The short answer: it’s very likely that life can be found on most planets, but it’s almost all microbial, such as single-celled organisms. The likelihood of other intelligent life is extremely low because of the large number of very rare events that must occur for it to develop. However, the universe is vast, and there are still many unknowns. Using data we can already observe here on Earth and from the universe around us, we can make informed estimates, and with new telescopes now coming online, we have better data than ever before.
Habitable Zone
Within our own solar system, that is the planets orbiting our Sun, we don’t expect to find any advanced life. Earth is the only planet that is at the ideal distance from the Sun, otherwise known as the Goldilocks zone (also called the habitable zone). The nearer planets are being cooked, and further planets are ice cold; plus their atmospheres are completely different with no oxygen* (Mars does have a very tiny amount of 0.1% oxygen), but despite this, many different types of microbial life could potentially exist on these planets. Even Venus or Mercury, while extreme, could host transient microbial life in the past or in extreme niches.
Mars Ancient River
Mars has strong evidence that microbial life may have existed in the past, as it has features that can only be explained with existence of life; its geological structure also indicates there were rivers, lakes and possibly oceans, and today it still has ice and signs of underground water, making it a key place to search for ancient bacteria. Fossilised microbes could still exist inside of the Martian rock.
Titan · Enceladus · Europa
Other planets within our system, such as the moons of Jupiter and Saturn (Europa and Enceladus), are also promising. Beneath their frozen surfaces are vast oceans of liquid water, kept warm by gravitational heating. The water within Enceladus gets so hot, it actually sprays water and organic chemicals into space. Even Titan, Saturn’s largest moon, has complex chemistry and liquid lakes, though made of hydrocarbons rather than water. These environments are extreme by Earth standards, but on our planet we know microbes can survive in boiling vents, deep underground and inside ice, so similar simple life elsewhere in our solar system is entirely plausible. There are planned missions to Titan.
Our own galaxy, the Milky Way, is a giant flat pancake of stars, 100,000 light-years across. A light-year is how far light travels in one year, which is nearly 10 trillion kilometres! The Milky Way has at least 100 billion stars. Stars are Suns just like ours, some smaller, some larger, some younger, some older, and we recently confirmed that most of them do have planets orbiting them, just like our Sun has Earth, Mars and others. Although intelligent life may exist within our galaxy, it would take thousands of years of travelling to get there.
Exoplanets
Planets around other stars are called exoplanets, and they are tough to see. It’s like trying to spot a tiny ant next to a bright streetlight from kilometres away. But our telescopes, like the Kepler space telescope, have gotten really good at this. Kepler looks for tiny dips in a star’s brightness when a planet passes in front of it, like a moth flying across a torch’s beam. This is called the “transit method”. Using this, Kepler found thousands of exoplanets, and we now think there are about 40 billion rocky planets in the habitable zone in our galaxy alone, but would have very different environments from Earth.
Drake Equation
To answer this question further about possibilities of other life, you need to know about something called the “Drake equation“; this is a mathematical formula that helps estimate the number of intelligent civilizations in our galaxy. If the universe is so huge, with billions of stars and planets, why haven’t we found alien life yet? What is needed for life to exist, like how many Earth-like planets might be out there, and how much time needs to pass for life to develop, etc. When we do all the calculations, the likelihood of intelligent life within our galaxy is estimated between 1 and 100 planets.
So why haven’t we heard from aliens? There are a few reasons this might be. First, life might be common, but creatures that can build spaceships or send radio signals might be very rare. On Earth, simple life such as bacteria emerged billions of years ago and spread even into the planet’s darkest and harshest environments. Intelligent life, by contrast, arrived only very recently. The transition from simple organisms to complex, thinking beings appears to require immense time and a great deal of chance. Even then, intelligent species may exist for only a few thousand years.
Even if intelligent aliens exist, they are too far away. Space is massive. The closest star to us, Proxima Centauri, is over 40 trillion kilometres away. Even if aliens were there, our telescopes can’t see their planet clearly or pick up their signals unless they’re deliberately sending something super strong our way. It’s like trying to hear a whisper between cities.
Some scientists think aliens might be staying quiet on purpose. This idea is called the “dark forest hypothesis”. Imagine the galaxy as a dark forest where everyone’s hiding because they’re scared of being noticed by someone dangerous. If aliens are smart, they might not want to broadcast their location, just like we wouldn’t shout in a spooky forest at night.
James Webb Space Telescope
Our best telescopes, like the James Webb Space Telescope, can now study the air around some exoplanets. For example, a planet called K2-18b, 120 light-years away, might have water and potentially chemicals like dimethyl sulfide, which on Earth can only be made by living things like ocean plankton. But the signal is weak, and we need more data to be sure. New telescopes, like the Extremely Large Telescope being built, will help us look closer in the next few years. These observations may provide the first hints of life beyond our solar system.
So, is there life out there? Probably, because there are billions of planets with the right conditions for life like ours. But intelligent life is going to be rare, and finding it is hard because space is just too big.
