Detecting multiple planets in Gaia data using Bayesian model selection
Abstract
Gaia will soon provide a census of cold giant planets. However, an astrometric signal contains the superimposed reflex motion of all companions, so planetary signals cannot be treated individually. We therefore investigate Gaia's planet-detection capabilities for 1-planet and 2-planet systems, with special attention to systems near 2:1 mean-motion resonance. Gaia can detect 2-planet systems, although the lower-S/N planet requires a higher S/N threshold, especially when the outer planet is more massive and near 2:1 commensurability. When fitting a 1-planet model to a circular 2-planet resonant system, the resulting fit shows biases in eccentricity (up to e = 0.8), mass (up to 100%), and inclination (up to 50 deg). The bias is maximised when the planets' astrometric signatures are similar, since the combined orbital signal deviates most from a single Keplerian orbit. Many of these systems can be identified as having 2 planets using Bayesian model selection. Still, in some cases the second planet is hidden and undetectable due to its S/N, leading to these biases for the outer planet's orbital parameters. When the outer planet is roughly over three times as massive as the inner planet, orbital approximations allow us to recover the astrometric resoeccentric degeneracy from the bias. When the planets are in dynamical resonance, apsidal precession observed over a long enough baseline (e.g. DR5) can contribute to breaking the degeneracy at short orbital periods. Overall, these biases might significantly affect constraints on planet formation, migration, and evolution models from Gaia population-wide demographics.
Related
- A pilot study on the CSST astrometric capability: Detecting astrometric binaries with Gaia synergy via simulated data
- Bayesian analysis of Gaia epoch astrometry and radial velocities with kima
- A Hierarchical Bayesian Differential Calibration of the Gaia DR3 Parallax Zero Point Using Galactic Globular Clusters
- Companion masses of Gaia DR3 AstroSpectroSB1 binaries: a Bayesian forward-modelling framework
- A comprehensive Gaia DR3 study of the open cluster NGC 6134: Bayesian distances, present-day mass function, mass segregation, and structural decomposition
BibTeX
@misc{greenfield2026detecting,
title = {{Detecting multiple planets in Gaia data using Bayesian model selection}},
author = {Emmanuel J. Greenfield and James E. Owen},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.07149},
eprint = {2610.07149},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.07149},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Detecting multiple planets in Gaia data using Bayesian model selection
AU - Emmanuel J. Greenfield
AU - James E. Owen
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2610.07149
UR - https://arxiv.org/abs/2610.07149
AB - Gaia will soon provide a census of cold giant planets. However, an astrometric signal contains the superimposed reflex motion of all companions, so planetary signals cannot be treated individually. We therefore investigate Gaia's planet-detection capabilities for 1-planet and 2-planet systems, with special attention to systems near 2:1 mean-motion resonance. Gaia can detect 2-planet systems, although the lower-S/N planet requires a higher S/N threshold, especially when the outer planet is more massive and near 2:1 commensurability. When fitting a 1-planet model to a circular 2-planet resonant system, the resulting fit shows biases in eccentricity (up to e = 0.8), mass (up to 100%), and inclination (up to 50 deg). The bias is maximised when the planets' astrometric signatures are similar, since the combined orbital signal deviates most from a single Keplerian orbit. Many of these systems can be identified as having 2 planets using Bayesian model selection. Still, in some cases the second planet is hidden and undetectable due to its S/N, leading to these biases for the outer planet's orbital parameters. When the outer planet is roughly over three times as massive as the inner planet, orbital approximations allow us to recover the astrometric resoeccentric degeneracy from the bias. When the planets are in dynamical resonance, apsidal precession observed over a long enough baseline (e.g. DR5) can contribute to breaking the degeneracy at short orbital periods. Overall, these biases might significantly affect constraints on planet formation, migration, and evolution models from Gaia population-wide demographics.
ER -