Science fiction is often predicated on the idea that there are many places in the universe where humans or humanoid life could comfortably exist. There are hundreds of planets in Star Wars and Star Trek that are just teeming with life. You can strut around like you own the place. But in our reality, we found only one place where anything living can do so comfortably and that’s Earth.
So far, we’ve not identified life and any other worlds, but that doesn’t mean we haven’t found other worlds that could potentially support life. Keep in mind, a planet that could support life is kind of the same as a place you could call a house. A shipping container could be a house, but you’re going to need to do some work to it. Likewise, a planet that could support life probably needs a bit of a spit polish before you’re raising pigs there and planting crops. Nevertheless, we have identified some potential possibilities. Emphasis on potential.
The Goldilocks Zone
One of the first things astronomers look for when trying to identify a planet that could support life is whether or not it exists in what is called the habitable zone or the Goldilocks Zone. This is, of course, inspired by Goldilocks and the Three Bears, where some porridge was too hot, some were too cold, but one was just right. You need a planet that’s in the just right zone, otherwise it will be too hot or too cold to support life as we know it.
The Goldilocks Zone is the area around a star where a planet can exist and support liquid water. So, again, it can’t be too hot or the water will evaporate and it can’t be too cold or the water will freeze. Earth is, of course, in the Goldilocks Zone around our sun. So you need something roughly equivalent to that, but of course, things change a little depending on the type of star and the size of the planet.
It’s worth noting that “as we know it” does a lot of heavy lifting in these determinations. There are many potential ways life could thrive in environments that we are not used to. Remember, even on Earth, life has found a way to exist around toxic hydrothermal vents at the bottom of the ocean. Stuff that would kill most things in our world. But some things have adapted and can’t survive. But for the purposes of trying to find a habitable planet, we’re going to stick with the basics.
It has been theorized that life could exist on planets that are outside of what we consider the Goldilocks Zone, even in our own solar system. For instance, liquid water may not be necessary for life, liquid methane could potentially sustain life. Titan, the moon of Saturn, has an abundance of liquid methane and there has been speculation it could perhaps support life. Even though it is much colder than Earth, it has been observed to have bodies of liquid like seas and rivers. The presence of life is unlikely, but more research needs to be done.
The Twilight Zone
The Goldilocks Zone is not the only zone out in space worth paying attention to. There’s also the Twilight Zone. You know some scientist was exceptionally excited when he got to name that.
The Twilight Zone exists on the planet, rather than out in space. It refers to a narrow band on a tidally locked planet. So imagine if Earth wasn’t rotating on its axis as it spun around the Sun. Just a stationary planet where it’s constantly night on one side and constantly day on the other. One side is freezing, one side is hot, and neither side can support life. But there is a band right around the center which is the Twilight Zone. The place where, in a very narrow range, conditions could allow for life to exist.
This is theoretical, of course, but like the Goldilocks Zone, it opens up the possibility that other planets might have conditions, however narrow, that could support life as we understand it.
So how does a planet even become tidally locked? In our own galaxy, there are a number of stars that are smaller than our sun. Because of that, a planet needs to be closer to it to be in the Goldilocks Zone. But when a planet gets too close to its star, it becomes gravitationally locked in place. That means it can’t rotate, it can only orbit, so one side is always facing that star. This is a tidally locked planet. And because it’s in the Goldilocks Zone, water can potentially exist in liquid form, but it would only be in that narrow band of The Twilight Zone.
It sounds kind of unbelievable, like science fiction stuff, but it’s more common than you might think. For instance, the moon always looks the same when you see it at night because it’s tidally locked to Earth. It orbits the planet, but it can’t spin on its axis.
By the Numbers
So, if a planet can only potentially host life if it’s within this Goldilocks Zone, how many planets are we talking about? Our solar system has one and we’re living on it. But there are about 3,200 stars with planets in orbit in the Milky Way Galaxy. That’s what we’ve discovered so far, anyway. And in the universe at large? There could be as many as 200 sextillion stars out there, each with its own system of planets.
If every star has a planet, then that’s 200 sextillion planets. But our star has 8 planets in orbit, which is actually the most we’ve ever discovered around a star so far. If other stars have even 4 planets, well, then that’s up to 800 sextillion worlds in the universe.
Scale it back a bit and researchers have estimated that our galaxy alone may have as many as 300 million potentially habitable planets. Data from the Kepler Telescope as well as the European Space Agency’s Gaia mission were used to come up with the potential numbers.
That 300 million is also not an optimistic estimate by any means. It’s considered to be fairly conservative, based on the possibility that only 7% of sun-like stars host potentially habitable planets. The real number could be quite higher as the average expected rate is 50%, meaning that 300 million could be 2.1 billion or so. Some of these planets are relatively close to us, but relatively are doing a lot of heavy lifting in that sentence. For instance, in interstellar terms, the nearest planet to us is extremely close, but it’s still 20 light-years away.
The closest star to us is Proxima Centauri at 4.3 light years away. With current technology, we could get there in 6,300 years. So our friendly planet 20 light years away might take closer to 30,000 years to reach. In other words, we won’t be visiting these planets any time soon.
If you go by the Drake Equation, an equation formulated by astronomer Frank Drake, to determine the number of habitable planets, things look a little different. Again, this is very speculative, but it multiples the average rate of star formation by the fraction of those stars that have planets by the average number of planets that can potentially support life per star by the average life of a planet. As you can see, some of those values are going to be imprecise. But, the answer, depending on how you like to do your math, is between 1.4 billion and 2.65 billion.
What a Planet Needs
Being in the Goldilocks zone doesn’t necessarily make a planet habitable. It just means it’s in the right area to be habitable. You really need to refine things before you can have life popping up. There are several things that a planet should have if it’s going to host life. Again, there are probably all kinds of circumstances we’ve never considered where different kinds of life could pop up. But for the sake of argument, we’re going with what we know.
Planetary scientist Alessandro Morbidelli believes seven factors need to be present on a planet in order for it to be hospitable to life. The first we’ve already covered. It has to be the right distance from its star.
The second factor is that it needs to be in the right orbit. If the orbit is too elongated rather than closer to a perfect circle, there will be stretches during the year when the temperatures vary drastically. Ultra cold winters, incredibly hot summers. The temperature range would make it unlikely life could survive with such orbits.
The next important thing is a stable rotational axis. Compared to Earth, Mars does not have a stable rotational axis, and that has led to it losing its atmosphere. Our axis remains fairly stable thanks to our moon.
The fourth identified factor is water. Yes, we already talked about how maybe other liquids could support life but, for an Earth-like situation, we need water. But not too much! Too much and you get layers of ice that prevent organic life from forming.
The composition of the atmosphere is also important. We are rich in nitrogen and oxygen. But if the planet formed faster, we could have had a hydrogen and helium-rich atmosphere like Neptune and not been able to support life.
The next factor is plate tectonics. You may not realize it, but plate tectonics maintain our climate. Volcanoes release greenhouse gases, rains wash them away, and the CO2 cycle continues. Planets like Venus don’t have that.
Last but not least, a planet needs a magnetic field. This is provided by the spinning, molten core of the earth. Because of it, we are protected from deadly charged particles flying through space that would kill all life on Earth by stripping our atmosphere if we didn’t have what is essentially a force field.
Another factor not listed in the seven, but pretty important, is the actual building blocks of life as we know it. Carbon, oxygen, nitrogen, hydrogen, phosphorus, and sulfur are what ensured life began on Earth.
How Do We Find Them?
So now we know what we’re looking for. How do we find them exactly? Well, it takes some work. Astronomers need to find a star, luckily there are a bunch of those out there. Then you need to look at what planets are in orbit around the star. You’re looking for something in that Goldilocks zone about the size of the Earth.
Because of the distances we are talking about, you can’t just point a telescope and look at an exoplanet like you are peeping on your neighbor with binoculars. Instead, we’re looking for dips in light from a star that indicates a planet may actually be there in front of it. That means you can only see them when they are orbiting around that side of their star.
Then, wavelengths must be analyzed to determine what kind of atmosphere we’re dealing with. Some planets are just gas, so we need to determine if this world is solid. Then the wavelengths of light absorbed or reflected by the planet help us understand its atmosphere because different atmospheres absorb or reflect light differently.
As we continue to discover these planets, it’s worth remembering that we’ve been broadcasting signals from our world for many years now. Scientists believe at least 29 potentially habitable planets would have received a signal from Earth by now.