The rotation of planets hosting atmospheric tides: from Venus to habitable super-earths

The rotation of planets hosting atmospheric tides: from Venus to habitable super-earths

Authors:

Auclair-Desrotour et al

Abstract:

The competition between the torques induced by solid and thermal tides drives the rotational dynamics of Venus-like planets and super-Earths orbiting in the habitable zone of low-mass stars. The tidal responses of the atmosphere and telluric core are related to their respective physical properties and strongly depend on the tidal frequency. The resulting torque determines the possible equilibrium states of the planet's spin. We compute here an analytic expression for the total tidal torque exerted on a Venus-like planet. This expression is used to characterize the equilibrium rotation of the body. Close to the star, the solid tide dominates. Far from it, the thermal tide drives the rotational dynamics of the planet. The transition regime corresponds to the habitable zone, where prograde and retrograde equilibrium states appear. We demonstrate the strong impact of the atmospheric properties and of the rheology of the solid part on the rotational dynamics of Venus-like planets, highlighting the key role played by dissipative mechanisms in the stability of equilibrium configurations.

Retrieval of Planetary Rotation and Albedo From DSCOVR Data for Direct Imaging of Exoplanets

Retrieval of Planetary Rotation and Albedo from DSCOVR data

Authors:

Kane et al

Abstract:

The field of exoplanets has rapidly expanded from the exclusivity of exoplanet detection to include exoplanet characterization. A key step towards this characterization will be retrieval of planetary albedos and rotation rates from highly undersampled imaging data. The Deep Space Climate Observatory (DSCOVR) provides a unique opportunity to test such retrieval methods using high cadence data of the sunlit surface of the Earth. There are two NASA instruments on board DSCOVR that can be used to achieve this task: the NASA instruments Earth Polychromatic Imaging Camera (EPIC) and the National Institute of Standards and Technology Advanced Radiometer (NISTAR). Here we briefly describe the properties of these instruments and the exoplanetary science that can be explored with their data products. These are described within the context of future NASA direct imaging missions for exoplanets.

Equilibrium Rotation of Semiliquid Exoplanets and Satellites

Equilibrium rotation of semiliquid exoplanets and satellites

Author:

Makarov

Abstract:

A wide range of exoplanet and exomoon models are characterized by a finite average rigidity and a viscosity much lower than the typical values for terrestrials. Such semiliquid bodies may or may not have rigid crusts with permanent figures. Unlike planets with solid mantles and Earth-like rheology, semiliquid bodies can be captured into stable pseudosynchronous spin resonance, where the average rate of rotation is higher than the synchronous 1:1 resonance. Two basic conditions are derived for capture of planets with a triaxial figure into pseudosynchronous rotation, one related to the characteristic tidal wave number (the product of the tidal frequency by the Maxwell time), and the other to the orbital eccentricity. If a semiliquid object does not satisfy either of the two conditions, it is captured into the synchronous resonance. For nearly axially symmetric bodies, only the first condition is in place, and the other is much relaxed, so they should predominantly be pseudosynchronous. It is also pointed out that the equilibrium pseudosychronous rotation rate can not reach the widely used asymptotic value from the constant time lag model but is in reality closer to the synchronous spin.

Rotation and Magnetic Field of T Class Brown Dwarf J1047539+212423 Measured

The Rotation Period and Magnetic Field of the T Dwarf 2MASSI J1047539+212423 Measured From Periodic Radio Bursts

Authors:

Williams et al

Abstract:

Periodic radio bursts from very low mass stars and brown dwarfs simultaneously probe their magnetic and rotational properties. The brown dwarf 2MASSI J1047539+212423 (2M 1047+21) is currently the only T dwarf (T6.5) detected at radio wavelengths. Previous observations of this source with the Arecibo observatory revealed intermittent, 100%-polarized radio pulses similar to those detected from other brown dwarfs, but were unable to constrain a pulse periodicity; previous VLA observations detected quiescent emission a factor of ~100 times fainter than the Arecibo pulses but no additional events. Here we present 14 hours of Very Large Array observations of this object that reveal a series of pulses at ~6 GHz with highly variable profiles, showing that the pulsing behavior evolves on time scales that are both long and short compared to the rotation period. We measure a periodicity of ~1.77 hr and identify it with the rotation period. This is just the sixth rotation period measurement in a late T dwarf, and the first obtained in the radio. We detect a pulse at 10 GHz as well, suggesting that the magnetic field strength of 2M 1047+21 reaches at least 3.6 kG. Although this object is the coolest and most rapidly-rotating radio-detected brown dwarf to date, its properties appear continuous with those of other such objects, suggesting that the generation of strong magnetic fields and radio emission may continue to even cooler objects. Further studies of this kind will help to clarify the relationships between mass, age, rotation, and magnetic activity at and beyond the end of the main sequence, where both theories and observational data are currently scarce.

Co-orbital Companions Split Spin-orbit Resonances

Spin-orbit coupling and chaotic rotation for eccentric coorbital bodies

Authors:

Leleu et al

Abstract:

The presence of a co-orbital companion induces the splitting of the well known Keplerian spin-orbit resonances. It leads to chaotic rotation when those resonances overlap.

Exoplanet Rotation has Big Impacts on Oceanic Heat Transport & Habitability

The Importance of Planetary Rotation Period for Ocean Heat Transport

Authors:

Cullum et al

Abstract:

The climate and, hence, potential habitability of a planet crucially depends on how its atmospheric and ocean circulation transports heat from warmer to cooler regions. However, previous studies of planetary climate have concentrated on modeling the dynamics of atmospheres, while dramatically simplifying the treatment of oceans, which neglects or misrepresents the effect of the ocean in the total heat transport. Even the majority of studies with a dynamic ocean have used a simple so-called aquaplanet that has no continental barriers, which is a configuration that dramatically changes the ocean dynamics. Here, the significance of the response of poleward ocean heat transport to planetary rotation period is shown with a simple meridional barrier—the simplest representation of any continental configuration. The poleward ocean heat transport increases significantly as the planetary rotation period is increased. The peak heat transport more than doubles when the rotation period is increased by a factor of ten. There are also significant changes to ocean temperature at depth, with implications for the carbon cycle. There is strong agreement between the model results and a scale analysis of the governing equations. This result highlights the importance of both planetary rotation period and the ocean circulation when considering planetary habitability.

beta Pictoris b's Fast Day

The fast spin-rotation of a young extrasolar planet

Authors:

Snellen et al

Abstract:

The spin-rotation of a planet arises from the accretion of angular momentum during its formation, but the details of this process are still unclear. In the solar system, the equatorial rotation velocities and spin angular momentum of the planets show a clear trend with mass, except for Mercury and Venus which have significantly spun down since their formation due to tidal interactions. Here we report on near-infrared spectroscopic observations at R=100,000 of the young extra-solar gas giant beta Pictoris b. The absorption signal from carbon monoxide in the planet's thermal spectrum is found to be blueshifted with respect to the velocity of the parent star by (-15+-1.7) km/sec, consistent with a circular orbit. The combined line profile exhibits a rotational broadening of 25+-3 km/sec, meaning that Beta Pictoris b spins significantly faster than any planet in the solar system, in line with the extrapolation of the known trend in spin velocity with planet mass.

Habitable Zone & Exoplanet Rotation Rate Interactions

Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate

Authors:

Yang et al

Abstract:

Planetary rotation rate is a key parameter in determining atmospheric circulation and hence the spatial pattern of clouds. Since clouds can exert a dominant control on planetary radiation balance, rotation rate could be critical for determining mean planetary climate. Here we investigate this idea using a three-dimensional general circulation model with a sophisticated cloud scheme. We find that slowly rotating planets (like Venus) can maintain an Earth-like climate at nearly twice the stellar flux as rapidly rotating planets (like Earth). This suggests that many exoplanets previously believed to be too hot may actually be habitable, depending on their rotation rate. The explanation for this behavior is that slowly rotating planets have a weak Coriolis force and long daytime illumination, which promotes strong convergence and convection in the substellar region. This produces a large area of optically thick clouds, which greatly increases the planetary albedo. In contrast, on rapidly rotating planets a much narrower belt of clouds form in the deep tropics, leading to a relatively low albedo. A particularly striking example of the importance of rotation rate suggested by our simulations is that a planet with modern Earth's atmosphere, in Venus' orbit, and with modern Venus' (slow) rotation rate would be habitable. This would imply that if Venus went through a runaway greenhouse, it had a higher rotation rate at that time.