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Planet known as GJ 357d deemed particularly intriguing

NASA discovers possibly habitable super-Earth 31 light-years away

US space administration’s exoplanet-hunting telescope finds new solar system, including 3 planets orbiting a star about a third of sun’s mass and 40% cooler

This illustration shows one interpretation of what new planet GJ 357 d may be like. (NASA's Goddard Space Flight Center/Chris Smith)
This illustration shows one interpretation of what new planet GJ 357 d may be like. (NASA's Goddard Space Flight Center/Chris Smith)

NASA’s exoplanet-hunting telescope, the Transiting Exoplanet Survey Satellite (TESS), has discovered a new solar system with at least three new planets including one that has shown potential for being habitable, the American space administration announced.

The three planets were discovered orbiting GJ 357, a red dwarf — a small and cooling star — 31 light-years away, relatively close in space terms, said Rafael Luque of Spain’s Institute of Astrophysics in the Canary Islands, the lead researcher in the discovery team.

The star is “about one-third the sun’s mass and size and about 40 percent cooler than our star,” NASA said.

The TESS cameras “caught the star dimming slightly every 3.9 days, revealing the presence of a transiting exoplanet — a world beyond our solar system — that passes across the face of its star during every orbit and briefly dims the star’s light,” NASA added.

The planet known as GJ 357d — the furthest away from the star — was particularly intriguing as researchers estimate it could be habitable. The other two, GJ 357b and GJ 357c are deemed too hot.

Signs of habitability in any planet include a rocky terrain, a size similar to Earth and a distance from their sun — the temperate “Goldilocks” zone neither too close nor too far — that allows the right temperature for liquid water, a key requirement for life.

Given its distance from its star, similar to that of Mars to our sun, researchers estimate the planet has temperatures of -53 degrees Celsius (-63.4 Fahrenheit), Luque told AFP.

“That seems a little cold at first,” he said.

But “if this planet had an atmosphere (unlike Mars), it could retain the heat it receives from its star, and water could be liquid.”

This diagram shows the layout of the GJ 357 system. Planet d orbits within the star’s so-called habitable zone, the orbital region where liquid water can exist on a rocky planet’s surface. (NASA’s Goddard Space Flight Center/Chris Smith)

Researchers also estimate GJ 357d could be roughly the same size as Earth or up to twice the size.

“The planet weighs at least 6.1 times Earth’s mass, and orbits the star every 55.7 days at a range about 20% of Earth’s distance from the sun. The planet’s size and composition are unknown, but a rocky world with this mass would range from about one to two times Earth’s size,” wrote Francis Reddy of NASA’s Goddard Space Flight Center, which oversees TESS as a NASA Astrophysics Explorer mission.

The findings were published on Wednesday, July 31, in the journal Astronomy & Astrophysics.

“GJ 357 d is located within the outer edge of its star’s habitable zone, where it receives about the same amount of stellar energy from its star as Mars does from the sun,” said co-author Diana Kossakowski at the Max Planck Institute for Astronomy in Heidelberg, Germany. “If the planet has a dense atmosphere, which will take future studies to determine, it could trap enough heat to warm the planet and allow liquid water on its surface.”

It is not the first potentially habitable planet to have been discovered close to us.

In 2016, the discovery of Proxima b at a mere four light-years from the solar system made waves.

But there is a hitch.

Proxima b and GJ 357d were discovered via so-called radial velocity, which involves looking for signs of a wobble in a star from the gravitational tug of an orbiting planet.

But Luque says the method is not precise enough to ascertain whether it actually is habitable.

As things stand, in order to measure its size, density and composition, the planet has to pass directly between its star and an observer, the so-called “transit” method, he says.

That has not been possible for Proxima b and other nearer potentially habitable planets, Luque says.

In the coming months, Luque and his team will be working to try and catch GJ 357d in “transit” to try and confirm it as a habitable planet.

“The probability that a planet passes in front of a star from our line of vision on Earth is pretty small,” he adds.

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