We love exoplanets here at DVICE. We just can't get enough of 'em. Over the last few years, astronomers have found a whole bunch of new worlds, some of which are potentially habitable by alien life, and a few of which may even be habitable by humans. But what exactly does "habitable" mean? What is it that we're looking for? The easy thing to say is that we're looking for a planet exactly like Earth, but really, it's a complicated question with a much more nuanced answer.
There are two distinct ways of thinking about a potential exoplanet when it comes to life. We can either look at an exoplanet and try to determine whether it's suitable for colonization by humans, or we can look at an exoplanet and try to determine whether it might be potentially inhabited by aliens. The former, of course, involves a much more stringent set of criteria, while the latter is as boundless as life can possibly get (and we have no idea what that might imply).
For the purposes of this article, we're going to be looking for an exoplanet that could support human life. And not just technically support human life (in the way that the International Space Station can technically support human life thanks to technology), but actually offer something to potential settlers, such that a colony could take advantage of the planet in some way as opposed to simply surviving there.
Now, all of the criteria for a potentially habitable exoplanet that we're going to be discussing should not be construed as absolutely necessary, and it's certainly true that there are lots of special cases (or even not-so-special cases) where a moon orbiting a gas giant (for example) might be a great place for life. However, the hunt for alien life or habitable planets is sort of like playing the lottery, except with the ability to cheat a little bit. There are billions or trillions of stars out there, and we haven't the faintest idea what the odds really are of finding something that'll work. All we can do is say, "here's what we think makes for the best places to start looking" and then look there, and that's what these criteria are all about: if we were going to send one ship to one alien solar system, here's the combination of features that we'd be looking for.
1. Galactic Location
Even before we start talking about stars or planets or solar systems, we need to talk about our galaxy. Just like a solar system, the Milky Way has a sort of habitable zone. There isn't a giant sun at the center, of course, but there is a giant black hole, and getting too close in to that would be a hostile environment, to say the least, with radiation spewing out everywhere. If you get too far from the galactic center, though, things start to get boring, and it's harder to get enough heavy elements together in one place to form rocky planets. Current estimates put the habitable zone of the Milky Way as starting between 25,000 light years from the galactic center, and extending 6,000 light years towards the edge.
It's also worth mentioning that our entire galaxy is spinning, and as such, our solar system itself is orbiting the center of the Milky Way. We're in a stable circular orbit (going around once every 200 million years), which is good, and for long-term habitation, we'd want a similar orbit, so as not to get sent through the galactic core.
2. Galactic Neighbors
Now that we've got a galactic region to start looking at, let's try to narrow it down further by striking some areas off of our potentially habitable list. The galaxy is a dangerous place, and there are some bad neighborhoods out there. For example, active galactic regions (like globular clusters) may contain stars likely to generate supernovae, hypernovae, or gamma ray bursts, any one of which (if within several thousand light years of an inhabited planet or pointed in the wrong direction) are capable of outright frying surface organisms and causing severe atmospheric damage over longer periods. While a gamma ray burst anywhere within the Milky Way could potentially cause mass extinctions on any planet within our galaxy that it's aimed at, we'd still be better off focusing on areas comfortably distant from things that appear to have violently exploded in the recent past.
3. The Right Star
The first thing to check out when searching for potentially habitable planets is their sun. Usually, this is the first place we have to look anyway, since it's all we can see from here at first glance. Stars come in all shapes and sizes, ranging from smaller than our sun to much, much bigger, and varying in temperature from a couple thousand degrees cooler than our sun to tens of thousands of degrees warmer. Our sun, by the way, is a class G main sequence star, and about 8% of all stars in the galaxy are similar to it.
Part of the reason that life has had such a good time of it here on Earth is that we have a very nice sun. It's been around for a long while, it's going to stay around for another long while, and it's not feisty in such a way that it's a terminal threat to our existence. So, a good star to look for would be one that's been stable for a billion years or so, and has some life left in it before it does anything crazy (like going nova or turning into a black hole, which would likely be unpleasant for any denizens of that particular solar system).
4. Habitable Zone
Even though there's a huge amount of variation between stars, each one has what's called a habitable zone, which is the region that's close enough to the star where there's enough available energy to melt ice into liquid water, but not so much energy that water boils off into space. This zone is relatively easy to calculate for every star that we can see, since it just depends on the energy output of that particular star, which we can measure from here.
Again, it's completely possible that planets and moons outside of a star's habitable zone might host liquid water thanks to energy from other sources, but we're trying to roll with the odds and find a planetary system that's most likely to host life, not just a specific moon or planet where life might be possible.
We'd love to find a solar system with a sun just like ours, there are lots of options when it comes to habitable zones. For example, red dwarf stars (which are much smaller and dimmer than our sun and the most common type of star in the galaxy by a huge margin) might offer habitable zones at about the range that Mercury orbits our sun, and there are similar zones (whether they're closer in or farther out) around other types of stars as well.
The easiest sort exoplanets for us to spot with our current technology are really, really big ones. This is simply because we're either finding them through their gravitational effects on their parent stars, or we're finding them when they partially eclipse the light from their parent stars, and in both of these cases, the bigger a planet is the more likely it is that we're going to see it.
Earth, of course, is very fortunate to have a gravity of just exactly 1g. Yes, I make this joke a lot (and it never fails that people end up taking me seriously), but it's especially pertinent when talking about a potentially habitable exoplanet, because none of these so-called "super-Earths" are going to be suitable for long-term habitation by humans. We're just not designed to deal with increased gravity. Finding a slightly smaller planet with 0.9g or something would be pleasant, but if we go too small, the planet won't have enough mass to hold onto an atmosphere and it won't exhibit any tectonic activity.
Now, if we're just talking about what planets might host life of some sort, well, that's a different matter. There's no reason why life couldn't evolve to deal with significantly higher or significantly lower gravity: in general, life is an adaptable critter, and especially if we're talking about very very small forms of life (say, lichens or microbes on up to something the size of a tardigrade), gravity just might not be that big of a deal.
While instability might be a driver for evolution, life in general is a huge fan of stability, which is why having a mostly circular orbit is important. Since solar energy drops off as the square of the distance a planet is from its star, even a modestly elliptical orbit can cause substantial variations in climate. So, even if the perihelion and aphelion are both within the star's habitable zone, anything living there would have to be able to survive some, shall we say, uncomfortable temperature swings.
Obviously, we need to find ourselves a planet that's made of dirt, or at least of rocks. It would be fantastic if there's more going on than that, and as far as developing a sustainable existence goes, it would likely be necessary for there to be some differentiation, with useful minerals near enough to the surface for a colony to access. Really, though, the point here is that we're not going to find a good place for a colony on any of those gas giants that we've already found hundreds of.
The other important part of the composition equation is water. As far as we know, water is required for all sorts of life, and it's definitely required for our sort. Having a planet in the habitable zone means that liquid water will be stable on the surface, but we'd need to have some water there in the first place for it to really count as habitable. Fortunately, water can be found all over the place in our solar system, so it's not much of a stretch to assume that many (if not most) exoplanets in the habitable zone might have water in their composition as well.
The only tricky bit about planets with lots of water is that water has the unfortunate tendency to complicate atmospheric heat, but that's a whole other issue.
Atmosphere is good for breathing. It's the best thing for breathing, really. Humans are designed to breathe oxygen along with other inert gases like nitrogen, and fortunately, oxygen looks to be one of the most common elements in our galaxy. In fact, we know that there's at least one exoplanet out there (a gas giant, but still) with a significant amount of oxygen in its atmosphere. Humans have no problem tolerating increased oxygen concentrations (up to a point), so we don't have to find a planet with an atmosphere that's exactly like Earth's for it to be breathable, it just has to be close.
A planet's atmosphere is also going to have a significant effect on surface temperatures, thanks to both carbon dioxide and water vapor. This is the problem with planets that have lots of surface water: things can get out of control on either end of the temperature spectrum. A planet that's cold, for example, will just get colder, since water that freezes into snow and ice reflects solar energy back into space. And on the other end of things, a planet that's a little bit too hot will eventually end up with a runaway greenhouse effect thanks to extra water vapor that gradually builds up in the atmosphere.
The upshot of all this is that depending on the atmospheric composition of an exoplanet, we may have to shift the habitable zone around a bit to be sure that it's actually a place where we'd be able to live. This could mean that an otherwise ideal planet might end up being not so ideal, or on the other hand, it's also possible that the right atmosphere could tip the balance in favor of a planet that's on the very edge of the habitable zone.
Speaking of tipping, let's talk about tilt. Earth has an axial tilt of 23 degrees (give or take), which is where we get our seasons from. It's thought that seasonality is important to the evolution of life, but as far as habitability goes, seasonality also helps to stabilize weather patterns, since without a significant axial tilt, solar radiation gets concentrated at the equator and warmth can't spread itself towards the poles very effectively.
While no tilt is probably workable but not great, and a moderate amount of tilt is a good thing, too much tilt is definitely bad. A high degree of tilt would have approximately the same effect as a highly elliptical orbit, which is to say, you'd get some absolutely brutal seasonal variations. This might be something that colonists would be able to survive, but they wouldn't be very happy, and any native forms of life would presumably find it difficult to adapt to such relentless environmental shifts.
Most planets in our solar system spin on an axis such that the side of the planet that faces the sun is continuously changing. Based on the planets in our solar system that don't spin this way, it seems reasonable to think that having relatively frequent days and nights is important to keep a planet with an atmosphere comfortable and stable.
Let's take a look at Mercury, for example. It takes two entire revolutions around the sun for Mercury to rotate just three times, which means that one side of it gets slowly deep-fried while the other side of it gets slowly frozen. Mercury, of course, is altogether too close to the sun to begin with, but the principle is the same: if all of your solar radiation gets concentrated on just one hemisphere, it's going to wreak havoc with your atmosphere.
The other reason why spin is important is because a planet needs to spin to generate a magnetic field.
11. Magnetic field
Earth's magnetic field is what protects us from the solar wind. Or, more importantly, it's what protects our atmosphere (including that all-important ozone layer) from being blown out into space. For a worst-case scenario, just take a look at Mars: it lost its magnetic field, followed by most of its atmosphere, and now it's freezing cold and mostly airless and won't be a nice place to live without some major terraforming. Plus, without a magnetic field, we'd have absolutely no idea which way north was.
We've been over how important axial tilt is, and it's been suggested that the Moon is what's responsible for keeping Earth's tilt steady. The Moon is also to blame for tides (both aquatic and geological), although these factors are more relevant for long-term evolutionary potential than for habitability. Without a moon, human settlers would probably be just fine.
13. Solar System Neighbors
We've got a lot going on in our solar system, and it may not be a coincidence that it's such a busy place. The large outer planets (like Jupiter and Saturn) may play an important role in the interception and swallowing up of inbound comets and asteroids in the present, whereas in the past, Jupiter probably helped send lots of water rich comets our way. Also, over the long term, Jupiter's gravity helps stabilize the orbit of the Earth.
Having Jupiters around does seem important, but it's also important that they stay out of the way. Specifically, far enough out of the habitable zone such that they don't disrupt planetary formation with their huge mass. This is what may have happened with the asteroid belt: thanks to Jupiter's meddling, all those rocks never got a chance to get together to form a planet.
Clearly, there are a lot of things that have to go right for humans to find the perfect home away from home, and we'd be very lucky if we manage to spot one close enough to Earth to be accessible as a vacation spot without warp drive. It may not be necessary to find the perfect home, though: Mars, for example, doesn't hit all of these points, but it's still a place where humans could likely establish a permanent settlement with the aid of currently available technology. And as technology improves, it's entirely possible that we'll be able to modify planets to suit us: first their atmospheres (by adding heat and gases by throwing comets at them), and eventually even their tilts, spins, and orbits. This is a long way off, for sure, but since it's going to take us a long time to a.) find and b.) travel to even the closest potentially habitable new world, the timing might just be perfect.