3. The first question to answer is whether a planet can actually support life.

support：to provide the necessary conditions so that life can exist（本文中）。More examples:

His brother supported him while he was at college. (support: to provide a living for sb, 养活某人)新概念英语培训班

The land is so poor that it cannot support any crops. (support: to provide the necessary conditions for crops to grow)

4. Imagine a star up to twenty times larger, brighter and hotter than our own sun.

up to: as much as or no more than，至多，多达，直到。More examples:

Up to three hundred people were on board the ship. 船上多达300人。

We teach learners up to intermediate level in our centre. 我们中心将初学者教到中级水平。

I know nothing about it up to now. 直到现在我对此事还一无所知。

Relatively few people are prepared to sacrifice up to a third of their holidays for the pleasure of travelling on a ship. (NCE3 Lesson44)

5. A planet would have to be a very long way from it to be capable of supporting life.

be capable of: having the ability to do sth. More examples:

His wife is a very capable woman.

When he's drunk he's capable of saying rude things. ( He is more likely to say rude things.)