Answer: The parallax angles are very small. Proxima Centauri is the parallax angle nearest to the star. It is less than one second or arc, which equals 1/3600 of a degree. A human finger is roughly 1 degree wide. The distances to stars are so large that conventional units such as kilometers or astronomical units are often too cumbersome to use. The nearest stellar system consists of the stars Proxima, alpha Centauri A and B and at least one planet Proxima b. The habitability of Proxima b and any other planets are likely to be significantly influenced by the orbital evolution of the system. To study the dynamical evolution of the system, we simulate the motions of Proxima and alpha Centauri A and B due to the perturbations from the Galactic tide and stellar encounters in a Monte Carlo fashion. From 100 clones, we find that 74% orbits of Proxima Centauri are bound to alpha Centauri A and B while 17% and 9% orbits become unbound in the simulations over the past and future 5 Gyr. If the system migrated outward in the Milky Way to its current location, more than 50% of clones could become unstable in backward simulations. The ratio of unstable clones increases with the simulation time scale and encounter rate. This provides some evidence for a capture scenario for the formation of the current triple system. Despite large uncertainties, the metallicity difference between Proxima and alpha Centauri A and B is also suggestive of their different origin. Nonetheless, further improvements in the available data and models will be necessary for a reliable assessment of the history of the Proxima-alpha Centauri system and its impact on the habitability of Proxima b.
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