MAUNA KEA, HI—Planet hunters using the W. M. Keck Observatory have identified at least six low-mass planets around two nearby, Sun-like stars. Two of the planets are five and 7.5 times the mass of Earth. These “super-Earths” are the first low mass planets found orbiting stars similar to the Sun.
The latest discoveries probe a new class of planets that are somewhat more massive than Earth but less massive than Uranus and Neptune, which suggests that low mass planets are quite common around nearby stars, said expert planet hunter Steven Vogt of the University of California, Santa Cruz (UCSC).
“The discovery of potentially habitable nearby worlds may be just a few years away,” he added.
Vogt and Paul Butler of the Carnegie Institution of Washington led the international team, which found the new planet systems by combining data gathered with the 10-meter Keck I telescope in Hawaii and the Anglo-Australian Telescope in New South Wales, Australia. The astronomers describe the new planetary systems in two papers that will appear in the Astrophysical Journal.
One of the new systems orbits the bright star 61 Virginis, which can be seen with the naked eye under dark skies in the spring constellation Virgo. This particular star, which is 28 light-years from Earth, stands out among hundreds of close stellar neighbors as being one of the best twins of the Sun in terms of age, mass and other properties. Vogt and his collaborators discovered at least three planets orbiting 61 Vir. They range in mass from roughly five to 25 times the mass of Earth and orbit the star in four, 38 and 124 days.
The second new system the team identified features a 7.5 Earth-mass planet orbiting HD 1461, another near-perfect solar twin located 76 light-years away. Lying in the constellation Cetus, HD 1461 can be seen with the naked eye in the early evening under dark-sky conditions. At least one planet orbits the star and two others are strong candidates.
The 7.5-Earth-mass planet, HD 1461b, has a mass nearly midway between the masses of Earth and Uranus and a period of 5.77 days. The researchers cannot tell yet whether HD 1461b is a scaled-up version of the Earth, composed largely of rock and iron, or whether it is similar to Uranus and Neptune and composed mostly of water.
The 61 Vir and HD 1461 planet detections add to a slew of recent discoveries that suggest that planets orbiting the Sun’s nearest neighbors are extremely common, and about half of all nearby stars have a detectible planet with mass equal to or less than Neptune’s, Butler said.
He explained that the new detections required state-of-the-art instruments and detection techniques. “We’ve found there is a tremendous advantage to be gained from combining data from the AAT and Keck telescopes, two world-class observatories,” he said. “It’s clear that we’ll have an excellent shot at identifying potentially habitable planets around the very nearest stars within just a few years.”
Currently, the inner planet orbiting the 61 Vir system has one of the lowest-amplitude planetary signals that have been identified with confidence, which means it is one of the more difficult planets to detect because it is so small, Butler added.
Based on extensive numerical simulations, a habitable Earth-like world could easily exist in the unexplored region between the newly discovered planets and the outer dust disk of 61 Vir, said Eugenio Rivera, lead author of one of the papers and a postdoctoral researcher at UCSC.
The team uses radial velocity measurements taken with Keck Observatory’s High Resolution Echelle Spectrometer, or HIRES, to detect a “wobble” induced in the star’s spectrum by the gravitational tug of an orbiting planet. This technique is coupled with photometric monitoring of the star to detect the transit of a planet in front of the star.
With improvements in the equipment and observing techniques, these ground-based methods are now capable of finding Earth-mass objects around nearby stars, said research collaborator Gregory Laughlin, also of UCSC.
“It’s come down to a neck-and-neck race as to whether the first potentially habitable planets will be detected from the ground or from space,” he said. “A few years ago, I’d have put my money on space-based detection methods, but now it really appears to be a toss-up. What is truly exciting about the current ground-based radial velocity detection method is that it is capable of locating the very closest, potentially habitable planets.”
The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i and is a scientific partnership of the California Institute of Technology, the University of California and NASA. For more information please call 808.881.3827 or visit http://www.keckobservatory.org.
The Anglo-Australian Telescope is a 3.9 meter telescope located at Siding Spring Observatory near Coonabarabran, Australia and is operated jointly by the United Kingdom and Australia.
This research was supported by the National Science Foundation and NASA. The Lick-Carnegie Exoplanet Survey Team has developed a publicly available tool, the Systemic Console, which enables members of the public to search for the signals of extrasolar planets by exploring real data sets in a straightforward and intuitive way. This tool is available online at www.oklo.org.