Free Astronomy Magazine July-August 2014

EXOPLANETS Already by the late ‘90s, it seemed rather clear that giant planets located in very close orbits (so-called hot Jupiters) were typically hosted by stars with high abun- dances of metals in their photosphere. (We remind that the photosphere is the visible “surface” of a star and that its chemical composition reflects quite closely that of the entire stellar globe.) The statistics were however extremely lim- ited due to a number of factors, starting with the difficulty of discovering giant planets, especially on not too tight orbits. The observational evidences, at least as re- gards to hot Jupiters, were however in line with the theoretical expectations. In fact, if the protoplanetary disks from which plan- ets take form are characterized by a high content of metals, the accretion processes are faster and this favours the “extra large sizes”, since planets manage to become giants before the winds and radiation of their newborn star have a chance to dissi- pate what remains of the disk. What was not yet known was if also the smaller-sized planets, especially the terres- trial type, could somehow reflect the level of metallicity of their stars. It seemed not, but the limited number of small planets known until a few years ago did not allow drawing definitive conclusions. To cast some light on the issue, it was necessary to have available a much more consistent sta- tistical sample than those previously consid- ered, and as soon as the quantity of dis- covered planets reached a reliable level, Buchhave and colleagues decided to tackle the problem, starting from the spectral analysis of 405 stars, orbited by a total of 600 relatively small planets. To the researchers' surprise, it emerged that the size distribution of those planets is by no means insensitive to the metallicity of the stars. In fact, if this latter is compa- rable to that of the Sun, the sizes of the planets are usually less than 1.7 times that of the Earth. For metallicity slightly higher than the solar one, the stars preferably host planets from 1.7 to 3.9 times larger than Earth, while bodies of still great- er size (but not giants) are more fre- quent around stars with enhanced me- tallicity. The most immediate conclu- sion that can be drawn from this di- vision into groups is that planets more similar to Earth in size preferably form around stars which metallicity is compa- rable to or slightly lower than that of the Sun – a fact not so obvious as it L ars Buchhave, a very active researcher in the field of extrasolar planets, who in recent years has been the co-au- thor of important discoveries. The latest is that of the high density of Kepler-10c. [Credit: Carl- sbergfondet] M ass-radius diagram for planets with a ra- dius of less than 4 Earth radii, whose mass has been de- termined with sufficient accuracy by means of mea- surements of the radial velocities (represented by circles, while Kep- ler-10b and c by squares), or mea- sured with transit timing variations (triangles). Kepler- 10c is the most dense among the super-Earths. [L. Buchhave et al.]