NASA’s Transiting Exoplanet Survey Satellite (TESS) has only been scanning the skies for about a year, but it has already identified several new candidate exoplanets. As astronomers were working to confirm one recent sighting, they happened upon something unexpected. That solar system, known as GJ 357, hosts not one but three exoplanets. What’s more, one of those planets is a super-Earth in the habitable region of the star.
TESS uses the transit method to spot exoplanets with its array of cameras, similar to the dearly departed Kepler satellite. It can scan multiple stars at the same time, watching for the telltale dips in light that indicate an exoplanet has passed in front of its host star. That’s what TESS saw in GJ 357, which is only 31 light-years away. To confirm the existence of the exoplanet dubbed GJ 357 b, astronomers used ground-based telescopes to measure the star’s radial velocity. This alternative method looks for tiny changes in light caused when a star “wobbles” in response to the gravity of orbiting planets.
The team confirmed GJ 357 b, but they also found two more exoplanets now known as GJ 357 c and GJ 357 d. GJ 357 is a small M-type dwarf star, which is 40 percent cooler than the sun. GJ 357 b orbits the star in just 3.9 days and is 22 percent larger than Earth. The surface equilibrium temperature (a measure of solar radiation only) is 490 degrees Fahrenheit (254 degrees Celsius), ruling out life as we know it. GJ 357 c is a bit farther out, but still too hot for life. It’s 3.4 times as massive as Earth and orbits ever 9.1 Earth days. The temperature here is 260 degrees F (127 degrees C).
Astronomers are most interested in GJ 357 d, which sits near the outer edge of the system’s habitable zone with a 55.7-day orbit around the star. It’s 6.1 times more massive than Earth, meaning it could be rocky or gaseous. If it’s a rocky planet, it would be about twice as large as Earth.
According to the team, GJ 357 d has an equilibrium temperature of -64 degrees F (-53 degrees C). That sounds too low to be even potentially habitable, but this is only a measure of solar radiation. Earth’s equilibrium temperature is -1 degree F (-18 degrees C), but the atmosphere increases the surface temperature. GJ 357 d gets about as much solar radiation as Mars, and a sufficiently dense atmosphere could allow liquid water to flow on its surface. Astronomers around the world plan to take a closer look at this nearby super-Earth over the coming years.