Habitable Zones of Host Stars During the Post-MS Phase

TL;DR

This paper investigates how the habitable zones of stars evolve during post-main sequence phases, identifying potential habitable planets and moons, including Mars, Jupiter, Saturn, and Titan, as stars become brighter and expand their habitable zones.

Contribution

It provides a detailed calculation of the shifting habitable zones during stellar evolution phases using Eggleton's code, highlighting potential habitability of planets and moons outside the main sequence.

Findings

Mars and Jupiter enter habitable zones during stellar evolution

Saturn's orbit becomes habitable for 137 million years during He-burning phase

Titan may become suitable for life during Saturn's habitable phase

Abstract

A star will become brighter and brighter with stellar evolution, and the distance of its habitable zone will become farther and farther. Some planets outside the habitable zone of a host star during the main sequence phase may enter the habitable zone of the host star during other evolutionary phases. A terrestrial planet within the habitable zone of its host star is generally thought to be suited to life existence. Furthermore, a rocky moon around a giant planet may be also suited to life survive, provided that the planet-moon system is within the habitable zone of its host star. Using Eggleton's code and the boundary flux of habitable zone, we calculate the habitable zone of our Solar after the main sequence phase. It is found that Mars' orbit and Jupiter's orbit will enter the habitable zone of Solar during the subgiant branch phase and the red giant branch phase, respectively. And the orbit of Saturn will enter the habitable zone of Solar during the He-burning phase for about 137 million years. Life is unlikely at any time on Saturn, as it is a giant gaseous planet. However, Titan, the rocky moon of Saturn, may be suitable for biological evolution and become another Earth during that time. For low-mass stars, there are similar habitable zones during the He-burning phase as our Solar, because there are similar core masses and luminosities for these stars during that phase.