Searching for moons with rings outside the Solar System 

The realm of exoplanets, i.e. planets orbiting a star other than the Sun, never ceases to amaze in terms of the richness and diversity of the worlds that inhabit them. Interestingly, the discovery of these new worlds does not necessarily come “out of the box” from the observations, as theoretical and computational models need to be developed by astrophysicists and applied to better understand the observations. Of course such models and predictions must be confronted to observations to confirm our hypotheses. Many exoplanets have been detected, but exomoons, exoplanetary rings, moonmoons (or submoons, small bodies that orbit the moons of exoplanets), and ploonets (or plunets, satellites of giant exoplanets that escape from their orbit to become a new world of the planetary system), are theoretical astrophysical objects that still await observational confirmation.

In this line, an international team of astrophysicists, led by Mario Sucerquia, FONDECYT postdoctoral researcher at the University of Valparaiso, and member of the Nucleus Millennium of Planetary Formation (NPF), proposed the existence of a new kind of astronomical object, the chronomoons (or chronomoons). These are satellites of the giant planets surrounded by complex ring systems, as described in a new research published in the prestigious journal Monthly Notices of The Royal Astronomical Society, UK.

The research also involved NPF director Amelia Bayo, research associates Jorge Cuadra and Johan Olofsson, and research adjoint researcher Matias Montesinos.

“Moons with rings are a typical alien landscape of science fiction stories and futuristic adventures of interstellar travelers. Despite this, prior to this work, there has been no attempt to study with scientific depth these hypothetical worlds, which can also offer us valuable information on the dynamic and morphological complexity of planetary systems. This intellectual vacuum, together with the notorious absence of chronomoons in the solar system, were the main motivations to investigate their possible existence, their formation and stability routes, and the possible observational traces that could reveal their presence”, explains Sucerquia.

To carry out the research, semi-analytical models describing the orbital migration of the satellites were used, in addition to numerical simulations designed to study the stability of the component particles of the rings under the influence of solar radiation and the gravitational perturbations of all the celestial bodies involved. The numerical simulations are computer approximations to perform highly complex calculations, where it is not possible, or too expensive, to solve the analytical expression.

Result of the numerical simulations made for an icy ring chronomoon. It shows the formation of gaps in the rings.

In addition to their stability, the researchers found a rather peculiar feature in the rings, which is a consequence of the periodic gravitational perturbations of the moon’s host planet on the particles that compose them. In the simulations they noticed that the rings of chrono moons are very prone to displaying gaps, which appear naturally. This is a phenomenon analogous to the so-called Cassini splits of Saturn’s rings, or the Kirkwood gaps in the asteroid belt of the Solar System.

According to the publication, chrono moons could have several ways of formation. Most of them require a violent interaction, such as collisions between moons. Subsequently, the debris from these collisions would be scattered in orbit of the surviving body forming ring-like structures, whose composition, morphology and size would depend on the physical properties of the objects involved. The researchers found that there are a number of physical conditions under which chronmoons can be stable over millions of years, and that the survival of rings composed of dust is more feasible than that of icy rings, but with the dust ones being more compact than their icy counterparts.

“The method of planetary transits (i.e., the detection of microeclipses in other stars), could reveal the existence of chrono moons through two effects to be taken into account. First, by having rings, the depth of the chrono-moon transits would be much greater than that of bodies without rings; that is, the rings would hide a larger area of the star, so that a detection could be erroneously interpreted as coming from an object much larger in size and without rings. However, in truth it would be a lighter object, which would induce in the planet a much smaller gravitational wobble than expected, altering to a lesser extent the periodicity and duration of the planet’s transits. These two effects, which are mutually incompatible, would be the footprint of a chrono-moon”, explains Sucerquia.

However, says the scientist, to detect a chrono-moon it is necessary to first detect a moon, and that is still a challenge.  “There are currently a handful of exo-moon candidates, but only the most unusual of them all seems to pass all the verification filters. According to the interpretation of the system’s light curves, Kepler 1625 b is composed of a planet whose mass resembles that of Jupiter, accompanied by a satellite as large as Neptune. The sizes of the bodies involved in this configuration are incompatible with current models of satellite formation, so the interpretation of these light curves is still a matter of debate among the planetary science community. The chronmoons offer another possible interpretation for these light curves, partly solving the problem of the moon’s size,” he says.

Scientific publication

 

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