It’s a bit narcissistic, our tendency to use the Earth as our gold standard for life-capable planets. After all, it’s literally our only data-point, as regards life in the universe; maybe slightly different conditions would be even more likely to produce life than those which gave rise to us. However, being that it is our only data point, it’s also true that Earth-like planets are the only logical targets when astronomers go looking for life.
So this week, when NASA announced that its Kepler planet-finder had located a uniquely Earth-like planet just a few hundred light-years away, it sparked a wave of interest. It’s not just about the cool-factor of finding our closest mirror image in our neighborhood of the Milky Way — it’s about finding a new best candidate for finding life elsewhere in the universe.
Kepler-186f is remarkably Earth-like, actually. It is just 10% larger than the Earth, and about 50% heavier — it’s almost certainly a rocky planet. The atmosphere is currently unknown, though with its mass it would be able to hold on to any atmosphere it did generate. Its orbit is very similar to Earth’s as well — but because its star is only about a third as bright it gets far less light than our planet. That places 186f near the outer, colder edge of its habitable zone, with a peak “high noon” intensity roughly equivalent to dusk on Earth.
The “habitable zone” varies from star to star, defined as the space around it within which liquid water can exist. That is, Kepler-186f is far enough from its star that water won’t boil away, as we see on Mercury, and close enough that it shouldn’t freeze totally solid, as on Pluto. It’s actually a bit more complicated than that; Venus, for example, would be in our star’s habitable zone if not for its thick, insulating atmosphere of carbon dioxide. This drives up the temperature and effectively gives Venus a different habitable zone than others planets.
Therefore, since it is near the edge of its own habitable zone, Kepler-186f would have planetary characteristics that dash all our high hopes for life, or even life-capable condition. Also remember that simply residing in a habitable zone does not guarantee that life will arise at all. Mars, for instance, orbits entirely within that space and holds no life, nor did it ever that we can find. Kepler-186f has a very average home star, a red dwarf of a type that dominates the galaxy — if life exists elsewhere in the Milky Way, it’s very likely that it orbits a very similar star.
We don’t even really know that this new planet has any water to keep in a liquid form. How planets acquire water is an open question, though they almost certainly generate some through natural chemical reactions, have some more deposited by comets and other interstellar travelers. What we do know from our few meager years of planet-hunting is that it’s fairly likely it has at least some water. If we find some, that should be a big enough discovery to warrant giving Kepler-186f a real name.
In future, an even better candidate that matches Earth’s physical properties this well and also orbits a more Sun-like star. Lower levels of sunlight mean both less heat and less mutation-inducing radiation like UV light and high-powered cosmic rays. Any analogy to photosynthesis would proceed much more slowly than on Earth, and biomass would likely be lower as a result. Though life could very conceivably supplement this with other sources of energy, like volcanic activity or deep sea fissures, a warm, bright star still gives us the best chance of finding what — or should I say, who – we’re looking for.