David Charbonneau of the Harvard Smithsonian Center for Astrophysics led a team of astronomers that discovered a super-Earth type exoplanet named GJ 1214b. They announced this discovery in the December 17, 2009 issue of Nature. GJ 1214b has large amount of water surrounding its rocky core.
A super-Earth planet is an extra solar planet between 1 and 10 times as massive as Earth. Only 28 of about 400 exoplanets discovered to date qualify as super-Earth type planets.
How Astronomers Discovered GJ 1214b
To discover the exoplanet GJ 1214b the team of astronomers used relatively small 16 inch (0.4 meter) telescopes equipped with astronomical CCD cameras. The astronomers used an array of eight small telescopes on Mount Hopkins Observatory near Tucson, Arizona called the MEarth array. The MEarth telescopes are small enough that many amateur astronomers could afford similar telescopes and instrumentation.
The Center for Astrophysics team is using the MEarth array to search for extra solar planets around M stars. Because M dwarf stars are among the smallest and coolest spectral class of stars, it is easier for astronomers to discover exoplanets around these stars.
The MEarth array accurately and repeatedly measures the brightness of M stars to search for transiting exoplanets. When planets orbit in front stars, astronomers say that planets are transiting. If planets were larger, they would eclipse the stars, however because transiting planets are so much smaller than the stars they only dim the star light a small amount.
Detecting transits of M stars is easier because they are cool and faint. Detecting planets transiting hotter more luminous stars would be harder because the dimming effect would be a much smaller percentage of the light from the star.
Astronomers used very accurate brightness measurements from the MEarth array of small telescopes to detect the exoplanet GJ 1214b around the M dwarf star GJ 1214.
How Astronomers Know GJ 1214b Has Water
The CCD cameras on the MEarth telescopes measure the periodic dimming that occurs when orbiting planets transit faint cool M dwarf stars. The amount of dimming during a transit tells astronomers how large the planet is. By studying transits the Center for Astrophysics astronomers deduced that GJ 1214b is 2.68 times as large as Earth.
After discovering GJ 1214b Charbonneau's team of astronomers used a larger telescope to study its spectra. Specifically they repeatedly measured the amount of Doppler shift in the spectra to find the radial velocity curve of GJ 1214b. A radial velocity curve for a star is a graph of its velocity towards or away from Earth as the planet and star orbit their mutual center of mass.
The radial velocity curve allows astronomers to calculate the mass of GJ 1214b. GJ 1214b has a mass of 6.55 times Earth's mass.
Density is the mass divided by the volume. After astronomers know the radius and mass of a planet, they can calculate the density. GJ 1214b is about 1.9 times the density of water. This density is consistent with a planet consisting of a very deep ocean surrounding a rocky core. Hence astronomers know that GJ 1214b is a very watery world.
This water is however very hot. With a 38 hour orbit, GJ 1214b is only 1.3 million miles from its parent star. Knowing the temperature and distance of the parent star allows astronomers to calculate that the surface temperature of GJ 1214b is 190 degrees Centigrade (370 degrees Fahrenheit). GJ 1214b is a hot steamy world.
Astronomers and biologists think that water is needed for life. It may seem that GJ 1214b is too hot to harbor life, but biologists have found that the superheated water near underwater volcanic vents is teeming with life.
Could GJ 1214b harbor similar life?
Further Reading:
Discovering the Extrasolar Planet CoRoT-9b
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
