[_0w8t]

Is it possible from geological evidence to confirm that Sun was dimmer billions years ago? I am asking as stability of Earth orbit cannot be taken for granted for such periods. For example, we could underestimate the effects of solar wind in past that could have pushed Earth, or the could be an interaction with passing close stars.

[cbkeller]

Ah, so there are many things that it is hard to be absolutely certain of in geology, but changing Earth's orbit is at least very very hard; even the kinetic energy from things like the Chixulub impact are far too small to have a significant effect. The "moon-forming impact" in the most common model of the origin of the moon might be more on the right order of magnitude, but there don't seem to have been any of those more recently than about 4.51 Ga. An astronomer could say more, but solar luminosity is also relatively well understood from studying other main-sequence stars of various ages.

The most common solutions involve high concentrations of organic greenhouse gases like methane as well as high CO2, but it's always possible there are other possibilities that have not yet been considered.

[_0w8t]

We do not have good models of the rate Sun has been losing hydrogen especially on scale of billions of years. So from that we do not have a precise answer how heavier was Sun in past. But heavier Sun implies that Earth was closer compensating for the dimmer younger Sun.

As I understand according to the current estimates this not enough to avoid the cold Earth problem, but there are way too much uncertainty. But if we do not have way to read the brightness from geology alone, that can be an answer.