Earth is so far the only planet in the universe which we know contains life. There are many other worlds in our own solar system that may contain life (or did at one time), but none confirmed. Part of the goal of the exoplanet exploration program is to determine how many worlds out there are capable of supporting life. This requires that we come up with some specific definition of a habitable world, and it is only natural that we will model that definition after our one data point – the Earth. But perhaps Earth is not the pinnacle of habitability.

Astrobiologists and exoplanet hunters have been considering this question for a while. A recent paper is the latest in the dialogue about what makes a planet habitable. Earth, of course, is perfect for Earth life, but it may not be the most habitable for life in general. By habitable astrobiologists mean that the world can not only contain life, but contain an abundance of life over a long period of time. So the authors of the recent paper set out to define optimal “superhabitability” and then find worlds in the exoplanet database that would best fit this definition.

I have to point out that still they were using Earth life as their standard. They are not considering, for example, hydrogen breathers or other exotic forms of life that would require entirely different environments. They are still building on the concept that life requires liquid water, and so the number one criterion is still a planet in the “Goldilocks” zone of its parent star that would allow for liquid water on the surface. The most superhabitable worlds, they argue, would be slightly larger than Earth, to maximize surface area for both oceans and land mass without having too much gravity. A larger world would also help the planet hold onto a sufficiently thick atmosphere for a long time.

A large planet has another potential advantage – it would remain geologically active for longer. The planet itself needs to retain some interior heat, which will only last for billions of years if there is radioactive material that will warm the planet as it decays. The Earth would have cooled long ago, for example, were it not for uranium, thorium, and other radioactive materials shedding heat at they decay. In any case, larger rocky worlds would cool more slowly.

This is also important because that would make it more likely to retain a magnetic field, which protects the world from external radiation and solar wind. Mars, for example, only has small regional magnetic fields, not a global field and nothing strong enough to protect it. As a result the solar wind has stripped Mars of most of its atmosphere. If life once existed on Mars, the lack of a global magnetic field doomed it.

The authors also argue that a slightly warmer average temperature would also be optimal for maximizing the density of life. Think of expanding the tropical rainforests to as much of the planet as possible. I see the logic here, but I think this criterion is a bit tricky. First, we have to consider the effect on the entire global system of warmer temperature. While this would expand the tropics away from the equator and shrink the frozen poles, would the equator at some point become too hot? What would the effect be on global weather? Also, the greatest density of biomass is not in the tropics. It’s in the temperate rainforests. This means they would have to choose biodiversity over biomass as the key criterion for “habitability”. This is not unreasonable, but it is not necessarily the objectively “correct” choice either.

There may be other aspects of weather that are more important, such as the ocean currents. Optimal ocean currents can cool the tropics and warm the poles, creating the maximal sweet spot possible. In any case, this is not something that we would know from the exoplanet database and I am not suggesting it as a criterion, just pointing out that average temperature is a bit crude for this purpose.

Another interesting criterion has to do with the parent star, rather than the world itself. Our sun, while fine for life, is perhaps not optimal. An optimal star would be stable even from the beginning of its life, would have a high metalicity (lots of heavier elements out of which life can form) and would have a long lifespan. Our sun meets the first two criteria, but has a lifespan of about 10 billion years. While this was certainly long enough for us to evolve, it took about 4 billion years for complex life to evolve. Perhaps, on average, it takes longer, and a star with a 10 billion year lifespan is cutting it a little close. The less mass a star has the longer it lives, so perhaps a smaller, cooler star would be best.

Red dwarfs can last hundreds of billions, even trillions, of years, but they have two big problems for life. The first is that they are very unstable when young, tending to flare and give off lots of coronal mass ejections. This would have the tendency of stripping any worlds in its Goldilocks zone of most of their atmospheres – not good for life. It’s possible a planet may migrate in from a safe distance after the star has settled down, but this makes the probability of such a world much lower. Second, the Goldilocks zone for these cool stars is close in, so close that worlds would likely become tidally locked. This means that one side of the world world always be facing the star, baking half the world and freezing the other half. This would leave only a strip of the planet in a comfortable temperature for life. However, some models show that a vibrant ocean current could distribute the heat very effectively and make such worlds more habitable. But certainly these would not be superhabitable worlds.

If red suns are too cool and unstable, and yellow suns are too short-lived, then orange suns would be just right. There are more of these in the galaxy than yellow suns, and they have lifespans from 20-70 billion years – more than enough time for complex life to evolve and flourish. They are warm enough that there is a generous Goldilocks zone at a sufficient distance not to cause tidal locking. Orange suns are definitely the sweetspot for life.

The authors of the recent study plugged all these criteria into the exoplanet database and came up with the 24 top candidates. Not one of them met all of their criteria for superhabitability, but one came close with four criteria. But certainly all of these planets are likely to be extremely habitable, even if they don’t meet all the criteria for optimal superhabitability. These would be good exoplanets to focus efforts on finding signatures of life.

Astronomers estimate that there may be as many as 6 billion Earth-like planets orbiting sun-like (G-type yellow) stars. They need to expand this estimate to include the orange K-type stars, which would likely increase the estimate by an order of magnitude or more. Of these a small percentage would be superhabitable, but that would still be a lot of exoplanets. And again, a world does not have to be superhabitable to be a good candidate for life. Regular habitable will do.