Searching for substellar companion candidates with <i>Gaia</i>

Context . The Gaia mission is expected to yield the detection of several thousands of exoplanets, perhaps at least doubling the number of known exoplanets. However, only 72 candidates have been reported with the publication of the Gaia third data release, or third Gaia data release (GDR3). Although a greater harvest of exoplanets is expected to occur with the publication of the astrometric time series in the DR4 in 2026, the GDR3 is already a precious database that can be used to search for exoplanets beyond 1 au. Aims . With this objective, we characterized multiple systems by exploiting two astrometric signatures derived from the GDR3 astrometric solution of bright sources with G &lt;16. We have the proper motion anomaly, or PMa, for sources also observed with H ipparcos and the excess of residuals present in the renormalized unit weight error ( ruwe ) and the astrometric excess noise (AEN). These astrometric signatures give an accurate measurement of the astrometric motion of a source seen with Gaia , even in the presence of non-negligible calibration and measurement noises. Methods . We introduce a tool called Gaia DR3 proper motion anomaly and astrometric noise excess, or GaiaPMEX for short, that is able for a given source to model the astrometric signatures that are hidden within the PMa, ruwe , and AEN by a photocenter orbit due to a companion with a certain mass and relative semi-major axis to the primary star (sma). GaiaPMEX calculates a confidence map of the possible companion’s mass and sma, given the actual measurements from GDR3, and H ipparcos , when available. This tool allowed us to determine for any source of interest if it may be a binary (or planetary) system and the possible companion’s mass and sma. Results . We find that the astrometric signatures can allow for identification of stellar binaries and hint toward companions with a mass in the planetary domain. The constraints on mass are, as expected, degenerate, but when allowed, coupling the use of PMa and ruwe or AEN, they may significantly narrow the space of solutions. Conclusions . Thanks to combining Gaia and H ipparcos , 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 of sma, exoplanets with a mass down to 0.1 M J are more favorably detected around M-dwarfs closer than 10 pc to Earth. 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 in the forthcoming DR4.