But in binary star systems, tidal forces between the stars can put the brakes on their spins, rapidly taming their youthful activity. Young stars spin quickly, slowing down as they age. The strength and frequency of these flares, however, is dependent on the starвЂ™s rotation rate: the faster it spins the more flares it produces. These gigantic flares can erode a young planetвЂ™s atmosphere, blast its water vapor off into space, and kill a potentially habitable world in its youth. On our own planet, these create dramatic northern and southern lights, bombard astronauts and satellites in dangerous amounts of radiation, disrupt communications, and pose risks to electronics and power grids.Д«ut many stars, especially in their youths have enormous flares that are risks not just to technology, but to life itself. It works, Mason says, by reducing the risk of hyper-intense solar flares. Paul Mason, an astrophysicist at New Mexico State University, Las Cruces, New Mexico, argues that there is a вЂњbinary habitability mechanismвЂќ that can sometimes enhance a planetвЂ™s chances of being habitable.

вЂњThere are times when there are no seasons, and others when seasons, on a time scale of a few tens of thousands of years.вЂќД«ut that doesnвЂ™t mean everything about binary star systems reduces their planetsвЂ™ habitability. вЂњSeasons around these binaries may be a lot more variable than on Earth,вЂќ he says. In binary star systems, this effect can be radically stronger, says Billy Quarles of Georgia Institute of Technology, Atlanta, Georgia, especially if the planetвЂ™s orbit doesnвЂ™t lie in the same plane as its starsвЂ™ orbits around each other.

On Earth, this change is limited to a 2.4-degree range, but thatвЂ™s enough for the changes to be linked to such major climate changes as ice ages. On Earth, this is 23ВЅ degrees, and gives rise to our seasons, as our planet turns first one hemisphere, then the other toward the Sun.Д«ut the force of other objectsвЂ™ gravities can cause this obliquity to change. The problem stems from the planetвЂ™s tilt, technically called its obliquity. They can also see dramatic changes in their seasonal cycles, with the differences between summer and winter sometimes being moderate, and sometimes extreme.

But if its вЂњclimate inertiaвЂќ is large, it might be able to tolerate longer excursions into regions of too much or too little heat, so long as it gets the right amount of energy, on average.Д«ut thatвЂ™s not the only problem Tatooine-like worlds must deal with. If the atmosphere is thin, the planetвЂ™s climate will react quickly to such excursions, and it wonвЂ™t be able to tolerate long ones. How much time a planet can tolerate outside of the habitable zone depends on how quickly its atmosphere reacts to changes in incoming sunlight, Eggl says. That said, a planet in a double-star system might still be habitable even it occasionally finds itself outside of the habitable zone, so long as it doesnвЂ™t stay there too long. The result, says Siegfried Eggl of the University of Washington, Seattle, is a significant restriction in the range of orbits that can allow planets to remain in the habitable zone вЂ“ never too hot and never too cold вЂ“ regardless of the changing positions of their stars.

With single-star systems like ours, itвЂ™s a simple concept, defined by the brightness of the star and how far you are from it.Д«ut on Tatooine-like worlds, the habitable zone varies as the stars move around each other, changing their distances from the planet and thereby changing the amount of stellar energy it receives вЂ“ an effect that can be particularly pronounced if the two stars arenвЂ™t of the same brightness. Being in the habitable zone isnвЂ™t the only requirement for a world to be habitable, but itвЂ™s long been considered an important starting point. The habitable zone is the region in a planetary system where an earthlike world would receive enough stellar energy for liquid water to exist on its surface. But even for planets in stable orbits, these stars can produce habitable zones that change dramatically as the stars move around each other. Partly thatвЂ™s because multiple stars can perturb a planetвЂ™s orbit, precluding any chance for life as we know it to survive. Life on these planets could have a hard go of it, however. вЂњMost stars are members of binaries ,вЂќ Manfred Cuntz, an astrophysicist at the University of Texas at Arlington, said at this this weekвЂ™s at AbSciCon 19 conference in Bellevue, Washington, US.ДЄnd, astronomers are learning, many of these binary-star systems have planets вЂ“ some circling a single star, and some circling both at once. In a finding thatвЂ™s great news for fans of Luke SkywalkerвЂ™s fictional home planet Tatooine, scientists say planets in multiple-star systems may be habitable вЂ“ though in keeping with TatooineвЂ™s hardscrabble image, it may be an uphill battle.ДЄstronomers have long known that multiple-star systems are common.

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