Extrasolar Carbon Planets

TL;DR

This paper proposes a new class of extrasolar planets rich in carbon compounds like silicon carbide, formed through a specific disk composition enhancement, with distinct spectral and surface characteristics.

Contribution

It introduces the concept of carbon-rich extrasolar planets formed from silicon carbide and other compounds, expanding planetary formation models.

Findings

Carbon planets form with a C/O ratio slightly above solar.

Spectra of hot carbon planets show low water vapor.

Cooler carbon planets may have hydrocarbon-rich surfaces.

Abstract

We suggest that some extrasolar planets <~ 60 Earth masses will form substantially from silicon carbide and other carbon compounds. Pulsar planets and low-mass white dwarf planets are especially good candidate members of this new class of planets, but these objects could also conceivably form around stars like the Sun. This planet-formation pathway requires only a factor of two local enhancement of the protoplanetary disk's C/O ratio above solar, a condition that pileups of carbonaceous grains may create in ordinary protoplanetary disks. Hot, Neptune-mass carbon planets should show a significant paucity of water vapor in their spectra compared to hot planets with solar abundances. Cooler, less massive carbon planets may show hydrocarbon-rich spectra and tar-covered surfaces. The high sublimation temperatures of diamond, SiC, and other carbon compounds could protect these planets from carbon depletion at high temperatures.