You've certainly heard that the Moon influences haircuts, pregnancies, people's personalities (for those who believe in astrology) and even our mood. Whether or not this is all true, we cannot be sure. However, in astronomical (rather than astrological) terms, she may have had an even greater responsibility: allowing us to exist on Earth.
This is what many astronomers have argued for a long time, based on scientific studies. Now, a new approach, published this week in the journal Nature Communications, goes further, revealing that our planet had the right dimensions to form such a large natural satellite, and that precisely the size of the Moon would have made life possible for here.
That study, carried out by researchers at the University of Rochester in New York, revealed that rocky planets with a diameter greater than 1.6 times that of Earth and icy planets with a diameter greater than 1.3 times that of our world are unlikely to create moons that would have these life-friendly effects on them.
Mild weather on Earth is favored by the Moon
Our Moon has a radius greater than a quarter of the Earth's radius. That's a much higher proportion than any other moon in our solar system and its host planet. Thanks to its large size compared to the planet, the Moon controls the length of Earth's day and governs ocean tides. It also stabilizes the planet's axis of rotation, which in turn stabilizes its mild climate, which is one of the key characteristics a world must have to support life.
According to scientists, the Moon was born from a cataclysmic collision of a nascent Earth with a Mars-sized world known as Theia. This impact awoke a huge amount of material, some of which turned into steam due to the heat generated by the event.
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For some time, this material circled the Earth in a disk similar to Saturn's ring system. The material in that disk, scientists believe, gradually gave rise to smaller moons, which later merged to form a large one.
So why can't larger planets achieve the same end result? According to the research, based on computer modeling, if larger worlds collide, the impact energy is such that all the ejected material vaporizes, not just some of it. And that makes a difference.
The large amount of vapor around the planet creates what is known as drag, which gradually slows the moons as they orbit the planet, causing them to fall to its surface, the study found. Drag is a phenomenon caused by friction forces, which act in a direction parallel to the surface of the object, and pressure forces, which act in a direction perpendicular to the surface of the object.
New findings could help scientists focus their search for habitable planets
"Our impact simulations show that terrestrial and icy planets larger than 1.3-1.6 [Earth's radius] produce vapor disks entirely, which fail to form a fractionally large moon," the researchers stated. “Our representation supports models of lunar formation that produce vapour-poor disks, and rocky, icy exoplanets whose radii are less than 1.6 [Earth's radius] are ideal candidates to host exomoons [moons orbiting planets outside our solar system] ] fractionally large”.
The findings from this study could help astronomers fine-tune their search for potentially habitable planets, as they simply have to focus on those that might have a large moon compared to their size.
"By understanding lunar formations, we have a better constraint on what to look for when looking for Earth-like planets," said Miki Nakajima, assistant professor of Earth and environmental sciences at the University of Rochester. “We expect exomoons to be everywhere, but so far we haven't confirmed any. Our constraints will be useful for future observations.”
More than 5,000 exoplanets have been discovered and none of them, so far, have been proven to have a moon, although scientists have found a few candidates.
Nakajima suggests that the reason for this absence of moons outside our solar system may simply be due to the size of the planets studied. "The search for exoplanets has typically been focused on worlds larger than six Earth masses," she said. "We are proposing that we should look at smaller planets instead because they are likely to be better candidates to host fractionally large moons."
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