Kepler-4b: Hot Neptune-Like Planet of a G0 Star Near Main-Sequence Turnoff

TL;DR

This paper reports the discovery and characterization of Kepler-4b, a hot Neptune-like exoplanet orbiting a near turnoff G0 star, with detailed measurements of its mass, radius, and host star properties from Kepler data and follow-up spectroscopy.

Contribution

It provides the first detailed characterization of Kepler-4b, including its mass, radius, and host star properties, demonstrating the capability of Kepler to find and analyze small exoplanets around evolving stars.

Findings

Kepler-4b has a mass of 24.5 Earth masses and a radius of 3.99 Earth radii.

The host star is near turnoff from the main sequence, with a mass of about 1.22 solar masses.

The planet's density is approximately 1.9 g/cm^3, slightly denser than Neptune.

Abstract

Early time-series photometry from NASA's Kepler spacecraft has revealed a planet transiting the star we term Kepler-4, at RA = 19h02m27.68s, Dec = +50:08:08.7. The planet has an orbital period of 3.213 days and shows transits with a relative depth of 0.87 x 10^{-3} and a duration of about 3.95 hours. Radial velocity measurements from the Keck HIRES spectrograph show a reflex Doppler signal of 9.3 (+1.1 -1.9) m/s, consistent with a low-eccentricity orbit with the phase expected from the transits. Various tests show no evidence for any companion star near enough to affect the light curve or the radial velocities for this system. From a transit-based estimate of the host star's mean density, combined with analysis of high-resolution spectra, we infer that the host star is near turnoff from the main sequence, with estimated mass and radius of 1.223 (+0.053 -0.091) solar masses and 1.487 (+0.071 -0.084) solar radii. We estimate the planet mass and radius to be 24.5 +/- 3.8 Earth masses and 3.99 +/- 0.21 Earth radii. The planet's density is near 1.9 g/cm^3; it is thus slightly denser and more massive than Neptune, but about the same size.