Whether or not life exists beyond Earth is a question that has persisted since humans first began to look up at the night sky.
Nowadays, the scientific pursuit in search of answers is known as SETI: the Search for Extra-Terrestrial Intelligence. SETI works under the premise that, if intelligent life exists out there, we should use science and technology to try and find it.
Do aliens exist?
It’s worth pointing out that, from a neutral, objective stance, there is no hard or even remotely convincing evidence that an advanced alien species has ever visited Earth.
But does that mean we should rule out life existing beyond Earth altogether?
Exoplanet scientists now infer that, for every star we can see in the night sky, there is on average at least one planet in orbit around it.
There could be at most 400 billion stars in the Milky Way alone, and ours isn't even a big galaxy. Our nearest major galactic neighbour, the Andromeda Galaxy, likely has one trillion stars.
And there could be two trillion galaxies in the observable Universe.
Think about how many planets there must be.
Some will be scorching hot gas giants, some freezing cold icy worlds.
Many will be rocky, Earth-mass planets orbiting close enough to their host stars that liquid water could pool on their surface.
They may have oceans and seas, just the right temperature for life to take hold the way it did on Earth.
It seems like life elsewhere in the Universe must surely be an inevitability. And yet, there is a distinct lack of any life, anywhere that we have been able to detect.
It seems inconceivable we could be the only planet to have ever hosted living beings, and yet for the time being at least, we must resign ourselves to that conclusion.