Saturn-mass planet in the habitable zone of M4V star HIP 57050 (Gliese 1148)

The Lick-Carnegie Exoplanet Survey: A Saturn-Mass Planet in the Habitable Zone of the Nearby M4V Star HIP 57050
http://arxiv.org/abs/1004.4608

Abstract wrote:

Precision radial velocities from Keck/HIRES reveal a Saturn-mass planet orbiting the nearby M4V star HIP 57050. The planet has a minimum mass of 0.3 Jupiter-mass, an orbital period of 41.4 days, and an orbital eccentricity of 0.31. V-band photometry reveals a clear stellar rotation signature of the host star with a period of 98 days, well separated from the period of the radial velocity variations and reinforcing a Keplerian origin for the observed velocity variations. The orbital period of this planet corresponds to an orbit in the habitable zone of HIP 57050, with an expected planetary temperature of approximately 230 K. The star has a metallicity of [Fe/H] = 0.32+/-0.06 dex, of order twice solar and among the highest metallicity stars in the immediate solar neighborhood. This newly discovered planet provides further support that the well-known planet-metallicity correlation for F, G, and K stars also extends down into the M-dwarf regime. The a priori geometric probability for transits of this planet is only about 1%. However, the expected eclipse depth is ~7%, considerably larger than that yet observed for any transiting planet. Though long on the odds, such a transit is worth pursuing as it would allow for high quality studies of the atmosphere via transmission spectroscopy with HST. At the expected planetary effective temperature, the atmosphere may contain water clouds.

Yeah, cold enoguh to retain liquid water. Would the presence of moons be possible at such tight orbital separation?

The radius of the Hill sphere is probably roughly 20 planetary radii (assuming the radius is somewhere in the Saturn-Jupiter range), so probably prograde stable orbits exist out to roughly 6-7 planetary radii, which is similar to Io's orbit around Jupiter

The paper does mention the prospects for moons, however if the ~0.02% ratio between satellite system mass and planet mass holds for this planet, you'd be looking at something with roughly twice the mass of our moon, which isn't a particularly promising situation for habitability.

Of course, if there do exist large habitable moons, the fact that it would be rotating with respect to the star implies very strong star-induced tides on the moon during its orbit around the planet. Probably large moons orbiting gas giants in M dwarf habitable zones would be very volcanically active.

How severely would the stellar tides affect a moon deeply rooted in the Hill sphere of the planet? Is there some literature on the subject?

Tidal force depends on the gradient of the gravitational field across the object, thus scales as m_star/d³.

So if we scale to the gradient of the Sun's gravitational field at 1 AU:

Sun on Earth = 1
Moon on Earth = 2.2
HIP 57050 on HIP 57050 b = 78
Jupiter on Io = 43000

Bear in mind that Io is mostly locked to Jupiter, with a small discrepancy due to perturbations from the other Galilean satellites inducing a small orbital eccentricity, whereas our hypothetical moon would be rotating with respect to the star, so you are not comparing like with like.

Also it is likely that the proximity of the star would prevent the moon's orbit from completely circularising, leading to Io-style tides induced by the gas giant.

Ah that makes sense. I wasn't sure if the m_star/d³ rule applied inside a planet's hill sphere or not.

Thanks. Clear enough. Much helpful for future "planet-building"

The CARMENES search for exoplanets around M dwarfs. Dynamical characterization of the multiple planet system GJ 1148 and prospects of habitable exomoons around GJ 1148 b
https://arxiv.org/abs/2002.00906

Quote :

Context. GJ 1148 is an M-dwarf star hosting a planetary system composed of two Saturn-mass planets in eccentric orbits with periods of 41.38 and 532.02 days. Aims. We reanalyze the orbital configuration and dynamics of the GJ 1148 multi-planetary system based on new precise radial velocity (RV) measurements taken with CARMENES. Methods. We combined new and archival precise Doppler measurements from CARMENES with those available from HIRES for GJ 1148 and modeled these data with a self-consistent dynamical model. We studied the orbital dynamics of the system using the secular theory and direct N-body integrations. The prospects of potentially habitable moons around GJ 1148 b were examined. Results. The refined dynamical analyses show that the GJ 1148 system is long-term stable in a large phase-space of orbital parameters with an orbital configuration suggesting apsidal alignment, but not in any particular high-order mean-motion resonant commensurability. GJ 1148 b orbits inside the optimistic habitable zone (HZ). We find only a narrow stability region around the planet where exomoons can exist. However, in this stable region exomoons exhibit quick orbital decay due to tidal interaction with the planet. Conclusions. The GJ 1148 planetary system is a very rare M-dwarf planetary system consisting of a pair of gas giants, the inner of which resides in the HZ. We conclude that habitable exomoons around GJ 1148 b are very unlikely to exist.