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A new trio of exoplanets could offer clues to how midsized planets form

The trifecta, discovered by NASA’s TESS, includes a “super-Earth” and two “sub-Neptunes” in a system called TOI-270.

ByKatherine J. WuNOVA NextNOVA Next
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NASA's Transiting Exoplanet Survey Satellite (TESS), shown here in a conceptual illustration, is identifying exoplanets orbiting the brightest stars just outside our Solar System. Image Credit: NASA's Goddard Space Flight Center

NASA’s planet-probing satellite, Transiting Exoplanet Survey Satellite (TESS), has had a very fruitful first year abroad.

According to a paper published Monday in the journal Nature Astronomy, TESS has discovered three petite new worlds orbiting a star just 73 light-years away. Each member of the trio is fairly middling as far as planets go, with diameters that sit between those of Earth and Neptune, the middle two members of our Solar System, in terms of size.

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Though it’s pretty unlikely that any of these new planets are fit for habitation, the discovery does more than fill a medium-sized hole in the hearts of exoplanet hunters. Studying Earth’s near neighbors could help scientists understand how planets of intermediate diameter form—and the newly uncovered system, named TESS Object of Interest (TOI) 270, might just be the perfect laboratory for the job, study author Maximilian Günther of MIT’s Kavli Institute for Astrophysics and Space Research said in a statement.

Midsized planets are conspicuously lacking in our own Solar System. Earth is the largest of the rocky inner planets, while Neptune is the runt of the gas giants that orbit farther from the Sun. And yet, Neptune has a diameter about four times that of Earth’s. This puzzling size gap persists in other parts of the known universe, too: data beamed back from Kepler, TESS’ predecessor satellite, showed there are very few exoplanets with a diameter that’s between 1.5 and 2 times Earth’s.

Some researchers have interpreted this jump in size as an indication that so-called super-Earths (no more than twice the size of Earth) and sub-Neptunes are formed by different processes. Confirming this theory requires collecting data on a lot of examples of both—which makes TOI-270 something of a missing link: Its planets straddle this suspicious gap, with diameters of 1.25, 2.13, and 2.42 times Earth’s.

If further observations show that TOI-270’s innermost super-Earth differs in composition from its two sub-Neptune neighbors, that’s good support for the two-process theory. But if all three planets are made up of similar stuff, they might instead share similar roots.

Because TOI-270 is within range of several powerful ground-based Earth telescopes, the researchers are hard at work planning follow-up observations on their new exoplanet trifecta. One thing researchers are unlikely to see, though, is any sign of life as we Earthlings know it. While the orbit of TOI-270’s outer sub-Neptune just barely grazes the edge of the system’s habitable zone, where it’s theoretically possible for life to exist, new data on the planet’s atmosphere suggest it isn’t hospitable, reports Dennis Overbye at The New York Times.

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An illustration of the TOI-270 system, located about 73 light-years away in the southern constellation Pictor. The system contains three planets of intermediate size. Image Credit: NASA’s Goddard Space Flight Center/Scott Wiessinger

But no matter the results, this is only the beginning for the team behind TESS. TOI-270’s trio of exoplanets are just a small taste of what’s to come. Since its launch in April 2018, TESS has definitively identified more than 20 exoplanets orbiting stars within about 300 light-years of Earth, with hundreds more awaiting confirmation—and the satellite is only at the midpoint of its mission.

As TESS Deputy Director of Science Sara Seager told Daniel Clery at Science, “We’re expecting thousands.”

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