Six super-Earths around two nearby sunlike stars

Planets orbiting the sun’s nearest neighbors appear to be extremely common, say astronomers.

An international team of planet hunters has discovered six low-mass planets – called “super-Earths” by astronomers – around two nearby sun-like stars. Two of the planets have masses only 5 and 7.5 times that of Earth.

“Super-Earths” this small may approach what we consider habitable. These astronomers say we may be nearing the discovery of even more Earth-like planets.

“These detections indicate that low-mass planets are quite common around nearby stars. The discovery of potentially habitable nearby worlds may be just a few years away,” said Steven Vogt, a professor of astronomy and astrophysics at University of California/Santa Cruz and principle investigator for the new Automated Planet Finder telescope.

Three of the newly-discovered planets orbit the bright star 61 Virginis. It is only 28 light-years away, and of the neighboring stars is the most similar to the sun in terms of age, mass, and other essential properties. Vogt and his collaborators have found that 61 Virginis hosts
at least three planets, with masses ranging from about 5 to 25 times the mass of Earth.

Vogt says that the Automated Planet Finder (APF) telescope recently constructed at Lick Observatory will be excellent for hunting new planets at 61 Virginis. “We’re very excited to continue monitoring this system using APF,” said Vogt.

A separate team of astronomers used NASA’s Spitzer Space Telescope to discover that 61 Virginis has a thick ring of dust roughly twice as far from the star as Pluto is from our sun. The dust is apparently created by collisions of comet-like bodies. Eugenio Rivera, a researcher at UCSC, says “Spitzer’s detection of cold dust orbiting 61 Vir indicates that there’s a real kinship between the Sun and 61 Vir.” Rivera’s calculations show that a habitable Earth-like world could easily exist in orbit around 61 Virginis.

Another stellar twin of the sun, HD 1461, located 76 light-years away, has a newly-discovered planet with a mass nearly midway between the masses of Earth and Uranus. The researchers cannot tell yet if the planet is a scaled-up version of Earth, mostly rock and iron, or whether, like Uranus and Neptune, it is composed mostly of water.

The 61 Virginis and HD 1461 sets of planets add to recent discoveries that have changed conventional thinking. In the past year, we have learned that planets orbiting the sun’s nearest neighbors are extremely common. According to Butler, it seems that fully one-half of nearby stars have a detectable planet with mass equal to or less than Neptune’s.

How are these discoveries made?

Discoveries like these were made by combining data from ground-based telescopes. Astronomers use radial velocity measurements to detect how a star “wobbles” as a planet orbits it. Delicate measurements of the stars’ brightness are taken. “We don’t see any brightness variability in either star,” said Gregory Henry of Tennessee State University. “This assures us that the wobbles really are due to planets and not changing patterns of dark spots on the stars.”

Such findings require state-of-the-art instruments and detection techniques, says Paul Butler of the Carnegie Institution of Washington. “The inner planet of the 61 Vir system is among the two or three lowest-amplitude planetary signals that have been identified with confidence. We’ve found there is a tremendous advantage to be gained from combining data, and 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.”

Improvements in equipment and observing techniques will enable ground-based methods to find objects as small as Earth around nearby stars, according to Gregory Laughlin, professor of astronomy and astrophysics at 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,” Laughlin 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 Lick-Carnegie Exoplanet Survey Team has developed a publicly available tool, the Systemic
Console, which enables you to explore real stellar data sets. You can read about the console program at www.oklo.org.

Deborah Byrd