Avoid Tidal Locking...

An artist's impression of Kepler-62f, a planet that is in the habitable zone of a star smaller and cooler than the Sun, located about 1200 light-years from Earth. (Courtesy: NASA Ames/JPL-Caltech)

Topics: Astronomy, Astrophysics, Exoplanets, Space Exploration

Even a thin atmosphere can keep a planet spinning freely, giving it a day-and-night cycle like Earth's, say astronomers in Canada and France. The result implies that many of the planets lying within the habitable zones of "dim suns" – the most common type of star – could have terrestrial-type climates.

"It was a surprise," says Jérémy Leconte, an astronomer at the University of Toronto. "We didn't expect that there would be such a strong effect."

Astronomers have so far discovered more than 1000 exoplanets – planets orbiting stars other than the Sun – and it is becoming clear that our tiny portion of the universe could contain vast numbers of such planets. What is not yet clear is how many of these planets could actually harbour life.

Most stars, known as orange and red dwarfs, are cooler, fainter and smaller than the Sun; so to stay warm, a habitable planet must huddle close to the star. But the closer a planet is to its sun, the stronger are the tidal forces that the star exerts on the planet. These tides can affect how fast the planet spins. In extreme cases, these tides are so strong that they produce "tidal locking", forcing the planet to spin as slowly as it revolves. This means that one side of the planet forever faces the star, while the other side forever faces away, creating a world with a permanent day side and a permanent night side. The night side may get so cold that air freezes there, robbing the planet of an atmosphere.

Physics World: Exoplanets could avoid 'tidal locking' if they have atmospheres
Ken Croswell