Saturday, August 20, 2022

Studing orbit stability to better understand planet formation process

 In a recent study published in Earth and Planetary Astrophysics, a team of researchers from the University of Texas at Arlington, Valdosta State University, Georgia Institute of Technology, and the National Radio Astronomy Observatory estimated how many moons could theoretically orbit the Earth while maintaining conditions such as orbital stability.  This study opens up the potential for a better understanding of planet formation processes that could also be applied to the identification of exolunes possibly also orbiting Earth-like exoplanets.

 "In previous work, I looked at the planet's packaging for the Alpha Centauri binary," said Dr. Billy Quarles, assistant professor of astronomy and astrophysics at Valdosta State University and co-author of the study.

 "In that case, I developed an estimate of the number of planets that might exist within the habitable zone of each star. In that scenario, the habitable zone provided natural boundary conditions, in which I could use the same dynamic formalism for the moons problem (Using the Earth-Sun system as a binary).

 To define the outer boundary, one of my co-authors developed a scheme that we could use. Combining these ideas, we expected that> 10 Ceres-, 6 Pluto- and 4 would be possible objects the size of the Moon (eg Table 2 of our article) ".

 While there are more than 200 moons in our solar system, only three orbit terrestrial (rocky) planets: our moon (Luna) around the Earth and Phobos and Deimos around Mars.  The remaining more than 200 moons orbit all the gas giants, to include Jupiter, Saturn, Uranus and Neptune.  As the study points out, this great difference is predictable because "they undergo different formation mechanisms and orbital evolution processes".

 The study examines how the maximum allowable number of moons that could exist around the Earth depends on the assumed size of the moons themselves.

 In this case, the researchers used objects the size of Ceres, Pluto and the Moon to determine how many of each could successfully orbit the Earth.  The results showed that orbital stability could be maintained with satellites up to 7 ± 1 mass of Ceres, 4 ± 1 mass of Pluto and 3 ± 1 moons of lunar mass.

 "The surprise was that the lower mass prototypes were more constrained, which we attribute to their higher probability of dispersion (due to lower inertia)," Quarles said.

 "The perturbations of nearby moons are sufficient to cause substantial dispersion in a few thousand years.

 We had to reduce the number of moons to explain it. "

 As seen with Jupiter's Galilean moons, small satellites orbiting a much larger planetary body can result in what is known as tidal warming, where the constant stretching and compression experienced by much smaller satellites leads to interesting results. , including volcanism on Io and an ocean hinterland on Europa.

 But could a multi-lunar system with Earth also experience these same results?

 "The tidal warming of the moons themselves may be possible, but the extent of the warming is not clear without detailed simulations," said Dr Quarles.

 "It is tempting to suggest that the innermost moon might resemble Io, but its tidal warming is in part due to mean motion resonances with the other Galilean moons. In our systems, mean motion resonances greatly destabilize the system. "satellites because the sun adds to each the increase in the eccentricity of the moon and its eventual dispersion".

 Along with the potential for tidal warming, this study also potentially expands the search for exolunes orbiting exoplanets.  Unfortunately, while the number of confirmed exoplanets is in the thousands, the number of confirmed exoplanets is currently less than a fraction of that number.

 "We currently have 2 candidate exolunes (Kepler-1625b-i and Kepler-1708b-i), but their respective host planets are similar to Jupiter," said Dr Quarles.

 "Candidate moons are even bigger than Earth."

 These more exotic cases may be easier to identify in a similar way.

 Hot Jupiters were easier to spot on smaller planets in the early days of exoplanets.

 However, more planet systems were discovered soon after.  first true exoplanets.

 We expect something similar for the eluunas.  When we have multiple candidate exolunes orbiting the same planet, our work will be more useful.

 The constraints we find are rather optimistic, where more realistic conditions are likely to limit the number of additional lunar exoplanets.  In photometric measurements, the objects in the background could mimic the transit signal of a candida exoluna and our work provides a physical basis for limiting the number of expected moons by testing different hypotheses. "

 Dr Suman Satyal, assistant adjunct professor of physics at the University of Texas at Arlington and lead author of the study, said that because the Earth can have more than one moon, this "increases the probability" of detecting exoons.  "This should give exmoon observers an idea of ​​the upper limit of the number of moons around a planet of Earth mass orbiting a sun-like star," he said.

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