Free Astronomy Magazine January-February 2021

21 JANUARY-FEBRUARY 2021 ASTRO PUBLISHING ing from a numerically smaller (about 250) but more diversi- fied sample of planets in terms of discovery techniques, since it additionally included the planetary transits. These first encouraging results spurred newworks, such as the one published at the end of November in the Monthly No- tices of the Royal Astronomical Society by Nanna Bach-Møller and Uffe Gråe Jørgensen (Niels Bohr Institute, University of Copenhagen). The two Danish researchers considered 1171 ex- oplanets in 895 systems and al- lowed for any method of dis- covery, a procedure that ex- cluded bias in the results due to different populations of planets. Furthermore, unlike previous works, each system was treated as a unit, using the value of the mean orbital eccentricity in the analysis in- stead of considering each planet out of its own context. This choice is justified by the fact that multiplicity and po- tential planet-planet interac- tions are properties of the planetary system as a whole, rather than of individual plan- ets. At the end of their analysis, Bach- Møller and Jørgensen confirmed the strong correlation between multi- plicity and eccentricity, finding also that this correlation follows a power law for all multiplicities greater than one. In other words, in all planetary systems that have two or more plan- ets (about 270 known systems) up to the maximum known today of eight confirmed planets (our system and the one of Kepler-90), the average value of eccentricity decreases in pro- portion to the increase in the num- ber of planets. We therefore have the confirmation that our solar sys- tem is not atypical, but that it is sim- ply at the limit of a scale that likely T his simple comparison between our inner solar system and the Upsilon Andromedæ system gives an idea of how different the orbits of the planets may appear depend- ing on the observer's position, a further complication for those who study them. [NASA, ESA and A. Feild (STScI)] extends farther than we can demon- strate today. Let’s consider, for exam- ple, the solar-type star HD 10180, which could host nine planets, six of which have already been confirmed. The only systems that seem to not re- spect the power law highlighted by the two researchers at the Niels Bohr Institute are those with only one (known) planet. From the 667 exam- ined single-planet samples, a certain dispersion of the orbital eccentricity values was evident. According to Bach-Møller and Jørgensen, this could be due to the fact that many of those systems actually contain other undis- covered planets. Detecting additional planets in those systems to remove them from the single-planet sample set would therefore be an interesting target for more in-depth research to improve the observed trend. Regardless of this anomaly, the re- searchers were able to estimate that the probability of a system consist- ing of eight or more planets is close to 1%, a value in good agreement with other recent predictions made through analysis based on independ- ent arguments. In conclusion, in the Milky Way, there could be at least a billion systems similar to ours, each with one or more planets likely placed in the habitable zone − a very interesting perspective from an as- trobiological point of view. !