Letter to the Editor

Formation of the terrestrial planets in the solar system around 1 au via radial concentration of planetesimals

¹ Division of Theoretical Astronomy, National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, 181-8588 Tokyo, Japan
e-mail: masahiro.ogihara@nao.ac.jp
² School of Arts & Sciences, University of Tokyo, 3-8-1, Komaba, Meguro, 153-8902 Tokyo, Japan
² Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Blvd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France

Received: 17 January 2018
Accepted: 29 March 2018

Abstract

Context. No planets exist inside the orbit of Mercury and the terrestrial planets of the solar system exhibit a localized configuration. According to thermal structure calculation of protoplanetary disks, a silicate condensation line (~1300 K) is located around 0.1 au from the Sun except for the early phase of disk evolution, and planetesimals could have formed inside the orbit of Mercury. A recent study of disk evolution that includes magnetically driven disk winds showed that the gas disk obtains a positive surface density slope inside ~1 au from the central star. In a region with positive midplane pressure gradient, planetesimals undergo outward radial drift.

Aims. We investigate the radial drift of planetesimals and type I migration of planetary embryos in a disk that viscously evolves with magnetically driven disk winds. We show a case in which no planets remain in the close-in region.

Methods. Radial drifts of planetesimals are simulated using a recent disk evolution model that includes effects of disk winds. The late stage of planet formation is also examined by performing N-body simulations of planetary embryos.

Results. We demonstrate that in the middle stage of disk evolution, planetesimals can undergo convergent radial drift in a magnetorotational instability (MRI)-inactive disk, in which the pressure maximum is created, and accumulate in a narrow ring-like region with an inner edge at ~0.7 au from the Sun. We also show that planetary embryos that may grow from the narrow planetesimal ring do not exhibit significant type I migration in the late stage of disk evolution.

Conclusions. The origin of the localized configuration of the terrestrial planets of the solar system, in particular the deficit of close-in planets, can be explained by the convergent radial drift of planetesimals in disks with a positive pressure gradient in the close-in region.