Earth-Sized 'Tatooine' Planets Could Be Habitable

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.

This artist's concept shows a hypothetical planet covered in water around the binary star system of Kepler-35A and B. Image credit: NASA/JPL-Caltech

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.

"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.

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.

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.

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. (NASA)

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