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But HD 40307g's host star is faint enough that the planet, with a year some 200 days long, falls well within the star's habitable zone.

The six planets exhibit minimum masses ranging from 3.5 Earth masses to 9.5 times the Earth's mass. The potentially habitable super Earth comes in at about seven Earth masses.

Based on calculations involving a similar, habitable-zone super Earth discovered by NASA's Kepler mission, Kepler 22b (which is also the other super Earth with a day-night cycle), the team posits that the planet could be a mini-Neptune, with a rocky core and thick atmosphere.

Other astronomers have been trying to confirm the existence of the initial three planets by hunting for the slight dimming a planet imparts to its star's light as it swings in front of the star during its orbit – and event known as a "transit." But no one has spotted anything yet.

Given that it is easier to spot transits for close-in planets than for more-distant planets, the chances of catching a transit for HD 40307g as well would seem remote. Astronomers are interested in such detections because a transit would not only provide confirmation of the planet's existence, but also allow astronomers to infer a great deal about the planet, including its density.