared by the astrophysics community for the National Academies, and which guides NASA and NSF investments in the field, made frequent mentions of exoplanet science, yet that community felt underserved by the report’s recommendations.

“Planets were everywhere in that report,” said Chas Beichman of the NASA Exoplanet Science Institute, “but I think many people in this room found that the recommendations were somewhat less that what we might have expected.”

Those recommendations included naming as its top-priority large mission the Wide-Field Infrared Survey Telescope (WFIRST), a mission with cosmology as its primary purpose. The survey recommended some technology development for exoplanet research, but another mission that had long been supported by exoplanet researchers, the Space Interferometry Mission, suffered what Beichman called “death by footnote” in the report.

Beichman argued that the exoplanet community suffered in the development of the 2010 decadal because of a lack of consensus within it about its priorities. “The community was evolving extremely rapidly,” he said, citing developments among a number of techniques to discover and study exoplanets. “Everybody wanted a piece of the pie, a pie that had not yet been made or baked.”

“This isn’t a zero-sum game,” said Roberge. “If you study habitable worlds, you will inevitably do all that other stuff, too.”

There was also, he said, tension between what NASA and scientists wanted. “The community was split between a NASA goal, directed by NASA Headquarters, to detect and do spectroscopy of nearby habitable Earths—that’s what the then-Origins program was studying—and more modest goals of comparative exoplanetology,” he said. “In that case, it was really impossible to arrive at a consensus.”

Beichman illustrated that point with a cartoon showing a firing squad, each person labeled with a particular exoplanet technique and technology. The firing squad, though, was arranged in a circle, firing inward. “It was not a pretty sight,” he said.

Astronomers want to avoid a similar situation in the 2020 decadal, which is closer than one might think. “If you start with the decadal survey and work backwards, and think about the things you need to do to be ready for the decadal survey and have this coordinated science vision, we need to be starting to do this process now,” Gaudi said. “Maybe we should have started it two years ago.”

NASA’s overall astrophysics program has already laid out one broad vision of its long-term future. In late 2013, NASA published a 30-year strategic roadmap report that identified both the key science goals and the missions—or, at least, general concepts of missions—needed to achieve those goals.

That study included science goals for exoplanet science, which Aki Roberge of NASA’s Goddard Space Flight Center said included three “quests” over the next three decades. One of those is cataloging the “exoplanet zoo” and their basic properties. The second quest is to better characterize them, while the final one is to look for life on these worlds.

While scientists have made great strides in that first quest, achieving the second and third ones will require missions beyond those active today or on the drawing board. One such concept included in the strategic roadmap is the Large Ultraviolet/Optical/Infrared (LUVOIR) Surveyor. It would be space telescope, likely with a mirror eight meters or larger in diameter, equipped with either a coronagraph or a starshade to block starlight and thus directly image exoplanets.

LUVOIR is not the only exoplanet mission included in the roadmap. Astronomers went even further out and proposed something called ExoEarth Mapper, which would have the ability to resolve Earth-like exoplanets. As envisioned, ExoEarth mapper would feature a number of large space telescopes arrayed over baselines of hundreds of kilometers in deep space, using interferometry to provide the images.

It will be towards the end of that 30-year timeframe of the roadmap report before something like ExoEarth Mapper is feasible, astronomers acknowledged. “This is obviously extremely challenging, especially at optical wavelengths,” Roberge said.

The discussion that followed those presentations at the ExoPAG meeting didn’t reach any consensus about what missions and technologies to pursue, although organizers said from the beginning that the discussion was intended to be the start of a longer-term process. The discussion did help identify where the fault lines might exist within the exoplanet community.

One of them is whether their pursuit of a truly Earth-like planet—one that is habitable, if not inhabited—should be a long-term goal. Some people at the meeting said that comparative exoplanetology, including study of a broad range of exoplanets and not just those that may be like the Earth, was an important goal, and wondered if the two efforts could coexist.

“This isn’t a zero-sum game,” said Roberge. “If you study habitable worlds, you will inevitably do all that other stuff, too. The spectroscopy of Earth twins provides all the other capabilities that you need to study a wide range of all kinds of planets, and planet formation topics as well.”

Others also cautioned against designing missions optimized primarily or exclusively for exoplanet studies, versus missions that could carry out a broader range of studies and thus attract support from other astronomers.

NASA associate administrator for science John Grunsfeld, who sat in on the ExoPAG meeting, advised for the broader approach based on experience with previous large space telescopes like Hubble. “Most of what those telescopes have discovered weren’t imagined at the time those telescopes were conceived,” he said.

Others at the meeting reminded scientists of the role groundbased telescopes and instruments play in the study of exoplanets, and the technical challenges those efforts face. Debra Fischer, a Yale University astronomer who is one of the pioneers in the use of the radial velocity technique that measures the Doppler shift of stars to look for the wobble created by orbiting exoplanets, argued for a renewed emphasis on increasingly precise spectrographs in order to detect more Earth-like planets.

Fischer warned that current spectrographs had precisions of about one meter per second, but needed to get down to about 10 centimeters per second. “It’s time for us to admit as a community that we are stuck. We’ve been stuck for a long time with a velocity precision of about a meter per second,” she said. She called for the development of high-resolution spectrographs that would allow astronomers to distinguish between Doppler shifts from orbiting planets and “stellar noise” and thus provide better overall prevision.

A cartoon from Chas Beichman’s ExoPAG presentation illustrates the infighting within the exoplanet science community during the 2010 decadal survey, with cosmologists, represented by “dark energy” to the side, ready to reap the benefits of that debate.

Concepts for future missions

Some missions already in development or planning will certainly be of help to exoplanet researchers. The James Webb Space Telescope (JWST), due for launch in late 2018, will be able to support exoplanet science, from spectroscopy of potentially Earth-like planets to direct imaging of some exoplanets.

“Even if we don’t get to fly this mission, we’ve done the community a service by showing that there does exist at least one probe mission concept in this $1 billion cost cap level,” Stapelfeldt said.

WFIRST, while originally seen as primarily a cosmology mission, will also support exoplanet science. NASA has been studying outfitting WFIRST—now likely to make use of one of two 2.4-meter telescopes donated to NASA by the National Reconnaissance Office—with a coronagraph that can block the light from a star, allowing for direct observations of orbiting planets that would otherwise be lost in the star’s glare.

WFIRST, though, hasn’t officially started as a program. An official “new start” for WFIRST is not expected until at least the fiscal year 2017 budget request, due out in just over a year, said Paul Hertz, director of NASA’s astrophysics division, at the AAS meeting. That decision will also require the concurrence of the administration.

Given concerns that the administration and/or Congress might be wary of starting another flagship-class mission so soon after JWST, because of its earlier budget and schedule problems, NASA has been performing studies of two “probe-class” exoplanet science missions that could be started should work on WFIRST be deferred. These would be smaller missions, with a total cost not to exceed $1 billion.

One of the concepts, called Exo-C, would involve a smaller space telescope similar in size to NASA’s Kepler mission, but equipped with a coronagraph. A second, Exo-S, would also use a similar space telescope but instead use a “starshade”: an external disk flying in formation with the telescope to block out light from the star.

Those studies are wrapping up, and for at least one of the concepts, the results look promising. “We have come up with what Paul Hertz asked us to do, which is to come up with a probe mission at the cost cap of $1 billion,” said Karl Stapelfeldt of NASA Goddard in a presentation about Exo-C at the ExoPAG meeting.

An independent cost estimate of an earlier iteration of the mission concept, performed by the Aerospace Corporation in September, came out to be $1.1 billion, he said. An updated design, submitted to Aerospace for costing in December, should be less, he said.

Exo-C would make use of the Kepler spacecraft bus, equipped with a 1.4-meter telescope and a coronagraph to allow direct imaging of exoplanets. The use of the Kepler-heritage technology, plus a launch on a SpaceX Falcon 9 rocket, allows the mission to fit into the $1 billion cost cap, he said. “Even if we don’t get to fly this mission, we’ve done the community a service by showing that there does exist at least one probe mission concept in this $1 billion cost cap level,” Stapelfeldt said.

Exo-S, which would fly a starshade 30 meters in diameter 30,000 kilometers from its telescope, has shown great technical promise, but may be more difficult to do within a single probe-class mission. Launching a starshade by itself to use in conjunction with another space telescope could be done well within the $1-billion limit, said Shawn Domagal-Goldman of NASA Goddard. “Unfortunately, a co-launched mission, where we launch the starshade and the telescope together, looks like they’d be safely over $1 billion together.”

He said that Exo-S, Exo-C, and the coronagraph-equipped version of WFIRST each have the ability to detect habitable exoplanets. “Essentially, you can view them as a lottery ticket” for potentially determining the habitability of an Earth-like exoplanet.

Astronomers are also starting to look beyond WFIRST, or probe-class exoplanet-specific missions, for missions that would get closer to the goals laid out in NASA’s strategic roadmap. At the AAS meeting, Hertz formally asked ExoPAG and two other program analysis groups for feedback on a shortlist of potential flagship missions to be considered for the next astrophysics decadal survey, to be published in 2020.

“It’s not too early” to start thinking about 2020 decadal survey, Hertz said. “We need to identify some missions for after WFIRST in order to start making technology investments.”

Hertz provided astronomers with NASA’s initial list of missions, taken from that roadmap document as well as the 2010 decadal: Far Infrared Surveyor, Habitable-Exoplanet Imaging Mission, Ultraviolet/Optical/Infrared Surveyor, and X-Ray Surveyor. He asked astronomers for feedback on which, if any, of those concept missions should be replaced, and if so, with what.

By the end of the year, Hertz said he plans to select three or four mission concepts for science and technology studies to flesh out their scientific utility, technology development needs, and, eventually, their cost. Those reports would be completed in time to deliver to the team working on the decadal survey, which will start work in 2018.

The studies, Hertz said, are intended “to make the 2020 decadal survey as effective as possible.” However, he said it will ultimately be up to the team working on the report to make use of—or ignore—those reports when developing their recommendations for large-scale missions.

Whichever mission does end up being selected as the top-priority flagship mission in the 2020 report still won’t fly for some time. Hertz indicated it’s unlikely such a mission would get a new start until the mid-2020s, as WFIRST approaches launch, and itself would not likely fly until some time in the 2030s.

Despite that long time horizon, Hertz said now was the time to start thinking about such missions and their technology requirements. “It’s not too early,” he said. “We need to identify some missions for after WFIRST in order to start making technology investments.”

That may make the upcoming decadal a key test of how unified the exoplanet science community really is. At the ExoPAG meeting, some scientists argued that they were not as fractured as they have been portrayed in some media accounts. “I think that opinion is largely outdated,” Gaudi said. “Most of us get along pretty well.”

“I think we’re all really going for the same goal,” he added. “We’re well on our way of achieving that consensus.”