Thus far, we’ve dealt with the three most probably inhabited alien worlds in our own solar system. Being in our own solar system, they all have some things in common; the same Sun, for example. Now it’s time to move into something Really Different.
As mentioned earlier, our sun outshines about 90% of the 100 billion stars in our galaxy. Main sequence G-type stars like our own (often referred to by the misnomer “yellow dwarf”) make up about 7 billion of these 100 billion stars. 3 billion are hotter and brighter than our Sun; the other 90 billion are cooler and dimmer. So in the search for life outside our solar system, determining the habitability of the vast majority of cooler, dimmer stars seems a good place to start.
The incredibly convenient picture at right (courtesy of a NASA artist; yes, “NASA artist” is a viable career choice) shows the Kepler-16 planetary system, also dubbed the “Tatooine” system by sci-fi junkies because it hosts the first known planet that actually has two suns. The brighter star is an orange dwarf, or a class K main sequence star. It’s substantially smaller and cooler than our own sun (or that of potentially earthlike Kepler-22b), but still much hotter and brighter than the smaller class M red dwarf seen passing in front of it in the picture.
The planet Kepler-16b, also known as Tatooine, is shown in the picture as a dark spot. It’s about the size of Saturn.
And that should give you some idea of the type of star we’re talking about here. Stars ranging from somewhat cooler than our own Sun to those barely larger than a gas giant–some have debated Jupiter’s possible status as a “brown dwarf,” a ball of gas big enough to produce substantial energy through its magnetic field, but not quite big enough to ignite nuclear fusion. Class M red dwarfs are the next step up from a brown dwarf; they might be so small as to have just twice Jupiter’s diameter.
Interestingly, Kepler 16-b orbits its red dwarf parent star at the outer edge of the habitable zone–this shows that planets can exist both within the habitable zone of a red dwarf, and in a binary star system at the same time. That’s kind of cool. But unfortunately, Kepler 16-b, having the mass of Saturn, is probably a gas giant and probably does sport below-freezing temperatures due to the whole “outer edge of the habitable zone” thing.
Now, onto the matter of the habitability of class K and class M stars in general. How would life on a planet in the habitable zone of such a star differ from our own?
The major difference between these stars and our sun, obviously, is that they’re smaller and cooler. This means that planets have to orbit much closer to be in their liquid water-friendly habitable zone.
Class K orange dwarfs, on average, are about 90% of the sun’s mass and 40% of its luminance. This means that, while the habitable zone would be substantially closer, it would fall somewhere between the orbits of Venus and Mercury around our own star; it wouldn’t be close enough to induce “tidal locking” that can have all sorts of messy side effects.
As a cool consequence, the sun on such a planet would not only appear orange, but it would appear enormous in the sky. The sky itself would likely be aqua to green; the Raleigh scattering that gives us our blue sky would still apply to a planet of sufficient atmosphere, but cooler stars produce exponentially less blue light, so green would be favored as the shortest wavelength present in abundance.
Class M red dwarfs are another matter. These are thought to compose about 75% of all stars in our galaxy (sunlike G-type stars are 7%; medium-cool K-type stars are 15%), so it would be nice if they could host life. But the obstacles are formidable. Red dwarfs are about 40% the size of our sun, but they give off only 4% of its heat and light (star-planet size comparison below by NASA, JPL/Caltech, 84user and Paul Stanifer). That means that to be in the habitable zone, a planet would likely have to orbit so close as to be tidally locked. That gets very sticky.
“Tidal locking” is what our Moon as with our Earth. It’s why we never see the Moon’s dark side; instead of rotating and having a day-night cycle, the same side always points toward us, and the same side always points away. Mercury is very close to having such a situation with our own Sun; it does rotate and have a “day/night cycle,” but the “day” on Mercury is barely shorter than the year. It does not revolve quickly.
A planet in the habitable zone around a red dwarf could have a full-out tidal locking scheme like our Moon, or a partial one like Mercury. Either way, there are some obvious issues. The “day” side is going to become extremely warm, while the “night” side might be in a perpetual deep freeze. Because of this, scientists used to think that planets orbiting red dwarfs would be nigh-uninhabitable. If the atmosphere didn’t freeze or boil off altogether from the extreme hot or cold of one side, only a thin “twilight band” between the day and night zones could support liquid water.
Recent models challenge this idea. Recent studies by Robert Haberle and Manoj Joshi of NASA’s Ames Research Center have shown that a planet possessing just 10% of Earth’s atmosphere may effectively distribute heat to its dark side. The extreme temperature differences between air and water on the day vs. night sides of the planet would create powerful convection currents; a tidally locked planet may actually be the theoretical extreme for the temperature differences that create winds. It would be windy.
It would also probably rain a lot. With the extreme heating of air and water on the planet’s day side, both air and clouds of vaporized water would be exported to the cooler night side very fast. And, as on Earth, hot, wet air cooling fast would result in storms. Big ones. Some models (like the Aurelia project) have a perpetual hurricane happening over oceans at the pinnacle of the heated day-side, while a torrential downpour happens at the pinnacle of the cooled night side.
These red dwarf-orbiting planets do not sound like a very friendly place. They certainly aren’t to human lifestyles or technology. But what about to life in general. Is a hurricane hostile to the evolution of bacterial life? Could life forms that have never known anything different adjust to such conditions quite handily?
As with all questions surrounding the origin of life, the answer to this one isn’t clear. On one hand, lightning strikes may have been a key player in catalyzing early organic chemistry reactions, so a perpetual thunderstorm might actually be a good thing. On the other hand, perpetual (unimaginably, unearthly) high winds and waves might create a chemical stew too turbulent for large structures to form.
The scientists who designed the inhabited simulation Aurelia proposed that life could develop in the sheltered nooks and crannies of the planet’s land and oceans. Caves and hydrothermal deep sea vents, for example, would receive plenty of warm and potentially biochemically-laden water. They would also be shielded from the brunt of the planet’s violent air and water currents. So could live evolve in one of these settings and spread outward, adapting appropriately as it went along?
We don’t know for sure if life could arise on planets orbiting red dwarfs. And we won’t know until we either find life, or a repletion of lifeless planets around these tiny stars. So we don’t know if life can exist around 75% of the stars in our galaxy.
But the odds for class K orange dwarfs, which are twice as common as our own sun, seem good. Between sunlike Class G main-sequence stars and cooler Class K main-sequence stars, we can estimate that about 22 billion stars in our galaxy have a substantial habitable zone in which life-bearing planets could exist. (Dizzying animation of Tatooine/Kepler 16-b’s orbital pattern by Silver Spoon; the Saturn-sized planet Kepler 16-b is colored purple.)
If we apply what we know from spectrological studies of stars overall, about 15% of all stars seem likely to have planets (the heavy elements that make up planets seem to be missing from their spectra, implying that they may have ended up elsewhere). That makes 3.3 billion potentially habitable stars in the Milky Way that probably have planets.
I think those odds are pretty good, don’t you?
Exciting new update: Astronomers See More Planets Than Stars In the Galaxy!