Does gravitational radiation impact the stellar habitable zone?

Abstract

Substellar objects of ultrashort periods (less than 50 min) orbiting central stars and many ultracompact star binary systems of even smaller periods (less than 10 min) have been optically detected in the last few years. Their gravitational luminosities can be of the same order or even much greater than their electromagnetic counterparts thereby also comprising a special class of targets for the Large Interferometer Space Antenna (LISA). In this paper, it is argued that the “gravitational illumination” of such systems may alter significantly the standard electromagnetic stellar habitable zone as long as a fraction of its gravitational energy is converted into heat. An analytical formula for the habitable zone is derived by assuming that the planet is simultaneously irradiated by electromagnetic and gravitational luminosities. The effect is quantified and some examples are discussed based on the available observations.