BY DR EMILY BALDWIN
Posted: 22 January, 2009
Astronomers at the Harvard-Smithsonian Center for Astrophysics have discovered a planet 4.7 times the size of Earth and 25 times more massive, earning it the nickname of a “super-Neptune”.
HAT-P-11b was discovered via the transit method, whereby a planet passes in front of its parent star, thus temporarily, but periodically reducing its brightness as seen by an observer in the same line of sight. In the case of HAT-P-11b, the periodic dimming, which reduced the stars luminosity by around 0.4 percent, was detected by a network of small, automated telescopes known as HATNet, which is operated by the Harvard-Smithsonian Center for Astrophysics in Arizona and Hawaii.
This artist's conception reveals the newly discovered super-Neptune planet orbiting a star 120 light years away from Earth. Normally blue in colour, its red hue is caused by the illumination from the nearby red dwarf star. Image: David A. Aguilar (CfA).
HAT-P-11b is the 11th extrasolar planet found by HATNet, and an important addition to the inventory of exoplanets since it is one of the smallest yet discovered by any of the several transit search projects underway around the world. Compared with our own Solar System’s planet Neptune, which has a diameter 3.8 times that of Earth and a mass 17 times Earth's, the new world is 4.7 times the size of Earth and has 25 Earth masses, placing it into the category of super-Neptunes.
Furthermore, the newfound world orbits very close to its star at a rate of one revolution every 4.88 days. As a result, it is baked to a temperature of around 600 degrees Celsius. The host star, HAT-P-11 is roughly three-quarters the size of our own Sun and somewhat cooler.
The astronomers who made the discovery say that there are indications of a second planet in the HAT-P-11 system, but more radial velocity data are needed to confirm this, and to subsequently determine its properties. The radial velocity technique allows a planet to be inferred by looking at the ‘wobble’ induced on its host star.
A separate team has already located another transiting super-Neptune, known as GJ436b, around a different star, which was discovered by a radial velocity search and later found to have transits. "Having two such objects to compare helps astronomers to test theories of planetary structure and formation," says Harvard astronomer Gaspar Bakos.
The HAT-P-11 system will come under further scrutiny once the forthcoming Kepler mission begins operations. Located in the constellation Cygnus, HAT-P-11 is favourably situated for Kepler to provide the finer details, and maybe even uncover more planets, in this extrasolar planetary system. "We expect Kepler to measure the detailed properties of HAT-P-11 with the extraordinary precision possible only from space," says Robert Noyes, another member of the super-Neptune discovery team.
Kepler is currently scheduled to launch on 6 March this year, and will seek out extrasolar planets using the transit technique. A lot of hopes are riding on this mission, for it has the potential to detect the first Earthlike world orbiting a distant star.