Free Astronomy Magazine March-April 2022

22 MARCH-APRIL 2022 ASTRO PUBLISHING In short, its size and positioning give hope that Proxima b is the closest exoplanet that has the essential re- quirements to host life. However, this scenario is made extremely un- likely by several factors. The first is the exuberant magnetic activity of Proxima Centauri, which continually storms the planet with deadly streams of ultraviolet and X-ray ra- diation. The second is the real mass of the planet itself, which might ac- tually be twice that of the Earth if the planet is far above or below the line of sight at the closest and furthest points from the Earth. In fact, when we talk about “minimum mass” from calculations of exo- planet orbits, we mean the mass the planet would have if its orbit led it to intersect the Earth-star junction - it is much more likely that that spe- cific configuration is not the actual one, making the mass of the planet only a bounded variable. Precisely knowing the mass of Prox- ima b would allow us to better frame the typology of the planet, but the known mass alone would not be sufficient to distinguish be- tween a small rocky planet, an ocean world, or a mini-Neptune. In other words, mass alone does not tell us whether a planet is about the T he relative sizes of several objects, including the three (known) members of the Alpha Centauri triple system and some other stars for which angular sizes were meas- ured with the Very Large Telescope Interferometer (VLTI) of the ESO Paranal Observa- tory. For compari- son, the Sun and Jupiter are also shown. [ESO] which was deter- mined to be equal to 1.3 times that of the Earth. From the orbital period, it was then possible to trace the star- planet distance, which is 0.048 as- tronomical units (7.2 million km or 4.5 million mi). These values were a sensation soon after their calcula- tion because they suggested that Proxima b may be a rocky planet that surely orbits in the habitable zone of Proxima Centauri, a zone that ex- tends from 0.023 to 0.054 astronom- ical units from the center of the star. Let’s recall that the term “habitable zone” means the region around a star where liquid water can exist on the surface of a planet. G uillem Anglada-Escudé, the astronomer who, with his team, discovered Proxima b. [Universitat de Barcelona] velocity variations, which mani- fested themselves by way of the aforementioned oscillations of the spectral lines. While magnetic activ- ity generates non-periodic random variations that do not move the geo- metric center of the star, a planet with an orbit not orthogonal to the line of sight physically moves the star towards the observer and in the opposite direction, generating a pe- riodic signal in the spectrum. The challenge of the Anglada-Escudé team was to be able to extract this signal (the orbital period) from the noise. The extent of the oscillation of the spectral lines, combined with the known mass of Proxima Cen- tauri, made it possible to calculate the minimum mass of the planet,