Oblique rings from migrating exomoons: A possible origin for long-period exoplanets with enlarged radii

Abstract

Context. The extremely low density of several long-period exoplanets in mature systems is still unexplained -- with HIP 41378 f being archetypical of this category. It has been proposed that such planets could actually have normal densities but be surrounded by a ring observed approximately face on, mimicking the transit depth of a puffy planet. This would imply that the equator of the planet is nearly perpendicular to its orbit plane, which is at odds with the formation process of gas giants. Yet, in the context of the Solar System planets, it has been shown that after gigayears of evolution, the tidal migration of a moon can naturally lead to a very tilted planet with a ring. Aims. As exomoons are expected to be ubiquitous around giant exoplanets, this mechanism may be responsible for the anomalous radii of some observed exoplanets. In preparation for the future discoveries of the PLATO mission, we present a simple method for checking the plausibility of this mechanism for a given exoplanet. Methods. Analytical formulas give the probability density function of the relevant precession harmonics of the planet. For each harmonic, simple criteria set the moon mass and other properties required for the mechanism to operate. Results. We applied this methodology to HIP 41378 f, and we show that in order to reproduce the observed configuration, a hypothetical former moon should have had a moon-to-planet mass ratio of a few times 1e-4 (i.e. roughly the mass of our Moon) and have migrated over a distance of a few planet's radii on a gigayear timescale. These orders of magnitude match the properties of moons expected to exist around gaseous exoplanets. Conclusions. We conclude that the migration of a former moon is a viable formation pathway for the proposed ring and tilt of HIP 41378 f. This example strengthens the ring hypothesis and motivates its application to other targets.

arXiv commentAccepted for publication in Astronomy and Astrophysics

References in corpus (53)

• The PLATO 2.0 Mission
• Architecture of Kepler's Multi-transiting Systems: II. New investigations with twice as many candidates
• Breeding Super-Earths and Birthing Super-Puffs in Transitional Disks
• Exoplanet Orbital Eccentricities Derived From LAMOST-Kepler Analysis
• On the Radii of Close-in Giant Planets
• On the Anomalous Radii of the Transiting Extrasolar Planets
• Heat transport in giant (exo)planets: a new perspective
• New constraints on Saturn's interior from Cassini astrometric data
• Satellite dynamics on the Laplace surface
• The Physics of Bodily Tides in Terrestrial Planets, and the Appropriate Scales of Dynamical Evolution
• Resonance locking in giant planets indicated by the rapid orbital expansion of Titan
• Evolution of Ohmically Heated Hot Jupiters
• AMD-stability and the classification of planetary systems
• Tidal evolution of hierarchical and inclined systems
• The Featureless Transmission Spectra of Two Super-Puff Planets
• NOTE: Explaining why the Uranian satellites have equatorial prograde orbits despite the large planetary obliquity
• On the Terminal Rotation Rates of Giant Planets
• Obliquity-Driven Sculpting of Exoplanetary Systems
• Re-inflated Warm Jupiters Around Red Giants
• Tidally-Induced Radius Inflation of Sub-Neptunes
• Deflating Super-Puffs: Impact of Photochemical Hazes on the Observed Mass-Radius Relationship of Low Mass Planets
• K2-97b: A (Re-?)Inflated Planet Orbiting a Red Giant Star
• A novel method for identifying exoplanetary rings
• Dusty outflows in planetary atmospheres: Understanding "super-puffs" and transmission spectra of sub-Neptunes
• Formation of Ultra-Short-Period Planets by Obliquity-Driven Tidal Runaway
• Five Planets Transiting a Ninth Magnitude Star
• A collisionless scenario for Uranus tilting
• Ring Formation around Giant Planets by Tidal Disruption of a Single Passing Large Kuiper Belt Object
• Effects of Unseen Additional Planetary Perturbers on Compact Extrasolar Planetary Systems
• Exploring Whether Super-Puffs Can Be Explained as Ringed Exoplanets
• Can planetary rings explain the extremely low density of HIP 41378f?
• Secular spin-axis dynamics of exoplanets
• The First Near-Infrared Transmission Spectrum of HIP 41378 f, a Low-Mass Temperate Jovian World in a Multi-Planet System
• A Featureless Infrared Transmission Spectrum for the Super-Puff Planet Kepler-79d
• Oscillations of Relative Inclination Angles in Compact Extrasolar Planetary Systems
• Tilting Ice Giants with a Spin-Orbit Resonance
• The large obliquity of Saturn explained by the fast migration of Titan
• Dynamics of Colombo's Top: Generating Exoplanet Obliquities from Planet-Disk Interactions
• A Discrete Set of Possible Transit Ephemerides for Two Long Period Gas Giants Orbiting HIP 41378
• Dynamics of Colombo's Top: Tidal Dissipation and Resonance Capture, With Applications to Oblique Super-Earths, Ultra-Short-Period Planets and Inspiraling Hot Jupiters
• Revisiting the HIP41378 system with K2 and Spitzer
• Ohmic Dissipation in Mini-Neptunes
• A transit timing variation observed for the long-period extremely low density exoplanet HIP 41378f
• The future large obliquity of Jupiter
• Boundary Layer Circumplanetary Accretion: How Fast Could an Unmagnetized Planet Spin Up Through Its Disk?
• Asteroseismology of the Multiplanet System K2-93
• Tilting Uranus via the migration of an ancient satellite
• Co-accretion + giant impact origin of the Uranus system: Tilting Impact
• Non-Trivial Oblique Spin Equilibria of Super-Earths in Multi-planetary Systems
• The past and future obliquity of Saturn as Titan migrates
• On the Terminal Spins of Accreting Stars and Planets: Boundary Layers
• Rossiter-McLaughlin detection of the 9-month period transiting exoplanet HIP41378 d
• Future destabilisation of Titan as a result of Saturn's tilting

Cited by in corpus (7)

• Stability and detectability of exomoons orbiting HIP 41378 f, a temperate Jovian planet with an anomalously low apparent density
• The "Drake equation" of exomoons -- a cascade of formation, stability and detection
• HIP 41378 observed by CHEOPS: Where is planet d?
• Exploring Exoplanet Dynamics with JWST: Tides, Rotation, Rings, and Moons
• The formation of transiting circumplanetary debris discs from the disruption of satellite systems during planet-planet scattering
• Planetary Obliquity Excitation Through Pre-Main Sequence Stellar Evolution
• The Coupled Tidal Evolution of the Moons and Spins of Warm Exoplanets