The Transiting Exoplanet Survey Satellite (TESS) will pick up where the Kepler telescope left off, finding thousands of planets that orbit other stars.



The mission will map 85 percent of the sky over the next two years.

The planets that TESS discovers will be prime candidates for future telescopes to study in the search for extraterrestrial life.

There are three basic ways to search for extraterrestrial life: flying spacecraft to planets and watery moons in the solar system to search for microbes, listening to the galaxy with large radio telescopes for signals from any technological civilizations, and probing planets that orbit other stars for signs of habitability.



That third field of research is a relatively new area of space science. But the search for habitable worlds is about to get a shot in the arm. NASA's next space telescope, the Transiting Exoplanet Survey Satellite (TESS), set to launch on Monday, April 16 on a SpaceX Falcon 9 rocket, will increase the catalog of known exoplanets by perhaps 400 percent.

This animation shows how a dip in the observed brightness of a star may indicate the presence of a planet passing in front of it, an occurrence known as a transit.

TESS is a survey telescope designed to scan 85 percent of the sky and measure the brightness of stars within about 300 light-years. When a planet passes in front of one of those nearby stars, the telescope will see a small, temporary dimming of the star's brightness.

The Kepler space telescope used the same "transit method" technique to find more than 2,500 exoplanets, plus more than 2,500 additional candidates waiting to be confirmed. About 30 of these worlds are rocky, roughly the size of Earth, and about the correct distance from their host stars to have liquid water. But Kepler will run out of fuel in the coming months, ending a nearly decade-long mission.



TESS is up next. George Ricker, principal investigator of the TESS mission and director of MIT's CCD imaging laboratory, tells Popular Mechanics that the new haul of planets—and especially nearby worlds—could be staggering. “It’s quite possibly going to be 10 to 20 thousand new planets."

Finding Planets Beyond the Solar System

The integration of the spacecraft with TESS’s four wide-field cameras, built at MIT, inside a thermal vacuum chamber at Orbital ATK where the survey satellite was tested for a month in December and January of 2017. MIT

"TESS is really a finder scope," Ricker says. "The main thing that we're going to be able to do is find a large sample from which the follow-up observations can be carried out in decades, even centuries to come."

In its search for new worlds, TESS will find dozens of rocky planets about the size of Earth—and even more bigger rocky planets known as a super-Earths—orbiting stars that are about the size of the sun. However, the telescope is optimized to find planets orbiting M-dwarf stars, also called red dwarfs. TESS was designed this way because Kepler has revealed that small and cool M-dwarf stars, like TRAPPIST-1, are the perfect places to search for planets.

"M-dwarfs actually have more planets per host than any solar-type stars, on average, by about a factor of two," Ricker says. Also, "it turns out that M-dwarfs are about 10 times more abundant in the galaxy—at least in the Milky Way—than are solar-type stars."

Planets orbiting M-dwarfs are easier to spot, too, because they orbit closer to their host stars. If a planet around an M-dwarf were to have life, it would need to be pretty close to the small star to get enough heat and energy, making it likely that TESS will spot the world during the telescope's initial two-year survey.



"The orbital year, as it were, for a planet that's in a habitable zone around an M-dwarf is only about 10 to 14 days," says Ricker. "So that means you get many more transits in a given observing time. All of those factors work to make M-dwarfs a more promising hunting ground."

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TESS is expected to find 3,000 to 4,000 planets orbiting M-dwarfs. It also will find gas giants bigger than Jupiter, so-called mini-Neptunes, and worlds far stranger than anything in our solar system, such as scorching-hot behemoths with titanium in their atmospheres. The study of exoplanets has revealed that our solar system is an anomaly, and most of the planets that orbit other stars don't look like the ones close to home. TESS will help astronomers get a better sense of how common various types of planets are across the galaxy.

Four Wide Eyes

To scan thousands of stars for transits, TESS will use four wide-field 16.8-megapixel cameras built at MIT that can each cover a square measuring 24 x 24 degrees across the sky. That's large enough to encompass an entire constellation. Together, the four cameras cover a 96-degree strip of the sky.

The space telescope will be placed in a high polar orbit, looping around the poles in a large ellipse that will take the telescope out about as far as the moon and then back in closer to Earth so it can downlink data to the Deep Space Network. TESS will observe one strip of sky for about 27 days before moving on to a new area, ultimately creating a map of 85 percent of the sky.

TESS will observe the southern skies first, and then the north. The survey strips will overlap near the celestial poles, creating pockets of sky that will have longer observation times. Conveniently, the patches of sky that will be observed the most by TESS are also in ideal viewing locations for the future James Webb Space Telescope, which will be able to study planets that TESS finds in more detail. MIT

The area to be observed by TESS is about 350 times larger than what Kepler saw. But with less time to collect light in any one part of the sky, TESS will need to look at stars that are closer and brighter. (By fixing its telescope on one patch of sky, Kepler could collect enough light to detect exoplanets out to about 3,000 light-years.) Even so, creating the first nearly full-sky map of nearby exoplanets will yield thousands of new worlds, all of which will be close enough to study in more detail, including the candidates for extraterrestrial life.

"The TESS survey should be comprehensive enough that we should be well positioned to essentially find all of these candidates, and certainly the ones that are most promising," Ricker says.



Finding Life on an Exoplanet

Red dwarfs are the easiest places to hunt for new planets, but would such a star make a good home system for aliens? The jury is still out.

Small red dwarf stars are volatile, especially during their early lives. A large flare was recently observed erupting from Proxima Centauri, a red dwarf that happens to be the closest star to the sun, only 4.2 light-years away. Flares like this could strip any orbiting planets (such as the planet around Proxima Centauri) of the gases that would otherwise form protective atmospheres. Also, planets orbiting red dwarfs tend to be tidally locked with their host stars, meaning the same side always faces the star, casting half the world in perpetual heat and brightness and the other half in endless frigid night.



But life could find a way. Red dwarfs have lifespans of trillions of years, making the earliest ones to form almost as old as the 13.8-billion-year-old universe—plenty of time for life to take root. Perhaps alien critters swim in the bottoms of the oceans on these worlds, or stay burrowed underground for protection from radiation. "To try to say you would exclude the possibility of life ever developing in those systems... most astrobiologists wouldn't go that far," Ricker says.

Diagram of the orbits of the rocky planets in the solar system, the seven TRAPPIST-1 planets, and Jupiter’s four largest moons. The TRAPPIST-1 planets, though they are about the size of Earth, orbit their small host star in about the same time that the Galilean moons orbit Jupiter. NASA/JPL-CALTECH

If life is hidden in the seas or buried underground, then it still might leave telltale signs of its existence in the atmosphere, such as higher-than-normal amounts of oxygen or methane. Earth has an atmosphere composed of 21 percent oxygen thanks to the work of photosynthesizing plant life, and for much of the planet's early history, microscopic life is known to have consumed and produced methane. The right ratio of methane and carbon dioxide has been identified as a particularly good biosignature, or a possible sign of life.

But TESS won't be looking for biosignatures. That task will fall to the next generation of telescopes—the much-delayed James Webb Space Telescope now scheduled to launch in 2020, and enormous ground-based observatories coming online in the mid- to late-2020s such as the 39.3-meter Extremely Large Telescope (ELT) and the 24.5-meter Giant Magellan Telescope (GMT).

These telescopes will be able to measure the spectra of light passing through exoplanet atmospheres as the planets transit their stars. The absorption of some wavelengths of starlight by a planetary atmosphere reveals its chemical composition, and possibly, a clue hinting at living organisms below.

The only way to be sure that an exoplanet harbors life, however, might be to fly there for a closer look.

Interstellar Probes

Imagine if we could send a small probe, no more than a computer chip with a smartphone-sized camera and mini transmitter, to a nearby star. Scientists say they may be able to use laser propulsion and light sails to accelerate these probes to around 20 percent the speed of light, cutting the trip to Proxima Centauri to just over 20 years. Breakthrough Starshot, funded by Russian billionaire Yuri Milner and partnered with Cornell University, is the most active group attempting to achieve interstellar laser flight.

Concept image of a spherical light sail being accelerated by laser propulsion from Earth. Michael Stillwell

"They don't seem crazy to me at all," says Ricker of proposals for interstellar probes. "They think that they're maybe 10-20 years away from being able to demonstrate those technologies, so maybe it'll take twice that long."

Scientists could make dozens or even hundreds of these mini-spacecraft to launch at essentially the same time. With the nearby exoplanets TESS will discover, astronomers will have plenty of flyby targets to choose from.

"There are 100 stars in a shell with a radius of 20 light-years [from Earth]," says Ricker. "If you can reach 20 percent of the speed of light... you can get there in 100 years. So 100 candidate planets could be investigated on a time horizon of the order of 100 years. So if this is something we can start out in 2100, then by 2200, you'd know the answer."

An interstellar mission is ambitious, but the laws of physics don't prevent one. If designers can overcome the engineering challenges—such as making the craft ablative so they can lose some bits to interstellar dust collisions and still function when they arrive—then little spacecraft could be zipping by other stars in a few generations.

"It's a long time, but you know, we spent a long time building cathedrals and doing all kinds of other things that took more than one human lifetime," says Ricker. "To me that's one of the most exciting things... that the objects that are going to be targets for these interstellar probes—there's a good chance that a sizable number are going to be objects that TESS discovered."

Illustration of the Transiting Exoplanet Survey Satellite (TESS) in front of a lava planet orbiting its host star. TESS will identify thousands of potential new planets for further study and observation. NASA/GSFC

Once TESS has completed its two-year survey, the telescope could receive a mission extension from NASA for another three years of planet hunting. After that, if the craft is healthy, the search could go on. Kepler had a primary mission of 3.5 years, but has been finding new worlds for nine years and counting. The TESS spacecraft was built to last decades, powering its electronics and telescopic cameras with solar panels.



"The special orbit that we designed for TESS allows us to avoid the need for using fuel for station keeping," Ricker says. "So the estimates that we've made, if we're fortunate and nothing happens during the mission, it could readily last 20 years."

In those two decades, TESS could pinpoint tens of thousands of planets. Perhaps one of them will prove to be a fellow pale blue dot.

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