Orbital Eccentricities Suggest a Gradual Transition from Giant Planets to Brown Dwarfs
Creators
-
1.
University of California, Los Angeles
-
2.
California Institute of Technology
Abstract
To date, hundreds of substellar objects with masses between 1 and 80 MJup have been detected orbiting main-sequence stars. The current convention uses the deuterium-burning limit, Mc ≈ 13 MJup to divide this population between giant planets and brown dwarfs. However, this classification heuristic is largely divorced from any formation physics and may not accurately reflect the astrophysical nature of these objects. Previous work has suggested that a transition from “planet-like” to “brown-dwarf-like” characteristics occurs somewhere in the range 1–10MJup, but precise the crossover mass and whether the transition is gradual or abrupt remains unknown. Here, we explore how the occurrence rate, host star metallicity, and orbital eccentricities vary as a function of mass in a sample of 70 Doppler-detected substellar objects (0.8 < Mc/MJup < 80) from the California Legacy Survey. Our population consists of objects near and beyond the water ice line (1 < a/AU < 10), providing valuable clues to the details of giant planet formation physics at a location in the protostellar disk where planet formation efficiency is thought to be enhanced. We find that occurrence rate, host star metallicity, and orbital eccentricity all change gradually across the mass range under consideration, suggesting that “bottom-up” core accretion mechanisms and “top-down” gravitational instability mechanisms produce objects that overlap in mass. The observed eccentricity distributions could arise either from different formation channels or from postformation dynamical interactions between massive substellar objects.
Copyright and License
© 2026. 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
We thank the anonymous referee for helpful comments which improved the quality of the manuscript. We are grateful for insightful conversations with Erik Ford and Ruth Murray-Clay which guided the interpretation of results.
Funding for this work was provided by a University of California, Los Angeles set-up award to E.A.P. and by the Heising-Simons Foundation Award 2022-3833. J.V.Z. was supported by NASA FINESST Fellowship 80NSSC22K1606.
This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This research has made use of the Astrophysics Data System, funded by NASA under Cooperative Agreement 80NSSC25M7105.
Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit 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.
The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.
Software References
astropy (Astropy Collaboration et al. 2018), numpy (C. R. Harris et al. 2020), scipy (P. Virtanen et al. 2020), pymc3 (J. Salvatier et al. 2016).
Files
Gilbert_2026_AJ_171_67.pdf
Files
(1.0 MB)
| Name | Size | Download all |
|---|---|---|
|
md5:9a509d6de530c1fb2ddf46a9c6ffdf23
|
1.0 MB | Preview Download |
Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2511.12816 (arXiv)
Funding
- University of California, Los Angeles
- Heising-Simons Foundation
- 2022-3833
- National Aeronautics and Space Administration
- 80NSSC22K1606
- National Aeronautics and Space Administration
- 80NSSC25M7105
- W. M. Keck Foundation
Dates
- Submitted
-
2025-09-18
- Accepted
-
2025-11-12
- Available
-
2026-01-05Published