A study by Dr. Siegfried Eggl of NASA’s Jet Propulsion Laboratory and Dr. Max Popp of Princeton University and the Max Planck Institute of Meteorology has found that an Earth-like circumbinary planet could be habitable if it were located within a certain range from its host stars.
This artist’s concept shows a hypothetical aquaplanet around the binary star system of Kepler-35. Image credit: NASA / JPL-Caltech.
Planets that orbit two stars are known as circumbinary planets, or sometimes ‘Tatooines,’ after Luke Skywalker’s home world in ‘Star Wars.’
Astronomers know that double stars can indeed support planets, although planets discovered so far in such systems are gaseous giants.
Dr. Eggl and Dr. Popp wondered: if an Earth-like planet were orbiting two stars, could it support life?
“The detection of planets orbiting close binary stars has fuelled a vivid debate on whether such environments can be suited for life as we know it,” the researchers explained.
“The most famous example for a circumbinary planet may still be the fictitious world in the ‘Star Wars’ universe named ‘Tatooine’ — a desert planet.”
“The presence of a second sun does not automatically mean that any world orbiting two stars has to be dry.”
It turns out, a circumbinary planet could be quite hospitable if located at the right distance from its host stars. In a particular range of distances from two Sun-like stars, a water-rich planet would remain habitable and retain its water for a long time.
“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,” Dr. Popp explained.
The researchers created a model for a planet in the Kepler-35 double-star system.
“The Kepler-35 binary-system consists of two stars — Kepler-35A and B — only slightly less luminous than the sun on a mutual orbit with a semimajor axis of 0.176 AU,” they said.
“An additional giant planet — Kepler-35b, a giant planet 8 times the size of Earth, with an orbit of 131.5 Earth days — has been discovered in this system.”
“However, we choose to neglect the planet in the remainder of this study as it would lead to more complex dynamical behavior of the additional, fictitious terrestrial world that is under investigation in this work.”
“We perform simulations of an aquaplanet (fully water-covered planet) on a variety of orbits to find habitable climates for fixed carbon dioxide concentrations both in the Kepler-35 and in our own Solar System for reference.”
The scientists examined how this planet’s climate would behave as it orbited the host stars with periods between 341 and 380 days.
They found that on the far edge of the habitable zone in the Kepler-35 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. The amount of water in the air makes a big difference,” Dr. Eggl said.
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.
The team’s climate model also found that, compared to Earth, a water-covered planet around two stars would have less cloud coverage. That would mean clear skies and striking double sunsets that, perhaps with less sand, are akin to the iconic scene of Luke Skywalker gazing upon the landscape of Tatooine as one star rises and another sets.
The findings were published in the April 6 issue of the journal Nature Communications.
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Max Popp & Siegfried Eggl. 2017. Climate variations on Earth-like circumbinary planets. Nature Communications 8, article number: 14957; doi: 10.1038/ncomms14957
This article is based on a press-release from NASA.
