One of the biggest questions of exoplanet astronomy is how many potentially habitable planets are out there in the galaxy. By one estimate the answer is 6 billion Earth-like planets in the Milky Way. But of course we have to set parameters and make estimates, so this number can vary significantly depending on details.

And yet – how many exoplanets have we discovered so far that are “Earth-like”, meaning they are a rocky world orbiting a sun-like star in the habitable zone, not tidally locked to their parent star, with the potential for liquid water on the surface? Zero. Not a single one, out of the over 5,500 exoplanets confirmed so far. This is not a random survey, however, because it is biased by the techniques we use to discover exoplanets, which favor larger worlds and worlds closer to their stars. But still, zero is a pretty disappointing number.

I am old enough to remember when the number of confirmed exoplanets was also zero, and when the first one was discovered in 1995. Basically since then I have been waiting for the first confirmed Earth-like exoplanet. I’m still waiting.

A recent simulation, if correct, may mean there are even fewer Earth-like exoplanets than we think. The study looks at the transition from a planet like Earth to one like Venus, where a runaway greenhouse effect leads to a dry and sterile planet with a surface temperature of hundreds of degrees. The question being explored by this simulation is this – how delicate is the equilibrium we have on Earth? What would it take to tip the Earth into a similar climate as Venus? The answer is – not much.

There have been studies modeling the process, but this is the first one to model the transition. What essentially happens is that there is a positive feedback loop. Increasing surface temperature increases the evaporation of water. Water vapor is itself a powerful greenhouse gas, which therefore increases the amount of warming, which causes further evaporation. Within a certain range of temperature, a range that Earth is within now, this process leads to a new equilibrium point of temperature. This is because the planet is still able to cool itself by radiating heat away into space. The hotter the planet becomes the more heat radiates away, until that equilibrium point is reach.

However, at some point the blanket of water vapor around the planet is so thick that the planet no longer loses heat in this way and there is nothing to stop the process I outlined above. This is the “runaway” heating point, which doesn’t stop until all the surface water has evaporated. We partly know this happens because that is what happened on Venus, with a surface temperature of  464 degrees C.

How much would the Earth have to heat in order to reach this point? That is what the new study addresses. They found that if the surface temperature of the Earth increased by a few tens of degrees due to increased solar radiance, then we would tip over into runaway heating. That is not something we need to worry about, at least no time soon. Even in a worst case scenario, AGW leads to a few degrees of warming, maybe 6 degrees C with tipping points and positive feedbacks. Also, the model used increased solar radiance as the initial cause of heating. They plan on doing a follow up study to see at what temperature we would hit runaway heating if the cause of initial heating were increased CO2. Still, it’s not reassuring that the range of temperatures that keep a planet in a habitable state is fairly narrow.

The biggest implication of this current model is on our search for Earth-like exoplanets. It may effect the estimate of the number of habitable worlds out there. Also, the researchers plan on figuring out if there are any signatures of a planet in equilibrium or in the grips of runaway heating, so that we can detect those signatures in exoplanets. This will further help us refine our estimates of Earth-like planets out there.

It would be nice if I live to see the confirmation of an Earth-like exoplanet clearly in a habitable zone of an orange or yellow star with liquid water on the surface. It is hard to extrapolate from zero. We may find that the Earth is far more rare and precious than we previously imagined.