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Slow and Steady: Astronomy Advisory Report Charts a Long Road for Exoplanet Science

The influential decadal survey champions the pursuit of potentially habitable worlds but offers few details for the near future



JPL/NASA

The study of planets outside the solar system was one of the hottest corners of the science world in the 2000s, a decade that saw the known tally of exoplanets increase by more than a factor of 10. By the end of 2009, more than 400 worlds had been discovered in the young field of study, and the scientists working on NASA's Kepler planet-hunting mission were preparing to announce the first discoveries of their recently launched spacecraft.

Just how the next decade will shake out for exoplanet scientists became a little clearer August 13 with the release of a report designed to guide the fields of astronomy and astrophysics through the 2010s. The so-called decadal survey, produced every 10 years by an expert committee convened by the National Research Council, ranks funding priorities that NASA, the National Science Foundation and the Department of Energy should follow.

The report, New Worlds, New Horizons in Astronomy and Astrophysics, flagged the discovery of nearby, potentially habitable exoplanets as one of three primary science objectives for the next decade. But rather than throwing its support behind an immediate follow-on mission to Kepler, which now appears poised to pile on hundreds of exoplanet discoveries in the coming years, the committee recommended rolling an exoplanet campaign into a proposed mission to study dark energy and to defer a dedicated, flagship exoplanet mission until the next decadal survey. That deferral means that one of the prime goals of exoplanet science—to find a potentially habitable world and inspect it for signs of life—appears to remain over the horizon.

Another specific blow dealt to exoplanet hunters by the new survey was a recommendation against pursuing the Space Interferometry Mission (SIM), a space observatory that could have launched before the end of the decade. SIM's charge was to make precision measurements of stellar positions, a task known as astrometry, and to use the motions of those stars under the gravitational influence of planets to seek out nearby Earths. The mission, then known as the Astrometric Interferometry Mission, was recommended by the 1990 decadal survey and re-endorsed in 2000, but the new report deemed it too costly and one-dimensional to be competitive.

"Personally, as a working astrometrist, I found the [SIM] outcome both unsurprising and irritating," says Fritz Benedict, an astronomer at The University of Texas at Austin. "Why unsurprising? Astrometry is a hard sell. We don't make pretty pictures. Why irritating? We would have probably found Earth-mass planets in Earth-like orbits around nearby sun-like stars." Kepler should be sensitive enough to detect Earth-like worlds, but the spacecraft is focused on surveying a large number of distant stars rather than a small number of local stars whose proximity would allow more detailed study of any planetary companions.

Once these Earth-like worlds are found, a proposed mission such as the Terrestrial Planet Finder (TPF) could scan those planets for chemical signatures such as oxygen and methane that might indicate the presence of life. That was the sequence recommended by an exoplanet task force of the Astronomy and Astrophysics Advisory Committee, which advises NASA and other government agencies. The task force's 2008 report endorsed a space astrometry mission as the most viable first step toward detecting possible living planets. But the new decadal survey concluded that the target worlds for a TPF-like mission could be identified from observatories on the ground, obviating the need for SIM, even though the sensitivity of those telescopes is currently far from sufficient to carry out such a survey.

TPF itself is barely mentioned by name in the new report, but its objectives loom large in the committee's recommendations. "The top priority for a medium-class mission is technology development for an exoplanet imaging mission, which is TPF," says James Kasting of Pennsylvania State University, who has worked on one of the proposed designs for a TPF. "The mission may not be called out explicitly, but the concept is."

The committee decided that TPF as it had been proposed was premature and overambitious, says astrophysicist Roger Blandford of Stanford University, who chaired the decadal survey panel. But he adds that "the scientific rationale for TPF—to find habitable Earth-like planets—is one of the main objectives of our report." With further study, a consolidated mission of the TPF class could be outlined by the middle of the decade. "The expectation is that a flagship mission will be proposed to the next decadal survey," Blandford says.

But the report recommends that NASA spend just $4 million per year at the outset to define the mission's requirements and technological milestones, with perhaps $100 million more to be spent later in the decade after selecting a viable technology for the task. "It would be great if in 2020 Terrestrial Planet Finder was the number-one recommendation, but they haven't put enough money in to actually develop it before the next survey," says Sara Seager, an astrophysicist at the Massachusetts Institute of Technology. "Without the big projects such as SIM or Terrestrial Planet Finder, you can't find Earth-like planets around nearby stars."

Despite the fact that the decadal survey scrapped SIM and essentially sent TPF back to the drawing board, exoplanet science scored a surprising coup with the committee's top-ranked space-based mission. The committee strongly backed an observatory called the Wide Field Infrared Survey Telescope (WFIRST), which would take the Joint Dark Energy Mission proposed by NASA and the Department of Energy and tack on an exoplanet census in the Milky Way's central bulge. WFIRST would detect planets by microlensing, a phenomenon in which the gravitational fields of distant worlds bend the light from background stars.

Like Kepler, WFIRST would paint with a fairly broad brush, determining demographic information of distant planetary systems rather than fine details of nearby planets. But the inclusion of a microlensing mission in a top-ranked mission is good news for proponents of the technique. Scott Gaudi of Ohio State University, who works on the Microlensing Follow-up Network (MicroFUN), says that WFIRST, combined with Kepler and other surveys, will help build a census of planets of all kinds throughout the galaxy. "That this can be done with essentially the same instrumentation that is required to explore dark energy is a fantastic coincidence and unique opportunity, and it's great that the panel recognized this," Gaudi says.

Just how the report will shape the decade remains to be seen. As Seager notes, there is plenty to keep exoplanet researchers busy even in the absence of a high-profile flagship mission. And a major discovery in the coming years could upend the field and supersede the recommendations of the decadal survey.

One thing that is not likely to change drastically in the new decade is funding. NASA and its partner agencies in astronomy and astrophysics have limited budgets with which to work, and some projects championed in the new survey are bound to fall by the wayside, just as past recommendations such as SIM now have. Despite SIM's dismissal, Benedict says he fully supports the decadal review process. "It is a truly rational response to a harsh reality," Benedict says. "There are not infinite funds with which to do the many exciting and useful future projects I and my colleagues have imagined."

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