It's a question that has puzzled scientists for decades – if there are aliens out there, why haven't they visited Earth yet?
Now, scientists believe they may have the answer to the so-called Fermi paradox.
Dr Michael Wong, of the Carnegie Institution for Science, and Dr Stuart Bartlett, of California Institute of Technology, claim that extraterrestrial civilisations may grow so large and technical that they hit crisis points, missing the chance to visit new worlds.
Does alien life exist?
No life beyond Earth has ever been found; there is no evidence that alien life has ever visited our planet.
However, this does not mean that the universe is lifeless other than on Earth, according to NASA.
The space agency says: 'While no clear signs of life have ever been detected, the possibility of extraterrestrial biology - the scientific logic that supports it - has grown increasingly plausible.'
One popular school of thought is that our own existence is evidence that there is certainly life on other planets, as the likelihood of Earth being a 'one-off' is almost zero.
However, one argument against this is - if there is extraterrestrial life, why have we not found any evidence for it?
The Fermi Paradox questions why, given the estimated 200 billion - 400 billion stars and at least 100 billion planets in our galaxy, there have been no signs of alien life.
The contradiction is named after its creator, Italian physicist Enrico Fermi, who first posed the question back in 1950.
In a new study, published in Royal Society Open Science, Dr Wong and Dr Bartlett set out to crack the 70-year-old puzzle.
The pair analysed previous studies which have shown that cities grow 'superlinearly' – at a faster rate than linear – thanks to their social nature.
This superlinear scaling results in crises called 'singularities', where population and energy demands supersede innovations, leading to the system's collapse.
For example, on Earth, the researchers say that ozone depletion, the escalation of weapons of mass destruction and whaling could have led to burnout, had they not been mitigated.
The researchers suggest burnout is likely to happen to alien civilisations too.
'We hypothesize that once a planetary civilization transitions into a state that can be described as one virtually connected global city, it will face an "asymptotic burnout", an ultimate crisis where the singularity-interval time scale becomes smaller than the time scale of innovation,' they wrote.
If alien civilisations can recognise they're heading towards this burnout, they may be able to put changes in place to prevent it, the researchers added.
However, this reprioritisation of resources could make the alien civilisations even more difficult to detect.
'We propose a new resolution to the Fermi paradox: civilizations either collapse from burnout or redirect themselves to prioritizing homeostasis, a state where cosmic expansion is no longer a goal, making them difficult to detect remotely,' the team wrote.
As civilisations near burnout, they may actually be the most detectable to us, according to the team.
'They would be altering their environments and dissipating free energy in a wildly unsustainable manner - fluctuations on the planetary scale that exhibit the largest signal-to-noise.
'This presents the possibility that a good many of humanity's initial detections of extraterrestrial life may be of the intelligent, though not yet wise, kind.
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