Possible Solutions to the Radius Anomalies of Transiting Giant Planets

Abstract

We calculate the theoretical evolution of the radii of all fourteen of the known transiting extrasolar giant planets (EGPs) for a variety of assumptions concerning atmospheric opacity, dense inner core masses, and possible internal power sources. We incorporate the effects of stellar irradiation and customize such effects for each EGP and star. Looking collectively at the family as a whole, we find that there are in fact two radius anomalies to be explained. Not only are the radii of a subset of the known transiting EGPs larger than expected from previous theory, but many of the other objects are smaller than the default theory would allow. We suggest that the larger EGPs can be explained by invoking enhanced atmospheric opacities that naturally retain internal heat. This explanation might obviate the necessity for an extra internal power source. We explain the smaller radii by the presence in perhaps all the known transiting EGPs of dense cores, such as have been inferred for Saturn and Jupiter. Importantly, we derive a rough correlation between the masses of our "best-fit" cores and the stellar metallicity that seems to buttress the core-accretion model of their formation. Though many caveats and uncertainties remain, the resulting comprehensive theory that incorporates enhanced-opacity atmospheres and dense cores is in reasonable accord with all the current structural data for the known transiting giant planets.Comment: 22 pages in emulateapj format, including 10 figures (mostly in color), accepted to the Astrophysical Journal (February 9, 2007); to appear in volume 661, June 200

Exoplanet Observations

“…Another interesting result in this area is that the heavy element abundances of transiting planets and their host stars seem to be correlated [22,137,138]. However, what is seen is not a simple linear style relationship, but an increase in the upper limit of the core mass distribution toward higher host star metallicities.…”

The EChO science case

“…On a large scale, the IR transit and eclipse spectra of hot-Jupiters seem to be dominated by the signature of water vapour (e.g. [10,21,26,34,38,47,48,50,54,74,91,111,[159][160][161][168][169][170][171]), similarly, the atmosphere of hot-Neptune HAT-P-11b appears to be water-rich [67]. The data available for other warm Neptunes, such as GJ 436b, GJ 3470b are suggestive of cloudy atmospheres and do not always allow a conclusive identification of their composition [22,65,70,88,117,156].…”

DISCOVERING THE GROWTH HISTORIES OF EXOPLANETS: THE SATURN ANALOG HD 149026b

“…Using nongray atmosphere models and highpressure equations of state, Fortney et al (2006) find that the HD 149026b's total heavy element abundance (core and envelope) could be anywhere between 60 and 93 M ⊕ . Exploring the possibility of heavy element-enriched planet atmospheres with metal abundances up to 10× Solar, Burrows et al (2007) derived a higher core mass of 80-110 M ⊕ . Finally, Carter et al (2009) used the grids of planetary cooling and contraction models calculated by Sato et al (2005) and Fortney et al (2007) in combination with the larger NICMOS-measured planet radius to derive a core mass of 45-70 M ⊕ .…”

“…Statistical analyses of the exoplanet population have shown that the probability of planet detection increases with stellar iron, silicon and nickel abundance Robinson et al 2006). Furthermore, the models of Guillot et al (2006) and Burrows et al (2007) suggest that planets with large amounts of heavy elements tend to be associated with metal-rich stars. We therefore expect any Saturn-like planet orbiting HD 149026 to contain a higher proportion of solids than Saturn itself.…”