If humans ever leave this solar system, they probably won't do it aimlessly. More likely they'll set a course for some distant waypoint, perhaps another solar system, to visit, study, or maybe even settle. And when they do, there's a good chance the destination they choose will have been discovered by NASA's new planet-hunting spacecraft.
Called the Transiting Exoplanet Survey Satellite, the instrument will soon hitch a ride to space aboard one of SpaceX's Falcon 9 rockets. There, from a highly unusual orbit, TESS will lead a search for planets beyond our solar system even more ambitious than the one mounted by its predecessor, the Kepler Space Telescope, by searching for rocky alien worlds in our immediate galactic vicinity—the kinds of places humans, or at least human-engineered probes, could reach within a human's lifetime.
Like Kepler, TESS is designed to detect small dips in the light emanating from stars. Those dips can serve as clues that an orbiting planet is moving across the face of its parent star, preventing some of its light from reaching the spacecraft in a phenomena astronomers call a transit.
Kepler's use of the transit method fundamentally changed our view of the universe. Thirty years ago, astronomers knew of only nine (now eight) planets—those which comprise our solar system. Throughout the '90s and early aughts, scientists found a handful of planets orbiting other stars; but as recently as a decade ago, it was still unclear whether so-called exoplanets were rare or commonplace in the galaxy. Also unknown was the prevalence of potentially habitable worlds—celestial bodies that, like Earth, were neither too hot nor too cold to harbor liquid, life-sustaining water.
But Kepler, which launched in 2009, changed that in a big way. It surveyed but a tiny patch of sky—but inside that tiny patch, Kepler has found, by the latest tally, more than 2,300 exoplanets, dozens of which could host liquid water. Based on Kepler's sampling, astronomers now believe that the Milky Way's planets could outnumber its stars, and that our galaxy could be home to billions of potentially habitable worlds.
And TESS? TESS is designed to find and study the exoplanets closest to Earth.
Kepler performed what astronomers call a deep, narrow survey of the heavens; it peered at a small segment of sky, at stars between several hundred and several thousand light years away. TESS's survey, in contrast, will be broad and shallow. It's designed to survey 85 percent of the sky—an area 400 times larger than the one monitored by Kepler—with four wide-field 16.8-megapixel optical cameras.
Each camera has seven lenses, which funnel light from the heavens toward four CCD image sensors. A single camera can cover a patch of sky 24 degrees wide by 24 degrees high. "It's a huge field of view—wide enough to fit the Orion constellation," says astrophysicist Padi Boyd, head of NASA's Exoplanets and Stellar Astrophysics Laboratory in the Astrophysics Science Division and the director of TESS's guest investigator program.
Stacking the images from all four cameras affords TESS a view of the sky 96 degrees high, enough to span an entire, 90-degree latitudinal segment of the northern or southern hemisphere. Every month, TESS will direct its gaze toward a different segment of sky and soak up all there is to see. Then it will rotate toward an adjacent segment and stare. Rotate and stare. Rotate and stare. In this fashion, TESS will scan most of the sky for the roughly 200,000 brightest, closest stars; any planets it identifies will be between just 10 and 300 light years from Earth. It will cover the southern hemisphere its first year in operation and the northern hemisphere during its second.
"Do that, and you've got a census of the solar neighborhood," says astrophysicist George Ricker, a senior researcher at the MIT Kavli Institute for Astrophysics and Space Research and leader of the TESS mission. He and his team expect to catalog some 20,000 new exoplanet candidates. They anticipate 500 of those will have radii less than twice that of Earth's—small enough, astronomers suspect, to be rocky, yet big enough to harbor an atmosphere. The proximity of these planets to Earth will enable researchers to study their masses and atmospheric compositions in follow-up studies, using ground-based instruments and future spacecraft like the James Webb Space Telescope.
But before TESS makes any of this possible, it'll need to enter into an unusual orbit around Earth. Its path will extend about as far out as the moon before sweeping back for a close pass of the planet once every 14 days. When TESS comes in close to Earth, it will beam down data at a higher bandwidth. When it's far away, it'll avoid radiation and temperature swings that can impede its performance. This highly elliptical orbit—which has never been attempted by a spacecraft—will enable TESS to have the best of both worlds.
"We are setting the stage for the future of exoplanet research—not just for the 21st century, but the 22nd century and beyond," says Ricker. "Even 1,000 years from now, TESS will be remembered for establishing the best and brightest systems in our solar neighborhood."