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With two suns in its sky, Luke Skywalker's home planet Tatooine in
"Star Wars" looks like a parched, sandy desert world. In real life,
thanks to observatories such as NASA's Kepler space telescope, we know
that two-star systems can indeed support planets, although planets
discovered so far around double-star systems are large and gaseous.
Scientists wondered: If an Earth-size planet were orbiting two suns,
could it support life?
It turns out, such a planet could be quite hospitable if located at
the right distance from its two stars, and wouldn't necessarily even
have deserts. In a particular range of distances from two sun-like host
stars, a planet covered in water would remain habitable and retain its
water for a long time, according to a new study in the journal Nature
"This means that double-star systems of the type studied here are
excellent candidates to host habitable planets, despite the large
variations in the amount of starlight hypothetical planets in such a
system would receive," said Max Popp, associate research scholar at
Princeton University in New Jersey, and the Max Planck Institute of
Meteorology in Hamburg, Germany.
Popp and Siegfried Eggl, a Caltech postdoctoral scholar at NASA's Jet
Propulsion Laboratory, Pasadena, California, created a model for a
planet in the Kepler 35 system. In reality, the stellar pair Kepler 35A
and B host a planet called Kepler 35b, a giant planet about eight times
the size of Earth, with an orbit of 131.5 Earth days. For their study,
researchers neglected the gravitational influence of this planet and
added a hypothetical water-covered, Earth-size planet around the Kepler
35 AB stars. They examined how this planet's climate would behave as it
orbited the host stars with periods between 341 and 380 days.
"Our research is motivated by the fact that searching for potentially
habitable planets requires a lot of effort, so it is good to know in
advance where to look," Eggl said. "We show that it's worth targeting
In exoplanet research, scientists speak of a region called the
"habitable zone," the range of distances around a star where a
terrestrial planet is most likely to have liquid water on its surface.
In this case, because two stars are orbiting each other, the habitable
zone depends on the distance from the center of mass that both stars are
orbiting. To make things even more complicated, a planet around two
stars would not travel in a circle; instead, its orbit would wobble
through the gravitational interaction with the two stars.
Popp and Eggl found that on the far edge of the habitable zone in the
Kepler 35 double-star system, the hypothetical water-covered planet
would have a lot of variation in its surface temperatures. Because such a
cold planet would have only a small amount of water vapor in its
atmosphere, global average surface temperatures would swing up and down
by as much as 3.6 degrees Fahrenheit (2 degrees Celsius) in the course
of a year.
"This is analogous to how, on Earth, in arid climates like deserts,
we experience huge temperature variations from day to night," Eggl said.
"The amount of water in the air makes a big difference."
But, closer to the stars, near the inner edge of the habitable zone,
the global average surface temperatures on the same planet stay almost
constant. That is because more water vapor would be able to persist in
the atmosphere of the hypothetical planet and act as a buffer to keep
surface conditions comfortable.
As with single-star systems, a planet beyond the outer edge of the
habitable zone of its two suns would eventually end up in a so-called
"snowball" state, completely covered with ice. Closer than the inner
edge of the habitable zone, an atmosphere would insulate the planet too
much, creating a runaway greenhouse effect and turning the planet into a
Venus-like world inhospitable to life as we know it.
Another feature of the study's climate model is that, compared to
Earth, a water-covered planet around two stars would have less cloud
coverage. That would mean clearer skies for viewing double sunsets on
these exotic worlds.
NASA's planet-hunting Kepler telescope is managed by NASA's Ames Research Center in Silicon Valley. JPL, a divison of Caltech, managed Kepler mission development.