Astronomers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., have discovered a “super-Earth” orbiting a red dwarf star 40 light-years from Earth. They found the distant planet with a small fleet of ground-based telescopes no larger than those many amateur astronomers have in their backyards. Although the super-Earth is too hot to sustain life, the discovery shows that current, ground-based technologies are capable of finding almost-Earth-sized planets in warm, life-friendly orbits.
A super-Earth is defined as a planet between one and 10 times the mass of the Earth. The newfound world, GJ1214b, is about 6.5 times as massive as the Earth. Its host star, GJ1214, is about one-fifth the size of our Sun. It is also much cooler, with a surface temperature of only about 4,900 degrees Fahrenheit and a luminosity only three-thousandths as bright as the Sun.
GJ1214b orbits its star once every 38 hours at a distance of only 1.3 million miles. Astronomers estimate the planet’s temperature to be about 400 F. Although the planet is as warm as an oven, it is still cooler than any other known transiting planet because it orbits a very dim star.
Since GJ1214b crosses in front of its star, astronomers were able to measure its radius, which is about 2.7 times that of Earth. This makes GJ1214b one of the two smallest transiting worlds astronomers have discovered. The resulting density suggests that GJ1214b is composed of about three-fourths water and other ices, and one-fourth rock. There are also tantalizing hints that the planet has a gaseous atmosphere.
“Despite its hot temperature, this appears to be a waterworld,” explains Zachory Berta, a graduate student at CfA who first spotted the hint of the planet among the data. “It is much smaller, cooler, and more Earthlike than any other known exoplanet.”
Berta adds that some of the planet’s water should be in the form of exotic materials like Ice VII—a crystalline form of water that exists at pressures greater than 20,000 times Earth’s sea-level atmosphere.
Astronomers found the new planet using the MEarth (pronounced “mirth”) Project—an array of eight identical 16-inch-diameter RC Optical Systems telescopes that monitor a pre-selected list of 2,000 red dwarf stars. Each telescope perches on a highly accurate Software Bisque Paramount and funnels light to an Apogee Alta U42 camera containing a charge-coupled device chip, which many amateurs also use.
“Since we found the super-Earth using a small ground-based telescope, this means that anyone else with a similar telescope and a good CCD camera can detect it too. Students around the world can now study this super-Earth!” says CfA’s David Charbonneau, lead author and head of the MEarth project.
MEarth looks for stars that change brightness. The goal is to find a planet that crosses in front of, or transits, its star. During such a mini-eclipse, the planet blocks a small portion of the star’s light, making it dimmer. Using innovative data processing techniques, astronomers can tease out the telltale signal of a transiting planet and distinguish it from “false positives” such as eclipsing double stars.
When astronomers compared the measured radius of GJ1214b to theoretical models, they found that the observed radius exceeds the model’s prediction, even assuming a pure water planet. Something more than the planet’s solid surface may be blocking the star’s light—specifically, a surrounding atmosphere.
The next step for astronomers is to try to directly detect and characterize the atmosphere, which will require a space-based instrument like NASA’s Hubble Space Telescope. GJ1214b is only 40 light-years from Earth, within the reach of current observatories.
“Since this planet is so close to Earth, Hubble should be able to detect the atmosphere and determine what it’s made of,” Charbonneau says. ”That will make it the first super-Earth with a confirmed atmosphere—even though that atmosphere probably won’t be hospitable to life as we know it.“
The discovery was published in the December 17 issue of the journal Nature.
Posted: 18 December 2009