Chapter 5 of 15
Astrometry
The star's small orbit also moves its position on the sky. Astrometry measures that tiny loop, which gives the orbit's tilt and so the planet's true mass. The signal grows with the planet's mass and orbit and shrinks with distance.
α = (Mp/M★)(a/d): the star's angular orbit, in arcseconds for a in AU and d in parsecs (Perryman et al. 2014). A wobble consistent with an orbiting companion becomes a planet only when its mass is pinned below the planetary limit, usually with RV as well, as for Gaia-4b (Stefánsson et al. 2025). Gaia ended science observations in January 2025; its fourth data release, planned for December 2026, carries the epoch astrometry.
The drawing is an illustration: its scales are chosen to show the three motions apart, and the planet's wobble is enlarged and labelled. It quotes no Gaia measurement precision: the project's research atlas holds no sourced per-measurement figure for Gaia DR3, so none is given here.
| Quantity | Value | From |
|---|---|---|
| Jupiter twin at 10 pc | 497 µas | [calc] |
| Earth twin at 10 pc | 0.30 µas | [calc] |
| GJ 896 Ab | 509 µas | exoplanet.eu Mp = 2.26 MJ, M★ = 0.43589 M☉, a = 0.64282 AU, d = 6.2565 pc [calc]; found by astrometry (Curiel et al. 2022) |
89 of the 8,049 planets with a stored distance in exoplanet.eu were found by astrometry (the first method listed). Show them on the Galaxy map ›
peak 21 (2026)
19882026