TESS Has Discovered Its Third Exoplanet, A Sub-Neptune World

Artist's impression of an exoplanet. NASA/JPL-Caltech

NASA’s Transitioning Exoplanet Survey Satellite, TESS, has discovered a dense and gaseous world located around a dwarf star 53 light-years away. This is the third exoplanet discovered by TESS since its launch last year and the one with the longest orbital period. The finding was presented at the American Astronomical Society meeting, which is taking place in Seattle this week.

The new planet is called HD 21749b and orbits a nearby dwarf star. It is classified as a sub-Neptune. It is three times the size of Earth but has a mass 23 times that of our planet. Its dimensions suggest that the planet is most likely gaseous with a thicker and warmer atmosphere than Neptune or Uranus.

It has a surface temperature of 148°C (300°F), which is surprisingly low for a planet orbiting so close to its star. Its year lasts just 36 days and given that its star is almost as bright as the Sun, the whole planet is very intriguing.

“It’s the coolest small planet that we know of around a star this bright,” lead discoverer Dr Diana Dragomir, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research, said in a statement. “We know a lot about atmospheres of hot planets, but because it’s very hard to find small planets that orbit farther from their stars, and are therefore cooler, we haven’t been able to learn much about these smaller, cooler planets. But here we were lucky, and caught this one, and can now study it in more detail.”

“We think this planet wouldn’t be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy,” Dragomir added. “The planet likely has a density of water, or a thick atmosphere.”

TESS hunts for exoplanets using the transition method, which involves looking for changes in brightness. It is studying the brightness of 200,000 stars to spot changes caused by potential planets passing in front of them. It is the same approach used by the now-defunct Kepler telescope, but it will cover four times the area. Researchers hope to also discover more Earth-like cool exoplanets.

During its two-year primary mission, researchers expect to find more than 20,000 exoplanets, over five times the number of exoplanets currently known. Among those, they estimate that 50 planets will be Earth or super-Earth size and prime for follow-up studies.

“We’ve confirmed three planets so far, and there are so many more that are just waiting for telescope and people time to be confirmed,” Dragomir said. “So it’s going really well, and TESS is already helping us to learn about the diversity of these small planets.”

The research is submitted for publication in the Astrophysical Journal Letters.

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