Astronomers Discover New Earth-size World Only 40 Light-Years Away

In the search for planets around other stars, astronomers often seek worlds that are most like our own. The discovery of Gliese 12b — the closest, transiting, temperate, Earth-size planet found to date — promises possibilities for understanding how terrestrial planets become habitable.

Gliese 12b orbits a cool red dwarf star around 40 light-years away — practically neighborly compared to other exoplanets — with a period of 12.8 days. Its distance from its host star means that its surface might be temperate enough for life, with a temperature of 107°F (42°C).

Temperate or not, Gliese 12b’s size is roughly the same as that of Earth or Venus. However, the difference between these two planets’ atmospheres — one of them welcoming, the other hellish — warrants caution before declaring Gliese 12b habitable.  What makes this exoplanet find unique  is that it might play a crucial role in understanding the formation and habitability of small, Earth-size planets.

Transiting, Temperate, and Terrestrial

Gliese 12b was initially detected by NASA’s Transiting Exoplanet Survey Satellite (TESS), a mission that searches for changes in stellar brightness and records transits that occur when a planet passes in front of its host star. After TESS captured a periodic dimming of the star Gliese 12, two teams of astronomers followed up with additional observations to confirm first, that the transits are real, and then to characterize the planet’s properties. Gliese 12b’s discovery was reported in two papers published on May 23rd.

“What’s most significant about this discovery is four-fold — the fact that it’s a nearby, transiting, temperate, Earth-size planet,” says Larissa Palethorpe (University of Edinburgh, UK), a graduate student who worked with one of the discovery teams, publishing in the Monthly Notices of the Royal Astronomical Society. “Each of these characteristics on their own make a planet exciting, but for a planet to satisfy all four of these criteria is extremely rare.”

The quest to find an Earth-like planet is motivated by our desire to determine how habitable planets form and evolve to potentially host liquid water and life. And one question that astronomers hope Gliese 12b will help answer is the puzzle of Earth and Venus’s atmospheres. Even though Venus receives only 1.6 times more radiation from the Sun than Earth does, it has developed a smothering atmosphere that seems to be inhospitable to life.

Although Gliese 12b is small, it’s also so close to Earth that it may be possible to obtain spectroscopic measurements of its atmosphere with the James Webb Space Telescope (JWST). Those observations could shed light on how smaller rocky planets form a habitable (or uninhabitable) environment.

“The Gliese 12 system is the second planetary system that is suitable for atmospheric transmission spectroscopy,” said Masayuki Kuzuhara (Astrobiology Center, Japan), who led the other team in publishing results inThe Astrophysical Journal Letters.

The first such system is TRAPPIST-1, which hosts no less than seven planets, three of them “potentially temperate Earth-size planets that are suitable for atmospheric transmission spectroscopy,” according to Kuzuhara. However, Gliese 12 offers an advantage that TRAPPIST-1 does not: It’s quiet.

Gliese 12: A Quiet Host

Red dwarf stars, or M stars, have small masses and radii and low luminosities, which means any planets orbiting in the “habitable zone” (the orbital space where a rocky planet might host liquid water on its surface) transit more deeply and more often. Gliese 12b’s orbit is a mere 0.07 astronomical unit from its host star. Yet even though it would be well inside Mercury’s orbit around the Sun, its star’s low luminosity means the planet could be comfortable for life.

However, radiation from active M stars can blast away any atmosphere from their planets. For example, the host star of the aforementioned TRAPPIST-1 system buffets its system with X-ray and UV emission that has likely stripped TRAPPIST-1 b of all air, while leaving the Venus-size TRAPPIST-1 c with, at best, a very thin atmosphere.

But not all M stars are created equal. “In contrast to the TRAPPIST-1 system, the central star's activity is relatively quiet,” Kuzukhara explained. “We can expect that the influence of extreme ultraviolet emissions and stellar wind from Gliese 12 to the planet is weaker than that from TRAPPIST-1 to the planets.”

X-ray observations have found Gliese 12 to be an inactive host star without atmosphere-stripping flares, which, if true for all the star’s 7 billion years of fusion, might mean that its planet could have an atmosphere after all.

Still More to Learn

Whether Gliese 12b is truly Earth-like remains to be seen. “We cannot fully rule out the possibility that it is a habitable planet,” Kuzuhara said. However, a thick atmosphere might make its surface too hot to retain liquid water. In any case, Kuzuhara and colleagues still plan to ask for observations of the planet with JWST.

For Palethorpe’s team, confirming radial velocity measurements is their next goal. As Gliese 12b orbits its host star, its own mass tugs on the star through mutual gravitational attraction, producing small wobbles that astronomers can measure. Taking these measurements will help determine the planet’s mass and average density, and narrow down what its interior structure might look like.

Whether Gliese 12b turns out to resemble a hot, inhospitable Venus, a temperate, life-sustaining Earth, or something entirely new, it is an exciting prospect for planet-hunters everywhere.