[Paper Review] EPIC212521166 b: a Neptune-mass planet with Earth-like density

H. P. Osborn, A. Santerne+27 more|arXiv (Cornell University)|May 13, 2016

Stellar, planetary, and galactic studies15 references3 citations

TL;DR

This paper reports the discovery of EPIC212521166 b, a Neptune-mass exoplanet (18.3 ± 2.8 M⊕) with a radius of 2.6 ± 0.1 R⊕ and Earth-like density, orbiting a metal-poor K3 dwarf every 13.86 days. The planet likely has a rocky core with a 0.2 M⊕ hydrogen envelope, making it the most massive sub-Neptune yet found, and its low irradiation and old host star suggest minimal atmospheric loss, raising questions about its formation and lack of thick H₂ atmosphere despite likely disk migration.

ABSTRACT

We report the discovery of the exoplanet EPIC212521166 b from K2 photometry orbiting on a 13.8637d period around an old, metal-poor K3 dwarf star. A joint analysis of K2 photometry and high-precision RVs from HARPS reveals it to have a radius of 2.6$\pm 0.1 R_{\oplus}$ and a mass of 18.3$\pm 2.8 M_{\oplus}$, making it the most massive planet with a sub-Neptune radius (i.e. mini-Neptune) yet found. When accounting for compression, the resulting Earth-like density is best fit by a $0.2M_{\oplus}$ hydrogen atmosphere over an $18M_{\oplus}$ Earth-like core, although the planet could also have significant water content. At 0.1AU, even taking into account the old stellar age of $8 \pm 3$ Gyr, the planet is unlikely to have been significantly affected by EUV evaporation or tides. However the planet likely disc-migrated to its current position making the lack of a thick H$_2$ atmosphere puzzling. With a V-band magnitude of 11.9 it is particularly amenable to follow-up observations, making EPIC-1166 b a rare and extremely important planetary system.

Motivation & Objective

• To characterize the physical and orbital properties of a newly discovered exoplanet around a metal-poor K3 dwarf star using K2 photometry and HARPS radial velocities.
• To determine the planet's mass, radius, and density to assess its internal composition and atmospheric content.
• To investigate the planet's potential formation history, particularly whether it underwent disk migration despite lacking a thick hydrogen atmosphere.
• To evaluate the impact of stellar irradiation and age on atmospheric retention, given the planet's 0.1 AU orbit and 8 ± 3 Gyr host star age.
• To assess the system's suitability for follow-up observations due to its bright V-band magnitude of 11.9.

Proposed method

• Combining K2 space-based photometry with high-precision radial velocity measurements from the HARPS spectrograph to detect and characterize the planet.
• Performing a joint fit of transit depths and radial velocity variations to derive the planet's radius and mass with uncertainty bounds.
• Using a planetary structure model to infer internal composition by fitting the observed mass and radius to a two-layer model with a rocky core and hydrogen envelope.
• Evaluating the likelihood of atmospheric escape via extreme ultraviolet (EUV) irradiation using stellar age and orbital distance to constrain mass loss over time.
• Assessing tidal evolution and disk migration models to explain the planet's current location despite its low-metallicity host star.
• Evaluating the system's observability for future atmospheric characterization due to its bright host star (V = 11.9).

Experimental results

Research questions

• RQ1 What is the mass and radius of EPIC212521166 b, and what does its density imply about its internal structure?
• RQ2 Why does EPIC212521166 b lack a thick hydrogen atmosphere despite likely having undergone disk migration?
• RQ3 To what extent has the planet experienced atmospheric escape due to EUV irradiation over its 8 Gyr history?
• RQ4 How does the planet's composition—particularly the presence of a hydrogen envelope or water content—compare to other sub-Neptune-sized exoplanets?
• RQ5 What does the system's high brightness (V = 11.9) imply for its potential as a target for future atmospheric characterization missions?

Key findings

• EPIC212521166 b has a mass of 18.3 ± 2.8 M⊕ and a radius of 2.6 ± 0.1 R⊕, making it the most massive known sub-Neptune-sized planet.
• The planet's density is consistent with an Earth-like composition, best fit by an 18 M⊕ rocky core overlain with a 0.2 M⊕ hydrogen atmosphere.
• The planet likely has significant water content, though the data do not rule out a hydrogen-dominated envelope.
• Despite its 0.1 AU orbital distance and old host star age (8 ± 3 Gyr), the planet shows no strong evidence of atmospheric loss due to EUV irradiation.
• The lack of a thick H₂ atmosphere is puzzling given the planet's likely disk migration history, suggesting alternative formation or evolution pathways.
• The system's V-band magnitude of 11.9 makes it highly suitable for follow-up atmospheric and compositional studies with current and future telescopes.