Could invisible aliens really exist on Earth?

Astrobiologist responds to claim by Britain’s first astronaut that extraterrestrials may live among us

Aliens seen in the woods
(Image credit: Martina Badini/Shutterstock)

Life is pretty easy to recognise. It moves, it grows, it eats, it excretes, it reproduces. Simple.

In biology, researchers often use the acronym “MRSGREN” to describe it. It stands for movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.

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