The Anglo-Australian planet search XXI : a gas-giant planet in a one year orbit and the habitability of gas-giant satellites

Abstract

We have detected the Doppler signature of a gas-giant exoplanet orbiting the star HD 38283, in an eccentric orbit with a period of almost exactly one year (P = 363.2 +/- 1.6 d, m sin i = 0.34 +/- 0.02 M-Jup, e = 0.41 +/- 0.16). The detection of a planet with period very close to one year critically relied on year-round observation of this circumpolar star. Discovering a planet in a 1 AU orbit around a G dwarf star has prompted us to look more closely at the question of the habitability of the satellites of such planets. Regular satellites orbit all the giant planets in our solar system, suggesting that their formation is a natural by-product of the planet formation process. There is no reason for exomoon formation not to be similarly likely in exoplanetary systems. Moreover, our current understanding of that formation process does not preclude satellite formation in systems where gas giants undergo migration from their formation locations into the terrestrial planet habitable zone. Indeed, regular satellite formation and Type II migration are both linked to the clearing of a gap in the protoplanetary disk by a planet, and so may be inextricably linked. Migration would also multiply the chances of capturing both irregular satellites and Trojan companions sufficiently massive to be habitable. The habitability of such exomoons and exo-Trojans will critically depend on their mass, whether or not they host a magnetosphere, and (for the exomoon case) their orbital radius around the host exoplanet.