The Stability of Orbits of Putative Earth-Mass Planets or Satellites of Giant Planets within Known Exoplanetary Systems

Abstract

The movement of the habitable zone of 55 stars from 0.5 to 1.5 Solar masses and 0.75% to 5% metallicity were modelled over their main sequence lifetimes. 156 stars known to have planetary systems had their mass and metallicity matched to their nearest model, giving approximate habitable zones, which were compared with their giant planet(s)’s orbital range and gravitational reach. Habitable zones lying outside a giant’s gravitational reach for one billion years (two billion years after star birth) could host an Earth-mass planet in a stable confined orbit long enough for life to develop. Low eccentricity giant orbits confined to the habitable zone could also host Earth-mass satellites. Results show 85 of 156 exosystems could house a habitable Earth-mass body over the last billion years, 113 could do so for at least a billion years at sometime during their main sequence lifetimes, excluding the heavy bombardment period. An orbital integrator computer program modelled orbits of Earth-mass planets in the habitable zones of x1 Gruis, HD 196050, HD 52265, 55 Cancri, and Earth-mass giant planet satellites in HD 23079 and HD 28185. The integrator’s predictions for satellite orbits were shown to comply with restricted three-body problem theory. ‘Earths’ were ‘launched’ at different distances from the star or planet and their orbital parameters monitored with time until the program concluded after either one billion years, 100 million years, or a cataclysmic event. Orbits lasting the full run time are assumed to remain stable for the star’s main sequence lifetime. Results reveal the possibility of habitable Earth-mass planets around HD 52265, 55 Cancri, and satellites around the giants of HD 23079 and HD 28185. The x1 Gruis system cannot host a habitable ‘Earth’ whereas HD 196050 could, provided the planet’s semimajor axis was at the inner 20% of the habitable zone width.