Aliens exist – they just haven’t visited Earth, a NASA veteran has claimed.

Dr Gentry Lee has worked at the US space agency since 1968, when he first got involved with the Viking mission to Mars.

He has since spent more than half a century designing probes to land on distant planets – but argues Earth has not yet been visited by other–worldly beings.

‘There exists nothing today that says any alien or any alien machine has ever landed on the planet Earth,’ he told the American Association for the Advancement of Science (AAAS) conference in Phoenix.

‘If you believe otherwise, you are being misled.’

According to the expert, in every case of a supposed UFO sighting or alien encounter, there is likely a simpler explanation for those phenomena.

But when it comes to distant planets, life has ‘just got to be there somewhere’.

He added: ‘We are going to find life of some kind somewhere else. The odds are overwhelming.’

Scientists generally believe that the best candidates for alien life are ‘Earth–like’ worlds orbiting distant stars in other parts of the galaxy.

One such contender is the Earth–sized planet TRAPPIST–1e, located just 40 light–years from Earth, which is located safely within its star’s habitable ‘Goldilocks zone’.

Another promising candidate is the planet K2–18b, which some studies suggest could be teeming with life.

Located 124 light–years from Earth in the constellation of Leo, K2–18b is a giant world covered entirely by oceans – making it what scientists call a ‘Hycean world’.

Even in our own solar system, Saturn’s moons Enceladus and Titan could have the right conditions for life to emerge and thrive.

Dr Lee also said that space scientists must be prepared to find alien forms that are very different from the life found on Earth.

He said that all life on our planet – ‘you, me, an elephant, slime mould, bacteria’ – relies on DNA to reproduce.

He added: ‘Extraterrestrial biologists would come to the Earth and would go back and report: “Not a terribly interesting planet. All life is the same. All of it reproduces in the same way using the same major [DNA] molecule.”‘

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