Twenty years ago you could count all the known planets in the universe on your fingers and toes and recite all their names from memory. Today you’d probably need a calculator and a spreadsheet: thousands of exoplanets—worlds orbiting other stars—fill our catalogues. Astronomers are now poised to find tens of thousands more.
Most of these strange new worlds are overheated, inhospitable and wildly diverse gas giants—“hot Jupiters” or “warm sub-Neptunes”—but some seem to be oversized versions of our own world—rocky, temperate “super-Earths.” A few almost mirror Earth in basic terms of size, mass and orbit. Statistics strongly suggest that planets must circle every star in the sky and even hint that the nearest Earth twin may be less than a dozen light-years away—practically right next door, in interstellar terms. And yet planet hunters still don’t know for certain whether such doppelgangers existWhen and how they finally find out may depend on decisions made in the near future.
Many of the most exciting discoveries to date have come from NASA’s planet-hunting Kepler mission, but that space telescope’s worlds have tended to be too dim and far away for easy follow-up investigation. Kepler’s latest mission phase, dubbed “K2,” is now looking for planets around closer, brighter stars. That search won’t hit its stride until 2017, however, when NASA plans to launch Kepler’s successor, the Transiting Exoplanet Survey Satellite (TESS). Like Kepler, TESS will search for shadowy worlds that “transit” across the faces of their suns. TESS will probably find thousands of nearby planets, hundreds of which may be small and rocky. A few of those small, rocky worlds will bask in sufficient starlight from the small, dim “M dwarf” stars they orbit to have liquid water on their surfaces.
That is, TESS should find a handful of nearby planets that could be habitable, if not even inhabited. To learn more, astronomers will have to remotely inspect each world's atmosphere, using spectroscopes to sniff out hallmarks of a clement climate such as water vapor and carbon dioxide, and perhaps even potential signs of life such as oxygen from photosynthetic plants or methane from bacteria.
In the 2020s a few ground-based “extremely large telescopes” with mirrors 30 meters or larger in diameter will seek to make some of these measurements for some of the very brightest and closest potentially habitable exoplanets that TESS and other missions may find. But consensus holds that studying such planets for signs of life is a task best done from space, high above the confounding influence of the Earth’s turbulent atmosphere.
Astronomers have already pushed the aging Hubble and Spitzer space telescopes to their limits to crudely inspect the air of some large, hot, uninhabitable worlds. Only one space observatory, NASA’s nearly $9-billion James Webb Space Telescope, slated for launch in 2018, has any hope of following up on TESS’s most tantalizing discoveries anytime soon—a point the telescope’s promoters rarely fail to emphasize when they communicate with the general public or with Congress.
Just how much hope Webb has depends on whom you ask, however. Jason Kalirai, Webb’s project scientist at the Space Telescope Science Institute is optimistic. “With [Webb] we can measure molecules like carbon dioxide, methane and water vapor in the atmospheres of warm super-Earths orbiting [M dwarf stars],” he says. Astronomer Jeff Valenti, also at the institute, is more uncertain of Webb’s prospects. “Nature has yet to speak on whether Webb can succeed or not in characterizing potentially habitable worlds around M dwarfs,” he says. “Will we try to do it? I’m pretty sure we’re gonna try. Will it be conclusive? I doubt it.”
Looking for life, finding hot air
With eagle-eyed infrared vision from its cryogenically cooled detectors and tremendous 6.5-meter mirror, Webb should be able to examine the atmospheres of some of TESS’s transiting planets. When a planet is in transit, and backlit by its star, Webb can attempt to study that world’s upper atmosphere by gathering the starlight that shines through, gaining information about the atmosphere’s composition and structure. Webb can also help estimate the planet’s dayside temperature by watching for telltale shifts in stellar brightness that occur when the planet is out of transit, such as when the world passes behind its star and is eclipsed.
The debate over just how far Webb can go in the search for life beyond the solar system hinges largely on luck. If TESS doesn’t find any tantalizing super-Earths around some of the nearest M dwarf stars to our sun, Webb won’t have much to work with. Webb was designed to study the universe’s first stars, not potentially habitable worlds, and its instruments were defined long before astronomers realized the potential of transiting exoplanets. Examining Earthlike planets around bigger, brighter, more sunlike stars will be out of Webb’s reach, requiring new, risky optical technologies that cannot be added to the telescope in time for launch.
The safest bet is that, for at least one potentially habitable super-Earth transiting a nearby M dwarf, Webb will be able to determine that the planet simply possesses an atmosphere. Even so, it might be a stretch to call any M dwarf world Webb will study “potentially habitable.” M dwarf stars pose a wealth of uncertainties and challenges for planetary habitability: Worlds in their habitable zones can be pummeled by very powerful stellar flares and may also be tidally locked, with one hemisphere continuously facing the star.
Simulations suggest that for Webb to readily detect water vapor on a rocky planet transiting an M dwarf, that planet would probably either need to be smothered beneath a thick blanket of hydrogen or to have a temperature in excess of 120 degrees Celsius. According to a new study from the Exoplanet Exploration Program Analysis Group (ExoPAG), an influential committee of scientists that helps NASA prioritize and plan missions, such measurements would require months of observation time per small, rocky world. Despite having what may be dismal prospects for life, each “potentially habitable” target Webb investigates would require a heavy investment of telescope time. “Even if we did detect water on one of TESS’s rocky transiting planets, that would probably be bad news for habitability,” says Nick Cowan, an astronomer at Amherst College in Massachusetts and lead author of the new study. An Earthlike planet with a sufficiently water-rich upper atmosphere for Webb to detect it would probably be in the process of losing that water to space and becoming a dry, hot world like Venus, Cowan says.
Given that Webb must be shared with the entire astrophysics community during its five-year primary mission, and that only perhaps 25 percent of its observing time would be available for studying transiting exoplanets, scientists are now debating just how much emphasis the telescope should place on studying borderline habitable worlds. After all, whereas Webb will struggle with those worlds, delivering low-quality observations that may raise more questions than answers, the telescope can more easily provide unprecedented new data about the formation and atmospheric evolution of larger, hotter, utterly uninhabitable planets. “TESS and Webb are taking us from a target-limited to a time-limited regime with transiting planets,” says ExoPAG chair Scott Gaudi, an astronomer at The Ohio State University. “It’s not that Webb can’t examine potentially habitable planets; it’s that it might take more time than we can realistically get…. If you add up all the time likely to be available, you can imagine either doing a large survey of many hundreds of hot Jupiters and warm sub-Neptunes or spending that time studying a few transiting, temperate terrestrial planets.”
In search of lost time
Figuring out how best to utilize Webb will be a delicate task for the telescope’s time-allocation committees, which will begin their deliberations well before the telescope’s 2018 launch. Although Webb could study many more uninhabitable planets than habitable ones, “in the case of many of these hot Jupiters, I think Webb is in some sense overkill,” Gaudi says. “It’s like using a sledgehammer to drive in a penny nail…. You could get the job done with something smaller. And there is the argument that you should use your best, most advanced and expensive facility for the things that it is uniquely capable of doing, even if it’s not perfect”—like observing super-Earths.
One potential solution to the impasse, according to ExoPAG, would be to outsource Webb’s potential studies of gas giants to a different space mission, a much smaller and cheaper space telescope designed solely to survey the atmospheres of hundreds of transiting hot Jupiters and warm sub-Neptunes. Such a mission could conceivably be built and launched by the mid-2020s, when Webb will be approaching its twilight years.
Astronomers have proposed such missions in the recent past, with less-than-stellar results. In 2013 NASA passed over a proposal called FINESSE (Fast Infrared Exoplanet Spectroscopy Survey Explorer) in favor of TESS. Last year the European Space Agency chose PLATO (Planetary Transits and Oscillations of Stars)—a mission much like Kepler—for a launch in 2024 rather than EChO (Exoplanet Characterization Observatory), a mission to survey gas giant atmospheres. Now that the harsh reality of Webb’s limited observing time is sinking in, both teams behind these proposals are reconsidering their chances. The European EChO team has already submitted a new proposal, dubbed ARIEL, and the American FINESSE team is rumored to be developing a new proposal as well.
“Part of the exoplanet community wants to find and study our nearest cousins, the Earth twins, to try to detect life on them, but there’s another facet of the community that loves and wants to understand the whole family of planets,” says Mark Swain, an expert in exoplanet atmospheres and FINESSE team lead at NASA’s Jet Propulsion Laboratory. “These are different points of view but maybe they end at the same place,” Swain adds. “I think ultimately to establish that life is present on a terrestrial planet orbiting another star, we’ll have to understand how its atmosphere works, and that requires putting it in the context of planetary atmospheres in general. A large-scale survey mission would provide that broad context.”
Whether or not a transit survey space telescope materializes to complement Webb, NASA’s post-Webb projects are already in motion. After Webb the agency plans to launch the 2.4-meter WFIRST (Wide-Field Infrared Survey Telescope) sometime in the 2020s to study dark energy and take snapshots of gas-giant exoplanets. Many planet hunters have pinned their hopes for finding alien Earths on whatever may follow WFIRST in the 2030s or beyond, dreaming of a 10- or 12-meter space telescope that would be purpose-built to directly image other Earthlike planets around large numbers of nearby sunlike stars. But optimizing a telescope for that herculean task would also compromise its ability to deeply investigate promising planets around smaller, dimmer stars. Such a telescope could in theory study hundreds of mirror Earths orbiting stars like our own, but at the cost of only probing perhaps a dozen potentially habitable worlds around the very nearest, brightest M dwarfs.
Consequently, the next great turf war in exoplanets may erupt over seeking true-blue Earth twins versus studying the more alien M dwarf worlds that Webb could begin to reveal. “I don’t think Webb will hit the ball out of the park in the search for life,” Amherst’s Cowan says. “It can get us on base with these M dwarf planets but there’s nothing in the works that can bring us home. Every next-generation mission is going after Earths orbiting sunlike stars instead. Imagine if we get tantalizing observations of these M dwarf planets and learn they have atmospheres, then just put them on the back burner for 30 years. That could be a problem.”