Where are the Very Short Period hot Neptunes?

The dearth of short-period Neptunian exoplanets - a desert in the period-mass/radius planes

Authors:

Mazeh et al

Abstract:

A few studies have noticed a significant dearth of Neptune-mass/radius exoplanets with orbital periods below 2--4 d. This cannot be explained by observational biases, as many Neptunian planets with longer orbital periods have been detected. The existence of this "desert" is similar to the appearance of the "brown-dwarf desert", which suggests different formation mechanisms of planets and stellar companions with short orbital periods. Similarly, the Neptunian desert could indicate different mechanisms of formation and evolution for hot Jupiters and short-period super-Earths. As done by Szab\'o & Kiss (2011), we study here the location and shape of the desert in both the period-mass and the period-radius planes, using the presently available large samples of planets. The desert in the period-mass plane has a relatively sharp upper edge, with planetary mass that is inversely proportional to the planetary orbital period, while the lower, somewhat blurred, boundary, is along mass that is apparently linearly proportional to the period. The desert in the period-radius plane of the transiting planets is less clear. It seems as if along the upper boundary the radius is inversely proportional to the period to the power of one third, while the lower boundary shows a radius that is proportional to the period to the power of two thirds. The combination of the two upper bounds of the desert, in the period-mass and period-radius planes, yields a planetary mass-radius relation of Rp/RJup≃(1.2±0.3)(Mp/MJup)0.27±0.11 for 0.1≲Mp/MJup≲1. The derived shape of the desert, which might extend up to periods of 5--10 d, could shed some light on the formation and evolution of close-in planets.