There is a conundrum among scientists about why we have not found alien life elsewhere in the universe. Known as the Fermi Paradox, it says that – theoretically – life should be abundant beyond our own solar system. But if that is the case, why haven't we found it yet?

At present, astronomers have identified some 3,500 exoplanets across more than 2,600 planetary systems. It is estimated the Milky Way alone is home to 100 billion exoplanets, with far more Earth-sized planets than massive ones like Jupiter. As a result, there could be billions of 'potentially habitable' exoplanets out there – meaning they have the potential to have liquid water on the surface.

So what are scientists looking for in exoplanets when considering their potential to host life? Robert Massey, Deputy Executive Director at the Royal Astronomical Society, tells IBTimes UK what is needed for life on another world:

If you want a planet that's habitable – and potentially inhabited – you need a lot of very specific conditions. At least we think so.

You need a planet around a star that is relatively stable, that has a long lifetime because that allows life to evolve. You also want a star that doesn't vary too much, because if your star changes its brightness and heat output radically over say a few years or even centuries, that's not going to be good for life. It's going to make it very hard for complex life to adapt and survive.

Artist impression of what it would be like to stand on the surface of the TRAPPIST-1 - a potentially habitable exoplanet. NASA/JPL-Caltech

You also need a world – we think – which is around a star with lots of metals in it. Metals in astronomy doesn't mean things like iron – it means everything heavier than helium. If you have those so-called metals, the heavier elements, you're more likely to be able to form rocky planets. The kinds of planets you could walk around on seem to be the sort of place where life could flourish based on our experience here on Earth.

You also need that planet to be in orbit around a star that is either single, or if it's a binary star – which is very common in the universe – that those stars are very close together, in which case a planet can have a stable orbit around them, or they're very far apart, in which the same thing applies. If these stars are separated by the same distance as the Earth and the Sun, it would be very hard for a planet to have a stable orbit. That would be quite bad for life as the planet might get thrown out of its star system or be subject to very extreme changes in climate.

Other factors include the famous 'Goldilocks zone' idea – that it has to be in the middle of the habitable zone. If the planet is outside of that it means it's either too cold for liquid water to form or it's too hot so it stays as water vapour and steam.

That again is not very good for advanced life evolving. Other factors beyond that include things like the planet not having extreme seasons. If you imagine a world like Uranus which is tipped right over so it has a tilt of more than 90 degrees, then its pole experiences 42 years of darkness and 42 years of daylight. That probably isn't going to be the kind of system that's good for life. There are ways around it – you could have a thick atmosphere that helps distribute the heat. But it probably isn't going to make life any easier.

It also helps if the planet is the right size. If a planet is too big and has too much mass it would have a strong gravitational field. That would mean it might end up like Jupiter in our own solar system with a very thick atmosphere, not much of a solid core, very hot in its interior. Very hard to imagine complex life surviving. Conversely if it's too small then it probably can't hold onto that atmosphere. If it's smaller than Mars, then that atmosphere would get stripped away very quickly.

Another related factor – one we benefit from on Earth – is a magnetic field. The advantage of that is it protects us from harsh radiation from the Sun and from interstellar space. High levels of radiation are very bad for life. We really don't want a planetary environment that is subject to that.

As well as having moderate seasons because the planet has a moderate tilt like Earth (so we don't get too many extreme seasons), it would also be very important the planet is not in an eccentric orbit. Now the orbit of the Earth around the Sun is roughly a circle. If you stretched it out too much you would have the issue that sometimes the planet would be very very cold, and potentially very hot. These extremes are not good for life. You want clement worlds. That means we need a circular orbits – at least for the kind of life we are familiar with to flourish.

earth from space
North America as seen from space Nasa

The other thing to think about is that although a very massive planet is probably not a good place for life, if you look at a planet like Jupiter it has a big retinue of moons so it's almost like a mini solar system. It is just possible that some of those moons, even in our own solar system like Europa, might host life in deep oceans under an icy crust. So we shouldn't be too worried if we don't find precisely Earth-like planets around other stars because there are other ways that life could get a foothold.

Of course the real problem we have is that the only example we have of life in the Universe – the only one we're aware of – is right here on Earth. Until we actually find life elsewhere in the universe, all of this is quite moot. It's very much a theoretical discussion. It's a discipline looking for extraterrestrial life so far with no data.

Having said that, if we did find it, it would probably rank as one of, if not the most exciting scientific discoveries of all time.