Hi Crystal,
This is actually a sort of complicated question, because the temperature of a planet depends on more than just its distance from the sun - it depends on its atmosphere too!

For instance, Venus is closer to the Sun than the Earth and just based on distance it should be about twice as hot as Earth is. However, it is actually about 3 times hotter than Earth. That is because it has a thick atmosphere containing lots of carbon dioxide (CO2) which holds in the heat.

Also, I am not quite sure how hot would be too hot... Hot enough to boil water would certainly be too hot, but I would guess the cut off, for humans anyway, would be lower than that.

Although we don't expect Earth to be getting any closer to the sun, this is still an important question. Astronomers think about them when they consider the possibilities of life on their planets. How likely is it? How close or far could a planet be from its star (or stars) and still have life? We are not sure yet.

Terry