Two of the extrasolar planets are considered super-Earths, more massive than Earth but less massive than Uranus and Neptune. Spotting true Earth-sized planets is challenging with current technology, but the presence of super-Earths suggests finding a world like ours is just a matter of time, researchers say.
"These detections indicate that low-mass planets are quite common around nearby stars," said study team member Steven Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz. "The discovery of potentially habitable nearby worlds may be just a few years away."
The astronomers are not sure if the super-Earths are rocky like our own world or if they have some other composition.
The team found the new planet systems by combining data gathered at the W. M. Keck Observatory in Hawaii and the Anglo-Australian Telescope (AAT) in New South Wales, Australia. They inferred the existence of the planets by noting the worlds' gravitational effects on the parent star's orbit. This method is called the radial velocity, or wobble, technique.
The objects have not been photographed.
Three of the exoplanets orbit the star 61 Virginis, which is virtually a twin of the sun and lies 28 light-years away in the constellation of Virgo. (At this time of year, Virgo can be seen rising a few hours before the sun.)
The researchers estimated the minimum mass of each planet as: 5.1 Earth masses for 61 Vir b, 18 Earth masses for 61 Vir c, and 23 Earth masses for 61 Vir d, according study team member Chris Tinney of the University of New South Wales.
"So the smallest one is in the super-Earth mass range, and is the first planet like this to be found around a sun-like star," Tinney told SPACE.com.
Other super-Earths have been found around stars that are cooler and redder than the sun, he said.
Tinney added, "This is exciting, because it demonstrates the ability of our team to find planets at these interesting, small masses around solar-mass stars. If we want to one day find habitable planets that are really like the Earth in systems that are really like ours, then those are the sorts of stars we need to be able to find low-mass planets around."
The second new system found by the team features a 7.5-Earth-mass planet orbiting HD 1461, another near-perfect twin of the sun located 76 light-years away in the constellation Cetus. The scientists say at least one and possibly two additional planets also orbit the star.
HD 1461 can be seen with the naked eye in the early evening under good dark-sky conditions.
The researchers say they aren't sure whether the planet, now called HD 1461b, is a scaled-up version of Earth, composed largely of rock and iron, or whether, like Uranus and Neptune, it is composed mostly of water.