CaltechAUTHORS
Published April 20, 2025 | Version Published
Journal Article Open

TOI-6324 b: An Earth-mass Ultra-short-period Planet Transiting a Nearby M Dwarf

Creators

  • 1. ROR icon University of Hawaii at Manoa
  • 2. ROR icon California Institute of Technology
  • 3. ROR icon Jet Propulsion Lab
  • 4. ROR icon University of Amsterdam
  • 5. ROR icon University of California, Los Angeles
  • 6. ROR icon University of California, Berkeley
  • 7. ROR icon University of Southern Queensland
  • 8. ROR icon University of Notre Dame
  • 9. ROR icon University of Chicago
  • 10. ROR icon University of Toronto
  • 11. ROR icon University of California, Irvine
  • 12. Flatiron Institute
  • 13. ROR icon Infrared Processing and Analysis Center
  • 14. ROR icon Massachusetts Institute of Technology
  • 15. ROR icon W.M. Keck Observatory
  • 16. ROR icon Harvard-Smithsonian Center for Astrophysics
  • 17. ROR icon University of Kansas
  • 18. ROR icon University of California, Santa Cruz
  • 19. ROR icon University of Geneva
  • 20. ROR icon Ames Research Center
  • 21. ROR icon Yale University
  • 22. ROR icon Lowell Observatory
  • 23. Schmidt Sciences
  • 24. ROR icon Goddard Space Flight Center
  • 25. ROR icon Macquarie University
  • 26. ROR icon Carnegie Institution for Science
  • 27. ROR icon Tsinghua University
  • 28. ROR icon Princeton University

Abstract

We report the confirmation of TOI-6324 b, an Earth-sized (1.059 ± 0.041 R⊕) ultra-short-period (USP) planet orbiting a nearby (∼20 pc) M dwarf. Using the newly commissioned Keck Planet Finder spectrograph, we have measured the mass of TOI-6324 b 1.17 ± 0.22 M⊕. Because of its extremely short orbit of just ∼6.7 hr, TOI-6324 b is intensely irradiated by its M dwarf host and is expected to be stripped of any thick H/He envelope. We were able to constrain its interior composition and found an iron-core mass fraction (CMF = 27% ± 37%) consistent with that of Earth (∼33%) and other confirmed USPs. TOI-6324 b is the closest to an Earth-sized USP confirmed to date. TOI-6324 b is a promising target for JWST phase-curve and secondary eclipse observations (emission spectroscopy metric = 25), which may reveal its surface mineralogy, day–night temperature contrast, and possible tidal deformation. From seven sectors of TESS data, we report a tentative detection of the optical phase-curve variation with an amplitude of 42 ± 28 ppm.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society.

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

R.A.L. acknowledges this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under grant No. 1842402 and grant No. 2236415. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

We thank Ellen Price for helpful discussions about tidal distortion.

A NASA Key Strategic Mission Support titled "Pinning Down Masses of JWST Ultra-short-period Planets with KPF" (PI: F. Dai) provided the telescope access and funding for the completion of this project.

This work was supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. Data presented herein were obtained at the W. M. Keck Observatory from telescope time allocated to the National Aeronautics and Space Administration through the agency's scientific partnership with the California Institute of Technology and the University of California. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

D.R.C. acknowledges partial support from NASA Grant 18-2XRP18_2-0007.

J.M.J.O. acknowledges support from NASA through the NASA Hubble Fellowship grant HST-HF2-51517.001, awarded by STScI. STScI is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

L.M.W. acknowledges support from the NASA Exoplanet Research Program (grant No. 80NSSC23K0269).

N.S. acknowledges support by the National Science Foundation Graduate Research Fellowship Program under grant Nos. 1842402 and 2236415 and the National Aeronautics and Space Administration (80NSSC21K0652).

This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).

This research was carried out, in part, at the Jet Propulsion Laboratory and the California Institute of Technology under a contract with the National Aeronautics and Space Administration and funded through the President's and Director's Research & Development Fund Program.

This research has made use of the Exoplanet Follow-up Observation Program (NexScI 2022) website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.

We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Resources supporting this work were provided by the NASA HighEnd Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products.

This Letter made use of data collected by the TESS mission and are publicly available from the Mikulski Archive for Space Telescopes (MAST) operated by the Space Telescope Science Institute (STScI). Funding for the TESS mission is provided by NASA's Science Mission Directorate. K.A.C. and C.N.W. acknowledge support from the TESS mission via subaward s3449 from MIT.

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Data Availability

All the TESS data used in this Letter can be found in MAST (MAST Team 2021). This work uses data supplied from the NASA Exoplanet Archive (ExoFOP 2019).

Facilities

Keck I/KPF, Keck I/HIRES, Keck II/NIRC2, TESS. -

Software References

arviz (R. Kumar et al. 2019), astropy (Astropy Collaboration et al. 2013, 2018), batman (L. Kreidberg 2015), dynesty (J. S. Speagle 2020; S. Koposov et al. 2024), emcee (D. Foreman-Mackey et al. 2013), exopie (M. Plotnykov & D. Valencia 2024), exoplanet (D. Foreman-Mackey et al. 2021b, 2024), isoclassify (D. Huber et al. 2017), Lightkurve (Lightkurve Collaboration et al. 2018), lmfit (M. Newville et al. 2014), Manipulate Planet (L. Zeng & D. Sasselov 2013; L. Zeng et al. 2016), MESA Isochrones and Stellar Tracks (MIST; J. Choi et al. 2016), pyMC3 (J. Salvatier et al. 2016), radvel (B. J. Fulton et al. 2018), SpecMatch-Emp (S. W. Yee et al. 2017), starry (R. Luger et al. 2019), superearth (D. Valencia et al. 2006), theano (Theano Development Team et al. 2016), wo¯tan (M. Hippke et al. 2019).

Files

Lee_2025_ApJL_983_L36.pdf

Files (1.4 MB)

Name Size Download all
md5:9f37350657c826f0e5ea72e62956c179
1.4 MB Preview Download

Additional details

Related works

Is new version of
Discussion Paper: arXiv:2502.16087 (arXiv)
Is supplemented by
Dataset: 10.17909/t9-nmc8-f686 (DOI)
Dataset: 10.26134/EXOFOP3 (DOI)

Funding

National Science Foundation
1842402
National Science Foundation
2236415
National Aeronautics and Space Administration
18-2XRP18_2-0007
National Aeronautics and Space Administration
HST-HF2-51517.001
Space Telescope Science Institute
National Aeronautics and Space Administration
NAS5-26555
National Aeronautics and Space Administration
80NSSC23K0269
National Aeronautics and Space Administration
80NSSC21K0652
National Aeronautics and Space Administration
80NM0018D0004
Jet Propulsion Laboratory
Massachusetts Institute of Technology
s3449
W. M. Keck Foundation

Dates

Accepted
2025-01-05
Available
2025-04-16
Published

Caltech Custom Metadata

Caltech groups
Astronomy Department, Infrared Processing and Analysis Center (IPAC), Division of Geological and Planetary Sciences (GPS), Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published