We used to think that we were at the centre of everything. That the Sun, planets and stars all rotated around the Earth. The Copernican revolution changed all that, relegating us to just another planet orbiting the Sun. We realised that the stars were just distant copies of the Sun. Much of modern astronomy hinges on the idea that there’s nothing special about our place in the universe. This is called the Copernican principle. This means that if there are planets here there should be planets in other galaxies too.
We also know that planets are a natural by-product of star formation – they form from the leftovers. Wherever we see stars, we also expect to see planets too. The growing field of astrochemistry has also provided evidence that other galaxies contain complex molecules required to fashion the building blocks of life. It is increasingly unlikely, it seems, that we are the only living planet in the entirety of the universe. Yet finding undeniable evidence that’s really the case is a significant challenge that the next generations of astronomers will try hard to meet. Whether we are alone is perhaps the biggest question scientists can ask.