Multiple rings in protoplanetary disks: one planet to govern them all

Recent observations of protoplanetary disks, these planetary systems in formation, frequently reveal sequences of dark and bright rings. While these structures are often attributed to multiple planets, numerical simulations show that they can be generated by a single Saturn moving jerkily towards its star. These results published in Monthly Notices of the Royal Astronomical Society (MNRAS) are the result of work carried out at the Institut de Recherche en Astrophysique et Planétologie (IRAP – CNRS/CNES/UT3 Paul Sabatier).

More and more planets are being detected around stars other than the Sun. There are even many cases of multiplanetary systems, whose planets formed during the early years of their star. Each protoplanetary disc that is frequently observed today is therefore a valuable photograph of a developing planetary system, and understanding them is a key issue in modelling the formation and evolution of planets.

Structures are very regularly observed in the light emitted by the dust of the protoplanetary discs. These structures, which often take the form of bright and dark rings, are fixed witnesses of dynamic mechanisms at work and have generated a great deal of activity on the part of the modelers of the protoplanetary discs. A common model suggests that each dark ring corresponds to the groove opened by a planet orbiting its star at a given distance. On the one hand, there is as yet no observational evidence of the presence of such planets, and on the other hand, the interaction between a planet and the gas in the protoplanetary disc can rapidly change the distance of the planet from its star. Taking into account this planetary migration can therefore provide more precise information about the impact of a planet on dust emission, especially in radio.

Above: light emitted by the dust of four protoplanetary discs observed with the ALMA radio telescope. Credit: Huang et al (2018). Sequences of bright and dark rings can be seen in these images. Bottom: Prediction of the radio emission of dust disturbed by the jerky motion of a giant planet in its disc. The alternation of bright and dark rings shows an observable contrast in intensity with the current sensitivity of ALMA.

Numerical simulations of a protoplanetary disk of gas and dust carried out at IRAP have shown that the intermittent migration of a Saturn-like planet in its disk is capable of generating successive rings of millimetre-sized dust. Complementary calculations of radiative transfer in dust, which model the interaction between the light rays of the central star and the dust in the disk, have also confirmed that the presence of these dust rings leads to an alternation of bright and dark rings in the radio emission. Moreover, the contrast in intensity obtained between bright and dark rings is detectable with the current sensitivity of the ALMA radio telescope.

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