Astronomers have, so far, discovered nearly 5,500 exoplanets—alien worlds orbiting alien stars—with over 7,000 candidate planets still waiting to be confirmed.
That’s a lot of planets.
We search for these distant worlds because we’re curious and we want to know what other planets are like. By understanding other worlds, we can better understand our own. But make no mistake: Scientists are humans, and we want the answer to one of the biggest questions of the scientific age: Are we alone?
We may get an answer soon. Our technology is just on the cusp of detecting exoplanetary biosignatures, telltale signs of life; for example, molecules in planetary atmospheres that could indicate the presence of biology. Until then, though, it’s useful to investigate everything we can about these planets and categorize them by their likelihood of supporting life.
A handy concept astronomers have for this is called the habitable zone. This is a region around a star where temperatures allow a planet to possibly harbor oceans, seas, or lakes of liquid water on its surface. Too close, and the stellar heat boils the water off. Too far and it freezes. In between these extremes, though, it could be just right. This is why some astronomers refer to it as the Goldilocks zone (though to be honest I’ve never cared for this moniker; at best it should be called the Baby Bear Zone. Naming it after the person who broke into that ursine home and usurped all their hard-won food and property smacks of colonialism).
As an idea it’s pretty useful. All life on Earth needs liquid water, and, since we’re unaware as yet of any other way life might occur, that’s a good place to start.
Measuring habitable zones, however, isn’t straightforward. Calculating the radiation a planet receives from its star is easy; that depends on well-understood physics. The hard part is the planet itself. A dark planet absorbs more light and heats up, while a lighter-toned one will reflect more light and be colder.
A planet’s atmosphere plays an even bigger role; if it’s loaded with greenhouse gases, then it needs to be farther out from the star to be clement. Just look at Venus, our sibling world in many ways, similar to Earth in size and mass, to see how important that is. Lead would melt on our evil twin’s surface because of its overwhelmingly thick atmospheric blanket of carbon dioxide; it’s not exactly an Earth-like planet.
So just finding a planet in its star’s nominal habitable zone, even a small (and presumably) rocky one world like our own, is no guarantee it will be, well, habitable. A lot more must be known, including whether it even has an atmosphere, and if it does what that’s composed of, and more. This is so complicated that astronomers argue over where the sun’s habitable zone even starts—and we’re literally inside it.
What’s more, the habitable zone may not be the only place where liquid water can exist in a solar system.
In the 1970s the Voyager 2 spacecraft flew by Jupiter’s moon Europa and saw surface features that hinted at the presence of a liquid water ocean beneath its frozen surface. We’ve since collected extremely compelling evidence of subsurface Europan liquid water, kept warm by the giant moon’s interaction with Jupiter’s immense gravity.
Europa isn’t the only ocean moon, either. In 2005, images from the Cassini spacecraft showed huge plumes of water erupting off the surface of Saturn’s icy moon Enceladus. Likely generated by Saturn’s tidal activity, similar to what warms Europa, these geysers point to the presence of huge pockets of subsurface liquid water, if not another ocean.
In fact, we know of quite a few rogue planets wandering through interstellar space, which were likely ejected from their original planetary systems as the worlds there first formed and gravitationally interacted with one another. The ones we find tend to be gas giants, even more massive than Jupiter. If they have icy moons, those too could be heated enough to have subsurface oceans. So you may not even need to have a star to have a habitable world!
To throw even more cold water on habitable zones, there are other liquids to ponder as well. Saturn’s huge moon Titan is too cold for liquid water on its surface, but Cassini observations in 2006 showed vast lakes of liquid methane on its surface. Methane is a carbon-based molecule, so many of the ingredients for life are perforce there. Who knows if there are alien fishies swimming in Titanic lakes a billion kilometers from the sun?
Needless to say, these are well outside the sun’s quote-unquote habitable zone, yet it’s possible that life may abound inside these prima facie frozen moons.
Clearly, the concept of a habitable zone is woefully incomplete to determine where life might exist. So is it time to kick it to the galactic curb?
Let’s not throw out the extraterrestrial baby with the subsurface bathwater! Some years ago, a team of planetary astronomers wrote that the term needs modifying, and suggested it be replaced with the “temperate zone.” I think this is a fine idea, still a highly useful one if we’re looking for Earth-like planets, which, to be clear, we are. It may not include frozen moons of gas giant planets, or worlds where life as we don’t know it may evolve, but as long as we are aware of these limitations it’s still handy. Renaming it would solve that.
The habitable zone, even by any other name, was never meant to be an ironclad rule. It was always a guideline, a conceptual idea to inform astronomers that they might be on to something interesting when a planet is found there. It’s not a device for measuring habitable worlds so much as a way to bookmark them for future observations.
It’s easy to want to draw lines in the sand—a planet this far from its star is inhospitable, but a planet that far is great—but nature almost never behaves that way. It generally works on a spectrum, with fuzzy borders and even larger overlaps. It’s always good to keep that in mind when reading about scientific discoveries.
A planet being in its star’s habitable zone may not be sufficient to be habitable, or even necessary, but it’s still a pretty good place to start when looking for life. We just need to make sure we don’t stop there.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.