Planet hunters have detected an extrasolar planet that is only four times the mass of Earth, making it the second smallest exoplanet ever discovered. Astronomers using the 10-meter Keck I telescope at the Keck Observatory in Hawaii found the un-poetically named HD156668b, which has a mass of roughly 4.15 Earth masses. It orbits its parent star in just over four days and is located roughly 80 light years from Earth in the direction of the constellation Hercules. This adds to the growing list of so-called “Super-Earths” now being found.
“This is quite a remarkable discovery,” said astronomer Andrew Howard of the University of California at Berkeley. “It shows that we can push down and find smaller and smaller planets.”
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. When the planet orbits around the back of the parent star, its gravity pulls slightly on the star causing the star’s spectrum to shift toward redder wavelengths. 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. Nearly 400 planets around other stars were discovered using this technique. But, the majority of these planets are Jupiter-sized or larger.
“It’s been astronomers long-standing goal to find low mass planets, but they are really hard to detect,” Howard said. He added that the new discovery has implications for not only exoplanet research but also for solving the puzzle of how planets and planetary systems form and evolve.
Astronomers have pieces of the formation and evolutionary puzzle from the discovery of hundreds of high-mass planets. But, “there are important pieces, we don’t have yet. We need to understand how low mass planets, like super-Earths, form and migrate,” Howard said.
The goal of the Eta-Earth Survey for Low Mass Planets, which was the brainchild of fellow planet hunter Geoff Marcy, also from UCB, to find these super-Earths. So far the survey has discovered two near-Earth-mass planets with more are on the way, Howard said.
Other collaborators included , Debra Fischer of Yale University, John Johnson of the California of Institute of Technology and Jason Wright of Penn State University.
The discovery was announced at the 215th American Astronomical Society meeting in Washington D.C.