Twenty-three years ago astronomers announced the discovery of 51 Pegasi-b, the first confirmed planet orbiting a Sun-like star. The tally of known extrasolar planets now stands at 3793 planets, 2837 planetary systems (http://exoplanet.eu/) with nearly 2423 more candidates waiting to be confirmed.
In this general framework, the present project will deal with 2 new, peculiar topics of exoplanet physics, shown to be of great interest by the rapidly growing observational evidence. One refers to the dynamical features of multiplanet extrasolar systems (i.e. several planets orbiting a star), the second to planets orbiting a binary or even multiple star system, both in isolation and in a stellar environment (a star cluster). With this project, we intend to investigate the dynamical properties of such systems both by analytical and numerical methods.
A few questions to answer are:
- is it a mere coincidence that the maximum number of planets observed so far around a single star is 7-8 ?
- is this limited number due to effects of the environment?
- has the external and internal (hosting star) tidal field played a role in the number and location of planets?
- if the answer to the previous question is positive, is this tidal field responsible also for placing planets of the famous TRAPPIST-1 system in orbital resonance?
- are circumbinary disks stable?
- how is it possible to form planet around a binary star?
We aim to answer these questions in the frame of the GAPS (Global Architecture of Planetary Systems), collaboration (http://www.oact.inaf.it/exoit/EXO-IT/Projects/Entries/2011/12/27_GAPS.html) and in collaboration with the foreign institutions indicated below.
Our specific task is the investigation of the evolution of orbits of protoplanets still in their birth discs and of multiple planets around a single and binary star.
These systems will be studied in isolation as well as in a more or less dense environment (an "open" or "globular" star cluster).
As mentioned above, the field of planetary dynamics has been little considered in the recent past because it seemed that most of the output has already been obtained. Celestial mechanics, which is the main theoretical tool, saw a large development leading to the great successes of the discovery of external planets of the solar system and the intra-solar system probes. Nowadays, with the ever rising number of exoplanets discoveries and with the increasing details in the information about their characteristics, it has become clear that an interpretation on the base of a study of their motion is getting more and more important. As an example: the distribution of exoplanets around their primary star shows massive planets (of Jupiter size) closer to the star than expected (semi-major axes less than Mercury's and hours to few days revolution period!) , both on the base of what happens in our solar system (giant planets have large orbits) and of theoretical thought about the difficulty of formation of giant planets in such a hot environment.
This feature of an excess of massive planets close to the star is confirmed also after disentangling the observational bias. The reasons for this peculiarity are just hypotheses. The main claim is that of a migration of the giant planet from the outskirt (where the environment is more favorable to gaseous giants formation) toward the parent star. But this phenomenon still needs a firm theoretical explanation. According to some authors, it is the residual gaseous disc after planet formation that may have caused the giant planets sink toward the system center. This is in principle possible, but it deserves a thorough demonstration which is, contemporary, compatible with the other observed feature (see [15] in the previous section) of a more numerous number of massive planets close to their stars in star cluster than in the environment.
Another new field of celestial mechanics, never studied before because there was no evidence of such kind of objects in the sky, would be the study of planetary motion around binary or even triple or quadruple stars. This may get far beyond the analytical treatment, which gives valid approximations only in the case of a single "mass-less" planet around a binary, so as to require a numerical treatment, in which the ASTRO group in Rome has a well established international reputation.
Given all this, we think the project is modern and timing. Consequently, we think that our group in Sapienza, in international collaboration with the Observatory of Leiden (NL), has the full possibility to take a lead in such field of investigation. This would bring, over a 2 year range, to at least 3 publications which should be state of the art in the field of multi-planet systems and planets around multiple stars.
Finally, we like to point attention on the availability of computational resources of our scientific group, whose experience in computational astrophysics is well established.