> If a planet absorbs more energy from its star than it's capable of radiating away, it will rise in temperature forever until and unless energy output increases above energy input.

That’s a really misleading way to describe what going on. The amount of energy radiated into space is a function of temperature, so increasing incoming radiation also increases outgoing radiation. 5% increase in luminosity is roughly 3.5C increase in earths average surface temperatures ignoring other effects.

However, you get a feedback loop as water vapor is a greenhouse gas which makes things hard to model. Here’s a study suggesting the tipping point is on 1 billion years https://www.sciencedaily.com/releases/2013/12/131216142310.h... but it’s not suggesting the oceans literally boil but rather a transition point where there’s a lot more water in the atmosphere.