Why is earth habitable planet?

Answer 1

We seriously lucked in. So many things about Earth are Just Right for habitation that in even one of them were changed a little bit, we'd all have to leave (if we could).

Here are just some:

1. Earth's Sun is the kind (about 4% of the stars are) that a) lives long enough for life to get underway and prosper and b) can have planets that aren't tidally locked, lilke Mercury.

2. Earth is just far enough away from a Sun of the right size that it can have lots of liquid water (not ice, not steam), plus some of both (ice & steam, or clouds).

3. Earth has a magnetic field that helps shield it from cosmic radiation.

4. Earth has a big "friend" in the planet Jupiter, that scoops up a lot of debris that might otherwise hit it.

5. Earth has a big Moon, which helps to stablize it.

6. Earth and the Sun live in a nice "suburban" part of the Galaxy, where there aren't too many stars to jerk us around with their gravities, but close enough in that we had instellar material to make a Sun and a few planets.

7. Earth is big enough to hold on to its atmosphere (unlike little Mars, which lost its long ago), but not so big that its atmosphere would crush things, like Juipiter, Saturn, Uranus and Neptune do.

8. Earth isn't so tectonically active that it has a runaway greenhouse effect like Venus.

9. Earth's Sun is a Population I star, which has some heavier elements good for making complex things, like plants and animals. We have some long-gone supernovas to thank for making these elements for us, because our Sun is too small to make them itself.