Astrophile: Two planets with two suns up odds for life

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Objects: An unusual exoplanet pair
Orbits: Circling two sun-like stars
Plot twist: Chance for a habitable moon

The sun rises on Kepler-47c's largest moon, turning the sky a rosy pink. The moon's host planet, a gas giant about the size of Uranus, hangs huge in the sky as always, its churning storms a constant sight for the inhabitants below. A single bright object ? the system's innermost planet ? is just visible near the sun, fading fast in the morning light.

Then the other sun rises.???????????????

This science-fiction scene could be playing out for real about 4900 light years from Earth, where a pair of planets jointly orbits both stars in a binary system.

Astronomers have previously found single planets with two suns, like Tatooine in Star Wars. But this is the first time a binary pair has been found to host a multi-planet system.

What's more, one of the planets is in the stars' habitable zone, the region around the suns where temperatures are just right for liquid water ? and therefore maybe life ? to exist on a planet's surface. The discovery boosts the odds of finding habitable planets in our galaxy.

"Binary stars are extremely common," accounting for about half the stars in the sky, says Dan Fabrycky of the University of California, Santa Cruz, who was not on the study team. "Ostensibly, you can now double the population of habitable planets."

Dubbed Kepler-47c, the new-found planet is almost certainly a gas giant, based on its estimated size. If it has any rocky moons, though, they could be ripe for life, like Star Wars' Endor or Avatar's Pandora.

One of the system's discoverers, Jerome Orosz of San Diego State University in California, thinks there's a strong case for this storyline.

"If I came back from France and told you that people there wore shoes, you would not be surprised. After all, people around here wear shoes. Likewise, nearly every planet in the solar system has moons. If you went to Kepler-47c and reported the existence of moons, it would not surprise me in the least."

Temperature swings

Orosz found the new worlds while looking at data from the planet-hunting Kepler space telescope, which searches for stars with planets that cross in front of them, or transit, as seen from Earth.

The team calculates that the inner planet, Kepler-47b, is about three times the width of Earth, and orbits its two suns once every 49.5 days. The outer planet, Kepler-47c, is larger ? about 4.6 times Earth's radius ? and orbits once every 303 days.

These orbits put the planets at safe distances from their chaotic parent stars, which are pulling each other around in a constant cosmic waltz.

Seen from a hypothetical moon, the suns would probably follow each other across the sky, rising or setting within 15 to 30 minutes of each other, assuming a 24-hour day, Orosz says.

There would be more daylight than darkness in general, though when the larger, brighter sun sets, the light might look a bit like the semi-darkness of a partial eclipse.

"Your seasons are going to be really odd," Orosz adds. "The distance between you and the big star changes constantly, so the amount of heat you get can change by several per cent over the course of a binary orbit, which is a week."

What that means for any potential Ewoks or Na'vi depends on the moon's atmosphere and how well it retains heat. But it could mean that a Monday feels like August in Miami, but Wednesday would feel like November in London, before temperatures become hot again by Sunday.

And if the moon ? like ours ? always showed the same face to its host planet, then for half the population the planet would be permanently visible in the sky, Fabrycky says.

"You know those beautiful pictures of Jupiter, with the swirls and jets?" he says. "They would see that all the time."

Journal reference: Science Express, DOI: 10.1126/science.1128380

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