An international team of astronomers has found that a nearby star previously thought to host two or three planets is in fact orbited by six or seven worlds, including an unprecedented three to five "super-Earths" in its habitable zone, where conditions could be right for life.

This is the first time that so many super-Earths - planets more massive than Earth but less than 10 times more massive - have been detected in the same system.

"It's exciting that we've found a nearby star that has so many planets in its habitable zone," said University of Washington astronomer Rory Barnes, lead U.S. author on the paper published June 20 in the journal Astronomy and Astrophysics. The paper's lead author is Guillem Anglada-Escude of the University of Gottingen, Germany.

GJ667C, part of a triple-sun system in the Scorpius constellation, is a low-luminosity "M-dwarf" star about one-third the mass of the Sun. At about 22 light-years distance from Earth, it is a relatively close celestial neighbor. (A light-year is about 5.9 trillion miles.)

Since such low-mass stars are inherently faint, their habitable zones - the swath of space that would allow an orbiting rocky planet to sustain liquid water on its surface - lie much closer to the star. The closeness of the habitable zone then makes it easier to find potentially habitable rocky planets around low-mass stars.

Astronomers have in recent years confirmed the existence of two planets orbiting GJ667C - including one super-Earth - as well as "tantalizing" but incomplete evidence for a third.

Additional observations enabled Anglada-Escude's team to detect additional planet candidates, bringing GJ667C's total companions to six, possibly even seven worlds.

Because the habitable zone is so close to the star, the planets' years are far shorter than the Earth's, between 20 and 100 days. "The close proximity of these planets in the habitable zone to the host star makes it likely they are 'tidally locked,' which in this case means the same hemisphere always faces the star," Barnes said. "Fortunately, we know that this state can still support life."

In addition to the three super-Earths in the habitable zone, two more orbit at its outskirts, and with the right properties, could also support life. One of these candidates, GJ667Ch, the researchers write, is only tentatively detected and will require further follow-up for confirmation.

The technique the researchers used, called Doppler spectroscopy, does not measure a planet's actual mass, but finds only its minimum mass. However, the only way the habitable zone can be so packed with planets is if their masses are smaller than 10 Earth masses.

"These planets are good candidates to have a solid surface and maybe an atmosphere like the Earth's, not something like Jupiter," Barnes said.

He added that the number of potentially habitable worlds will be substantially greater if astronomers can expect to find several around each low-mass star like GJ667C.

"Instead of observing 10 stars to look for a single potentially habitable planet, we now know we can look at just one star and find several of them," Barnes said.

Doppler and other surveys show that systems with multiple super-Earths may be fairly common in the cosmos, and that small planets may be abundant around cool M-dwarf stars.

The discovery provides not only a better understanding of the star in question, but also a clue that these worlds may be the first members of an emerging population of M-dwarf stars with multiple low-mass planets in their habitable zones. The Sun's neighborhood may contain many rocky planets in habitable zones.

Other coauthors are Mikko Tuomi and Hugh R. A. Jones of the University of Hertfordshire; Enricho Gerlach of the University of Turku, Finland; Rene Heller of the Leibniz Institute for Astrophysics Potsdam in Germany; James Jenkins of Universidad de Chile; Sebastian Wende1 and Steven S. Vogt of the University of California, Santa Cruz; Paul Butler of the Carnegie Institution; and Ansgar Reiners of the University of Gottingen.