- Title: Astrometric Detection of Giant Planets Around Nearby M Dwarfs: The Gaia Potential
- Authors: A. Sozzetti, P. Giacobbe, M. G. Lattanzi, G. Micela, R. Morbidelli, and G. Tinetti
- First Author’s Institution: INAF – Osservatorio Astrofisico di Torino, Pino Torinese, Italy
The Gaia Mission
It’s not too hard to measure the position of a star on the sky: just look up. But the projection of a star’s position on the sky only provides a position in two dimensions (astronomers typically use right ascension and declination, the astronomical analogues to longitude and latitude, respectively). What remains is the distance to the star. To measure such distances, astronomers observe the position of a star at multiple epochs. As the Earth moves around the Sun, the relative positions of stars on the sky change due to a change in viewing angle, a phenomenon known as parallax.
Gaia, by producing observations of a billion stars across our side of the galaxy, will usher in a revolution in astrometry and galactic science. The authors of today’s paper, however, focus on another potential application: planet hunting. As we’ve noted previously, astrometry is most sensitive for detecting giant planets a few AU from their host stars, where both radial velocity and imaging surveys are incomplete. These authors note that recent studies suggest around 6.5±3.0% of M dwarf stars (stars smaller than 0.6 solar masses) host Jupiter-mass planets, and set out to determine if Gaia could detect such companions.
As a planet and star orbit their center of mass, both objects trace ellipses on the sky. Careful observations of a star over many epochs allow this ellipse to be separated from the parallactic motion Gaia was designed to detect. Since it will probe each star in its sample 50 times, Gaia should collect enough data for these two movements to be easily separated. To test this, the authors injected artificial Jupiter-mass planets around each of the 3150 M dwarf stars within 33 parsec. They assigned random orbital elements for each planet’s orbital parameters and let the planet orbit the star, then “observed” its position on the sky 50 times, estimating the expected performance of the Gaia pipeline. Finally, the authors compared the parameters of the planets detected by the Gaia software to the known parameters of the planets they injected.Expected Results
By testing their injected planets with the Gaia pipeline, the authors find they recover the majority of these fake companions. They detect excess variability in the parallactic signal due to orbital motion around 85% of the sample!. As expected (right), they are most successful at finding Jupiters around the most nearby stars (where the parallactic signal is large and easily separable) and the smallest stars (where the Jupiters are comparatively massive, so the elliptic signal is large and easily separable). For more than 80% of these stars, they find they can measure the planet’s mass to within 10%.Not all M dwarfs host giant planets, but Sozzetti et al. estimate that, from the 3150 most nearby M dwarfs, Gaia will detect about 100 new giant planets. Moreover, from the sample of M dwarfs within 100 pc, the authors suggest Gaia will find 2500 new planets. Unlike Kepler, which generally only provides estimates of planet radii and orbital periods, many of the Gaia planets will have their full orbits determined. These will be the first planets detected by astrometry (although a few planets detected via other methods have been confirmed by astrometry. The Gaia mission will also allow for unprecedented analyses of planet formation, evolution, dynamics, and populations around the most common types of stars in our galaxy.