Free Astronomy Magazine March-April 2019

MARCH-APRIL 2019 oxygen only subsequently by photosynthetic processes? This point is fundamental for planetary protection purposes. We find the an- swer in our knowledge of stellar evolution. The time a protostar takes to con- tract from an interstellar cloud of gas and reach the so-called “main sequence” is inversely proportional to its mass. For Sun-like stars, this period extends to about 10 million years and ends before any planets are fully formed. For smaller mass stars, such as red dwarfs, times can stretch up to several hundred million years. The planets that are orbiting around the latter, in the fu- ture habitable zone, will experience a prolonged greenhouse effect due to the larger initial size of their star. The water present in the planetary stratospheres will be photo-dissociated into pure hydrogen and oxygen, with the hydrogen gas dis- persed into space due to its lightness, and the oxygen gas left to accumulate in the at- mosphere. If this process were the rule (the topic is still under discussion), the number ! T his infographic shows with re- markable clarity the fundamental stages of geology and biology on Earth, highlighting events of great importance in the evolution of the biosphere. On the left, Claudius Gros, the promoter of the Genesis proj- ect. [Institut für Theoretische Physik, Goethe- Universität] of planets compatible with the Genesis Proj- ect would be very high. It is estimated that there can be a habitable planet every five red dwarfs, and that these stars make up 85% of the stellar population of the entire galaxy. Therefore, there may exist tens of millions of “fertilizable” planets, some of which are a few light-years from Earth. All this is positive for Gros, but not for those who trust precisely on a red dwarfs’ plane- tary systems to discover the first traces of ex- traterrestrial life in the form of biomarkers, among which oxygen seemed to be the most interesting. But for the Genesis Proj- ect, there is also some bad news. A study re- cently conducted by Abraham Loeb and Manasvi Lingam (Harvard University) shows that red dwarfs’ planets might not receive enough radiation from their stars to allow photosynthesis: the flow of UV rays would be insufficient to maintain a biosphere sim- ilar to Earth’s. Perhaps, it is no coincidence that the only inhabited planet we know or- bits around a yellow dwarf.