It takes a special type of planet to make a moon

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This artist’s concept depicts two celestial bodies crashing into each other, creating a disc of mostly molten, partially vaporized rock that eventually became the moon.
NASA/JPL-Caltech

Based on analyzes of lunar rock samples that Apollo and other space missions have brought back to Earth, scientists know more about Earth’s moon than any other planetary body. However, questions continue to vex scientists regarding how a planet forms a moon and why some planets have moons while others do not.

In a new paper published in Nature Communications, researchers from the University of Rochester, the University of Arizona and the Tokyo Institute of Technology have made an important discovery that helps unlock the mysteries of the moon. The new study proposes that planets must be of a certain composition and within a limited mass range for moons to form.

“The question we decided to answer is: can all planets form substantial moons?” said the study’s co-author Erik Asphaugprofessor at the Arizona Lunar and Planetary Laboratory.

As scientists continue to explore the possibility of other life in the universe, the research has important implications for determining the factors that make a planet habitable. A planet may not need a big moon to be habitable, but Earth’s moon is vitally important in shaping life as we know it: it controls day length and tides. oceans, which affect the biological cycles of the planet. The Moon also contributes to Earth’s climate by stabilizing its axis of rotation, providing an ideal environment for life to grow and evolve.

“By understanding lunar formations, we have a better constraint on what to look for when looking for Earth-like planets,” said the study’s lead author, Miki Nakajima, assistant professor of science at Earth and Environment at the University of Rochester. “We expect exomoons (moons orbiting planets outside our solar system) to be everywhere, but so far we haven’t confirmed any. Our constraints will be useful for future observations.”

Earth is the only planet known to harbor life and has a number of characteristics that make it unique. These include active plate tectonics, a strong magnetic field, and a fairly large moon. More than a quarter the size of Earth, the moon raises tides in the oceans that are important for life, so researchers speculated that a moon could be a potentially beneficial feature for harboring life on d other planets.

It is generally believed that Earth’s large moon was generated by a collision between proto-Earth and another planet about 4.5 billion years ago. The collision resulted in the formation of a disc of mostly molten, partially vaporized rock that eventually became the moon.

This illustration shows the relative sizes of the planets needed to form moons

For rocky planets larger than Earth by 60% in size, or larger than six times the mass of Earth (the upper left planet), and for icy planets more than 30% larger than Earth, or an Earth-mass (the first two planets on the right), it is not possible to form large moons by giant impacts, according to the new study. Such “super-Earths” do not exist in our solar system, so the prediction applies to astronomical searches for planets around other stars. The prediction is consistent with planet-moon systems in the solar system, including Earth-Moon (left) and Pluto-Charon (right) that are shown here.
Nakajima et al.

To find out if other planets can form similarly sized moons, the researchers ran 3D computer simulations, starting with a number of hypothetical planets of varying masses. Infant planets either had an architecture similar to Earth’s—namely, a mantle of rock and an iron core—or icy planets in which the mantle is water ice and the core is rock.

They found that if the planet is too massive, giant impacts produce discs made entirely of vapor. This is because impacts between more massive planets are more energetic and occur at a higher speed. Over time, the vapor disk from such an impact cools and liquid moons – the building blocks of moons – form.

“These growing moonlets experience a strong drag force as they pass through the vapor disk, which slows them down,” Asphaug said. “In a short time, they succumb to the gravity of the planet and fall into it, without making a moon.”

Once the disc has cooled and its vapor has begun to dissipate, the moons face less drag and have a better chance of turning into moons. However, at that time a significant part of them were lost to the planet. The researchers conclude that a disk composed only of vapor is not capable of forming large moons. The disc must initially start at low vapor content for the moonlets to develop without experiencing a strong gas drag.

“Giant impacts work great for creating moons, but only up to a point,” Asphaug said.

The findings provide important benchmarks for researchers studying planets in other solar systems, the authors said. In particular, some planets are too massive to form a sizeable moon. Computer simulations have shown rocky planets more than six times the mass of Earth – so-called super-Earths – end up with vapor-only discs, meaning a large moon couldn’t develop . The same is true for planets with a composition rich in water or ice whose mass exceeds that of the Earth; they too will end up with vapor-only discs and no moon.

Thousands of exoplanets have been discovered, but astronomers have yet to definitively spot a moon orbiting a planet outside our solar system.

“We are still a long way from learning the different types of moons that orbit exoplanets that exist,” Asphaug said.

“The search for exoplanets has focused on large planets, generally larger than 1.6 Earth radii (60% larger than Earth),” Nakajima said. “We propose that instead we should look at smaller planets because they are likely better candidates for hosting fractional moons.”

The research is based on computer simulations that are the result of efforts since the 1980s to understand how Earth’s moon formed from a giant impact at the end of Earth’s formation, Asphaug said. .

“Until now most of the attention has been on the formation of Earth’s moon, but we think the moon started out as an ocean of flying magma, not primarily vapour,” he said. he stated, referring to the disc of molten rock around the early Earth. “These moons may have accreted into the major body we know and love.”

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