Six new planets have been discovered orbiting two nearby Sun-like stars, including two 'super-Earths' with masses 5 and 7.5 times the mass of Earth.

The detections were made using data from the W. M. Keck Observatory in Hawaii and the Anglo-Australian Telescope (AAT) in New South Wales, Australia as part of the Lick-Carnegie Exoplanet Survey Team led by Steven Vogt and Paul Butler of the University of California, Santa Cruz (UCSC). They use the radial velocity technique from ground-based telescopes to detect the 'wobble' induced in a star by the gravitational tug of an orbiting planet. These observations were complemented with precise brightness measurements using robotic telescopes in Arizona by Gregory Henry of Tennessee State University.

“We don’t see any brightness variability in either star,” says Henry. “This assures us that the wobbles really are due to planets and not changing patterns of dark spots on the stars.”

Three of the new planets, ranging from 5 to 25 times the mass of Earth, orbit the nearby star 61 Virginis, a naked eye star just 28 light years away that resembles the Sun in terms of properties such as age and mass. In a separate study using NASA's Spitzer Space Telescope, astronomers also discovered that 61 Vir harbours a thick dust ring at a distance roughly twice as far from 61 Vir as Pluto is from our Sun, likely created by collisions of comet-like bodies in the cold outer reaches of the system.

“Spitzer’s detection of cold dust orbiting 61 Vir indicates that there’s a real kinship between the Sun and 61 Vir,” says Eugenio Rivera of UCSC. Rivera computed an extensive set of numerical simulations to find that a habitable Earth-like world could easily exist in the as-yet unexplored region between the newly discovered planets and the outer dust disc.

“These detections indicate that low-mass planets are quite common around nearby stars,” says Vogt. “The discovery of potentially habitable nearby worlds may be just a few years away.” Vogt is the principal investigator for the new Automated Planet Finder (APF) Telescope and says that 61 Vir will be an excellent target for further study with the APF.

“The inner planet of the 61 Vir system is among the two or three lowest-amplitude planetary signals that have been identified with confidence,” says Butler. “We’ve found there is a tremendous advantage to be gained from combining data from the AAT and Keck telescopes, two world-class observatories, 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.”

The second system resides 76 light years away in the the constellation Cetus. HD 1461b hosts a 7.5 Earth-mass planet and possibly two other planets. The 'super-Earth' orbits its star once every six days and has a mass roughly half way between the Earth and Uranus, but further study will be required to determine which planet it most resembles in terms of composition.

The team say that since these planets have orbits close to their stars they would be too hot to support liquid water and life, but the discoveries point the way to finding similar planets around M-dwarf stars that are typically less than half the mass of the Sun and output less than two percent the Sun’s energy. “These sorts of planets around M-dwarfs actually would be in a liquid water zone,” says Butler. “So we are knocking on the door right now of being able to find habitable planets.”

In the last decade-and-a-half over 400 exoplanets have been detected, and while the majority of them have been massive Jupiter-like planets, more recent discoveries are forming a new and exciting trend towards uncovering the first Earth-like planets.

Two papers describing the new planets have been accepted for publication in the Astrophysical Journal.