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D. Valencia, R. J. O'Connell (Earth and Planetary Sciences, Harvard University), D. D. Sasselov (Harvard-Smithsonian Center for Astrophysics)
In June of 2005 the first Super-Earth was discovered orbiting an M4V star with a minimum mass of 6 times the mass of the Earth and a period of 1.94 days. The composition of this planet is unknown; we may assume that it does not differ too much from an Earth-like composition. In such close proximity to its star the surface temperature could range between 460 and 650 K. Additionally, tidal effects can be expected to be large in planets with such low period orbits and tidal heating can affect the thermal state of a planet.
Our work focuses on computing the internal structure of terrestrial planets with masses from 1 to 10 Earth-masses (Super-Earths). We derive scaling relationships with mass, internal structure parameters and thermal state . We obtain a scaling relationship for the radius of R~ M(0.265-0.271) and average density ~ M(~0.2) for planets with a composition and temperature structure similar to Earth's. Tidal heating and surface temperature have little effect in the scaling relationships compared to pressure effects. Using parameterized convection calculations, we find that the internal temperature beneath the boundary layer depends very weakly on mass. We determine how the surface temperature might extend the existence of a magma ocean in the early evolution.
Liquid water can exist at temperatures above T=400K and high pressures (above 10 MPa) allowing for the possibility of a water layer on top of a rocky core for some planets in close orbits. We explore the effects of a water/icy layer above a rocky core as well as other types of compositions in determining the internal structure and scaling relations. The pressure regime will determine the coefficient in the exponent of R~M(\beta). A planet with the same mass as Earth and 20% of its mass as an ice layer would have a radius about 1.15 times larger than Earth's.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.