Less than a year after launch, TESS is already finding bizarre worlds

The first few exoplanets nabbed by the telescope are unlike any yet seen

TESS telescope

WHOLE NEW WORLDS  The TESS exoplanet hunting telescope caught eight new planets in its first four months of observing. Three are shown in this artist’s illustration.

NASA’s Goddard Space Flight Center (edited by MIT News)

SEATTLE — The next generation exoplanet hunter is coming into its own. NASA’s Transiting Exoplanet Survey Satellite, or TESS, has already found eight confirmed planets in its first four months of observing — and some are unlike anything astronomers have seen before.

“The torrent of data is starting to flow already,” TESS principal investigator George Ricker of MIT said January 7 in a news conference at a meeting of the American Astronomical Society.

TESS launched in April and began science observations in July (SN: 5/12/18, p. 7). It was designed to be a follow-up to the prolific Kepler space telescope, which went dark in October after almost a decade of observing (SN Online: 10/30/18). Like Kepler, TESS searches for planets by watching for dips in starlight as planets cross, or transit, in front of their stars.

Unlike Kepler, which stared unblinkingly at a single patch of sky for years, TESS scans a new segment of sky every month. Over two years, TESS will cover the entire 360 degrees of sky visible from Earth’s orbit.

In the first four segments, TESS has already spotted eight confirmed planets and more than 320 unconfirmed candidates, said Xu Chelsea Huang of MIT. And several of them are downright strange.

Pi Mensae c
WATER WORLD The exoplanet Pi Mensae c, as seen in this artist’s illustration, has the same density as pure water. TESS/MIT/NASA
Take the third-found planet, HD 21749b . Only 52 light-years away, it has the lowest temperature known for a planet orbiting a bright, nearby star, astronomers reported at the meeting and in a paper posted at arXiv.org on January 1.

That makes it a great candidate for follow-up observations with future telescopes like the James Webb Space Telescope, scheduled to launch in 2021. Webb will use starlight filtering through the atmospheres of planets like this one to measure those atmospheres’ properties and search for signs of life (SN: 4/30/16, p. 32).

“If we want to study atmospheres of cool planets, this is the one to start with,” Huang said.

“Cool” is a relative term. This particular planet is still probably too hot and gassy to host life. Its orbit takes 36 Earth days, the longest known orbital period for planets transiting bright stars within 100 light-years of the sun.

That leaves it at a distance from the star that should heat the planet’s surface to about 150° Celsius, too hot for liquid water. And at 2.84 times Earth’s size and 23.2 times Earth’s mass, its density suggests it must have a thick atmosphere, unlike Earth’s life-friendly one.

But it’s still worth checking out, says astronomer Diana Dragomir of MIT, a member of the TESS team. Despite its heat, this planet is “tepid” compared with most of the scorched worlds whose atmospheres astronomers can probe right now, she says, so closer to an Earthlike system. Smaller, cooler, more Earthlike worlds are few and far between, and may not orbit such bright stars.

Finding more longer-period planets “helps you explore the diversity of planets that are out there,” says astronomer Paul Dalba of the University of California, Riverside, who studies exoplanet atmospheres but was not involved in the TESS discovery. Because TESS spends such a short stretch of time looking at each segment of the sky, astronomers expect most of its planets to have shorter years than an Earth month. “The fact that we’re already getting one that’s longer period I think is just really exciting, showing that TESS isn’t just for the shortest-period exoplanets.”

LHS 3844b
LAVA LAND The exoplanet LHS 3844b, illustrated, is slightly bigger than Earth, and orbits its star every 11 hours. It’s probably a lava world, scientists think. TESS/MIT/NASA
The other planets in TESS’s first haul are equally exotic. TESS’s first find, Pi Mensae c, was reported in September ( SN Online: 9/18/18 ). The planet orbits its star every 6.27 days, and is about 2.14 times Earth’s size and 4.8 times Earth’s mass, giving it a density similar to pure water.

The weirdest thing about that super-Earth is the company it keeps, Huang said. Previous observations showed that the star Pi Mensae also has a planet 10 times the mass of Jupiter that orbits every 5.7 years. That planet, Pi Mensae b, revolves on a wildly eccentric orbit, swinging between the distance of Earth and the distance of Jupiter from its star.

“This is the most extreme system we know of that has this type of architecture,” Huang said.

Theories of how planets develop such wonky orbits suggest that this super-Jupiter should have booted Pi Mensae c out of the system (SN: 5/12/18, p. 28). “We are really surprised that the inner super-Earth actually survived that disruptive event,” Huang said. “It’s a mystery we really want to understand.”

The second planet found by TESS, LHS 3844b, has a radius just 1.3 times Earth’s. But it swings around its star every 11 hours, giving it a surface temperature of about 540° C, Huang said. “It’s likely a lava world.”

TESS has completed about one-twelfth of its first sky survey, but Ricker is already writing proposals to extend its initial two-year mission. TESS’s orbit is held stable by the moon’s gravity, so it doesn’t need to spend any fuel to stay put. The fuel on board, used to change the direction the telescope points, is enough to last for 300 years.

“The orbit itself was designed to be extremely stable on timescales of decades to centuries,” Ricker said. “TESS is really going to be an important part of our astronomical efforts for the next decade and for more to come.”

Editor’s note: This story was updated January 29, 2019, to correct the description of the planet LHS 3844b’s orbit. It orbits a star, not a planet.

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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