Free Astronomy Magazine September-October 2021

11 SEPTEMBER-OCTOBER 2021 ASTRO PUBLISHING T his plot captures one of the nearest rocky exoplanets, dubbed HD 219134b, in the act of passing in front of its star. The data were obtained in infrared light using the NASA Spitzer Space Telescope. By carefully measuring the brightness of the star over several hours, Spitzer easily detected the faint de- crease in light that occurred when the planet’s disk blocked a tiny portion of the star’s light. Even though the planet is 1.6 times the size of Earth, it still only accounts for less than a 0.04% reduction in the total light from the star during its transit. [NASA/JPL-Caltech] Conversely, the stars larger and hot- ter than the Sun (within certain lim- its) guarantee their eventual planets the radiation necessary to develop a biosphere, but often their existence is not long enough to allow for the evolution of complex life forms. In short, only stars between F5V and K5V spectral types offer the ideal conditions for the development of a biosphere like ours. Outside of this range, everything becomes more difficult. Nonetheless, even consid- ering only these spectral types, we could have millions of biospheres in the Milky Way. But let us not delude ourselves that, by the law of large numbers, some of them are very similar to ours. In fact, the more sci- entists compare the Earth with other potentially habitable (by us) planets, the more we realize how unlikely the latter might actually be. In addition to orbiting at the right distance from their stars, these plan- ets must have an atmosphere with the right density, pressure and com- position. They must also have a magnetic field and, therefore, a ro- tating molten core, as well as plate tectonics and, therefore, continents on the surface. For life to evolve in the direction of high complexity, it is also essential that the host planet’s rotation axis is as stable as possible for billions of years, a situ- ation that has occurred on Earth thanks to the presence of the Moon. How many other planets have both a biosphere and an equally large natural satellite, originating from a fortuitous collision between proto-