Planets without moons
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Planets Without Moons: Insights from Recent Research
Compact Planetary Systems and Moonless Worlds
Exoplanetary Systems and Moon Presence
Recent discoveries in exoplanetary science have revealed a significant number of compact planetary systems, where multiple planets orbit within approximately 0.5 astronomical units (au) of their host star. These systems are particularly common around low-mass stars, which often have planets situated within the habitable zone1. However, the presence of moons in these compact systems is constrained by several factors, including the Hill radius and Roche limit. These constraints suggest that many planets in such systems, including those in the well-studied TRAPPIST-1 system, are likely to be moonless1.
Habitability and the Role of Moons
The habitability of a planet can be influenced by the presence of a substantial moon. For Earth, the Moon's tidal effects help stabilize axial tilt variations and promote the formation of tidal pools, which are considered beneficial for the development of life1. In compact planetary systems, the limited space and gravitational interactions make it challenging for moons to form or remain stable, thus many of these planets may lack moons, potentially impacting their habitability1.
Moons and Submoons: Stability and Formation
Stability of Submoons
In our Solar System, giant planets have large moons, but these moons do not have their own moons, known as submoons. Research indicates that submoons can only remain stable around large moons with wide-separation orbits. Tidal dissipation destabilizes submoons around smaller moons or those too close to their host planet2. This phenomenon explains why most moons in our Solar System do not have submoons, with only a few exceptions like Saturn's Titan and Iapetus, Jupiter's Callisto, and Earth's Moon being capable of hosting long-lived submoons2.
Implications for Exomoon Studies
The study of submoons also extends to exoplanetary systems. For instance, the exomoon candidate Kepler-1625b-I, based on its mass and orbital separation, could theoretically host submoons. However, its large orbital inclination might challenge the dynamical stability of such submoons2. The presence or absence of submoons can provide valuable insights into the processes of satellite formation and evolution in planetary systems2.
Conclusion
The research on compact planetary systems and the stability of moons and submoons highlights the complexities of satellite formation and stability. Many planets in compact systems, such as those around low-mass stars, are likely to be moonless due to gravitational constraints. This absence of moons could influence the habitability of these planets. Additionally, the study of submoons offers further understanding of the dynamics within planetary systems, both in our Solar System and beyond.
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Most relevant research papers on this topic
Worlds without Moons: Exomoon Constraints for Compact Planetary Systems
Most planets in compact planetary systems, such as TRAPPIST-1, are likely to be worlds without moons due to constraints on Hill radii and Roche limit considerations.
Can moons have moons?
Submoons can survive only around large, wide-separation orbits, with few exceptions, such as Saturn's moons and Earth's Moon.
Looking for planetary moons in the spectra of distant Jupiters.
Future space observatories could detect Earth-sized moons on Jupiter-sized planets by detecting their near-infrared spectral contrast, which could reveal potential habitats for life.
Habitable moons around extrasolar giant planets
Rocky moons orbiting extrasolar giant planets could potentially be habitable, if they orbit within the 'habitable zone' and have sufficient atmosphere and magnetic field strength.
Highly CitedMoon could be a planet under new definition
The new definition proposes that the moon could be considered a planet in its own right, if it meets the criteria of being more or less round, massive enough to have accreted or flung away objects in its orbital neighborhood, and not just round and massive enough to have a ring of objects orbiting it
Discovery of five irregular moons of Neptune
Five irregular moons with diameters of 30 to 50 km have been discovered, indicating that Neptune may have captured these moons from heliocentric orbits.
Moon-packing around an Earth-mass Planet
The maximum number of moons that can stably orbit an Earth-mass planet depends on the assumed masses of the satellites, with up to 7 Ceres-mass, 4 Pluto-mass, and 3 Luna-mass moons possible.
In Situ Formation of Icy Moons of Uranus and Neptune
Ice giants like Uranus and Neptune can form icy moons in gaseous circumplanetary disks within a couple of 105 years, potentially leading to a larger population of exomoons than previously thought.
Formation, habitability, and detection of extrasolar moons.
Extrasolar moons in the range of 0.1-0.5 Earth mass are potentially habitable, can form within circumplanetary debris and gas disks, and are detectable with current technology.
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