Although several thousands of exoplanets have now been detected and characterized , observational biases have led to a paucity of long-period , low-mass exoplanets with measured masses and a corresponding lag in our understanding of such planets .
In this paper we report the mass estimation and characterization of the long-period exoplanet Kepler-538b .
This planet orbits a Sun-like star ( V = 11.27 ) with M _ { * } = 0.892 ^ { +0.051 } _ { -0.035 } M _ { \odot } and R _ { * } = 0.8717 ^ { +0.0064 } _ { -0.0061 } R _ { \odot } .
Kepler-538b is a 2.215 ^ { +0.040 } _ { -0.034 } R _ { \oplus } sub-Neptune with a period of P = 81.73778 \pm 0.00013 d. It is the only known planet in the system .
We collected radial velocity ( RV ) observations with HIRES on Keck I and HARPS-N on the TNG .
We characterized stellar activity by a Gaussian process with a quasi-periodic kernel applied to our RV and cross correlation function full width at half maximum ( FWHM ) observations .
By simultaneously modeling Kepler photometry , RV , and FWHM observations , we found a semi-amplitude of K = 1.68 ^ { +0.39 } _ { -0.38 } m s ^ { -1 } and a planet mass of M _ { p } = 10.6 ^ { +2.5 } _ { -2.4 } M _ { \oplus } .
Kepler-538b is the smallest planet beyond P = 50 d with an RV mass measurement .
The planet likely consists of a significant fraction of ices ( dominated by water ice ) , in addition to rocks/metals , and a small amount of gas .
Sophisticated modeling techniques such as those used in this paper , combined with future spectrographs with ultra high-precision and stability will be vital for yielding more mass measurements in this poorly understood exoplanet regime .
This in turn will improve our understanding of the relationship between planet composition and insolation flux and how the rocky to gaseous transition depends on planetary equilibrium temperature .