The invasion of a free-floating planet and the number asymmetry of Jupiter Trojans

¹ School of Astronomy and Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, PR China
e-mail: ljian@nju.edu.cn
² Key Laboratory of Modern Astronomy and Astrophysics in Ministry of Education, Nanjing University, Nanjing 210023, PR China
³ Department of Mathematics, Northwestern University, 2033 Sheridan Road, Evanston, IL 60208, USA
⁴ New York University Abu Dhabi, PO Box 129188 Abu Dhabi, United Arab Emirates
⁵ Center for Astro, Particle and Planetary Physics (CAP3), New York University Abu Dhabi, PO Box 129188 Abu Dhabi, United Arab Emirates
⁶ University of Occupational and Environmental Health, 1-1 Iseigaoka, Yahata, Kitakyusyu 807-8555, Japan
⁷ Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan

Received: 23 February 2023
Accepted: 25 April 2023

Abstract

Context. This paper extends our previous study of the early evolution of Jupiter and its two Trojan swarms by introducing the possible perturbations of a free-floating planet (FFP) invading the Solar System.

Aims. In the framework of the invasion of a FFP, we aim to provide some new scenarios to explain the number asymmetry of the L4 and L5 Jupiter Trojans, as well as some other observed features (e.g. the resonant amplitude distribution).

Methods. We investigate two different cases: (i) the indirect case, where Jupiter experiences a scattering encounter with the FFP and jumps outwards at a speed that is sufficiently high to make the L4 point temporarily disappear, resulting in a change in the numbers of the L4 (N₄) and L5 (N₅) Trojan swarms; (ii) the direct case, in which the FFP traverses the L5 region and affects the stability of the local Trojans.

Results. In the indirect case, the outward migration of Jupiter can be fast enough to make the L4 islands disappear temporarily, inducing an increase in the resonant amplitude of local Trojans. After the migration is over, the L4 Trojans come back to the reappeared and enlarged islands. As for the L5 islands, they always exist but expand even more considerably. Since the L4 swarm suffers less excitation in the resonant amplitude than the L5 swarm, more L4 Trojans are stable and could survive to the end. In the direct case, the FFP could deplete a considerable fraction of the L5 Trojans, while the L4 Trojans at large distances are not affected and all of them could survive.

Conclusions. Both the indirect and direct cases could result in a number ratio of R₄₅ = N₄/N₅ ~ 1.6 that can potentially explain the current observations. The latter has the advantage of producing the observed resonant amplitude distribution. To achieve these results, we propose that the FFP should have a mass of at least of a few tens of Earth masses and its orbital inclination should be allowed to be as high as 40°.