An artist’s conception depicts GJ 357 d orbiting its host star. (Cornell University Illustration / Jack Madden)
Astronomers are sharing a flood of findings from NASA’s Transiting Exoplanet Survey Satellite, or TESS, including the detection of a potentially habitable super-Earth far beyond our solar system.
The planet is said to circle an M-type dwarf star called GJ 357, about 31 light-years from Earth in the constellation Hydra. Known as GJ 357 d, the world is at least six times more massive than Earth — and orbits the star every 55.7 days, at a distance that’s only 20% as far away as Earth is from our own sun.
With that orbit, GJ 357 d would be broiling-hot if it were in our solar system. But its parent star is so much dimmer than our sun that the super-Earth could conceivably be just warm enough to have liquid water. That characteristic serves as the definition for habitable zones around alien suns.
“This is exciting, as this is humanity’s first nearby super-Earth that could harbor life – uncovered with help from TESS, our small, mighty mission with a huge reach,” astronomer Lisa Kaltenegger, who’s the director of Cornell University’s Carl Sagan Institute, said in a news release.
The findings relating to the GJ 357 system are detailed in research papers published by the journal Astronomy & Astrophysics and the Astrophysical Journal Letters.
Detecting GJ 357 d was a complex operation. It started with data from the TESS satellite, which was launched just last year and scans the skies for the telltale signs of planets crossing in front of their parent stars. The same transit-tracking technique was used by NASA’s now-defunct Kepler telescope to identify thousands of candidate planets.
TESS’ readings indicated that the dwarf star harbored a super-close-in planet dubbed GJ 357 b, which makes a complete orbit in just 3.9 Earth days. Assuming that the planet lacks an atmosphere, scientists estimated GJ 357 b’s equilibrium temperature to be in the range of 490 degrees Fahrenheit (254 degrees Celsius).
“We describe GJ 357 b as a ‘hot Earth,’ ” Enric Pallé, an astrophysicist at the Institute of Astrophysics of the Canary Islands, said in a NASA news release. “Although it cannot host life, it is noteworthy as the third-nearest transiting exoplanet known to date and one of the best rocky planets we have for measuring the composition of any atmosphere it may possess.”
To get a better fix on the toasty planet, astronomers turned to ground-based observations. Those observations were analyzed using a different method, which measures the ever-so-slight wobbles in a star’s position caused by the gravitational tug of its planets.
In addition to confirming GJ 357 b’s existence, the analysis determined that two other planets orbited the star in farther-out orbits. One was GJ 357 c, a too-hot planet that’s at least 3.4 times as massive as Earth and orbits the star every 9.1 days. The other was GJ 357 d.
If GJ 357 d lacks an atmosphere, its equilibrium temperature would be uncomfortably chilly, around 64 degrees below zero F (-53 degrees C). But if it has an atmosphere, as expected for a rocky planet at that distance, temperatures would be more moderate — and the prospects for life would be much sunnier.
“With a thick atmosphere, the planet GJ 357 d could maintain liquid water on its surface like Earth, and we could pick out signs of life with telescopes that will soon be online,” Kaltenegger said.
The studies about GJ 357 and its planets were published in conjunction with the first TESS Science Conference, conducted this week at MIT in Cambridge, Mass. The TESS mission is led and operated by MIT, and managed by NASA’s Goddard Space Flight Center. More than a dozen other partners contribute to the mission.
Earlier in the week, the TESS team reported the detection of three exoplanets circling TOI 270, another M-type dwarf star that’s about 73 light-years away in the constellation Pictor. (For what it’s worth, TOI stands for “TESS Object of Interest.”)
The innermost planet, TOI 270 b, appears to be a hot super-Earth that’s similar to GJ 357 b. In a paper published by Nature Astronomy, scientists say the other two planets, TOI 270 c and d, are about half the size of Neptune — and are likely to be similar to that gas giant in composition.
Such “mini-Neptunes” don’t exist in our solar system.
“An interesting aspect of this system is that its planets straddle a well-established gap in known planetary sizes,” said Fran Pozuelos, a postdoctoral researcher at the University of Liège in Belgium.
“It is uncommon for planets to have sizes between 1.5 and two times that of Earth for reasons likely related to the way planets form, but this is still a highly controversial topic,” Pozuelos said in a NASA news release. “TOI 270 is an excellent laboratory for studying the margins of this gap and will help us better understand how planetary systems form and evolve.”
All this is just the start: TESS’ primary mission is due to last another year, and the data analysis could go on for years longer.
In addition to Pozuelos, the authors of the Nature Astronomy paper, “A Super-Earth and Two Sub-Neptunes Transiting the Nearby and Quiet M Dwarf TOI-270,” are Maximilian Günther and Ian Waite.
Pallé and Kaltenegger are among 76 authors of the Astronomy & Astrophysics paper, “Planetary System Around the Nearby M Dwarf GJ 357 Including a Transiting, Hot, Earth-Sized Planet Optimal for Atmospheric Characterization.” Principal author is Rafael Luque.
In addition to Kaltenegger, Luque and Pallé, the authors of the paper in the Astrophysical Journal Letters, “The Habitability of GJ 357 d: Possible Climates and Observability,” include Jack Madden, Zifan Lin, Sarah Rugheimer, Antigona Segura and Néstor Espinoza.