Free Astronomy Magazine January-February 2019

10 JANUARY-FEBRUARY 2019 The combination of new measurements with the archive datasets clearly showed a signal with a periodicity of 233 days, and also revealed a fainter long-term modula- tion. The 233-day periodicity originates from a variation in the radial velocity of just 1.2 m/s, a shift that could easily be produced by the photospheric activity. However, the re- searchers are 99% confident they can rule out this possibility because Barnard’s Star shows an extremely low level of magnetic activity, a minimum X-ray flow, an imper- ceptible emission in H-alpha, and negligible chromospheric emission. Independent photometric and spectroscopic monitoring has indicated a rotation period of the star of 140 ± 10 days, which leads the researchers to exclude the possibility that the 233-days signal is in any way related to surface structures, such as unexpectedly long-lasting active regions. The fact that the star is between 6 and 11 billion year old also favors a very mild magnetic activity, unable of producing persistent phenomena for decades. For all these reasons, Ribas and colleagues have come to the conclusion that this signal is more easily explained if produced by a planetary companion, with a minimum mass of 3.2 Earth masses, placed on a low- eccentricity orbit, and with a semi-major axis greater than about 0.4 AU. This dis- tance in the Barnard’s Star system corresponds roughly to the so-called “snow line”, where volatile ele- ments such as water are present in the solid- state. A planet orbit- ing in this region would only receive 2% of the energy the Earth receives from the Sun and, accordingly, the sur- face temperature of Barnard’s Star b should range be- tween -150°C and -170°C. The pre- dicted temperature of this planet is therefore not habit- able for life as we know it, although this could possibly be mitigated by the pres- ence of a thick atmos- phere. The mass could also represent an addi- tional hurdle: the calcu- lated value of 3.2 Earth masses must be considered as a minimum limit, valid for a planet which, although non- transiting in front of and behind the disk, is very close to the visual line in the points of maximum ap- proach and maximum dis- tance from the observer. As the deviation from this line increases, the value of the mass grows at the same pace. In the most favorable of possible configurations, Barnard’s Star b weighs a lit- tle more than three times that of Earth. For this rea- son, it falls into the category of super-Earths, the type of planet that is most fre- quently discovered in orbit his animation shows the stars closest to the Sun and highlights the position of Barnard’s Star. [ESO/L. Calçada/ Vladimir Roma- nyuk (spaceengine.org ). Music: Astral Electronics]