Researchers Max Popp and Siegfried Eggl haven't found such a planet yet. But they have been able to model it to suggest this possibility for telescopes so they can search for these types of systems in the future.
Previously, circumbinary systems -- where a planet orbits two stars
-- have been found, but those planets were similar to the large gas planets in our solar system, such as Neptune or Jupiter.
Popp and Eggl took the previously discovered Kepler 35 system, comprised of two stars that are orbited by the giant planet Kepler 35B. They removed 35B from the equation and substituted their own Earth-sized, water-covered planet to see how it might interact with the two stars.
"The planet in our model looks a bit like Kevin Costner's 'Waterworld,'" said Eggl, a postdoctoral researcher at the California Institute of Technology and NASA's Jet Propulsion Laboratory. "It has the size and atmosphere of our Earth, but it is fully covered with water. We decided that it makes more sense to study a planet with a global ocean, than to distribute continents at random or mimic the way the Earth looks like today."
In order for the planet to sit within the habitable zone of two stars and compensate for twice the power output, Popp and Eggl had to place the planet further away than the Earth is from the sun. Too close and it would end up like Venus
. Venus once had liquid on the surface
and lost it due to a runaway greenhouse effect in the atmosphere, where heat from the sun became trapped close to the surface and evaporated the liquid there.
That liquid water is not only important for life as we know it, but also drives important geological and geochemical processes, all of which are crucial to establish and maintain habitable conditions, Eggl said.
But if the planet is too far out, it would freeze over and look like the ice planet "Hoth," also in the Star Wars saga.
In between these two extremes is the "Goldilocks" zone, where things are just right and the planet is in the habitable zone of both stars, meaning it would retain the liquid water on its surface and potentially support life.
What would it be like?
Having two suns means that there would be longer periods of dawn and dusk on the planet as both stars rise and set, Eggl said. Interestingly enough, the researchers also discovered that there would be less cloud cover because of the two suns, with clearer skies to enjoy such a dramatic beginning and end to each day.
"Having two suns in the sky dancing around each other is a feature that is hard to overlook," Eggl said. "I guess such a planet would make an excellent place for a romantic getaway."
But the actual climate conditions would experience a shift similar to what can happen when you travel from one extreme region on Earth to a different one, like going from a tropical rain forest to a dry desert.
Because of the two stars, this planet would not enjoy a circular orbit like the Earth moves around the sun. Rather, it would change shape with time, wobbling and receiving different amounts of light. This would cause a variation in surface temperature and shift the habitable zone.
Before this study, it wasn't known that a planet's climate variability would be connected to the motions of two stars and the planet's shifting orbit, Eggl said.
When the planet is closer to its stars, the climate would be similar to Earth, with stable temperatures and rainfall. But the further away the planet moves during its orbit, the less water vapor is in the atmosphere. This can lead to extreme temperature changes and "a more volatile climate," Eggl said.
"They can cause a rapid succession of seasons of variable duration -- not unlike the unpredictable winters in the TV series 'Game of Thrones,'" Eggl said.
The quest to find these planets
Popp and Eggl think the odds of finding a planet like this are quite good, based on their research. Having two sun-like stars increases the size of the habitable zone for the planet. And given the way the stars interact with each other, less radiation is directed toward the planet as well.
"Finding and confirming circumbinary planets, however, is somewhat more difficult than detecting planets around single stars," Eggl said. "We hope that we can convince the community that those systems are worth the extra effort."
They are developing tools to help other astronomers know where to look for potentially habitable worlds. And both are excited at the prospect that what was once science fiction is now becoming science fact.
"Science fiction gave many valuable ideas of what habitable climates could look like in the past when planetary climates were not well understood," said Popp, associate research scholar at Princeton University and the Max Planck Institute of Meteorology. "In recent years, three-dimensional climate simulations have allowed scientists to gain a better understanding of the range of possible habitable climates. So I hope that the scientific insights on habitable climates will now make their way back into film and literature."
And given that we are in the golden age of exoplanet discovery, "it is easy to forget that we are just at the beginning of our quest to find habitable worlds, especially in multiple star systems," Eggl said.
Even though a planet like the one in Popp and Eggl's model hasn't been found yet, they believe it will happen.
"And frankly I would love to go there, too!" Eggl said. "Imagine what it would be like to see two suns in the sky orbiting each other. The view must be stunning! Granted, those systems are most likely several tens of light years away -- too far for an afternoon walk. Still, I trust in the human ability to come up with creative solutions to this problem. After all, humans have no wings and still people fly all over the globe. Anything is possible, if we understand nature well enough. That is why fundamental science is so important."