Earth is teeming with life. In fact, your body alone harbors millions of lifeforms. Everywhere you look on our planet you can find life. From the harshest deserts to the deepest depths you can find some form of life. Yet, as soon as you leave the planet you seem to leave all that life behind. Or at least so it seems. For life to exist, you need to meet a very specific set of requirements. Requirements Earth has all met. But scientists all around the world are busy looking for life outside of that on Earth.

There are several ways to look for alien life. You could look for life that is similar to that on Earth, or you can look for intelligent life. Let’s first look at searching for any kind of life that is similar to life on Earth. At the backbone of life on Earth is a molecule called dna. dna is a fairly complex molecule made up of Hydrogen, Oxygen, Nitrogen, Carbon and Phosphorus. If alien life is made up out of a molecule similar to dna you can try to search for it by searching for liquids, specifically water. Water allows for all of the chemical reactions needed to form dna. Other liquids might also do the trick, but water seems to be the best bet.

Searching for water

Since water is the prime ingredient needed for Earth-like life to form it’s currently astrobiology’s focus when looking for extraterrestrial life. We might not even have to leave our solar system to find other locations with liquid water that could harbour life. There are several locations fairly nearby with water.

Mars has water, mostly in the form of ice. But there is evidence that Mars once had large bodies of liquid water. This means that Mars may have been fairly similar to Earth at some point and might have even harboured life.

Europa, a moon of Jupiter, has a crust of ice. There is evidence that underneath this layer of ice lies an ocean of liquid water.

Something similar is true for Enceladus, a moon of Saturn. There is evidence for an ocean underneath a layer of ice on the moon’s south pole. From this ocean, water vapor jets are spouting into space.

The problem with these locations is that, while they do have water, they are nothing like Earth. Again, to find life in space you want to look for something that looks as much like Earth as possible. Mars is an oxygen-deprived wasteland, Enceladus and Europa are freezing cold. That’s why it might be a good idea to also look outside of the borders of our solar system.

Looking for planets like Earth outside of the solar system is incredibly difficult. At these distances you can really only make out stars. One of the most common way of looking for planets around stars is by looking at the star’s brightness. If a planet rotates around a star, the star dims whenever the planet gets between us and the star. By measuring a star’s brightness over time you can detect these planets. Another popular method for detecting planets is by looking at a star’s movement. If a planet rotates around a star the star wobbles slightly. You can compare this to a hammer throw at the Olympics. The athlete rotates a weight around himself and while doing so his body moves away from the weight slightly.

Using these methods many planets have already been discovered in the past few years, some of which are not unlike Earth in size and distance from their star. The Kepler telescope, which is floating out in space, has been the main instrument behind these discoveries. Currently there have been well over 950 confirmed planet discoveries in the Kepler mission. Most of these planets are nothing like Earth, but some are. One of these Earthlike planets might even have life on its surface.

The Kepler space telescope, just before launch

The Kepler Telescope, before its launch. Image by NASA

With our current technology it would be incredibly difficult to look closer at these planets and see if they have life on them. But finding these planets proofs that planets are actually quite common in our universe and with an estimated 3⨯1023 stars in the observable universe it’s a safe bet that life is more common than we currently know it to be. In future missions stronger space telescopes should be able to get a better idea of what these planets are like and if they are habitable.

Intelligent life

Mars, Europa, Enceladus and Earth-like planets are good contenders for finding some form of alien life. Most likely in the form of micro-organisms. But what if you wanted to find life that’s more like us humans? What if you wanted to find “intelligent” life? In order to this, you need to look for something completely different. Geology is not enough, you need to find evidence of civilization or scientific progress.

Perhaps the most well-known group searching for intelligent life in outer space is seti. A non-profit organization that looks for alien intelligence by looking for radio signals. The idea here is that any civilization that is sufficiently advanced will be sending out some form of radiowaves and these radiowaves will stand apart from the normal background noise you’ll find in space. These radiowaves will either be repetitive or just really loud. As it stands right now seti has yet to find any signal that meets these criteria, but has stated that they expect to find something in the next few decades.