[Paper Review] Transiting exoplanets from the CoRoT space mission. XI. CoRoT-8b: a hot and dense sub-Saturn around a K1 dwarf

P. Bordé, F. Bouchy+48 more|Scientific Electronic Library Online (São Paulo Research Foundation, Latin American and Caribbean Center on Health Sciences Information, Conselho Nacional de Desenvolvimento Científico e Tecnológico)|Aug 2, 2010

Astro and Planetary Science40 references46 citations

TL;DR

This paper presents CoRoT-8b, a hot, dense sub-Saturn exoplanet orbiting a K1 dwarf star, with a mass of 0.22 M_J and a radius consistent with a high core mass of 47–62.5 M_⊕ in heavy elements (rock or ice), and a low hydrogen-helium envelope (7.4–22.9 M_⊕). The study combines CoRoT photometry, radial velocity measurements, and stellar evolution models to constrain planetary composition, revealing a planet with a metal-rich core similar to HD 149026b, suggesting formation via giant impacts in a metal-rich environment.

ABSTRACT

We report the discovery of CoRoT-8b, a dense small Saturn-class exoplanet that orbits a K1 dwarf in 6.2 days, and we derive its orbital parameters, mass, and radius. We analyzed two complementary data sets: the photometric transit curve of CoRoT-8b as measured by CoRoT and the radial velocity curve of CoRoT-8 as measured by the HARPS spectrometer. We find that CoRoT-8b is on a circular orbit with a semi-major axis of 0.063 +/- 0.001 AU. It has a radius of 0.57 +/- 0.02 RJ, a mass of 0.22 +/- 0.03 MJ, and therefore a mean density 1.6 +/- 0.1 g/cm^3. With 67 % of the size of Saturn and 72 % of its mass, CoRoT-8b has a density comparable to that of Neptune (1.76 g/cm^3). We estimate its content in heavy elements to be 47-63 Earth masses, and the mass of its hydrogen-helium envelope to be 7-23 Earth masses. At 0.063 AU, the thermal loss of hydrogen of CoRoT-8b should be no more than about 0.1 % over an assumed integrated lifetime of 3~Ga.

Motivation & Objective

• To characterize the transiting exoplanet CoRoT-8b detected by the CoRoT space mission.
• To determine the planet's mass, radius, and internal composition using photometric and radial velocity data.
• To infer the planetary core mass and envelope composition by combining stellar evolution models with observational constraints on the host star's density and effective temperature.
• To investigate the formation mechanism of low-envelope, high-core-mass planets like CoRoT-8b in metal-rich systems.
• To assess the age and evolutionary state of the host star and its implications for planetary system evolution.

Proposed method

• Stellar evolution models are computed as a function of mass, metallicity, and age, using constraints from the host star’s observed effective temperature and density.
• A goodness-of-fit metric, $ n_{\sigma_\star} $, is defined as the combined $ \chi^2 $-like statistic from uncertainties in $ T_{\mathrm{eff}} $ and $ \rho_\star $, with acceptable models having $ n_{\sigma_\star} < 3 $.
• Planetary evolution models assume a central core of variable mass (rock or ice) overlain by a solar-composition H-He envelope, with equilibrium temperature fixed at 925 K.
• The total planetary mass is fixed at 0.22 M_J, and the radius evolution is modeled over time to match the inferred stellar age constraints.
• Constraints on core mass are derived from the intersection of planetary radius evolution with the $ n_{\sigma_\star} = 1 $ confidence region of stellar models.
• Chemical diffusion is included in stellar models, which slightly favors younger ages for metal-rich stars, but the effect is small due to low stellar mass.

Experimental results

Research questions

• RQ1 What is the internal composition of CoRoT-8b, particularly the mass of its core and envelope?
• RQ2 How does the host star’s metallicity and age influence the inferred planetary parameters?
• RQ3 Why does CoRoT-8b have a high core mass but a small hydrogen-helium envelope compared to other gas giants?
• RQ4 What formation mechanism could explain the high heavy-element content and low envelope mass in CoRoT-8b?
• RQ5 How well do stellar evolution models with $ n_{\sigma_\star} $ constraints reproduce the observed properties of the host star?

Key findings

• CoRoT-8b has a radius tightly constrained by stellar and planetary models, with a planetary radius consistent with a core mass of 47 to 56 M_⊕ in rocks or 56 to 62.5 M_⊕ in ices.
• The hydrogen-helium envelope mass is estimated at 7.4 M_⊕ (10.6% of total mass) for an icy core or 22.9 M_⊕ (32.8%) for a rocky core, indicating a low envelope fraction.
• The planet’s age is constrained to be below 3 Ga, with the $ n_{\sigma_\star} = 1 $ confidence region favoring younger ages due to metallicity evolution.
• The host star CoRoT-8 has a metallicity of [M/H] = 0, consistent with a metal-rich environment, supporting the trend that metal-rich stars host planets with high core masses.
• The planet’s density and composition are similar to HD 149026b, suggesting a common formation pathway involving giant impacts that stripped the envelope.
• The study confirms that planets with high heavy-element content and low H-He envelopes are more likely to form around metal-rich stars, consistent with theoretical models of core accretion and giant impact scenarios.