astrometry
/ uh-STROM-uh-tree /
Like the radial-velocity method, astrometry hunts for a planet by watching the star wobble — but instead of measuring the star's speed toward and away from us, it measures the star's actual position drifting across the sky. A star with a planet does not trace a straight path among the background stars; it wiggles, tracing a tiny corkscrew or loop as it and its planet swing around their common balance point. Astrometry is the art of measuring star positions so precisely that this minuscule wobble in the sky becomes visible.
The wobble is breathtakingly small. The Sun, seen from a nearby star, would appear to loop by an angle of well under a thousandth of an arcsecond because of Jupiter — far finer than the apparent width of a human hair seen from a hundred kilometers. Measuring positions to that level demands extraordinary stability, which is why astrometry has been the hardest of the major methods to make work, and why it really came into its own only with space missions free of the blurring atmosphere. Unlike radial velocity, astrometry measures the full size of the wobble on the sky, so it can give a planet's true mass, not just a minimum.
Astrometry is best at planets that are far from their stars (which make the widest, slowest wobbles) and around nearby stars (whose wobbles look biggest). That complements the transit and radial-velocity methods, which favor close-in planets. The Gaia spacecraft, charting the precise positions of well over a billion stars, is expected to reveal many thousands of planets this way over its mission. The method's challenge remains its brutal precision requirement: any tremor, drift, or imperfection in the instrument can swamp a signal smaller than almost anything else in astronomy.
From 30 light-years away, Jupiter would make the Sun appear to swing by under a milliarcsecond — about the angle a coin's thickness covers seen across a continent. Catching that wobble is what the Gaia mission's billion-star census is designed to do, and it should harvest thousands of planets from the wiggle alone.
Astrometry tracks a star's tiny looping path on the sky to weigh its hidden planet.
Astrometry can give a planet's true mass (it sees the full sky wobble), but its precision demands are so severe that it has historically lagged behind transits and radial velocity.