Nicole Thom

Date of Award

January 2012

Document Type


Degree Name

Master of Science (MS)


Space Studies

First Advisor

Michael Gaffey


The distributions of the semi-major axes and masses of close-in planets (planets within 0.1 AU of their host stars) provide clues to their origin. Over billions of years, the mass and orbital distance are constantly evolving. Tidal forces and atmospheric mass loss, driven by stellar ultraviolet flux, can influence the observed planetary distribution. Coupling these effects can lead to a greater understanding of how the observed distribution was shaped and may help to explain the gaps in the distribution of mass and semi-major axes for these close-in planets.

To study the effects of mass loss and tides, we applied a numerical model to many hypothetical populations of close-in planets and compared these hypothetical populations to the observed population. The evolutionary paths determined by the model depend on two as of yet poorly-constrained factors: the tidal dissipation factor (Q*) and the heating efficiency (ε). By statistically comparing the observed distribution of close-in exoplanets with the hypothetical population, modeled under different ε and Q'* conditions, these values are tested. Under all conditions the two populations were statistically dissimilar, indicating that the population was either not initially evenly distributed, or that there is another important factor in planetary evolution.