The sun is a main sequence star. From the billions of examples of these stars we can see that the sun will get hotter and bigger as it ages. In 1/2 billion years it will be too hot here on earth. Eventually the sun will become so large that it will fill the sky and it will eat both Mercury and Venus, and maybe earth, too.
This should not be a revelation. The idea that the sun will burn out in 5 billion years is common enough, but that earth will be too hot long before then is probably not as commonly known.