The billionaires Jeff Bezos and Elon Musk both have companies dedicated to space travel — Blue Origin and SpaceX, respectively.

Bill Gates, however, is spending his billions on solving some of the world's biggest problems: disease, sustainable-energy solutions, and climate change. To that end, he studies the works of the acclaimed energy scientist and researcher Vaclav Smil, who has written dozens of books on those subjects.

Smil had a scathing criticism of efforts to leave Earth: "You have to be a delusional Elon Musk to think that we can terraform Mars and leave this planet," Smil said in the new Netflix documentary on Bill Gates. "We will never leave this planet."

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Elon Musk has aspirations to reach the stars — to "extend consciousness beyond Earth" — and he's got a company dedicated to just that: SpaceX.

That company just revealed its plans for reaching Mars, which Musk intends to make livable for humans.

However, he's got at least one major detractor: the acclaimed energy scientist Vaclav Smil, who's most well-known as the author of dozens of complex books on a variety of scientific subjects. He called Musk's plans "delusional" in the recently released documentary on Bill Gates, "Inside Bill's Brain: Decoding Bill Gates."

Gates is a fan of Smil, and he's read every one of Smil's books.

"If it's about health areas, energy, climate change. Quite a few that, if there's a good book, I'm gonna make sure to read it," Gates said in the documentary when asked how he chooses what books to read.

"In the area of energy, Vaclav Smil has written every one of these books," Gates said in the documentary, standing in his library in front of a shelf of Smil's books. "He spent his whole life building a framework of energy by studying every little topic."

Vaclav Smil's many books in Bill Gates' library. Netflix

Smil is interviewed in the documentary, and it turns out he isn't a fan of Musk's plans to advance humanity in the stars — when he's asked whether people should worry about climate change, he says, "Absolutely, because it's the only planet we have, you know. You have to be a delusional Elon Musk to think that we can terraform Mars and leave this planet. We will never leave this planet."

Representatives for Musk and SpaceX did not respond to Business Insider's requests for comment.

Read more: Elon Musk doubled down on his theory on why nuking Mars would be a good idea

Musk has repeatedly spoken of plans to move humanity to the stars and, specifically, to make Mars a habitable planet through terraforming.

In August, he tweeted "Nuke Mars" — a reference to his idea that shooting nuclear weapons at the two poles of Mars would help to warm that planet and create an atmosphere that's livable for humanity.