Intriguing Earth-sized planet discovered 40 light years away

artist’s impression of the planet and star
artist’s impression of the planet and star
Two international teams of astronomers, including one co-led by a PhD candidate based at UCL and the University of Edinburgh, have discovered a planet only 40 light years away whose size is between that of Earth and Venus.

The researchers used observations by NASA’s TESS (Transiting Exoplanet Survey Satellite). Multiple factors make it a candidate for further study using NASA’s James Webb Space Telescope.

TESS stares at a large swath of the sky for about a month at a time, tracking the brightness changes of tens of thousands of stars at intervals ranging from 20 seconds to 30 minutes. Capturing transits  - brief, regular dimmings of stars caused by planets passing in front of the stars  - is one of the mission’s primary goals.

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% of the Sun’s size, with about 60% of the Sun’s surface temperature.

The newly discovered world, named Gliese 12 b, orbits every 12.8 days and is Earth’s size or slightly smaller  - comparable to Venus. Assuming it has no atmosphere, the planet has a surface temperature estimated at around 107 degrees Fahrenheit (42 degrees Celsius).

Both research teams suggest that studying Gliese 12 b may help unlock some aspects of our own solar system’s evolution.

Larissa Palethorpe, a doctoral candidate at UCL’s Mullard Space Science Laboratory and the University of Edinburgh and co-lead of one of the teams, said: "It is thought that Earth’s and Venus’s first atmospheres were stripped away and then replenished by volcanic outgassing and bombardments from residual material in the solar system.

"Whilst the Earth remained habitable, Venus did not as the runaway greenhouse effect led to a complete loss of water. Because Gliese 12 b is between Earth and Venus in temperature, its atmosphere could teach us a lot about the habitability pathways planets take as they develop."

Co-author Dr Vincent Van Eylen (Mullard Space Science Laboratory at UCL) said: "Gliese 12 b is an incredibly exciting planet because its size is identical to that of Earth. Even though Gliese 12 b is about 15 times closer to its star than Earth is to our Sun, because it orbits such a small star the temperature on the planet may be quite similar to that on Earth.

"That doesn’t necessarily guarantee that the planet is habitable, but it does make it a great place to start looking.

"Fortunately it’s also a very nearby star, so we will learn much more about the planet and its atmosphere with telescopes like JWST in the next years."  

Astronomers say that the diminutive sizes and masses of red dwarf stars make them ideal for finding Earth-size planets. A smaller star means greater dimming for each transit, and a lower mass means an orbiting planet can produce a greater wobble, known as "reflex motion," of the star. These effects make smaller planets easier to detect.

The lower luminosities of red dwarf stars also means their habitable zones - the range of orbital distances where liquid water could exist on a planet’s surface - lie closer to them. This makes it easier to detect transiting planets within habitable zones around red dwarfs than those around stars emitting more energy.

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

The paper co-led by Larissa Palethorpe and Shishir Dholakia, a doctoral candidate at the University of Southern Queensland in Australia, was published in the Monthly Notices of the Royal Astronomical Society. Another paper, co-led by Dr Masayuki Kuzuhara at the Astrobiology Center in Tokyo and Dr Akihiko Fukui at the University of Tokyo, was published on the same day in The Astrophysical Journal Letters.

Dr Kuzuhara said: "We’ve found the nearest, transiting, temperate, Earth-size world located to date. Although we don’t yet know whether it possesses an atmosphere, we’ve been thinking of it as an exo-Venus, with similar size and energy received from its star as our planetary neighbour in the solar system."
  • Top: Gliese 12 b, which orbits a cool, red dwarf star located just 40 light-years away, promises to tell astronomers more about how planets close to their stars retain or lose their atmospheres. In this artist’s concept, Gliese 12 b is shown retaining a thin atmosphere. Credit: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)
  • Middle: This artist’s concept compares Earth with different possible Gliese 12 b interpretations, from one with no atmosphere to one with a thick Venus-like one. Follow-up observations with NASA’s James Webb Space Telescope could help determine just how much atmosphere the planet retains as well as its composition.  Credit: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)

Mark Greaves

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