Searching for substellar companion candidates with Gaia. I. Introducing the GaiaPMEX tool

TL;DR

GaiaPMEX introduces a Bayesian, grid-based approach to infer companion mass ($M_c$) and semi-major axis ($sma$) from Gaia DR3 astrometric proxies—astrometric excess noise ($\text{AEN}$), RUWE, and the proper motion anomaly ($\text{PMa}$). By simulating photocenter orbits under a wide range of companion properties and instrument noises, the method builds likelihoods from transformed variables $\text{mse}^{1/3}$ and $\text{pma}^{2/3}$ and performs a Bayesian inversion to produce 2D posterior maps over $(\log M_c, \log sma)$ for each target. The maps reveal characteristic short-, long-, and equal-mass branches in the mass-sma plane, and demonstrate that combining AEN/RUWE with PMa significantly narrows degeneracies, enabling robust constraints and even minimum-mass estimates in favorable regimes. Illustrative cases (GJ 832, HD 114762, HD 81040, AF Lep, Sirius B, Beta Pictoris) show that GaiaPMEX can recover known companions, constrain new candidates, and quantify Gaia’s current sensitivity to exoplanets and brown dwarfs, with strongest prospects for nearby M-dwarfs within 1–10 au. The approach sets the stage for exploiting the forthcoming DR4 time series to refine and confirm planetary signals identified through astrometric signatures.

Abstract

The Gaia mission is expected to yield the detection of several thousands of exoplanets, perhaps at least doubling the number of known exoplanets. Although the harvest is expected to occur when the astrometric time series will be published with DR4 at the eve of 2026, the DR3 is already a precious database to search for exoplanet beyond 1 au. With this objective, we characterized multiple systems by exploiting two astrometric signatures derived from the DR3 astrometric solution of bright sources (G<16). We have the proper motion anomaly, or PMa, for sources also observed with Hipparcos, and the excess of residuals in the RUWE and the astrometric excess noise (AEN). Those astrometric signatures give an accurate measurement of the astrometric motion of a source seen with Gaia, even in the presence of calibration and measurement noises. We found that they can allow identifying stellar binaries and hint to companions with a mass in the planetary domain. We introduce a tool called GaiaPMEX, that is able, for a given source, to model its astrometric signatures, by a photocenter orbit due to a companion with certain mass and semi-major axis (sma). Comparing to their actual measurements from the DR3 and Hipparcos, GaiaPMEX calculates a confidence map of the possible companion's mass and sma. The constraints on mass are, as expected, degenerate, but when allowed, coupling the use of PMa and RUWE, may significantly narrow the space of solutions. Thanks to combining Gaia and Hipparcos, planets are expected to be most frequently found within 1-10 au from their star, at the scale of Earth-to-Saturn orbits. In this range, exoplanets with mass down to 0.1 MJup are more favorably detected around M-dwarfs closer than 10 pc. Some fraction, if not all, of companions identified with GaiaPMEX may be characterized in the future using the astrometric time series that will be published with the DR4.

Figures (30)

• Figure 1: Illustration of the equality of pma modulo $\Omega$ between two systems with the same central star and a companion on a long-period orbit but with different values of sma and mass. For a given sma and a given mass of the companion (left panel) the PMa is directed toward the companion. There exists a smaller sma and a larger mass for which the $\Vert{\rm \glsxtrshort{pma}}\Vert$ is the same (right panel) but the orientation at equal $\Omega$ is different. Nevertheless, it is possible to align the pma on the same pa (pa) by rotating the system by some $\Delta\Omega$.
• Figure 2: Median formal error distribution $\sigma_{\rm formal}$ with respect to magnitude and color (top) and ra and dec (bottom) in the g3 database of sources brigther than $G$=16.
• Figure 3: Median attitude excess noise distribution with respect to ra and dec in the g3 database of sources brighter than $G$=16.
• Figure 4: Maps of the calibration noise with respect to both $G$-mag and $Bp-Rp$ color. Left: for the 5p dataset. Right: for the 6p dataset.
• Figure 5: Schematic representation of the orientation of one of Gaia's detectors (red arrow) compared to a star's direction (green arrow). The solid circle represents the celestial sphere as seen from the Gaia center of mass, and the dashed-line circle represents the celestial equator. The two quadrilaterals represent Gaia's preceding (light red) and following (light yellow) fov detectors. On the bottom right, we show the possible location of the star on the detector and the $\eta$ angle that is measured projected along the al axis ($u_{\rm \glsxtrshort{al}}$). Arbitrary north and east directions are shown with the definition of the pa of the al direction. They are not intended to exactly correspond to the top-left drawing but allowed us to define $\theta_{\rm AL}$, the eastward-oriented angle between $u_{\rm \glsxtrshort{al}}$ and the north.