Habitable Planets May Not Look Exactly Like The Earth

When I was a kid we had Star Trek reruns showing twice a day. These were, without a doubt, the most important hours of my day. One thing that came from watching the Enterprise zoom around the galaxy so much was the recognition that there were a whole lot of "class M planets" out there. Back in the day it was never clear (to me) exactly what a class M planet was, but in general, it seemed to be a place you could beam down to and not explode, be crushed or breathe poisoned air.

In other words, it was habitable.

With the gold rush of exoplanet discoveries these days, astronomers are learning to think deeply and broadly about habitable planets in a way that far outstrips the old Star Trek. It's now understood that almost every star in the galaxy will host planets, and perhaps 20 percent of those planets will be in the right place for life to start. But if you're thinking these planets are going to be similar to Earth, think again.

There's a new "M" in town when it comes to habitability: M-dwarf stars and their planets.

People often talk about the sun as an average star. But that's not really true. There are far more stars with mass, size and brightness below that of our good old sol.

Called M-dwarfs, these stars win with ubiquity what they lack in grandeur. That is what makes them important for habitable planet hunting. To understand the role of M-dwarf stars, I asked two experts in exoplanet studies, University of Washington professor Vikki Meadows and MIT professor Sara Seager, to explain.

As Meadows puts it:

"These planets are likely to be the first place we'll search for evidence of life on a planet outside our solar system. M-dwarf planets are the most common type of potentially habitable world in the universe, which means they are more likely to be nearby to us and so easier to study for signs of life with currently planned astronomical missions."

But there is a twist. Astronomers think in terms of habitable zones — the ring of orbits around a star where a planet will be neither too hot (too close) nor too cold (too far) for life as we know it to exist. M-dwarf stars are so faint and cold that their habitable zones are tiny — mostly orbits smaller than Mercury's (which is the innermost world in our own solar system).

With their tiny orbits, planets around M-dwarfs could be as different from Earth as ... well ... night and day. That's because many M-dwarf planets might not have night-and-day cycles.

M-dwarf planets on circular orbits would mostly become tidally locked, meaning the same side will always face toward the star (just as the moon always shows Earth the same face). You might think having one hemisphere always in the dark while the other's always baking underneath a red "sun" would make for a lousy place to call home. But as Meadows and Seager explain, things might not be so bad on tidally locked M-dwarf planets. Meadows explains:

"As long as the planet has an ocean and a dense enough atmosphere, they will likely circulate and carry heat around. So temperatures can be evened out on the day-night sides, despite the planet's lack of rotation. The constant sunlight or constant darkness is also unlikely to make life impossible. We have plants on Earth that can grow happily without a day-night cycle. Tundra plants that grow pole ward of the Arctic Circle in summer are a case in point. So a synchronously rotating planet has some challenges, and might not be a veritable Eden, but it's not guaranteed that it is a wasteland, either."

For Seager it is exactly the challenge posed by tidal locking and M-dwarf planets that showed how much broader the concept of habitability needed to become:

"[Those] atmosphere models (specifically atmospheric circulation) showed that nature is smarter than we are. As long as the basics are met like a liquid water environment, temperatures suitable for covalent bonds between atoms (for enable complex molecules), an energy source and conditions for Darwinian evolution, there really shouldn't be a problem. We need to stop imposing our limited understanding [of habitability] on the rest of the universe."

Many M-dwarf planets will not be tidally locked if their orbits, perturbed by other worlds, become elliptical. But, tidally locked or not, planets orbiting M-dwarfs will face other challenges too. Being so close to their star means that explosive flares occurring on the stellar surface will shower planets in the habitable zone with energetic particles. Meadows points to other kinds of variables that must be considered:

"The spectrum of light coming from the star will be very different, too, with a smaller proportion of light in the visible range where Earth plants do photosynthesis and more in the infrared where the planet absorbs radiation."

So what will these differences mean for the evolution of life on M-dwarf planets? Will they preclude life forming? Or will they mean a richer set of possibilities than we can, so far, imagine? As Meadows sees it:

"These extra influences on the environment of the planet don't preclude habitability, but they do make our assessments of whether or not a planet is likely to be able to support life a lot more challenging, and interesting!"

Whatever you might think the answer is, Seager points out that the chance to find out for ourselves is coming up soon:

"We have a shot at finding a pool of rocky planets transiting M stars with the MIT-led NASA mission TESS (launch 2017) and then studying the atmospheres via transmission spectroscopy with the JWST (launch 2018)."

Stay tuned.

You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4.