The planets xkcd forgot

 A recent cartoon/poster on xkcd tries to estimate what the population of habitable zone planets in our stellar neighborhood looks like. Unfortunately, despite labeling the poster as “all habitable zone planets”, there are a couple of very important omissions.  The center of the picture should look like this:

When discussing the habitable zone, and how it applies to exoplanets, one needs to remember that the definition of habitable zone is sufficiently wide that it covers both Mars and Venus, the closest planets to Earth.  In fact, despite discovering thousands of exoplanets and exoplanet candidates, we still do not have any planets as earthlike as Venus. It is hard to say much about exo-Mars equivalents, as exoplanet detection technology has trouble finding a planet that small and far from its host star.

Most of the planets shown in the chart have not been discovered yet.  Even among those which have, very little data about the planets is available.  It will be years, perhaps even decades, before we have the technology to pick an exo-Earth from an exo-Venus. But framing the exo-planet debate as an Earth versus Venus relative distribution would be a mistake.  Chances are, the vast majority of these planets are completely unlike either planet.

Our solar system is strange.  It is missing the most abundant type of planet in our galaxy- those which are larger than Earth, but smaller than Uranus.  These worlds are often, albeit deceptively, referred to as “super-earths”.  But as Systemic has shown, those which we have data for are not only completely different to anything in our solar system, they are often quite different from each other. 

The omission of Venus and Mars is therefore important, because it gives the false impression that planets in the habitable zone are going to be Earthlike.  Neither of the habitable zone planets in our solar system are particularly Earthlike, and everything we know about exoplanets so far suggests that they will be far stranger still.