Could Tidal Heating Explain the DIfference Between Super Earths and Mini Neptunes?

Tidal Heating of Young Super-Earth Atmospheres

Authors:

Ginzburg et al

Abstract:

Short-period Earth to Neptune size exoplanets (super-Earths) with voluminous gas envelopes seem to be very common. These gas atmospheres are thought to have originated from the protoplanetary disk in which the planets were embedded during their first few Myr. The accretion rate of gas from the surrounding nebula is determined by the ability of the gas to cool and radiate away its gravitational energy. Here we demonstrate that heat from the tidal interaction between the star and the young (and therefore inflated) planet can inhibit the gas cooling and accretion. Quantitatively, we find that the growth of super-Earth atmospheres halts for planets with periods of about 10 days, provided that their initial eccentricities are of the order of 0.2. Thus, tidal heating provides a robust and simple mechanism that can simultaneously explain why these planets did not become gas giants and account for the deficit of low-density planets closer to the star, where the tides are even stronger. We suggest that tidal heating may be as important as other factors (such as the nebula's lifetime and atmosphere evaporation) in shaping the observed super-Earth population.

Exomoon Climate Models with the Carbonate-Silicate Cycle and Viscoelastic Tidal Heating

Exomoon Climate Models with the Carbonate-Silicate Cycle and Viscoelastic Tidal Heating

Authors:

Forgan et al

Abstract:

We presence of tidal heating, eclipses and planetary illumination in the exomoon energy budget combine to produce both circumstellar and circumplanetary habitable regions. Analytical calculations suggest that the circumplanetary habitable region is defined only by an inner edge (with its outer limits determined by orbital stability). Subsequent calculations using 1D latitudinal climate models indicated that the combined effect of eclipses and ice-albedo feedback can produce an outer edge to the circumplanetary habitable zone. But is this outer edge real, or an artefact of the climate model's relative simplicity?

We present an upgraded 1D climate model of Earth-like exomoon climates, containing the carbonate-silicate cycle and viscoelastic tidal heating. We conduct parameter surveys of both the circumstellar and circumplanetary habitable zones, and we find that the outer circumplanetary habitable edge remains provided the moon's orbit is not inclined relative to that of the planet. Adding the carbonate-silicate cycle pushes the circumplanetary habitable zone outward, by allowing increases in atmospheric partial pressure of carbon dioxide to boost the greenhouse effect. Viscoelastic tidal heating widens the habitable zone compared to standard, fixed-Q models. Weakening the tidal heating effect due to melting allows moons to be habitable at higher eccentricity, and pushes the inner circumstellar and circumplanetary habitable zone boundary inward.

Tidal Heating Could Dessicate Habitable Zone Exoplanets Around M Dwarfs

Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions

Authors:

Driscoll et al

Abstract:

The internal thermal and magnetic evolution of rocky exoplanets is critical to their habitability. We focus on the thermal-orbital evolution of Earth-mass planets around low-mass M stars whose radiative habitable zone overlaps with the “tidal zone,” where tidal dissipation is expected to be a significant heat source in the interior. We develop a thermal-orbital evolution model calibrated to Earth that couples tidal dissipation, with a temperature-dependent Maxwell rheology, to orbital circularization and migration. We illustrate thermal-orbital steady states where surface heat flow is balanced by tidal dissipation and cooling can be stalled for billions of years until circularization occurs. Orbital energy dissipated as tidal heat in the interior drives both inward migration and circularization, with a circularization time that is inversely proportional to the dissipation rate. We identify a peak in the internal dissipation rate as the mantle passes through a viscoelastic state at mantle temperatures near 1800 K. Planets orbiting a 0.1 solar-mass star within 0.07 AU circularize before 10 Gyr, independent of initial eccentricity. Once circular, these planets cool monotonically and maintain dynamos similar to that of Earth. Planets forced into eccentric orbits can experience a super-cooling of the core and rapid core solidification, inhibiting dynamo action for planets in the habitable zone. We find that tidal heating is insignificant in the habitable zone around 0.45 (or larger) solar-mass stars because tidal dissipation is a stronger function of orbital distance than stellar mass, and the habitable zone is farther from larger stars. Suppression of the planetary magnetic field exposes the atmosphere to stellar wind erosion and the surface to harmful radiation. In addition to weak magnetic fields, massive melt eruption rates and prolonged magma oceans may render eccentric planets in the habitable zone of low-mass stars inhospitable for life.

A Better Model for Exomoon Tidal Heating

Viscoelastic Models of Tidally Heated Exomoons

Authors:

Dobos et al

Abstract:

Tidal heating of exomoons may play a key role in their habitability, since the elevated temperature can melt the ice on the body even without significant solar radiation. The possibility of life is intensely studied on Solar System moons such as Europa or Enceladus, where the surface ice layer covers tidally heated water ocean. Tidal forces may be even stronger in extrasolar systems, depending on the properties of the moon and its orbit. For studying the tidally heated surface temperature of exomoons, we used a viscoelastic model for the first time. This model is more realistic than the widely used, so-called fixed Q models, because it takes into account the temperature dependency of the tidal heat flux, and the melting of the inner material. With the use of this model we introduced the circumplanetary Tidal Temperate Zone (TTZ), that strongly depends on the orbital period of the moon, and less on its radius. We compared the results with the fixed Q model and investigated the statistical volume of the TTZ using both models. We have found that the viscoelastic model predicts 2.8 times more exomoons in the TTZ with orbital periods between 0.1 and 3.5 days than the fixed Q model for plausible distributions of physical and orbital parameters. The viscoelastic model gives more promising results in terms of habitability, because the inner melting of the body moderates the surface temperature, acting like a thermostat.

ExoPlanetary Habitability Tied to System Dynamics

Dynamics of exoplanetary systems, links to their habitability

Authors:

Bolmont et al

Abstract:

Our knowledge of planets' orbital dynamics, which was based on Solar System studies, has been challenged by the diversity of exoplanetary systems.

Around cool and ultra cool dwarfs, the influence of tides on the orbital and spin evolution of planets can strongly affect their climate and their capacity to host surface liquid water.

We illustrate the role of tides and dynamics with the extreme case of planets orbiting around brown dwarfs. In multiple planet systems, the eccentricity is excited by planet-planet interactions. Planets are therefore heated up from the inside by the tidally-induced friction. This process can heat a habitable zone planet to such a level that surface liquid water cannot exist.

We also talk about the newly discovered potentially habitable Earth-sized planet Kepler-186f. Given the poorly estimated age of the system, the planet could still be evolving towards synchronization and have a high obliquity or be pseudo-synchronized with a zero obliquity. These two configurations would have a different effect on the climate of this planet.

Exoplanets With Large Companions Will Remain Habitable Longer

Having a companion in old age is good for people — and, it turns out, might extend the chance for life on certain Earth-sized planets in the cosmos as well.

Planets cool as they age. Over time their molten cores solidify and inner heat-generating activity dwindles, becoming less able to keep the world habitable by regulating carbon dioxide to prevent runaway heating or cooling.

But astronomers at the University of Washington and the University of Arizona have found that for certain planets about the size of our own, the gravitational pull of an outer companion planet could generate enough heat — through a process called tidal heating — to effectively prevent that internal cooling, and extend the inner world's chance at hosting life.

UW astronomer Rory Barnes is second author of a paper published in the July issue of the Monthly Notices of the Royal Astronomical Society. The lead authors are graduate student Christa Van Laerhoven and planetary scientist Richard Greenberg at the University of Arizona.

Tidal heating results from the gravitational push and pull of the outer companion planet on its closer-in neighbor, Barnes said. The effect happens locally, so to speak, on Jupiter's moons Io and Europa. The researchers showed that this phenomenon can take place on exoplanets — those outside the solar system — as well.

Using computer models, the researchers found the effect can occur on older Earth-sized planets in noncircular orbits in the habitable zone of low-mass stars, or those less than one-quarter the mass of the Sun. The habitable zone is that swath of space around a star just right to allow an orbiting rocky planet to sustain liquid water on its surface, thus giving life a chance.

"When the planet is closer to the star, the gravitational field is stronger and the planet is deformed into an American football shape. When farther from the star, the field is weaker and the planet relaxes into a more spherical shape," Barnes said. "This constant flexing causes layers inside the planet to rub against each other, producing frictional heating."

The outer planet is necessary, Barnes added, to keep the potentially habitable planet's orbit noncircular. When a planet's orbit is circular, the gravitational pull from its host star is constant, so its shape never changes, and there is no tidal heating.

And so, the researchers conclude, any discoveries of Earth-sized planets in the habitable zone of old, small stars should be followed by searches for outer companion planets that might improve the inner world's chance at hosting life.

link.

Tidal Heating of Multilayered Terrestrial Exoplanets

TIDAL HEATING IN MULTILAYERED TERRESTRIAL EXOPLANETS

Authors:

Henning et al

Abstract:

The internal pattern and overall magnitude of tidal heating for spin-synchronous terrestrial exoplanets from 1 to 2.5 RE is investigated using a propagator matrix method for a variety of layer structures. Particular attention is paid to ice-silicate hybrid super-Earths, where a significant ice mantle is modeled to rest atop an iron-silicate core, and may or may not contain a liquid water ocean. We find multilayer modeling often increases tidal dissipation relative to a homogeneous model, across multiple orbital periods, due to the ability to include smaller volume low viscosity regions, and the added flexure allowed by liquid layers. Gradations in parameters with depth are explored, such as allowed by the Preliminary Earth Reference Model. For ice-silicate hybrid worlds, dramatically greater dissipation is possible beyond the case of a silicate mantle only, allowing non-negligible tidal activity to extend to greater orbital periods than previously predicted. Surface patterns of tidal heating are found to potentially be useful for distinguishing internal structure. The influence of ice mantle depth and water ocean size and position are shown for a range of forcing frequencies. Rates of orbital circularization are found to be 10-100 times faster than standard predictions for Earth-analog planets when interiors are moderately warmer than the modern Earth, as well as for a diverse range of ice-silicate hybrid super-Earths. Circularization rates are shown to be significantly longer for planets with layers equivalent to an ocean-free modern Earth, as well as for planets with high fractions of either ice or silicate melting.