Essentially, they are ordered by
points / age^2 * a-lot-of-penalties
A more detailed (old) discussion is in: http://www.righto.com/2013/11/how-hacker-news-ranking-really... HN discussion: https://news.ycombinator.com/item?id=6799854 (920 points, 1240 days ago, 190 comments)
The algorithm is tweaked from time to time without warning, so many details may have changed since this was posted ~3 years ago. But the main idea is still there.