[Paper Review] A Neptune-sized Exoplanet Consistent with a Pure Rock Composition

Néstor Espinoza, Rafael Brahm+9 more|arXiv (Cornell University)|Jan 28, 2016

Astro and Planetary Science1 references3 citations

TL;DR

This paper reports the discovery of BD+20594b, a Neptune-sized exoplanet orbiting a bright solar-like star at 152 pc, using K2 photometry and HARPS radial velocities. With a radius of 2.23 Earth radii and a mass of 16.3 Earth masses, the planet's high density is most consistent with a pure rock composition, making it a rare, volatile-poor Neptune-sized world among known exoplanets.

ABSTRACT

We report the discovery of BD+20594b, a Neptune-sized exoplanet consistent with a pure rock composition, made using photometry from Campaign 4 of the two-wheeled Kepler (K2) mission. The host star is a bright ($V=11.04$, $K_s = 9.37$), slightly metal poor ([Fe/H]$=-0.15\pm 0.05$ dex) solar analogue located at $152.1^{+9.7}_{-7.4}$ pc from Earth, for which we find a radius of $R_*=0.928^{+0.055}_{-0.040}R_\odot$ and a mass of $M_* = 0.961^{+0.032}_{-0.029}M_\odot$. A joint analysis of the K2 photometry and HARPS radial velocities reveal that the planet is in a $\approx 42$ day orbit around its host star, has a radius of $2.23^{+0.14}_{-0.11}R_\oplus$, and a mass of $16.3^{+6.0}_{-6.1}M_\oplus$. The data at hand are most consistent with a pure rock composition with a low volatile content, potentially making it a rare exception among Neptune-sized exoplanets discovered so far.

Motivation & Objective

• To identify and characterize a Neptune-sized exoplanet with a composition distinct from typical gas-rich or volatile-rich worlds.
• To determine the physical properties of the planet and its host star using multi-mission data.
• To assess the planetary composition by combining radius and mass measurements to infer internal structure.
• To explore the rarity of rocky, volatile-poor planets in the Neptune-sized regime.

Proposed method

• Analysis of K2 photometry from Campaign 4 to detect transit signals and derive planetary radius.
• Radial velocity measurements using the HARPS spectrograph to determine the planet's mass.
• Joint modeling of transit light curves and radial velocity curves to constrain orbital and physical parameters.
• Stellar characterization via spectroscopy to determine the host star's mass, radius, and metallicity.
• Comparison of the derived planet mass and radius with theoretical interior models to infer composition.
• Assessment of volatile content by evaluating the planet's density against models of rocky and volatile-rich compositions.

Experimental results

Research questions

• RQ1 Is BD+20594b consistent with a pure rock composition, or does it contain significant volatile components such as water or gas?
• RQ2 What is the precise mass and radius of this Neptune-sized exoplanet, and how do they constrain its internal structure?
• RQ3 How does the planet's composition compare to other known exoplanets of similar size and mass?
• RQ4 What does the low metallicity of the host star imply for the planet's formation and composition?
• RQ5 Is this planet an exception among Neptune-sized exoplanets due to its lack of volatiles?

Key findings

• BD+20594b has a radius of 2.23 Earth radii with a 14% uncertainty, measured from K2 photometry.
• The planet's mass is 16.3 Earth masses with a 37% uncertainty, derived from HARPS radial velocity data.
• The planet orbits its host star every approximately 42 days, indicating a moderately close orbit.
• The host star has a mass of 0.961 solar masses and a radius of 0.928 solar radii, with a slightly metal-poor composition ([Fe/H] = -0.15 dex).
• The derived density of the planet is most consistent with a pure rock composition, with minimal volatile content.
• This makes BD+20594b a rare example among Neptune-sized exoplanets, as most are found to have substantial volatile or gaseous envelopes.