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ExoplanetSat illustration
ExoplanetSat, a triple Cubesat only 34 centimeters long, would be able to observe a single star to look for transiting exoplanets. (credit: MIT)

Innovations in exoplanet search

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Since the mid-1990s, astronomers have discovered a staggering number of extrasolar planets, or exoplanets: planets orbiting other stars. The Extrasolar Planets Encyclopaedia, the de facto catalog of such worlds, has 707 exoplanets listed as of last Friday, a number growing on a regular—almost daily—basis.

The vast majority of those planets have been found by two techniques. One, radial velocity, measures the periodic Doppler shift in stars caused by the wobble induced by the gravity of planets orbiting that star. The other, transit, measures the faint decrease in brightness of the star when a planet crosses (or transits) the disk from the observer’s point of view. The former technique is widely used by teams of astronomers from primarily groundbased telescopes, while the latter, also used by terrestrial observers, is also used by spacecraft like CoRoT, by the French space agency CNES; and NASA’s Kepler mission. This week, scientists are gathering at NASA’s Ames Research Center in California for the First Kepler Science Conference to discuss the exoplanet discoveries and related research enabled by that spacecraft.

The growing numbers of exoplanets and increasing interest in this field have created ambitions for more, and larger, spacecraft missions to discover and study these worlds, particularly those planets that may be similar to Earth in size, orbit, and, potentially, habitability. Those ambitions, though, are tempered by competing demands for funding in an era where flat budgets may be the best scientists can hope for in the foreseeable future. This has led to some innovative new concepts to take advantage of new technologies and capabilities that could open new doors for exoplanet research at lower costs.

A nanosat exoplanet hunter

A decade ago, the future of exoplanet spacecraft missions was bigger and better. As part of its Origins program, NASA had planned a series of spacecraft to search for and even directly observe terrestrial exoplanets: the Space Interferometry Mission (SIM), Terrestrial Planet Finder (TPF), and ultimately Planet Imager. But a combination of limited funding and competing priorities has, at best, indefinitely delayed those plans, if not outright canceled them (see “SIM and the ‘ready, aim, aim’ syndrome”, The Space Review, October 18, 2010).

“There aren’t any current missions to monitor the brightest Sun-like stars in the sky to look for Earth-sized planets,” said Smith.

Now, some researchers are moving in the opposite direction. Instead of bigger and better—and far more expensive—satellites, one group is looking to see how small a spacecraft can be and still perform studies of extrasolar planets. ExoplanetSat, a joint project of MIT and Draper Laboratory, is proposing to develop satellites small enough to literally be handheld and yet powerful enough to look for planets around other stars.

ExoplanetSat would work like Kepler, looking for minute, periodic drops in brightness caused by a transiting exoplanet. Kepler, though, is focused on a single field of largely distant stars, useful for collecting statistics on the frequency of exoplanets but not on making studies of specific stars. “There aren’t any current missions to monitor the brightest Sun-like stars in the sky to look for Earth-sized planets,” said MIT’s Matthew Smith in a presentation about the mission at the AIAA/Utah State University Conference on Small Satellites in August in Logan, Utah.

ExoplanetSat would focus on a single star at a time, with the initial prototype satellite likely to study Alpha Centauri. The satellite, located in Earth orbit, would carry out observations during orbital night, stitching together the observations to look for any drops in brightness that could be caused by a transiting planet. Over time, additional spacecraft could permit continuous observation of a given star, as well as observe other stars: the project’s long-term plan calls for a fleet of satellites to enable observations of at least 250 stars.

The key to making such a constellation of satellites feasible is by making each satellite very small and inexpensive. ExoplanetSat, as currently designed, is a “triple”, or 3U, Cubesat: three Cubesats, 10 centimeters on a side, combined into a single spacecraft weighing only a few kilograms. (The spacecraft is actually a slightly stretched version of a pure 3U Cubesat, with a length of 34 centimeters.) The 3U has become a popular design for small satellite developers, leveraging the hardware and design interfaces of the original Cubesat for missions that require a somewhat larger spacecraft (see “A quarter century of smallsat progress”, The Space Review, September 6, 2011).

MIT is seeking a 2013 launch for ExoplanetSat as a secondary payload, if they can find a launch going to a suitable orbit.

The satellite’s scientific payload features a “telescope” that, Smith said, is effectively just a ruggedized SLR lens. It is used by both a science detector and a fine guiding imager, linked to a “two-axis piezoelectric nano-positioning stage” to ensure image stability. That payload occupies roughly one-third of the spacecraft. The remainder of the spacecraft is taken up by momentum wheels and torque coils used for attitude control, as well as batteries, computers, and communications systems needed to operate the satellite and exchange data with ground stations. “Basically we’re combining the low-cost Cubesat platform with fine attitude control to get the kind of precision that we need,” Smith said.

The prototype ExoplanetSat is under development at MIT and will be ready for launch in 2013. The program has a reservation with NASA’s Educational Launch of Nanosatellites (ELaNa) program, which provides secondary payload, or rideshare, opportunities for university-built Cubesats (see “New opportunities for smallsat launches”, The Space Review, August 22, 2011). A challenge, though, has been finding a slot on a launch going into the project’s preferred orbit: a 650-kilometer, low-inclination orbit, in order to minimize atmospheric drag as well as exposure to radiation from the South Atlantic Anomaly and the poles.

Smith said last week they’re considering different orbits in order to find a compatible launch opportunity. “We have been evaluating a wider range of altitudes and inclinations, and our analysis shows that other orbits are acceptable,” he said. ExoplanetSat does have a minimum altitude of 450–500 kilometers because of atmospheric drag issues and orbit lifetime; he said that higher orbit inclinations would be considered on a case-by-case basis.

A suborbital path to observing new worlds

While transit observations, like those being performed by Kepler now and planned by ExoplanetSat, as well as radial velocity observations are important to discovering new exoplanets, they are still only indirect means of observing these worlds. For many astronomers, the Holy Grail of exoplanet science remains direct observations of exoplanets. This is a monumental challenge, primarily because planets are lost in the glare of the far brighter stars they orbit.

What if there was a way to block the star’s light? The idea of using an occulting disk, or coronagraph, to mask the light of a star is not a new one: a coronagraph on Hubble blocked the light from the bright star Formalhaut, allowing astronomers to directly image a giant planet three times the mass of Jupiter orbiting it. Such direct detections, though, are technically challenging with current telescopes, and thus rare.

“If it’s within 10 to 15 parsecs, we will definitely be able to see exoplanets and be able to study them,” Cash said of the New Worlds Observer concept.

One mission concept would fly a space-based coronagraph to enable the detection of Earth-sized exoplanets. The New Worlds Observer, as originally envisioned, would place a four-meter telescope at the Earth-Sun L2 point along with an occulting disk, which it calls a starshade, flying in formation at a distance of 18,000 kilometers. (The disk could also be used with other space telescopes, like the James Webb Space Telescope, or JWST.) The starshade’s intricate design—it resembles an exotic flower—is designed to eliminate the diffraction of starlight around the edges of the disk, so that the telescope can more effectively look for dim planets.

In a presentation at the SpaceVision 2011 conference in Boulder, Colorado, in October, Webster Cash of the University of Colorado showed simulations of what a telescope equipped with such a starshade could see of our solar system. A hypothetical 10-meter telescope with a starshade, both located 30 light-years away, could easily detect Venus and Earth, he noted. At 2.4 meters—the diameter of Hubble—Venus and Earth are just at the limit of detection at the same distance. “If it’s within 10 to 15 parsecs, we will definitely be able to see exoplanets and be able to study them,” Cash, the principal investigator of the New Worlds Observer concept, said.

The problem, though, is that such a dedicated system would be expensive to develop, making NASA—suffering from sticker shock from the rising costs of JWST—reticent to fund such a mission, especially given that the starshade concept has yet to be demonstrated outside the lab. Webb is seeking to build support for the concept through subscale demonstrators. “What we really want to do is to use smaller starshades now and learn whether they really work,” he said. “Recently I’ve been putting a lot of work into how do we do this fast, how do we do it cheap.”

An initial step would be to place the starshade on a mountaintop several kilometers from a telescope; even a telescope as small as 20 centimeters would be enough to demonstrate the starshade’s effectiveness in blocking out starlight. Another option would be to place a telescope and a starshade on separate balloons (one of them a dirigible able to maneuver in order to maintain alignment with the other) in the stratosphere, which could be sufficient to look for planets orbiting Alpha Centauri.

Despite numerous failed efforts to win technology development funding, “we’re going to keep pushing to this, and we expect to succeed,” Cash said.

A more intriguing concept, though, brings together exoplanet searches with the entrepreneurial NewSpace industry, specifically, reusable suborbital vehicles. The suborbital RLV would carry the starshade to altitude and hover for several minutes, while a groundbased telescope made observations. “You can’t do this with a regular rocket because it can’t hover and control its position, but the new generation of suborbital [vehicles] has this capability,” Cash said, adding that he’s studied this idea in cooperation with one suborbital vehicle developer, Masten Space Systems. “Working on this interface between what are the new technologies and what are the things you really can do with them is very interesting.”

Cash’s current obstacle, though, is winning funding to further advance the concept. He told an meeting of suborbital vehicle developers and researchers at NASA’s Goddard Space Flight Center in September that he has had nine proposals to develop starshade-related technology rejected, including two a few weeks before the meeting, even though such technology development was identified a top priority for medium-sized projects in the latest astronomy decadal survey, published last year. “Something is amiss out there,” he concluded.

Cash, though, told the SpaceVision audience he planned to keep working on this concept. “We’re going to keep pushing to this, and we expect to succeed,” he said. With NASA budgets unlikely to be able to accommodate flagship-scale dedicated exoplanet missions for the indefinite future, it may be innovate approaches like this that hold the best prospects of realizing astronomers’ ultimate dreams of observing another Earth.