The planet, dubbed GJ 667Cc, orbits a red dwarf star 22 light-years from Earth, in the constellation Scorpio. A binary pair of orange dwarf stars are part of the same system.
The new planet has a mass 4.5 times that of Earth and orbits its host star every 28 days.
The red dwarf is relatively dim, so the planet receives slightly less light from its star than Earth does from the sun. But most of the star's light is infrared, so the planet should absorb more of its incoming energy than Earth does from sunlight.
That means if the planet has a rocky surface—which is predicted for planets less than ten times Earth's mass—and an atmosphere, it could support liquid water and maybe life, said co-discoverer Guillem Anglada-Escudé, who conducted the work while at the Carnegie Institution for Science in Washington, D.C.
"If it has an atmosphere, it's probably reddish all the time, because the star is really red," Anglada-Escudé said. "It would be like being evening all the time."
For any hypothetical observers on the surface, the binary stars in the distance would be "very prominent in the sky, and it would be an exotic thing."
Rocky Planet Around Unexpected Star
Anglada-Escudé and colleagues found the new planet using public data from the European Southern Observatory, which hosts telescopes that can measure wobbles in a star's orbit caused by a planet's gravitational tug.
The new super-Earth was somewhat unexpected, because some planetary-formation models say that metal-poor stars such as GJ 667C shouldn't have terrestrial planets around them.
In stellar terms, metals are elements heavier than hydrogen and helium. Such heavy atoms—including carbon, oxygen, and nitrogen—are the "building blocks" for rocky planets. If a young star has fewer metals, the theory goes, so does its disk of planet-forming debris.
Still, the results might not be that surprising, said Aki Roberge of NASA's Goddard Space Flight Center, who wasn't part of the study team.
"We know it's more likely to have a gas giant planet around a metal-rich star, but we don't really know if that holds to [lower mass, rocky planets], because we haven't found enough of them yet," Roberge said.
But smaller objects, such as asteroids and comets, have been found around low-metal stars, so "there doesn't appear to be any favorability for being a low- or high-metallicity star," she said.
For metal-poor stars, "maybe it's easier to form smaller things, [like] small rocky bodies, [than] to form a massive giant planet."
New Super-Earth May Be First of Many
Study co-author Anglada-Escudé, who is now a postdoc at the University of Gottingen in Germany, would like to eventually confirm that GJ 667Cc is in fact a potentially habitable super-Earth.
That would require a transit observation, when astronomers measure the dimming of the host star's light as the planet passes in front of the star, as seen from Earth.
Transit data can help astronomers determine a planet's density—and thus its composition—and possibly observe its atmospheric characteristics.
With our current view of the red dwarf, a transit of GJ 667Cc has about a one-percent chance of happening, he said.
But so far, planets outside our solar system have been discovered in so many different configurations that it's possible GJ 667Cc is the first of many super-Earths orbiting metal-poor stars, Anglada-Escudé said.
"What we expect with new instruments coming online is we can find 20 or 30 of these objects" in the near future, he said. "So within two or three years, one of them has to transit."
The new super-Earth will be described in an upcoming issue of the Astrophysical Journal Letters.