Keywords

We apply a combination of N-body modeling techniques and automated data fitting with Monte Carlo Markov Chain uncertainty analysis of Keplerian orbital models to RV data to determine long-term stability of the planetary system GJ 581. We find that while there are stability concerns with the four-planet model as published by Forveille et al., when uncertainties in the system are accounted for, particularly stellar jitter, the hypothesis that the four-planet model is gravitationally unstable is not statistically significant. Additionally, the system including proposed planet g by Vogt et al. also shows some stability concerns when eccentricities are allowed to float in the orbital fit, yet when uncertainties are included in the analysis, the system including planet g also cannot be proven to be unstable. We present revised reduced χ² values for Keplerian astrocentric orbital fits assuming four-planet and five-planet models for GJ 581 under the condition that best fits must be stable, and we find no distinguishable difference by including planet g in the model. Additionally, we present revised orbital element estimates for each, assuming uncertainties due to stellar jitter under the constraint of the system being gravitationally stable.

The GJ 581 system has been amply studied since its discovery in 2005: the number of known planets in the system has increased and their orbital parameters are among the most precisely determined for radial-velocity-detected exoplanets. We have acquired MOST space-based photometry during 2007 and 2009, with the aims of measuring the stellar variability and searching for transits of GJ 581e, respectively. We quantify our sensitivity to shallow transit signals using Monte Carlo simulations, and perform a transit search within the 3σ transit windows corresponding to both the circular and Keplerian orbit ephemerides. Our analysis rules out transits for a planet with an orbital period of 3.15 days (GJ 581e) having a radius larger than 1.62 R_⊕ (or a density lower than 2.39 g cm⁻³ for an orbital inclination of 90°) to 2σ confidence. Thus, if the planet transits, we can exclude hydrogen, helium, and water theoretical model compositions. The MOST photometry also allows us to rule out transits of GJ 581b within the Keplerian orbit-derived transit window for impact parameter values smaller than ∼0.4 and confirm previous results which exclude transits for this planet within the circular orbit-derived transit window, for all plausible interior compositions. We find that the stellar brightness of GJ 581 is stable to within 1%, a characteristic which is favorable to the development of life in the habitable zone of the system. In the 2009 photometry, we detect a stellar signal with a period of 5.586 ± 0.051 days, which is close to the orbital period of GJ 581b (P = 5.37 days). However, further monitoring of the system is necessary to verify the nature of this variation.

GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be 0.299 ± 0.010 R_☉. Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581's effective surface temperature T_EFF = 3498 ±  56 K and its luminosity L = 0.01205 ±  0.00024 L_☉. From these measurements, we recompute the location and extent of the system's habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.

The habitable zones (HZs) of main-sequence stars have traditionally been defined as the range of orbits that intercept the appropriate amount of stellar flux to permit surface water on a planet. Terrestrial exoplanets discovered to orbit M stars in these zones, which are close-in due to decreased stellar luminosity, may also undergo significant tidal heating. Tidal heating may span a wide range for terrestrial exoplanets and may significantly affect conditions near the surface. For example, if heating rates on an exoplanet are near or greater than that on Io (where tides drive volcanism that resurfaces the planet at least every 1 Myr) and produce similar surface conditions, then the development of life seems unlikely. On the other hand, if the tidal heating rate is less than the minimum to initiate plate tectonics, then CO₂ may not be recycled through subduction, leading to a runaway greenhouse that sterilizes the planet. These two cases represent potential boundaries to habitability and are presented along with the range of the traditional HZ for main-sequence, low-mass stars. We propose a revised HZ that incorporates both stellar insolation and tidal heating. We apply these criteria to GJ 581 d and find that it is in the traditional HZ, but its tidal heating alone may be insufficient for plate tectonics.