With better detectors and improved techniques, the hunt for exoplanets—planets outside our Solar System—has switched from merely finding new planets to cataloging their variety. As of March 20, 762 exoplanets are listed in the database at the Extrasolar Planets Encyclopaedia, but additions are frequent enough that I don’t expect that number to be meaningful for long. Of course, many people are understandably interested in finding the most Earthlike planets, but there’s also some excitement in discovering worlds in environments very different from our own.

Take, for example, the exoplanets known as Kepler-16b (“Tatooine”, which I wrote about on this blog), Kepler-34b, and Kepler-35b (which I covered for Ars Technica). These three planets are circumbinary: they each orbit a binary, two stars instead of one. These three star systems have several things in common: the stars are in tight mutual orbits, while the planets follow a larger path encompassing both of them. Systems where the binary has a larger separation are far less likely to have stable planet orbits: in that kind of situation, the planet will orbit chaotically and eventually be ejected from the system. (This may also happen even in tight binaries, though based on my understanding, they’ll stick around much longer—possibly as long as the stars themselves survive. In fact, our own Solar System may not be stable forever, though it’s nothing to lose sleep over.)

None of the three confirmed circumbinary planets are particularly Earthlike, but if you’re like me, you’re still curious about what life in one of these systems might be like—if an appropriate planet can even exist in a system like that. So, let’s think about the implications for habitability might be. To quote from my earlier post “What Does Habitable Mean?“:

So what do we mean by the “habitable zone”? Simply put, this is the region of a star system where the amount of heat received by a planet is sufficient for liquid water, but not so much as to boil that water away.

The primary factors for habitability are (in summary)

1. the temperature of the star, or in this case, stars in the plural
2. the physical sizes of the host stars
3. the albedo of the planet (how much light it reflects back into space)
4. the atmospheric composition, which includes fun stuff like the greenhouse effect
5. how far the planet is away from the host stars, which is the determining factor for the habitable zone

Circumbinary planets have slight complications, but for close binaries like Kepler-35b, it’s hard to determine exactly what effect that has on habitability. To illustrate that, I’ve created a short animation. Here’s a planet orbiting a Kepler-35-like binary star system at the distance Earth orbits the Sun:

Unlike in the image that started this post, the relative sizes of the stars and their orbit is correct here. This imaginary planet orbits at the same distance Earth does from the Sun and has the same albedo. Cloudtop temperature assumes the atmosphere doesn’t react slowly to sunlight (as it does in real life), and nothing is assumed about internal heating (via greenhouse effect or whatever) – this is a deliberately simple model. Earth’s cloudtop temperature, marked with the red line, is obviously much colder than Earth’s real atmosphere, for example, but it shows how the imaginary planet compares.

The Kepler-35-like stars orbit each other with a period of about 21 Earth days, and there is a noticeable variation in the amount of light our imaginary planet receives during that time. The eclipses are of relatively short duration, but depending on the atmosphere, it might still have an effect; the slower variation just from the stars changing positions over time may be more interesting, as it would overlay any seasonal variation the planet has. Earth’s seasons are due to the tilt of our axis, but with larger changes in sunlight over time, things might be different on the imaginary world.

Consider the examples of Earth and Venus. Earth’s atmosphere is composed mostly of nitrogen and oxygen, neither of which is a greenhouse gas, but the presence of water vapor and carbon dioxide keeps Earth from freezing over. The air circulates a lot with atmospheric heating and cooling, and the oceans, which are ginormous heat reservoirs, help influence the global atmospheric conditions. Earth’s axis is tilted 23.5° from the plane of orbit, which creates the seasons. In contrast, Venus’ thick atmosphere is mostly carbon dioxide, so it retains heat very well. (As we’re seeing on Earth, even a small increase in greenhouse gases can have a noticeable effect on climate!) Venus has no significant axial tilt, so it has no seasons, and it’s significantly closer to the Sun to begin with. As a result of all these conditions, Venus experiences little day-night variation in temperature and no seasonal variation: every day and every night are hot enough to melt lead.

As the two suns set on our imaginary planet, we can imagine what its inhabitants would consider “normal” compared to our standard of normal. They might think planets in orbit around single stars are the weird ones, and ponder what life might be like under the light of only one sun.