Image: Astrometry is the method that detects the motion of a star by making precise measurements of its position on the sky. For more on this, see Kervella et al., вЂњStellar and Substellar Companions of Nearby Stars from Gaia DR2,вЂќ Astronomy & Astrophysics Volume 623, A72 (March 2019). Gaia also found evidence for a possible gas giant around Epsilon Indi. Thus the missionвЂ™s detections of velocity anomalies at BarnardвЂ™s Star, Tau Ceti and Ross 128. The Gaia mission makes the case that astrometry is already in play in the exoplanet hunt.

Instrumentation-induced errors unfortunately made these detections unlikely.Д«ut astrometry has major advantages if we can reach the necessary accuracy, which is HorzempaвЂ™s point. Kaj Strand, likewise working at Sproul Observatory at Swarthmore, thought he had detected a planet orbiting 61 Cygni all the way back in 1943. Long-time Centauri Dreams readers will recall Peter Van de KampвЂ™s work at Swarthmore on what he believed to be a planet detection around BarnardвЂ™s Star, but we can also mention Sarah LippincottвЂ™s efforts at the same observatory on Lalande 21185. The problem has always been the level of precision needed to detect such minute variations. But radial velocity examines Doppler effects in a starвЂ™s spectrum as the star moves toward and then away from us, while astrometry looks for tiny changes in the position of the star in the sky, especially the kind of periodic shift that implies an otherwise unseen planet. When I talk to people about detecting exoplanets, I find that many confuse astrometry with radial velocity, for in loose explanatory terminology, both refer to measuring the вЂwobbleвЂ™ a planet induces on a star. IвЂ™m homing in on astrometry itself in this post rather than the mission concept, for the technique may be coming into its own as an exoplanet detection method, and IвЂ™m interested in new ways to exploit it.ДЄstrometry is all about refining our measurement of a starвЂ™s position in the sky. In a white paper submitted to the Decadal Survey on Astronomy and Astrophysics ( Astro2020), Philip Horzempa (LeMoyne College) suggests using technology originally developed for the NASA Space Interferometry Mission (SIM), along with subsequent advances, in a mission designed to exploit astrometry as an exoplanet detection mode.

Definition of astrometry