A Possible Carbon-rich Interior in Super-Earth 55 Cancri e

Abstract: Terrestrial planets in the solar system, such as the Earth, are oxygen-rich, with silicates and iron being the most common minerals in their interiors. However, the true chemical diversity of rocky planets orbiting other stars is yet unknown. Mass and radius measurements are used to constrain the interior compositions of super-Earths (exoplanets with masses of 1 - 10 Earth masses), and are typically interpreted with planetary interior models that assume Earth-centric oxygen-rich compositions. Using such models, the super-Earth 55 Cancri e (mass of 8 Earth masses, radius of 2 Earth radii) has been suggested to bear an interior composition consisting of Fe, silicates, and an envelope (>= 10% by mass) of super-critical water. We report that the mass and radius of 55 Cancri e can also be explained by a carbon-rich solid interior made of Fe, C, SiC, and/or silicates and without a volatile envelope. While the data allow Fe mass fractions of up to 40%, a wide range of C, SiC and/or silicate mass fractions are possible. A carbon-rich 55 Cancri e is also plausible if its protoplanetary disk bore the same composition as its host star, which has been reported to be carbon-rich. However, more precise estimates of the stellar elemental abundances and observations of the planetary atmosphere are required to further constrain its interior composition. The possibility of a C-rich interior in 55 Cancri e opens a new regime of geochemistry and geophysics in extraterrestrial rocky planets, compared to terrestrial planets in the solar system.

• The paper models 55 Cancri e's interior using precise mass and radius data to propose a carbon-rich composition.
• It explores interior models incorporating iron, silicon carbide, and carbon to explain diverse mass-radius relationships.
• Results imply that a carbon-rich interior could fundamentally alter our understanding of rocky exoplanet geophysics.

A Possible Carbon-Rich Interior in Super-Earth 55 Cancri e

The composition of rocky exoplanets, often referred to as super-Earths, remains largely speculative due to the often limited observational data available. This paper investigates the potential for a carbon-rich interior in the super-Earth 55 Cancri e, presenting an alternative perspective to the traditionally Earth-centric oxygen-rich model typically employed in planetary interior studies.

Background and Rationale

55 Cancri e is a transiting super-Earth orbiting a G8V star in a five-planet system. Precise measurements of both its mass (8.37 M$_\oplus$) and radius (2.04 R$_\oplus$) have been obtained, providing an opportunity to constrain its interior composition. Traditionally, the planet has been considered to have a composition similar to that of terrestrial planets in the solar system—dominated by iron, silicates, and a substantial envelope of supercritical water. However, given the carbon-rich nature of its host star, the authors propose that the interior may be constituted of carbon-rich materials, specifically iron, silicon carbide (SiC), and carbon, without the presence of a volatile envelope.

Methodology

The authors employ planetary interior models that simulate the potential compositions of 55 Cancri e, using its known mass and radius as constraints. Two classes of models are explored: one composed of iron, SiC, and carbon, and another of iron, MgSiO$_3$, and carbon. The models solve equations governing the internal structure of solid planets, akin to those used for silicate and iron-rich Earth-like compositions but adapted for carbon-rich conditions.

Findings

1. Interior Compositions:

• The visible radius data allows a broad range of carbon-rich interior configurations. For example, with iron mass fractions up to 41%, the remaining composition could alternate among SiC or carbon configurations.
• Considering the gray radius, including potential atmospheric contributions, models suggest a reduced iron fraction that demands more carbon and SiC content.

2. Mass-Radius Relations:

• Mass-radius relations for homogeneously composed planets demonstrate that a carbon and SiC centers can explain the radius of 55 Cancri e without necessitating an atmospheric envelope.
• Depending on the assumptions regarding interior composition, significant variation in theoretical mass-radius relations arises, offering a renewed perspective on super-Earth characterizations.

Implications and Future Work

The concept of a carbon-rich interior augments our understanding of potential planetary compositions that diverge markedly from Earth's. Should 55 Cancri e possess such an interior, this would offer insights into geophysical processes significantly different from those observed on Earth, reflecting the high thermal conductivities characteristic of carbon and SiC.

Further scrutiny of this hypothesis will require a more meticulous determination of elemental abundances in 55 Cancri e’s host star, which would refine the understanding of its primordial protoplanetary disk. Additionally, future spectroscopic observations could ascertain whether gas-phase absorption lines influence the planet's gray radius, indicating the presence of an atmosphere.

Conclusion

This exploration into the potential carbon-rich nature of 55 Cancri e elucidates new possibilities within exoplanet science, emphasizing the diversity and complexity of planetary systems outside our own. This study beckons further observational and theoretical investigations that could redefine existing paradigms surrounding the formation and evolutionary processes of rocky planets in the universe.