Tuesday marks the 77th birthday of Stephen Hawking, the world-renowned physicist and author of "A Brief History of Time." Hawking was born on Jan. 8, 1942, and had made groundbreaking studies about parts of the cosmos like black holes.

For years, Hawking's fought with ALS, also known as Lou Gehrig's disease, after being diagnosed more than a half-century ago. Typically given just 10 years to live from the time of diagnosis, Hawking survived for more than five decades fighting the disease. He died March 14, 2018. 

To celebrate Hawking's birthday, here are some of his famous quotes on aliens and life on Earth.

1. "Although the chance of a disaster to planet Earth in a given year may be quite low, it adds up over time, and becomes a near certainty in the next thousand or ten thousand years."

2. "We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the Universe. That makes us something very special."

3. "Even if it turns out that time travel is impossible, it is important that we understand why it is impossible."

4. "I am discounting reports of UFOs. Why would they appear only to cranks and weirdos?" 

5. "Human colonization on other planets is no longer science fiction. It can be a science fact... If humanity is to continue for another million years, our future lies in boldly going where no one else has gone before."

6. "The radiation left over from the Big Bang is the same as that in your microwave oven but very much less powerful. It would heat your pizza only to minus 271.3 degree celsius — not much good for defrosting the pizza, let alone cooking it."

7. "Because there is a law such as gravity, the universe can and will create itself from nothing."

8. "No one undertakes research in physics with the intention of winning a prize. It is the joy of discovering something ,no one knew before."

9. "Look up at the stars and not down at your feet. Try to make sense of what you see, and wonder about what makes the universe exist. Be curious."

10. "I believe alien life is quite common in the universe, although intelligent life is less so. Some say it has yet to appear on planet Earth."

Stephen Hawking 2 In this photo, Stephen Hawking delivers a lecture in Cape Town, South Africa, May 11, 2008. Photo: Getty Images