Assessing the massive young sun hypothesis to solve the warm young Earth puzzle

A moderately massive young Sun has been proposed to resolve the so-called faint young Sun paradox. We calculate the time evolution of the solar mass that would be required by this hypothesis using a simple parameterized energy-balance model for Earth's climate. Our calculations show that the solar mass-loss rate would need to have been 2-3 orders of magnitude higher than at present for a time on the order of - 2 Gyr. Such a mass-loss history is significantly at variance ( both in the timescale and in the magnitude of the mass-loss rates) with that inferred from astronomical observations of mass loss in younger solar analogs. While suggestive, the astronomical data cannot completely rule out the possibility that the Sun had the required mass-loss history; therefore, we also examine the effects of the hypothetical historical solar mass loss on orbital dynamics in the solar system, with a view to identifying additional tests of the hypothesis. We find that ratios of planetary orbital spacings remain unchanged, relative locations of planetary mean motion and secular resonances remain unchanged, but resonance widths and the sizes of the Hill spheres of all planets increase as the Sun loses mass. The populations and dynamics of objects near resonances with the planets, as well as those of distant irregular satellites of the giant planets, may contain the signature of a more massive young Sun. Planetary and satellite orbits provide a few tests, but these are weak or non-unique.