Washington: Astronomers have spotted an extrasolar planet with a mass just four times that of Earth.
Experts from the California Institute of Technology (Caltech) and other institutions made the finding using the highly sensitive 10-meter Keck I telescope atop Hawaii`s Mauna Kea.
The planet orbits its parent star HD156668 about once every four days.
The planet dubbed HD 156668b is the second-smallest world among the more than 400 exoplanets (planets located outside our solar system) known to man.
The discovery was made using the radial velocity or wobble method, which relies on Keck`s High Resolution Echelle Spectrometer (HIRES) to spread light collected from the telescope into its component wavelengths or colors, producing a spectrum.
As the planet orbits the star, it causes the star to move back and forth along our line of sight, which causes the starlight to become redder and then bluer in a periodic fashion.
The color shifts give astronomers the mass of the planet and the characteristics of its orbit, such as how much time it takes to orbit the star.
John A Johnson, assistant professor of astronomy at Caltech and codiscoverer of the new planet said: "If the stars themselves have imperfections and are unstable, their wobbling would cause jumps in velocity that could mimic or hide the existence of a planet."
He added: "We have been doing simulations to understand the astrophysics of these imperfections, and how to distinguish them from the signals from a planet. We hope to use these simulations to design even better observing strategies and data-analysis techniques."
However, he explained that discovery of a planet that is comparable in size to Earth and found within the habitable zone, "will require a great deal of work."
He added: "If we could build the best possible radial-velocity instrument tomorrow, we might have answers in three years, and a solid census of Earthlike planets within a decade. We``ll need gigantic leaps in sensitivity to get there, and we``re hot on the trail."
Johnson is also presently working on building a new camera for the 60-inch telescope that will allow astronomers to search for the passages-or transits-of low-mass planets like HD156668.
He concluded: "If we catch the planet in transit, we can measure the planet``s radius and density, and therefore address the question of whether the planet has a composition more like Earth, with a solid surface and thin atmosphere, or is a miniature version of Neptune, with a heavy gaseous atmosphere."
The discovery made with support from NASA``s Eta-Earth Survey for Low-Mass Planets was announced at the 215th American Astronomical Society meeting in Washington, D.C.