Dan Vergano, USA TODAY
A nearby star appears to be circled by six planets, including one "Super Earth" world that orbits at distances that could allow the existence of oceans, astronomers report.
Astronomers had earlier reported that three planets closely orbit the star, HD 40307, which is about 43 light years away (one light year is about 5.9 trillion miles). Now in the forthcoming Astronomy & Astrophysics journal report led by Mikko Tuomi of the United Kingdom's University of Hertfordshire, the team reports three more planets larger than Earth - but not as big as Jupiter - likely orbit the star.
One, dubbed HD 40307g, weighs at least seven times more than Earth and orbits the star on a 200-day "year" in the region amenable to liquid oceans on planets. (If planets orbit too close to their stars, they're likely too hot to support oceans.) "The star HD 40307, is a perfectly quiet old dwarf star, so there is no reason why such a planet could not sustain an Earth-like climate," says study co-author Guillem Anglada-Escude of the University of Goettingen, in a statement.
In the study, the team re-analyzed data revealing gravitational wobbles seen in the light coming from the star that might reveal the presence of planets tugging on HD 40307, which is about 77% as massive as the sun.
MIT astronomer Sara Seager, who was not part of the discovery team, notes the authors describe the planet as a more tentative "candidate" planet in the study, calling for further observations to confirm its existence. "To me this discovery stands out as adding to the tip of the iceberg, or adding to the tipping point where we will get to an almost everyday occurrence of finding potentially habitable planets!" Seager says, by e-mail. "More seriously, we are definitely heading towards a statistical picture of how many potentially habitable planets are out there..."
While the detection looks solid, "the planet is a little too hefty to be the whole enchilada of an Earth-like planet," says University of Toronto astronomer Ray Jayawardhana, author of Strange New Worlds: The Search for Alien Planets and Life beyond Our Solar System. Instead, a planet seven times heavier than Earth or more seems more likely to resemble the gas-clouded planets Neptune and Uranus in our solar system. "We are getting very close to finding another planet, or planets, like Earth," he says.
Because HD 40307 is relatively close, the discovery team suggests that its possible planets may be good targets for future space telescopes that target worlds orbiting nearby stars.