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.
Confirmation will be a challenge without space-based telescopes designed to image planets directly, the team acknowledges. HD 40307g could be the first planet spotted by any such future observatories. Because it is so far from its host star, HD 40307g's reflected light is less likely to be swamped by the light from its star.
Yet even without the prospect of immediate confirmation, the team is confident it has picked up signatures from bona fide planets.
With the team's confirmation of the star's inner three planets and the discovery of three more candidates, "I see no reason to doubt the existence of these planets," says Mikko Tuomi, a researcher at the University of Hertfordshire in Britain and a team leader. The system wouldn't exist if the planets' true masses weren't close to those the team calculates as minimums, he explains. Otherwise the system would have been unstable.
The discovery of these three additional planets highlights the often unsung roll data-analysis software plays in the planet-detection business, astronomers say.