For a long time, it looked as though Earth was an outlier. While our watery rock is teeming with life of all kinds, the rest of our solar system (and beyond) appears to be a baron wasteland devoid of biological beings. However, the more we learn about space and the mechanisms of life, the more evident it becomes that life could have evolved independently elsewhere. Of course, the jury is still out on whether life actually has developed outside of Earth, but that doesn't mean we can't speculate on where we're most likely to find it. So let's take a look at the planets and moons in our solar system that represent the biggest candidates for life.
Mars has always occupied a special place in our culture, with media like The Martian and War of the Worlds. But could it inhabit life? Maybe. Mars is the fourth planet from our Sun and is often called The Red Planet due to its dusty desert surface that contains iron oxide. We know that the planet was much more Earth-like in the past and even had liquid oceans. However, today the planet is cold and dry.
Crucially, Mars is also the most explored planet in our solar system, and there currently exist many more projects for Mars exploration in the works. While life on the surface of Mars seems unlikely from what we've studied so far, it's possible that life could exist below the surface in deep aquifers. Lending further credence to this theory, scientists have discovered strange fluctuations of methane gas on the planet. Geological processes could explain these fluctuations, but they could also be a byproduct of life 1 .
At first look, Venus appears an unlikely candidate for life. After all, it's the hottest planet in our solar system, with surface temperatures of 475 degrees Celsius (or around 900 degrees Fahrenheit). In other words, temperatures are hot enough to melt lead!
However, there are still some reasons to believe life is not only possible but could potentially already exist on the fiery planet. Around 50 kilometers up from the surface, temperatures range from 30 to 70 degrees Celsius. These temperatures are mild enough to sustain microbial life, even at the high end of that temperature range. Additionally, that far up, atmospheric pressures are similar to Earth's surface.
Some stranger aspects of Venus also suggest life could be present. For example, we see dark streaks in the atmosphere that don't budge even in hurricane-force winds. Although these streaks are most likely ice crystals or something similar, it's possible they could be colonies of microbial life.
In September 2020, Scientists revealed they had detected Phosphene on Venus. Phosphene is considered a biosignature (a chemical that provides evidence for life) because it's essential to life, and no known geological processes can create it. However, recently these claims have been called into question, and scientists are much less sure Phosphene was detected at all 2.
Jupiter's moon Europa is arguably the strongest candidate for life in our solar system and presents an exciting research prospect for future space missions. Europa's surface is mostly water ice around 15 to 25 kilometers thick (10 to 15 miles). However, we have evidence that there are liquid water oceans up to 150 kilometers deep under this icy shell!
But why is Europa such a good candidate for life? Well, it has the right mix of conditions. It has water and an energy source in the form of heat from tidal flexing and geysers that spew ocean material into space (it's active). Additionally, strange colored patterns we see surrounding the geysers on Europa look eerily similar to microbial life patterns we see around geysers here on Earth 3.