Free Astronomy Magazine January-February 2021

20 JANUARY-FEBRUARY 2021 ASTRO PUBLISHING orbit from the sum of all of the early collisions during its formation will re- main substantially unchanged and therefore high. As the number of planets increases, the interactions will become more and more fre- quent and the planets may either be expelled from the system or settle on different orbits. Since gravitational interactions cause planets to lose en- ergy, more planets lead to more in- teractions, resulting in eventually more circular orbits. Is there a correlation between the multiplicity (number) of planets in a system and the final eccentricity of their orbits? This possibility was first evaluated in a 2014 paper (Davies et al.) and empirically tested in a 2015 paper (Limbach & Turner), taking over 400 exoplanets discovered by the radial velocity method as a refer- ence sample. On that occasion, a strong correlation (or anti-correla- tion, if you prefer) was highlighted between multiplicity and eccentric- ity. A similar study presented in 2017 (Zinzi & Turrini) substantially con- firmed that type of correlation, start- teract gravitationally with each other as they reach signifi- cant masses. The re- sult is a dispersion of planetesimals on de- cidedly more eccen- tric orbits, which in astronomically short timespans cause colli- sions and aggrega- tions that originate solid planets and the nuclei of gaseous ones. After the gas from the disk has been cap- tured by the young planets or dispersed by stel- lar radiation, the dominant mecha- nism for deter- mining the final eccentricities of the orbits are the planet-planet in- teractions them- selves. It is easy to understand that if a system con- sists of only one planet, the initial eccentricity of its A possible interpretation of the phe- nomenon was proposed in 2008, with the publication of a couple of works that identified the dynamic interaction between planets as a reg- ulator of orbital eccentricity. Sub- sequent works have improved our knowledge of that process, which be- gins very early on with the formation of planetesimals within the proto- planetary discs. These building blocks of the protoplanets form on rela- tively circular orbits but begin to in- T he authors of the work discussed in the article: Nanna Bach-Møller and Uffe Gråe Jørgensen. [Niels Bohr Institute, University of Copenhagen] T he percentage of planetary systems with a given multiplic- ity. The probability fits to an exponential function. Mean multiplicity estimated to ~ 2.5. [Bach-Møller & Jørgensen]