Meet TESS, the Next Generation Exoplanet Hunter

Kepler was the planet counter. TESS will help us tell their stories.

Exoplanet hunter TESS may just find "all the worlds we've been dreaming about," says Sara Seager. (NASA)
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Our solar system only has eight planets (sorry, Pluto), but countless others orbit other stars in the Milky Way. So far, astronomers have not been able to scrutinize them in great detail—including searching them for signs of life—but that is about to change. The hunt for alien worlds is set to take another leap forward Monday with the launch of NASA’s Transiting Exoplanet Survey Satellite.

The satellite, known as TESS, will search for new planets around nearby bright stars, with a goal of finding planets similar to Earth in size and makeup. It is set to launch Monday night at 6:32 p.m. Eastern [Update: The TESS launch was delayed two days, but went off smoothly on Wednesday, April 18] from Kennedy Space Center.

TESS begins its mission as its predecessor, the vaunted Kepler Space Telescope, is running out of fuel and nearing its end. Kepler revolutionized the search for planets around stars other than our own, finding some 5,000 planets and planet candidates. Thanks to Kepler, astronomers now believe that most stars come with planets.

But if Kepler was a census mission, TESS will serve as an in-depth demographer, taking the measure of specific star and planet types. It is looking specifically for planets around nearby, bright stars, which other telescopes on the ground or in space can study more readily.

“A lot of the stars that Kepler found exoplanets around were extremely faint and really far away, which made them really difficult to follow up on from the ground. So TESS came about to be even more useful to the broader astronomical community,” says Natalia Guerrero, deputy manager of the TESS Objects of Interest project.

Much like Kepler, TESS will find exoplanets by looking for telltale blips in a star’s brightness that indicate a planet passing in front of it. Astronomers can use these transits to tell a planet’s size. To figure out a planet’s mass, astronomers need to take its spectrum, but TESS won’t be able to do this. Follow-up measurements are therefore key to its overall mission.

“By doing those measurements, we hope to actually identify all the worlds we’ve been dreaming about,” MIT astronomer Sara Seager, TESS’ deputy director of science, said in a news conference about the launch. That includes “hot super-Earths that may have liquid lava lakes. Or water worlds that may be 50 percent or more by mass water, like scaled-up versions of Jupiter’s icy moons. Or rocky worlds, of all different kinds and even maybe ones that have thin atmospheres reminiscent of Earth’s.”

Using its four cameras, the refrigerator-sized TESS will build a map of 85 percent of the sky, a field of vision 350 times that of Kepler’s. TESS’s search will include 200,000 of the brightest nearby stars, and it is expected to find some 20,000 new worlds during its two-year main mission. Its observational sweet spot is for red dwarf stars, which are about half the size of our sun. Proxima Centauri, the nearest star to the solar system, is one such star and it has at least one exoplanet.

While looking for exoplanets, TESS will also see other unrelated phenomena, like possible supernovae or other fast-changing objects. Guerrero says the TESS team is working on ways to share those findings quickly and publicly. Unlike many major science missions, TESS won’t have any proprietary data; everything it collects will stream into a data archive that any scientist can use right away.

“When TESS was being built, one of its taglines was ‘it’s the people’s telescope,’ ” Guerrero says. “We’re trying really hard to stay true to that goal.”

Once Guerrero’s Objects of Interest team figures out which stars to follow up, other observatories will look at them in more detail. For instance, the James Webb Space Telescope, now scheduled for launch in 2020, will scrutinize some of TESS’s planets to look for signs of life. Determining where to point JWST is one of the main goals for TESS, said MIT astronomer George Ricker, TESS principal investigator.

Monday’s launch marks the first time NASA has used the SpaceX Falcon 9 rocket for a science mission. After the launch, TESS will refine its orbit for two months before it starts collecting data. It will sail around Earth every 13.7 days, in a highly elliptical orbit that maximizes the amount of sky it can see and equals half the orbital period of the moon. The moon’s gravity will stabilize TESS without needing extra fuel, which could prolong the mission’s life.

Kepler changed the field of exoplanets, and TESS could usher in another sea change, Guerrero says.

“The headlines have changed over the past couple of years, from ‘Exoplanet Found,’ to ‘Multiple Exoplanets Found.’ We think it’s going to change again now, to ‘New Thing About Exoplanets Has Been Found,’ ” she says.

About Rebecca Boyle

Rebecca Boyle is an award-winning freelance journalist based in St. Louis, Mo. She is a contributing writer for The Atlantic and her work regularly appears in New Scientist, FiveThirtyEight, Quanta and other publications for adults and kids.

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