Habitable alien planets may be less common than previously believed

If there’s one modern astronomical concept that has successfully permeated the mainstream media, it’s the “Goldilocks zone.” Not only is it associated with the search for alien life, one of the more popular topics in science, it’s also broadly evocative and easy to understand. To host life, a planet can’t be too hot or too cold, but has to be just right. Classically, whether or not a planet is considered to be in the Goldilocks zone has derived from the amount of radiation the planet receives and the amount of internal convection in the mantle beneath the surface. Now, research published in Science Advances claims that convection of the molten mantle is not a meaningful contributor — which changes our idea of where life might be able to get started.

One of the major assumed preconditions for life is the existence of liquid water, which is only possible on planets for a certain temperature range. This can be affected by everything from the level of reflectivity of the surface to the amount of greenhouse gas in the atmosphere. But in general it derives from the size and power of the host star and the distance between that star and the planet of interest. We’re beginning to see that hot planets might keep stable deposits of liquid water in shadowed craters, or cold ones near heated portions of the crust. Mars, ours closest neighbor, is showing just how cleverly water can hide on alien planets.

Host stars lined up with their exoplanet in silhouette. All here were found by the Kepler space observatory.

But things aren’t always that simple. This week saw the release of evidence that the Venus-like planet GJ 1132b is likely not habitable; though there is a high level of oxygen in the atmosphere, that’s a counter-intuitive sign of the lack of water. The planet is so hot (more than double the boiling point of water) and so bathed in radiation that water molecules get broken into their constituent hydrogen and oxygen atoms. The lighter hydrogen leaves the atmosphere more quickly, leading to a relative buildup of oxygen — what might in one context reveal the existence of life, in this case shows that it could never exist.

The researcher on this new habitability study, Jun Korenaga, claims that his new calculations show that mantle convection should not be able to “self regulate” the planet’s temperature, meaning that one mechanism long thought to normalize planetary evolution might not actually exist. In Korenaga’s words, the “evolution of terrestrial planets is… likely to be far from thermal equilibrium, and be sensitive to the peculiarities of their formation histories.”

Most scientists believe that, if there’s life out there, it’ll probably be on Earth-like planets in the habitable zone — like Kepler-62f (artist’s impression).

What does that mean? It means that even if you have free convection within the planet, and you’re sitting right in the Goldilocks zone, you’re not guaranteed to be habitable. Rocky planets like Earth are likely formed due to multiple massive collisions, and such a haphazard method of accretion can lead to any number of different starting points for planets. The starting temperature has to be right at the beginning, they say, or it won’t slowly become that way over time. This is quite a departure from what most astronomers had assumed.

But don’t fret! There are still bound to be countless genuine candidates in the Milky Way alone — unless, of course, continued research just keeps chipping away at the number. For now though, the galaxy still looks like it could be teeming with possible life.