Three newly discovered exoplanets could help researchers redefine the shaky line between rocky and gaseous planets, according to new observations from NASA’s Transiting Exoplanet Survey Satellite (TESS). TESS, which marks its first year of operations this month, spotted the trio of planets some 73 light-years away from Earth. The exoplanets are of a type that does not exist in our solar system, being between the Earth and Neptune in size.
That makes the closely packed system, known as TOI-270, a good bet for answering long-standing questions about how such “super-Earths” or “mini-Neptunes” form. The system is within range of ground-based telescopes and soon-to-be-launched orbiting instruments such as NASA’s James Webb Space Telescope (JWST). “This will be one of the key systems for JWST to study,” says Michaël Gillon of the University of Liège in Belgium, who is following up on the TESS discovery.
Like a predecessor satellite, Kepler, TESS spots planets when they pass in front of their home star and cause its brightness to dip. But where Kepler discovered more than 2300 confirmed exoplanets staring at a distant starfield, TESS is aiming to find choice candidates closer to home that are ripe for further study. Halfway through its primary 2-year mission, TESS has found 24 confirmed exoplanets. Nine hundred more candidates await follow-up studies, says TESS Deputy Director of Science Sara Seager of the Massachusetts Institute of Technology (MIT) in Cambridge. “We’re expecting thousands,” she says.
But TESS Object of Interest (TOI) 270 is a rare find. “It’s a perfect laboratory,” says Maximilian Günther of MIT, lead author of the paper describing the system, published today in Nature Astronomy. Planetary scientists have separate theories describing the formation of our solar system’s rocky inner planets and outer gas giants, but they struggle to account for these midsize planets around other stars. “We still don’t understand how these sub-Neptunes and super-Earths form,” Günther says.
One theory suggests these two types of midsize planets are formed by the same process, except that the rocky super-Earths have had their atmospheres blown away by their stars. But that can’t explain a puzzling gap observed by Kepler: There are very few planets with sizes between 1.5 and 2 Earth radii. That suggests separate formation processes for super-Earths and mini-Neptunes. TOI-270, with one planet at 1.25 Earth radii and two others at 2.13 and 2.42, bridges the gap. “There are only a handful of systems showing these two types of planets spanning the radius gap, and none around stars as bright,” he says. If further observations find all three are similar in make-up, that suggests they formed the same way. If the innermost one is distinct, that supports two different formation processes.
Efforts are now underway to measure the planets’ masses so that researchers can calculate their density and determine whether they are rocky or gaseous. Because TOI-270 is so close and its star so bright, those masses can be measured in two different ways. Researchers can study how planets tug at the star, making it move back and forth, or they can measure tiny delays or advances in when the planet passes the star, caused by gravitational interactions of the closely packed planets. “It’s quite exceptional to be able to measure masses by two different methods,” Günther says.
The outermost of the three planets skirts the edge of the system’s habitable zone—where liquid water could exist and life arise—but both of the mass-measuring methods could hint at whether there is a farther rocky planet inside that zone. Planned observations of the system with the Hubble and Spitzer space telescopes may also reveal what the atmospheres of the three planets are like.
TESS, which watches a 90° strip of sky for nearly a month before shifting to the next strip, has found more than just exoplanets. It has snapped previously unknown comets in our solar system and exocomets around Beta Pictoris, 63 light-years away. It has also bagged a dozen supernovae, sometimes capturing the evolution of the cosmic blast over days or weeks, providing valuable data for astronomers. And it has detected some 800 stellar flares around other stars, akin to the solar storms on our sun that cause electromagnetic trouble on Earth. When looking for potentially habitable planets around other stars, understanding the star’s flaring behavior is key. “We don’t know what it will do to the planetary environment,” Seager says.
On 18 July, TESS completed its survey of the Southern Hemisphere and flipped over to begin work on the north. “TESS is doing amazingly well and clearly fulfilling its scientific goal,” says Didier Queloz of the University of Cambridge in the United Kingdom.