Science Gazette

Moons might provide insight on what makes planets livable

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Because the moon is so crucial to life on Earth, scientists believe that a moon may be a useful feature for sustaining life on other worlds. Other planets have moons, but Earth’s is unique in that it is big in comparison to the size of the planet; the moon’s radius is more than a quarter of Earth’s, a far higher ratio than most moons to their planets. According to new studies, this difference is crucial.

The moon plays a critical role in shaping Earth into the globe we know today: it regulates the length of the day and ocean tides, all of which have an impact on the biological cycles of lifeforms on our planet. The moon also helps to maintain Earth’s temperature by stabilizing the planet’s spin axis, providing an optimal environment for life to develop and evolve.

Because the moon is so crucial to life on Earth, scientists believe that a moon may be a useful feature for sustaining life on other worlds. Other planets have moons, but Earth’s is unique in that it is big in comparison to the size of the planet; the moon’s radius is more than a quarter of Earth’s, a far higher ratio than most moons to their planets.

That difference is crucial to Miki Nakajima, an assistant professor of earth and environmental sciences at the University of Rochester. And she and her colleagues from the Tokyo Institute of Technology and the University of Arizona examined moon formations in a recent research published in Nature Communications, concluding that only specific kinds of planets may develop huge moons in relation to their home planets.

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“We have a greater limit on what to look for when looking for Earth-like planets since we understand moon formations,” Nakajima adds. “We anticipate exomoons [moons circling planets beyond our solar system] to be abundant, but we have yet to find any. Future observations will benefit from our restrictions.”

The genesis of the moon’s orbit around the Earth

Many scientists have long assumed that Earth’s huge moon was formed by a 4.5 billion-year-old collision between proto-Earth (Earth in its early stages of evolution) and a massive, Mars-sized impactor. The impact caused a partly vaporized disk to form around Earth, which later grew into the moon.

Nakajima and her colleagues used computer simulations to see whether other planets might generate moons as massive as Earth’s. They used a variety of hypothetical Earth-like rocky planets and ice planets of varied sizes. They wanted to see whether the simulated collisions resulted in partly vaporized disks, similar to the disk that created Earth’s moon.

The researchers discovered that rocky planets with a mass more than six times that of Earth (6M) and ice planets with a mass greater than one Earth mass (1M) create completely evaporated disks rather than partly vaporized disks, which are incapable of creating fractionally massive moons.

“We discovered that if the planet is too huge, these collisions form totally vapor disks,” Nakajima adds. “This is because encounters between massive planets are typically more energetic than impacts between tiny planets.”

After an impact that creates a vaporized disk, the disk cools down and liquid moonlets — the building components of a moon — emerge. The expanding moonlets in a completely evaporated disk encounter significant gas drag from vapor, and descend onto the planet extremely swiftly. Moonlets, on the other hand, do not experience as much gas drag when the disk is just half evaporated.

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“As a consequence,” Nakajima explains, “we infer that a fully gaseous disk is incapable of generating fractionally big moons.” “In order to form such moons, planet masses must be less than the criteria we determined.”

 

The hunt for planets that are similar to Earth is ongoing

The limits established by Nakajima and her colleagues are crucial for astronomers studying our cosmos; although hundreds of exoplanets and putative exomoons have been discovered, no moon circling a planet beyond our solar system has been conclusively discovered.

 

This study might help them figure out where to look

As Nakajima puts it, “The hunt for exoplanets has traditionally been limited to planets with a mass greater than six Earth masses. Instead, we propose that we look at smaller planets, since they are likely to be better prospects for hosting fractionally massive moons.”

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