HD156668b – Hercules Constellation exoplanet discovered

Hercules Constellation hosts the latest exoplanet which was discovered recently. The new planet is four times the mass of Earth making it the second smallest extrasolar or exoplanet to have been found till date.

Picture: NASA rendering of HD156668b exoplanet in Hercules constellation

Called rather unpoetically as HD156668b, it sits in a star system which is about 80 light years from the Earth in the constellation of Hercules. The Hercules Constellation exoplanet has a mass of roughly 4.15 Earth masses.

An extrasolar planet or exoplanet orbits a star other than the Sun and is beyond the solar system. HD156668b from the Hercules Constellation is the latest extrasolar planet to join the ranks of the so-called ‘super-Earths’. A super-Earth has mass between that of the Earth and the Solar System’s gas giants like Jupiter, Saturn, Uranus or Neptune. The smallest extrasolar planet currently known is called Gliese 581 e. It has a mass that is nearly twice that of Earth and orbits a planet-filled star system about 20.5 light-years away in the constellation Libra.

HD156668b from Hercules Constellation was discovered by astronomers using the twin 10-metre Keck I telescopes atop Mauna Kea in Hawaii. They found that the planet orbits its parent star once every four days or so. According to Andrew Howard of the University of California at Berkeley, who found the planet along with his colleagues, it was a “wobble” they found in its parent star that led to the discovery of this planet.

The method used to detect the Hercules Constellation planet measures the effects of a planet’s gravity on its parent star. The researchers used the radial velocity or wobble method, using Keck’s High Resolution Echelle Spectrograph, or HIRES instrument, to spread light collected from the telescope into its component wavelengths or colors. As the planet passes behind its stellar parent as seen from Earth, its gravity tugs slightly on the star, causing the starlight to shift to a redder wavelength in the light spectrum.

When the planet orbits in front of the star, it pulls the star in the other direction. The star’s spectrum shifts toward bluer wavelengths.
The color shifts give astronomers the mass of the planet and the characteristics of its orbit, such as the time it takes to orbit the star.

Apart from the Hercules Constellation  exoplanet, more than 400 extrasolar planets have been discovered using the same method. But most of those planets are giants similar in mass to Jupiter.

Howard said it has been the long-standing goal among astronomers to find low-mass planets similar to Earth, though their size makes them difficult to detect. The astronomers consider this as a remarkable discovery as it showed that they could “push down and find smaller and smaller planets”. Small planets are extremely hard to detect, he said.

Howard, along with his colleagues from the California Planet Search team (CPS) Geoff Marcy of UCB, Debra Fischer of Yale University, John Johnson of the California of Institute of Technology and Jason Wright of Penn State University participated in the study. The Hercules Constellation planet discovery was announced at the 215th meeting (January4-7, 2010) of the American Astronomical Society in Washington. Marcy has created the Eta-Earth Survey for Low Mass Planets to hunt for super-Earths. The survey has found two such planets, Howard said.