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Using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS) and many other facilities, two international teams of astronomers have discovered a planet between the sizes of Earth and Venus just 40 light-years away. A number of factors make it a suitable candidate for further study using NASA’s James Webb Space Telescope.

TESS stares across much of the sky for about a month at a time, tracking changes in the brightness of tens of thousands of stars at intervals of 20 seconds to 30 minutes. Capturing transits — brief, regular dimmings of stars caused by the passage of orbiting worlds — is one of the mission’s primary goals.

“We have discovered the closest, transiting, moderate Earth-sized world found so far,” said Masayuki Kuzuhara, a project assistant professor at the Tokyo Astrobiology Center who led a research team with Akihiko Fukui, a project assistant professor at The University of Tokyo. “Although we don’t yet know if it has an atmosphere, we thought of it as an exo-Venus, with a similar size and energy received from its star as our planetary neighbor in the Solar System.”

The host star, called Gliese 12, is a cool red dwarf located almost 40 light-years away in the constellation Pisces. The star is only about 27% the size of the sun, with about 60% the surface temperature of the sun. The newly discovered world, called Gliese 12 b, orbits every 12.8 days and is about the size of Earth or slightly smaller – comparable to Venus. Assuming no atmosphere, the planet’s surface temperature is estimated to be about 107 degrees Fahrenheit (42 degrees Celsius).

Astronomers say the tiny size and mass of red dwarf stars make them ideal for finding Earth-sized planets. A smaller star means more dimming for each transit, and a lower mass means an orbiting planet can cause a larger wobble, known as the star’s “reflex motion.” These effects make smaller planets easier to detect.

The lower brightness of red dwarf stars also means that their habitable zones — the range of orbital distances where liquid water can exist on a planet’s surface — lie closer to them. This makes it easier to detect transiting planets in the habitable zones around red dwarfs than those around more energetic stars.

The distance separating Gliese 12 and the new planet is only 7% of the distance between Earth and the sun. The planet receives 1.6 times as much energy from its star as Earth does from the sun, and about 85% of what Venus experiences.

“Gliese 12 b represents one of the best targets for investigating whether Earth-sized planets orbiting cool stars can retain their atmospheres, a crucial step in advancing our understanding of the habitability of planets in our galaxy,” said Shishir Dholakia, PhD student at the Center for Astrophysics at the University of Southern Queensland in Australia. He leads a different research team with Larissa Pailthorpe, a PhD student at the University of Edinburgh and University College London.

Both teams suggest that studying Gliese 12 b could help unlock some aspects of the evolution of our own solar system.

“The first atmospheres of Earth and Venus are thought to have been removed and then rebuilt by volcanic gases and bombardment of remnant material in the Solar System,” Palethorpe explained. “Earth is habitable, but Venus is not due to its complete loss of water. Because Gliese 12 b is between Earth and Venus in temperature, its atmosphere can teach us a lot about the habitable paths planets take as they evolve.”

One important factor in preserving the atmosphere is the turbulence of its star. Red dwarfs tend to be magnetically active, resulting in frequent, powerful X-ray bursts. However, the two teams’ analyzes concluded that Gliese 12 showed no signs of extreme behavior.

A paper led by Kuzuhara and Fukui appears The Astrophysical Journal Letters. Dholakia and Palethorpe’s findings were published in Monthly Notices of the Royal Astronomical Society On the same day.

During a transit, the host star’s light passes through each atmosphere. Different gas molecules absorb different colors, so the transit provides a set of chemical fingerprints that can be detected by telescopes like Webb.

“We know of only a few temperate Earth-like planets that are both close enough to us and meet other criteria needed for this kind of study, called transmission spectroscopy, using current facilities,” said Michael McElwain, a research astrophysicist at Goddard on NASA Space Flight Center in Greenbelt, Maryland and co-author of the paper Kuzuhara and Fukui. “To better understand the diversity of atmospheres and evolutionary outcomes for these planets, we need more examples like Gliese 12 b.”

TESS is a NASA Astrophysics Explorer mission managed by NASA Goddard and managed by MIT in Cambridge, Massachusetts. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA’s Ames Research Center in California’s Silicon Valley; the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes and observatories from around the world are participating in the mission.