Although most stars have at least one gravitationally bound companion, only about 4% of the more than 5000 extrasolar planets discovered to date reside in a binary star system. One possible reason the figure is so low is that the presence of additional stars stunts planetary formation or ejects nascent worlds into interstellar space. But selection bias also makes at least some contribution, because of the way astronomers tend to detect exoplanets. For example, the radial-velocity method, which measures a planet’s tug on its host star through Doppler spectroscopy, doesn’t work on binary systems that have two stars with very little angular separation.
To assemble a more accurate census of planets in multiple-star systems, astronomers are looking to other exoplanet-detection methods. Salvador Curiel of the National Autonomous University of Mexico and colleagues turned to a technique that has rarely been used for exoplanet searches: astrometry, the measurement of the positions and motion of stars. By combining fresh radio observations of a nearby binary system with archival optical data going back to 1941, the researchers not only revealed the existence of a Jupiter-mass planet but also precisely mapped the three-dimensional orbital motion of the primary star and its stellar and planetary companions.
Curiel and colleagues are not the first to use astrometry for exoplanet hunting: Some of the first searches in the mid 20th century relied on measuring stellar positions and detecting the gravitational influence of an orbiting planet. But astronomers today have access to resources that those early planet hunters could only have dreamed of. Curiel and his team used the Very Long Baseline Array (VLBA), a network of US radio telescopes whose observations are combined to achieve milliarcsecond resolution, to observe GJ 896AB, a binary located about 20 light-years away. GJ 896A and GJ 896B are red dwarfs weighing in at about 0.44 and 0.17 solar masses, respectively. The researchers complemented the short-term, high-resolution VLBA position data of GJ 896AB with archival data from the US Naval Observatory spanning from 1941 to 2017.
With observational constraints in place, Curiel and colleagues modeled the orbits of both stars with and without planetary companions. The analysis revealed a slight wobble in the orbit of GJ 896A, presumably caused by a shift in the position of the system’s center of mass as a planet circled the star. The best fit includes a planet more than twice the mass of Jupiter that orbits GJ 896A every 284 days. In a final step, Curiel and colleagues produced a 3D map of the orbits of all three massive objects in the GJ 896AB system. Intriguingly, the planet travels in a tilted, retrograde orbit, moving in a direction opposite that of its host star’s rotation and of GJ 896B’s orbit about its heftier sibling. Such a configuration is difficult to explain with the prevailing theory that the planet would form within the same rotating protoplanetary disk that surrounded the two stars in their infancy.
Further study of the system and its orbital mechanics could help astronomers determine the likelihood and mechanisms of planetary formation in multiple-star systems. And astrometry is likely to yield even more discoveries of planets around all kinds of stars. Aside from the VLBA and other networked radio telescopes, the Gaia spacecraft is delivering precise position measurements for more than a billion stars in the Milky Way. (S. Curiel et al., Astron. J. 164, 93, 2022.)
https://physicstoday.scitation.org/do/10.1063/PT.6.1.20220909a/full
2022-09-09 16:30:35Z
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