Earth-mass exoplanets orbiting close to low-mass parent stars likely have magnetic fields that protect them from stellar radiation, says a duo of astronomers at the University of Washington in Seattle, WA.
This artist’s conception shows a terrestrial exoplanet. Image credit: NASA Ames / JPL-Caltech.
Low-mass M-class stars are among the most common in the Universe. Exoplanets orbiting near such stars are easier for scientists to target for study because when they transit their parent star, they block a larger fraction of the light than if they transited a more massive star. But because such a star is small and dim, its habitable zone also lies relatively close in.
And an exoplanet so close to its star is subject to the star’s gravitational pull, which could cause it to become tidally locked, with the same side forever facing its host star, as the Moon is with the Earth. That same gravitational tug from the star also creates tidally generated heat inside the planet, or tidal heating.
In a study published online in the journal Astrobiology, Dr Peter Driscoll and Dr Rory Barnes sought to determine the fate of such exoplanets across time.
“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,” they said.
The astronomers combined models of orbital interactions and heating with those of thermal evolution of planetary interiors.
“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.”
Their simulations ranged from one stellar mass – stars the size of our Sun – down to about one-tenth of that size.
“There has been a general feeling in the astronomical community that tidally locked planets are unlikely to have protective magnetic fields and therefore are completely at the mercy of their star,” Dr Barnes said. “This research suggests that assumption false.”
“Far from being harmful to a planet’s magnetic field, tidal heating can actually help it along – and in doing so also help the chance for habitability.”
“This is because of the somewhat counterintuitive fact that the more tidal heating a planetary mantle experiences, the better it is at dissipating its heat, thereby cooling the core, which in turn helps create the magnetic field.”
The scientists also found that the tidal heating process is more extreme for planets in the habitable zone around very small stars, or those less than half the mass of the Sun.
For planets in eccentric, or noncircular orbits around such low mass stars, they found that these orbits tend to become more circular during the time of extreme tidal heating. Once that circularization takes place, the planet stops experiencing any tidal heating at all.
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Driscoll P.E. & Barnes R. 2015. Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions. Astrobiology, 15 (9): 739-760; doi: 10.1089/ast.2015.1325
