How Big Can a Habitable Planet Be?

Exoplanets with liquid oceans may not be capable of supporting life if they are too big, according to new calculations

One of the great discoveries in modern science is the presence of planets around other stars. Many of these exoplanets seem just like the planets in our Solar System and most astronmers think it is only a matter of time before we discover some like Earth.

That raises the extraordinary possibility that life might have evolved in these places too, especially on exoplanets in the so-called habitable zone where the temperature is just right for water to exist in liquid form.

So it’s understandable that astrobiologists are chomping at the bit to study these places in more detail.

There’s a problem, however. Given the rate at which astronomers are discovering new exoplanets—they currently know of over 1000—the likelihood is that in just a few years astrobiologists will be overwhelmed with habitable candidates.

So how should they prune this group so that they concentrate on planets with the greatest potential? Today, Yann Alibert at the University of Bern in Switzerland says that the radius of these planets places an important but previously unrecognised constraint on the possibility of life. And because of this, astrobiologists should discount exoplanets that are too big, even if they have liquid water oceans.

Alibert’s basic premise is that while it is not easy to define the conditions necessary for life, it is straightforward to define conditions in which life cannot flourish. And by excluding planets that are not habitable, astrobiologists can concentrate on those that have the greatest potential.

So what makes a planet uninhabitable? Alibert says the most basic condition is the presence of a carbon cycle that buffers the temperature of the planet against changes (if its parent star becomes hotter or colder for example).

On Earth, the carbon cycle involves the capture of carbon dioxide as it dissolves in the ocean. This then reacts with silicates on the ocean floor producing carbonates that are eventually subducted into the Earth’s core. The high temperatures here breakdown the carbonates, releasing the carbon dioxide again through volcanic activity.

The key is that this process regulates the atmospheric temperature. If the temperature drops, less carbon dioxide dissolves leaving more in the atmosphere and raising the temperature again. And vice versa if the temperature rises.

This cycle is crucial in preventing runaway temperature changes that can turn a planet into a giant snowball or a steaming dessert.

It’s easy to imagine that any Earth-like body with planet-wide oceans would have a similar carbon cycle. But Alibert says that’s not the case.

He points out that the size of the planet places an important constraint on this process. Bigger planets will have more gravity and this determines both the atmospheric pressure and the pressure at the bottom of the oceans. If this is too large, ice will form at the bottom of the oceans preventing liquid water from interacting with the silicate material that makes up the ocean floor.

And when that happens, the carbon cycle immediately shuts down leaving the planet at the mercy of any temperature changes.

Alibert goes on to calculate the radius of an Earth-like planet above which a carbon cycle cannot operate. The critical threshold turns out to be about twice the radius of Earth (although this depends on a variety of factors such as the mass,density and make up of the planet).

Alibert’s claim is that although there is no guarantee that planets in the habitable zone that are less than twice the radius of Earth will actually be habitable, those that are bigger than this threshold are not habitable.

That’s a potentially useful way to prune the list of planets that astrobiologists should study.

But is Alibert’s reasoning sound? Is the fundamental assumption correct that a carbon cycle is the crucial ingredient of habitability?

Planetary geologists might say that there could be many ways a planet could regulate its own temperature without a carbon cycle. For example, the Jovian moon Europa is thought to hold a liquid ocean covered in ice. The temperature of the water is maintained by tidal heating from the friction caused by the constant flexing of the moon by Jupiter’s gravity.

So if Europa maintains its temperature in that way, why not Earth-like exoplanets too?

So while Alibert’s approach may help to prune the list of possible habitable exoplanets, it’s hard to imagine that it’s good general rule. And astrobiologists may find good reason to ignore it when it comes to deciding which exoplanets to study in more detail in the not too distant future.

Ref: arxiv.org/abs/1311.3039 : On the Radius of Habitable Planets