Astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) say they've spotted a super-Earth water world orbiting a red type M star some 40 light years from Earth.
GJ 1214 is a dim M4.5 red dwarf in the constellation Ophiuchus with an apparent magnitude of 14.7. It is located at a distance of approximately 40 light years from Earth. It is about one-fifth as large as the Sun with a surface temperature estimated to be 3026 K, (4,900 °F, 2700 °C). Its mass is 0.157 M_{\odot} and radius is 0.211 R_{\odot} and a luminosity of 0.00334 L_{\odot}. Its age is 6 billion years.
Stellar classification M4.5: Class M is by far the most common class. About 76% of the main sequence stars in the solar neighborhood are Class M stars. Although most Class M stars are red dwarfs, the class also hosts most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The late-M group holds hotter brown dwarfs that are above the L spectrum. This is usually in the range of M6.5 to M9.5. The spectrum of an M star shows lines belonging to molecules and all neutral metals but hydrogen lines are usually absent.
GJ 1214 b - is circling dim host red dwarf star GJ1214 every 38 hours at a distance of just 1.01967 x 10^10 meters (0.06816±0.0019 AU). The star's modest surface temperature of 2,700°C, though, means that GJ 1214 b itself is a balmy 200°C.
The planet has a mass and radius of 6.5 and 2.7 times that of Earth, respectively. The density obtained from these figures "suggests that GJ 1214 b is composed of about three-fourths water and other ices, and one-fourth rock".
Depending on how reflective the planet's atmosphere is, it may have temperatures as high as 555K, or as low as 393K—the latter figure is only 20°C above the boiling point of water.
At approximately 393–555 K (120–282 °C or 248–540 °F), it may be cooler than any known transiting planet. The temperature range depends on the planet Bond albedo, with 555 K being the upper limit for an albedo of 0 (totally dark) and 393 being the value for an albedo of 0.75, analogous to Venus.
The planetary mass and radius are consistent with the planet being composed prevalently (~75%) of water and ~25% of rock, possibly covered by an hydrogen and helium atmosphere making up to 0.05% of the mass of the planet. Due to the hydrostatic pressure, some of the planet's water could be in the form of ice VII.
Some of the planet’s water should be in the form of exotic materials such as Ice VII (seven), a crystalline form of water that exists at pressures greater than 20,000 times Earth’s atmosphere.
Due to the estimated old age of the planetary system and the calculated hydrodynamic escape rate, Charbonneau et al. conclude that there has been a significant atmospheric loss during the lifetime of the planet and any current atmosphere cannot be primordial.
The boiling point of water rises with pressure, so evidently GJ 1214 b has a surface_pressure greater than 16 Bar, at which water boils at 200°C
Based on the first paper to come out of the project, everything worked precisely as planned. Every 1.58 days, the light from the M-dwarf GJ 1214 displayed a dip in its luminosity of about 1.3 percent. The team apparently alerted the HARPS project, which detects exoplanets by looking for red and blue shifts that their gravitational pull creates in the light from their host stars.
The HARPS instrument was able to confirm the presence of a planet and, based on how it tugged the host star around, estimated its mass: 6.55 times that of the Earth. We've also got a good sense of the radius of GJ 1214, which allows us to estimate the planet's radius, based on how much of the star's light it's occluding. Combine that radius (2.7 Earth radii) with the mass, and you have a rough estimate of the planet's density. It turns out to be less than half as dense as Earth.
All indications are that the host star is old, though, so it appears that GJ 1214 b is at a stable, or at least long-lived, point in its evolution.
Assuming that the planet has an atmosphere dominated by hydrogen, the authors calculate that 0.16 percent of the light from GJ 1214 will pass through the atmosphere during transit events. As such, GJ 1214 b may provide one of the best opportunities we have to study the contents of an exoplanet's atmosphere in the near future.
“If you want to describe in one sentence what this planet is, it’s a big, hot ocean,” said David Charbonneau of the Harvard-Smithsonian Center for Astrophysics.
The CfA's findings are published in the 17 December issue of Nature.
