# GJ 367b: A dense ultra-short period sub-Earth planet transiting a nearby red dwarf star

**Authors:** Kristine W. F. Lam, Szilárd Csizmadia, Nicola Astudillo-Defru, Xavier Bonfils, Davide Gandolfi, Sebastiano Padovan, Massimiliano Esposito, Coel Hellier, Teruyuki Hirano, John Livingston, Felipe Murgas, Alexis M. S. Smith, Karen A. Collins, Savita Mathur, Rafael A. Garcia, Steve B. Howell, Nuno C. Santos, Fei Dai, George R. Ricker, Roland Vanderspek  
**arXiv:** [2112.01309](https://arxiv.org/abs/2112.01309)  
**Date:** 2021-12-02  
**Summary page:** https://www.summarizepaper.com/en/paper/2112.01309v1/

## Key points

- Ultra-short-period (USP) exoplanets provide insights into planet formation and evolution
- GJ 367b is a USP planet with a period of 7.7 hours
- GJ 367b has a radius of $0.718 \pm 0.054$ Earth-radii and a mass of $0.546 \pm 0.078$ Earth-masses
- GJ 367b has a bulk density similar to that of iron, with an iron core radius fraction of $86 \pm 5\%$
- GJ 367b may have formed through collisions between protoplanets or accretion from metal-rich material, similar to Mercury's formation process
- The discovery of GJ 367b adds to our understanding of planetary systems and their formation mechanisms, particularly for USP planets with extreme conditions compared to those found in our own solar system
- Further studies on USP planets will continue to shed light on the processes involved in their formation and evolution, leading to a better understanding of how planets form throughout the universe

## Summary

Ultra-short-period (USP) exoplanets, which have orbital periods shorter than one day, provide valuable insights into the formation and evolution of planets. In a recent study, high precision photometry and radial velocity observations were used to detect and characterize the USP planet GJ 367b. The planet orbits a nearby red dwarf star every 7.7 hours and has a radius of $0.718 \pm 0.054$ Earth-radii, making it a sub-Earth. Its mass is $0.546 \pm 0.078$ Earth-masses, resulting in a bulk density of $8.106 \pm 2.165$ g cm$^-3$, similar to that of iron. An interior structure model predicts that GJ 367b has an iron core radius fraction of $86 \pm 5\%$, which is comparable to Mercury's interior structure. This finding suggests that GJ 367b may have formed through a similar process as Mercury, possibly involving collisions between protoplanets or accretion from metal-rich material. The discovery of GJ 367b adds to our understanding of the diversity of planetary systems and their formation mechanisms, particularly for USP planets like this one with extreme conditions compared to those found in our own solar system. Further studies on USP planets will continue to shed light on the processes involved in their formation and evolution, ultimately leading to a better understanding of how planets form throughout the universe.