Planets in multiple-star systems are habitable?

In a finding that’s great news for fans of Luke Skywalker’s fictional home planet Tatooine, scientists say planets in multiple-star systems may be habitable – though in keeping with Tatooine’s hardscrabble image, it may be an uphill battle.

Astronomers have long known that multiple-star systems are common.

“Most stars are members of binaries [other than the coolest dwarf stars],” Manfred Cuntz, an astrophysicist at the University of Texas at Arlington, said at this this week’s at AbSciCon 19 conference in Bellevue, Washington, US. 

And, astronomers are learning, many of these binary-star systems have planets – some circling a single star, and some circling both at once. 

Life on these planets could have a hard go of it, however. 

Partly that’s because multiple stars can perturb a planet’s orbit, precluding any chance for life as we know it to survive. But even for planets in stable orbits, these stars can produce habitable zones that change dramatically as the stars move around each other. 

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. Being in the habitable zone isn’t the only requirement for a world to be habitable, but it’s long been considered an important starting point.

With single-star systems like ours, it’s a simple concept, defined by the brightness of the star and how far you are from it. 

But 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 – an effect that can be particularly pronounced if the two stars aren’t of the same brightness. 

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 – never too hot and never too cold – regardless of the changing positions of their stars.

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 doesn’t stay there too long.  

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. 

If the atmosphere is thin, the planet’s climate will react quickly to such excursions, and it won’t be able to tolerate long ones. But if its “climate inertia” 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.

But that’s not the only problem Tatooine-like worlds must deal with. They can also see dramatic changes in their seasonal cycles, with the differences between summer and winter sometimes being moderate, and sometimes extreme.

The problem stems from the planet’s tilt, technically called its obliquity. 

On Earth, this is 23½ degrees, and gives rise to our seasons, as our planet turns first one hemisphere, then the other toward the Sun.

But the force of other objects’ gravities can cause this obliquity to change. On Earth, this change is limited to a 2.4-degree range, but that’s enough for the changes to be linked to such major climate changes as ice ages.

In binary star systems, this effect can be radically stronger, says Billy Quarles of Georgia Institute of Technology, Atlanta, Georgia, especially if the planet’s orbit doesn’t lie in the same plane as its stars’ orbits around each other.

“Seasons around these binaries may be a lot more variable than on Earth,” he says. “There are times when there are no seasons, and others when seasons [are larger], on a time scale of a few tens of thousands of years.” 

But that doesn’t mean everything about binary star systems reduces their planets’ habitability. 

Paul Mason, an astrophysicist at New Mexico State University, Las Cruces, New Mexico, argues that there is a “binary habitability mechanism” that can sometimes enhance a planet’s chances of being habitable. 

It works, Mason says, by reducing the risk of hyper-intense solar flares. 

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. 

But many stars, especially in their youths have enormous flares that are risks not just to technology, but to life itself. These gigantic flares can erode a young planet’s atmosphere, blast its water vapor off into space, and kill a potentially habitable world in its youth.

The strength and frequency of these flares, however, is dependent on the star’s rotation rate: the faster it spins the more flares it produces.

Young stars spin quickly, slowing down as they age. But in binary star systems, tidal forces between the stars can put the brakes on their spins, rapidly taming their youthful activity. The result, Mason says, is that in some circumstances, “conditions for life are enhanced.”

Conditions around these stars, he says, could therefore be “better than earth-like” – at least compared to what Earth experienced in its first billion years. “The Earth suffered a lot of water [and] atmosphere loss,” he says.

In fact, he says, whatever other issues they pose, binary star systems could actually provide the best niches for life in the galaxy.

Luke Skywalker would undoubtedly agree.