Most of the exoplanets will be orbiting stars called red dwarfs, much smaller and cooler than the sun. They make up the vast majority of stars in our neighborhood (and in the universe) and presumably lay claim to most of the planets.
Like Kepler, Tess will hunt those planets by monitoring the light from stars and detecting slight dips, momentary fading indicating that a planet has passed in front of its star.
The mission’s planners say they eventually expect to catalog 20,000 new exoplanet candidates of all shapes and sizes. In particular, they have promised to come up with the masses and orbits of 50 new planets that are less than four times the size of the Earth.
Most of the planets in the universe are in this range — between the sizes of Earth and Neptune. But since there are no examples of them in our own solar system, as Dr. Seager notes, “we don’t know anything about them.”
Are they so-called “superearths,” mostly rock with a veil of atmosphere, or “mini-Neptunes” with small cores buried deep inside extensive balls of gas?
Data from Kepler and astronomers suggests that the difference is mass: fertile rocks often are less than one and a half times the size of the Earth, while barren ice clouds often are bigger. Where the line really is, and how many planets fall on one side or the other, could determine how many worlds out there are balls of freezing vapor or potential gardens.
“We need to make precise mass measurements,” said David Latham of the Harvard-Smithsonian Center for Astrophysics, who is in charge of organizing astronomers to follow up the Tess observations.
Credit NASA GSFC
To that end, the team has procured 80 nights of observing time a year for the next five years on a spectrograph called Harps North, which resides on an Italian telescope on the island of La Palma in the Canary Islands, a part of Spain off the coast of Africa.
Harps — for High Accuracy Radial velocity Planet Searcher — can measure the mass of a planet by how much it makes its home star wobble as it goes around in an orbit. Such measurements, if precise enough could help distinguish the composition and structure of these bodies.
Tess is one of NASA’s smaller missions, with a budget of $200 million; by comparison, Kepler had a budget of about $650 million.
Recently Tess, partly clad in shiny aluminum foil, stubby solar panels folded modestly against its side, was sitting on a round pedestal inside a plastic tent. The tent occupied one corner of a cavernous “clean room” in a remote building on the scrubby outskirts of the space center here, amid palms and canals and flocks of cormorants.
The spacecraft is about the size of a bulky, oddly shaped refrigerator, festooned not with magnets but with mysterious nozzles and connectors. Four pairs of blue-clad legs were sticking out from underneath the pedestal, as if high-tech mechanics were working under a car.
The engineers were taping plaques to the bottom of the spacecraft, including a memory chip containing drawings by schoolchildren who had been asked to imagine exoplanets might look like.
Standing to the side, in a “bunny suit” of protective material that left only his bespectacled eyes visible, Dr. Ricker was staring into the tent at his new spacecraft, as if he were watching his car get fixed, and exchanging rocket talk with the engineers who had designed and built it.
Dr. Ricker has been a rocket scientist, building astronomical satellites to be shot into space, for pretty much his entire career as a researcher at M.I.T.’s Kavli Institute for Astrophysics and Space Research.
Most of his previous projects involved measuring X-rays or gamma rays from various snaps, crackles and pops in the cosmos, most recently the High Energy Transient Explorer, used to study the cataclysms known as gamma-ray bursts.
Asked if planets represented a departure for him, Dr. Ricker shrugged, “Not so much.” All his work has involved delicate measurements of things changing, what he called “time-domain astronomy.”
The key to this work is to maintain very stable and sensitive detectors — the imaging chips that are elite relatives of the sensors in your smartphone — so that they can reliably record the changes in brightness, just a few parts per million, that signal a planet passing by its star.
Dr. Ricker said he and his colleagues had started “noodling” about a planet-finding mission back in 2006. After they lost out in a competition for NASA’s Small Explorers program, which are less expensive missions, the scientists re-entered a competition for a larger mission in 2010 — and won.
They had gone to great lengths to design a compact spacecraft that would fit the rockets NASA used for Small Explorers, and so were nonplused when NASA selected SpaceX’s Falcon 9, which can carry a much larger payload, to launch the Tess mission.