The search for extrasolar planets (EXOPLANETS) is one of the most rapidly expanding fields in astrophysics. Thanks to recent space-based efforts the number of detected planets in extrasolar stellar systems has increased dramatically, and more than 900 objects have been identified as of today, see e.g. http://exoplanet.eu/catalog/. With candidates identified in the so-called habitable zone (HZ)[1] the question of extraterrestrial life becomes very actual. Here we discuss ideas that could lead to infer the presence (or the past existence) of an extraterrestrial civilization by the detection of artifacts in orbit around the host planet. [1] The theoretical band around a star where a planet could orbit and host liquid water

Thermohaline mixing has recently been proposed to occur in low-mass red giants, with large consequence for the chemical yields of low-mass stars. We investigate the role of thermohaline mixing during the evolution of stars between 1$\mso$ and 3$\mso$, in comparison with other mixing processes acting in these stars. We use a stellar evolution code which includes rotational mixing, internal magnetic fields and thermohaline mixing. We confirm that during the red giant stage, thermohaline mixing has the potential to decrease the abundance of ³He, which is produced earlier on the main sequence. In our models we find that this process is working on the RGB only in stars with initial mass $ \le 1.5\mso$. Moreover we report that thermohaline mixing is also present during core He-burning and beyond, and has the potential to change the surface abundances of AGB stars. While we find rotational and magnetic mixing to be negligible compared to the thermohaline mixing in the relevant layers, the interaction of thermohaline motions with the differential rotation may be essential to establish the timescale of thermohaline mixing in red giants. To explain the surface abundances observed at the bump in the luminosity function, the speed of the mixing process needs to be more than two orders of magnitude higher than in our models. However it is not clear if thermohaline mixing is the only physical process responsible for these surface-abundance anomalies. Therefore it is not possible at this stage to calibrate the efficiency of thermohaline mixing against the observations.

We study the convection zones in the outer envelope of hot massive stars which are caused by opacity peaks associated with iron and helium ionization. We determine the occurrence and properties of these convection zones as function of the stellar parameters. We then confront our results with observations of OB stars. A stellar evolution code is used to compute a grid of massive star models at different metallicities. In these models, the mixing length theory is used to characterize the envelope convection zones. We find the iron convection zone (FeCZ) to be more prominent for lower surface gravity, higher luminosity and higher initial metallicity. It is absent for luminosities below about $10^{3.2}\lso$, $10^{3.9}\lso$, and $10^{4.2}\lso$ for the Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the Hertzsprung-Russell diagram for three metallicities, and compare this with the occurrence of observational phenomena in O stars: microturbulence, non-radial pulsations, wind clumping, and line profile variability. The confirmation of all three trends for the FeCZ as function of stellar parameters by empirical microturbulent velocities argues for a physical connection between sub-photospheric convective motions and small scale stochastic velocities in the photosphere of O- and B-type stars. We further suggest that clumping in the inner parts of the winds of OB stars could be caused by the same mechanism, and that magnetic fields produced in the FeCZ could appear at the surface of OB stars as diagnosed by discrete absorption components in ultraviolet absorption lines.