Gas giants in hot water: inhibiting giant planet formation and planet habitability in dense star clusters through cosmic time

Abstract

I show that the temperature of nuclear star clusters, starburst clusters in M82, compact high-z galaxies, and some globular clusters of the Galaxy likely exceeded the ice line temperature (T_Ice ~ 150-170 K) during formation for a time comparable to the planet formation timescale. The protoplanetary disks within these systems will thus not have an ice line, decreasing the total material available for building protoplanetary embryos, inhibiting the formation of gas- and ice-giants if they form by core accretion, and prohibiting habitability. Planet formation by gravitational instability is similarly suppressed because Toomre's Q > 1 in all but the most massive disks. I discuss these results in the context of the observed lack of planets in 47 Tuc. I predict that a similar search for planets in the globular cluster NGC 6366 ([Fe/H] = -0.82) should yield detections, whereas (counterintuitively) the relatively metal-rich globular clusters NGC 6440, 6441, and 6388 should be devoid of giant planets. The characteristic stellar surface density above which T_Ice is exceeded in star clusters is ~6 x 10^3 M_sun/pc^2 f_{dg, MW}^{-1/2}, where f_{dg, MW} is the dust-to-gas ratio of the embedding material, normalized to the Milky Way value. Simple estimates suggest that ~5 - 50% of the stars in the universe formed in an environment exceeding this surface density. Caveats and uncertainties are detailed.