AN AMERICAN astronaut who spent 230 days in space has warned the future is not so bright for humanity - and claims aliens do exist.

But who is Leroy Chiao?

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The engineer, former NASA astronaut, entrepreneur and speaker was born in Milwaukee, Wisconin and raised in Danville, California.

The 56-year-old, whose parents are from Taiwan, has a Ph.D. in chemical engineering from the University of California Santa Barbara, and was selected by NASA in 1990 to become an astronaut in July 1991.

He's been to space four times - most recently in 2004 - and has completely four space walks. Interestingly, the father-of-two was also the first person to vote in a US presidential election while in space.

He quit NASA in 2005, and now works in the private sector.

But why is he so convinced that we're not alone in the universe?

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Writing for , Chiao says he believes it would be "arrogant" to think life only exists on Earth - and that life on Earth will eventually die out, either from natural causes or of our own doing.

He says his view is a result of what's sometimes called the "Overview Effect" - where an astronaut's perception of the Earth changes as a result of seeing it from space, where it appears insignificant and fragile in comparison with the rest of the universe.

But Chiao also points to some scientific evidence that there could be other life forms out there.

Proxima b is a planet which orbits the red dwarf star Proxima Centauri - the closest star to the sun. It's around 25 trillion miles away from Earth - but could also be home to life, according to Chiao, as it is the right distance from from Proxima Centauri to support liquid water.

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However, years on the planet only last 11 days - and the radiation environment is thought to be harsher than Earth - but Chiao says scientists still think some sort of life could live there.

The NASA Distinguished Service Medal recipient also mentions Mars, where scientists recently discovered evidence of liquid water - as he believes it is more likely we could find evidence of microbial life under Mars' surface.

Meanwhile, although he's certain species on Earth are not the only life form in the universe, he says even sending a spacecraft to Proxima Centauri would take almost 80,000 years - so we won't be able to easily find answers.