There has always been some doubt as to whether Proxima Centauri, [closest star to our solar system] was part of the system of Alpha Centauri A and B: Recent findings and high precision radial velocity measurements have now confirmed this within any reasonable doubt. https://arxiv.org/pdf/1611.03495v1.pdf Proxima’s orbit around α Centauri: ABSTRACT: Proxima and αCentauri AB have almost identical distances and proper motions with respect to the Sun. Although the probability of such similar parameters is in principle very low, the question whether they actually form a single gravitationally bound triple system has been open since the discovery of Proxima one century ago. Owing to recent high precision radial velocity measurements and the revision of the parameters of the αCen pair, we show that Proxima and αCen are gravitationally bound with a high degree of confidence. The orbital period of Proxima is approximately 600 000 years, with a moderate excentricity of 0.42+0.07 −0.08. Proxima comes within 5.3 +1.2 −0.9 kAU of αCen at periastron, and the apastron occurs at 12.9 +0.3 −0.1 kAU. This orbital motion may have influenced the formation or evolution of the recently discovered planet orbiting Proxima as well as circumbinary planet formation around αCen. Conclusions: Using recent high accuracy RV measurements and astrometry, we show with a high level of confidence that Proxima is gravitationally bound to αCen and orbits the pair on a moderately eccentric, very long period orbit. This conclusion is particularly valuable for the modeling of this star as it means that the three stars are coeval and share the same initial metallicity. Due to the very weak gravitational interaction between Proxima and αCen, Beech (2009, 2011, 2015) proposed that this system could be a test case for the Modified Newtonian Dynamics (MOND) theory (Milgrom 1983; Bekenstein 2004). Such a wide multiple system may have formed during the dissolution of their original star cluster (Kouwenhoven et al. 2010). In spite of its large semi-major axis, the statistical dissolution time of the Proxima- αCen system is expected to be much longer than 10 Gyr (Jiang & Tremaine 2010; Bahcall et al. 1985). The orbital motion of Proxima could have played a role in the formation and evolution of its planet (Anglada-Escudé et al. 2016). Conversely, it may also have influenced circumbinary planet formation around αCen (Worth & Sigurdsson 2016). A speculative scenario is that Proxima b formed as a distant circumbinary planet of the αCen pair, and was subsequently captured by Proxima. Proxima b could then be an ocean planet resulting from the meltdown of an icy body (Brugger et al. 2016). This could also mean that Proxima b may not have been located in the habitable zone (Ribas et al. 2016) for so long as the age of the αCen system (5 to 7 Gyr; Miglio & Montalbán 2005; Eggenberger et al. 2004; Kervella et al. 2003; Thévenin et al. 2002).
