# Alpha Centauri: Disc Dynamics, Planet Stability, Detectability
## 5 Discussion and Conclusions
### Disc Evolution and Planet Formation in Alpha Centauri  
The way discs evolve in binaries is key to understand the distribution of available material to form planets. In particular, circumstellar discs are heavily truncated by the periodic perturbations of the outer stellar companion. In the case studied here (\(a_{\rm b}=23.3\) au, \(e_{\rm b}=0.51947\)), the discs were 10 au in size initially and ended up with radii of about 3 au after tidal truncation. This corresponds to about 12% of the binary semimajor axis, given that the binary is eccentric. As a reminder, for circular binaries, the discs are truncated between 33% and 50% \(a_{\rm b}\)[22]. Regardless of the specific values of the disc masses, a circumstellar  
Figure 12: Temporal evolution of the radial velocity for each star in the Alpha Cen triple stellar system: A (**left**), B (**centre**), and C (**right**). The upper panels display the system’s evolution with orbiting planets. The middle panel presents the star’s radial velocity in the absence of planets, while the lower panel shows the result of subtracting the two aforementioned signals. disc in a binary exhibits a steeper surface density profile compared to a disc around a single star. This effect unavoidably accelerates the radial inward drift of dust towards the central star [31]. In other words, there is less time available to transform dust into planetesimals. However, we also note that since more material is concentrated in a more compact disc, this naturally leads to higher values of surface density. This could help to reach higher values of the dust-to-gas ratio within the inner disc regions, which may fuel streaming, self-induced, and/or magneto-hydrodynamical instabilities [32], for a review. Interestingly, at the onset of disc formation, it is possible to exchange material from one disc to the other as tidal truncation operates. This implies that some material from the outer regions of the disc around B may land onto the disc around A (and vice versa). The fact that this alien material was well distributed within both discs (see Figure 4) means that solid bodies with different compositions (e.g., ices and silicates) are more efficiently mixed in binary systems--as opposed to discs around single stars. This could potentially accelerate the growth of dust particles within the disc, but this statement remains highly speculative at this stage. The detailed study of dust dynamics in the circumstellar discs of Alpha Cen is left for future work. At any rate, in order to efficiently transform planetesimals into planets within discs, the eccentricity oscillations of the disc must remain under a certain threshold. This is key to ensure that solid bodies do not collide at destructive relative speeds [33; 34; 35]. The detailed modelling of the disc around Alpha Cen B considering a broad variety of disc parameters [19] revealed that two eccentricity oscillation regimes are expected to occur. However, the mean eccentricity of the discs eventually reached a quasiseady value around 0.05-0.1, with little or no disc precession at all (depending on disc viscosity). Therefore, although the discs in Alpha Cen are part of a multiple stellar system, their eccentricities were shown to remain fairly low, which should have a limited impact on planet formation within the gaseous disc. However, as the disc evolves, the gas is progressively removed from the disc, and dust particles decouple from the gaseous disc6. When this occurs, the average disc eccentricity increases, thereby rendering collisions between dust particles and solid bodies more destructive [19]. So, the most favourable scenario to form planets in Alpha Cen is during the early gas-rich phase and at stellocentric distances below 3 au. Given the potential mass reservoir of several tens of Earth masses, this means that one or two S-type multiplanetary systems (of rocky planets) may have formed in Alpha Cen AB.