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Register a new userSpecial cases : moons, rings, comets, trojans
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Non-planetary bodies provide valuable insight into our current under- standing of planetary formation and evolution. Although these objects are challeng- ing to detect and characterize, the potential information to be drawn from them has motivated various searches through a number of techniques. Here, we briefly review the current status in the search of moons, rings, comets, and trojans in exoplanet systems and suggest what future discoveries may occur in the near future.
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Each of the giant planets within the Solar System has large moons but none of these moons have their own moons (which we call ${it submoons}$). By analogy with studies of moons around short-period exoplanets, we investigate the tidal-dynamical stability of submoons. We find that 10 km-scale submoons can only survive around large (1000 km-scale) moons on wide-separation orbits. Tidal dissipation destabilizes the orbits of submoons around moons that are small or too close to their host planet; this is the case for most of the Solar Systems moons. A handful of known moons are, however, capable of hosting long-lived submoons: Saturns moons Titan and Iapetus, Jupiters moon Callisto, and Earths Moon. Based on its inferred mass and orbital separation, the newly-discovered exomoon candidate Kepler-1625b-I can in principle host a large submoon, although its stability depends on a number of unknown parameters. We discuss the possible habitability of submoons and the potential for subsubmoons. The existence, or lack thereof, of submoons, may yield important constraints on satellite formation and evolution in planetary systems.
Saturns mid-sized moons (satellites) have a puzzling orbital configuration with trapping in mean-motion resonances with every other pairs (Mimas-Tethys 4:2 and Enceladus-Dione 2:1). To reproduce their current orbital configuration on the basis of Crida & Charnozs model of satellite formation from a hypothetical ancient massive rings, adjacent pairs must pass 1st-order mean-motion resonances without being trapped. The trapping could be avoided by fast orbital migration and/or excitation of the satellites eccentricity caused by gravitational interactions between the satellites and the rings (the disk), which are still unknown. In our research, we investigate the satellite orbital evolution due to interactions with the disk through full N-body simulations. We performed global high-resolution N-body simulations of a self-gravitating particle disk interacting with a single satellite. We used $N sim 10^5$ particles for the disk. Gravitational forces of all the particles and their inelastic collisions are taken into account. As a result, dense short-wavelength wake structure is created by the disk self-gravity and global spiral arms with $m sim$ a few is induced by the satellite. The self-gravity wakes regulate the orbital evolution of the satellite, which has been considered as a disk spreading mechanism but not as a driver for the orbital evolution. The self-gravity wake torque to the satellite is so effective that the satellite migration is much faster than that was predicted with the spiral arms torque. It provides a possible model to avoid the resonance capture of adjacent satellite pairs and establish the current orbital configuration of Saturns mid-sized satellites.
The only discovery of Earth Trojan 2010 TK$_7$ and the subsequent launch of OSIRIS-REx motive us to investigate the stability around the triangular Lagrange points $L_4$ and $L_5$ of the Earth. In this paper we present detailed dynamical maps on the $(a_0,i_0)$ plane with the spectral number (SN) indicating the stability. Two main stability regions, separated by a chaotic region arising from the $ u_3$ and $ u_4$ secular resonances, are found at low ($i_0leq 15^circ$) and moderate ($24^circleq {i_0}leq 37^circ$) inclinations respectively. The most stable orbits reside below $i_0=10^circ$ and they can survive the age of the Solar System. The nodal secular resonance $ u_{13}$ could vary the inclinations from $0^circ$ to $sim 10^circ$ according to their initial values while $ u_{14}$ could pump up the inclinations to $sim 20^circ$ and upwards. The fine structures in the dynamical maps are related to higher-degree secular resonances, of which different types dominate different areas. The dynamical behaviour of the tadpole and horseshoe orbits, reflected in their secular precession, show great differences in the frequency space. The secular resonances involving the tadpole orbits are more sensitive to the frequency drift of the inner planets, thus the instabilities could sweep across the phase space, leading to the clearance of tadpole orbits. We are more likely to find terrestrial companions on horseshoe orbits. The Yarkovsky effect could destabilize Earth Trojans in varying degrees. We numerically obtain the formula describing the stabilities affected by the Yarkovsky effect and find the asymmetry between the prograde and retrograde rotating Earth Trojans. The existence of small primordial Earth Trojans that avoid being detected but survive the Yarkovsky effect for 4.5,Gyr is substantially ruled out.
The most distant Kuiper belt objects exhibit the clustering in their orbits, and this anomalous architecture could be caused by Planet 9 with large eccentricity and high inclination. We then suppose that the orbital clustering of minor planets may be observed somewhere else in the solar system. In this paper, we consider the over 7000 Jupiter Trojans from the Minor Planet Center, and find that they are clustered in the longitude of perihelion $varpi$, around the locations $varpi_{mbox{{J}}}+60^{circ}$ and $varpi_{mbox{{J}}}-60^{circ}$ ($varpi_{mbox{{J}}}$ is the longitude of perihelion of Jupiter) for the L4 and L5 swarms, respectively. Then we build a Hamiltonian system to describe the associated dynamical aspects for the co-orbital motion. The phase space displays the existence of the apsidally aligned islands of libration centered on $Deltavarpi=varpi-varpi_{mbox{{J}}}approxpm60^{circ}$, for the Trojan-like orbits with eccentricities $e<0.1$. Through a detailed analysis, we have shown that the observed Jupiter Trojans with proper eccentricities $e_p<0.1$ spend most of their time in the range of $|Deltavarpi|=0-120^{circ}$, while the more eccentric ones with $e_p>0.1$ are too few to affect the orbital clustering within this $Deltavarpi$ range for the entire Trojan population. Our numerical results further prove that, even starting from a uniform $Deltavarpi$ distribution, the apsidal alignment of simulated Trojans similar to the observation can appear on the order of the age of the solar system. We conclude that the apsidal asymmetric-alignment of Jupiter Trojans is robust, and this new finding can be helpful to design the survey strategy in the future.
The exquisite photometric precision of the Kepler space telescope now puts the detection of extrasolar moons at the horizon. Here, we firstly review observational and analytical techniques that have recently been proposed to find exomoons. Secondly, we discuss the prospects of characterizing potentially habitable extrasolar satellites. With moons being much more numerous than planets in the solar system and with most exoplanets found in the stellar habitable zone being gas giants, habitable moons could be as abundant as habitable planets. However, satellites orbiting planets in the habitable zones of cool stars will encounter strong tidal heating and likely appear as hot moons.
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