A new planet—probably a rocky super-Earth—has been found squarely within its star's habitable zone, making it one of the best candidates yet to support life, its discoverers say.
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.
(Related:
"'Tatooine' Planet With Two Suns Could Host Habitable Moon?")
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."
(Relaed:
"New Planet May Be Among Most Earthlike—Weather Permitting.")
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.
(Related:
"Three Theories of Planet Formation Busted, Expert Says.")
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.
(Also see
"Nearby 'Super Earth' May Have Oceans, Thick Atmosphere.")
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.
Nyuk Nyang
