Alien Life May Exist Beyond the Habitable Zone

As NASA’s Kepler mission began spotting planets outside our solar system, scientists were excited to announce the discovery of a planet orbiting within a star’s habitable zone. Also referred to as the “Goldilocks zone,” the habitable zone is the distance from the star that is neither too hot nor too cold for liquid water to exist on the planet’s surface—if it has water, so goes the reasoning, it could have life.

But a paper just published in the journal Astrobiology points out that the name is a bit of a misnomer. “To be habitable, a world (planet or moon) does not need to be located in the stellar habitable zone, and worlds in the habitable zone are not necessarily habitable,” the paper states. Without abandoning the water-indicates-life paradigm completely, the researchers take the habitable zone to task. They argue that outside the habitable zone, there could planets even more amenable to life. We just have to know what to look for.

While we don’t have an example of a planet supporting life outside of the habitable zone, we have empirical evidence that orbiting within the habitable zone means, necessarily, that liquid water exists on the planet. Mars, for instance, is in the habitable zone around our Sun. And even though Mars may not be as totally frozen or arid as was once thought, if water on Mars is ever in a liquid form, it isn’t ever for very long. What’s more, Mars doesn’t have enough of an atmosphere, ozone layer, or adequate magnetic field for life as we know it to flourish on its surface.

What’s more, there are more sources of heat than just the star your planet is orbiting. Jupiter’s moon Europa, for example, may have a liquid ocean below its icy crust that is kept liquid via “tidal heating.” The pull from Jupiter varies thanks to Europa’s non-circular orbit around the gas giant, and the tidal bulge goes up and down as the moon is pulled on by Jupiter and its other large moons. This flexing, scientists think, could be heating Europa from within. As such it’s one of the likeliest places in the solar system that could have water, and thus life.

Life on Europa would have to contend with being bombarded with radiation thanks to running through Jupiter’s radiation belt. It also couldn’t rely on the Sun like much of life on Earth does, to say nothing of flourishing where the temperature is on average around -170 degrees below zero.

The researchers from McMaster and Webster State Universities argue that rigidly anthro- and geocentric views on how a habitable planet should look could cause scientists to overlook habitable planets that could be hiding in plain, if telescopic, sight.

Instead of focusing on the distance between the star and the planet, the researchers suggest that other factors be considered when looking for viable candidates: the age of the planet is important, the paper contends, so looking for smaller stars that are longer burning. A slightly more massive planet could be more viable.

The Kepler mission found exoplanets was by looking for their transits in front of their stars. Determining if the discovered planets are in the habitable zone is really just the first step that leads to follow-ups. This paper in Astrobiology argues that the follow-up should look at factors such as the planet’s mass, its surface area, the magnetic shielding, and on.

The researchers set out to address the “Rare Earth” hypothesis, which says that life emerged on Earth was the product of a really unique confluence of events, and they seem to agree—the “occurrence of another truly Earth-like planet is trivially impossible.” However, they also “hold that this argument does not constrain the emergence of other inhabited planets.” The habitable zone isn’t the be and end all of what’s habitable.