Categories: LifemoonsPlanets

Massive Rocky Planets Probably Don’t Have big Moons

The Moon has orbited Earth since the Solar System’s early days. Anyone who’s ever spent time at the ocean can’t fail to notice the Moon’s effect. The Moon drives the tides even in the world’s most remote inlets and bays. And tides may be vital to life’s emergence.

But if Earth were more massive, the Moon may never have become what it is now. Instead, it would be much smaller. Tides would be much weaker, and life may not have emerged the way it did.

The widely-believed Giant Impact Hypothesis explains the Moon’s formation. A Mars-sized protoplanet named Theia crashed into the young Earth, sending molten material into space. That material formed a disk around Earth which eventually coalesced into the Moon.

A new study examines the process laid out in the Giant Impact Hypothesis and shows that rocky planets more massive than Earth might not form large life-enabling satellites like Earth’s Moon.

The Moon formed as a result of a collision between the Earth and a protoplanet named Theia, according to the widely-believed Giant Impact Hypothesis. Credit: NASA/GSFC

The paper is “Large planets may not form fractionally large moons.” The lead author is Miki Nakajima, assistant professor of Earth and Environmental Sciences at the University of Rochester. Nature Communications published the paper.

The Moon is critical to life on Earth. The tides it causes are critical because life may have originated in intertidal zones. Intertidal zones are some of the most biologically diverse and nutrient-rich regions on Earth, partly because tides deliver regular quantities of ocean nutrients to the regions. Four billion years ago, Earth already had oceans, and the Moon may have been half the distance away that it is now. So the tides would have been even more extreme.

Strong tides create an extreme environment, forcing creatures to adapt. This pressure helped drive speciation, which pushes populations to develop into new species. The Moon also acts as a ballast by stabilizing Earth’s spin axis. That stabilization helps moderate Earth’s climate, enabling life to evolve into more and more complex forms. The Moon has undoubtedly shaped the course of life on Earth, of which we are the most complex expression. Thanks, Moon.

We may have Earth itself to thank for having just the right size moon. Our Moon is relatively large compared to Earth, and in the context of other planet-moon relationships in the Solar System. The other planets in the Solar System have much smaller moons relative to their masses. According to the authors of this new paper, only certain types of planets—and specific sizes—can form moons the size of Earth’s moon. And that has clear implications for the emergence of life elsewhere in the Universe, just as it has on Earth.

“By understanding moon formations, we have a better constraint on what to look for when searching for Earth-like planets.”

Miki Nakajima, lead author, University of Rochester.

“By understanding moon formations, we have a better constraint on what to look for when searching for Earth-like planets,” lead author Nakajima said in a press release. “We expect that exomoons [moons orbiting planets outside our solar system] should be everywhere, but so far, we haven’t confirmed any. Our constraints will be helpful for future observations.”

Earth’s Moon is large, but it’s not the largest. There are four moons in the Solar System larger than our Moon. But they orbit Jupiter and Saturn, which are far more massive than Earth. So the Earth-Moon relationship is unique because the Moon is fractionally large. Why is the Moon fractionally large?

The impact between Earth and Theia generated a rotating disk of material around Earth. Much of it eventually coalesced into the Moon, but some material fell back to Earth. If Earth had been more massive, the Moon might look very different or may never have formed at all. Much of this has to do with the vapour content of the disk of material.

The disk of debris created by an enormous impact is different depending on the nature of the planet. Different planets with different masses and compositions produce disks with varying amounts of vapour. And the vapour content can have a powerful effect on the fate of the debris disk.

If there’s more vapour, there’s more drag in the disk. As tiny moonlets began to form in the rotating disk, the gas drag from the vapour would pull the moonlets down to crash into Earth. They’d never get a chance to combine into the larger Moon, and instead would become a part of Earth.

If there’s not too much vapour, the drag is weaker, and tiny Moonlets can eventually create a Moon much like ours. But what are the mass ranges and other characteristics that lead to large moon formation? We know of thousands of exoplanets, and we assume many of them have moons. But moons are much harder to see. If we understand what sizes of planets produce larger moons beneficial for life, we have a better chance of identifying exoplanets that may harbour life.

An artist’s illustration of the Kepler 1625 system. The star in the distance is called Kepler 1625. The gas giant is Kepler 1625B, and the exomoon orbiting it is unnamed. The suspected moon is about as big as Neptune, but is a gas moon. Astronomers found evidence of the moon’s existence in 2017, but the original discoverers say that the detection is likely an error. So far we know of no exomoons for certain. Image: NASA, ESA, and L. Hustak (STScI)

Scientists suspect that giant impacts like the one between Earth and Theia are common in young solar systems. Young solar systems are unruly, and there are multitudes of objects moving around. Orbits haven’t settled yet, and larger planets can migrate towards and away from their stars, disturbing protoplanets and sending them smashing into each other. To find out what types of impacts likely produce larger moons in young solar systems, the team of researchers turned to computer simulations.

The amount of vapour in a Moon-forming disk is a critical factor. It’s called the vapour mass fraction (VMF.) A VMF of 1.0 means a debris disk is all vapour, while a VMF of 0.0 means there’s no vapour. Previous research into the VMF of disks showed that a moon-forming disk that’s too vapour rich might be too unstable to form a Moon because they lose too much mass in a short time. But this is the first study to examine gas drag in moon-forming vapour-rich disks in detail.

The authors simulated collisions between rocky planets and impactors and icy planets and impactors. They didn’t simulate collisions between rocky planets and icy impactors or icy planets and rocky impactors, because those types of collisions are far less likely. The authors note that while the impact dynamics are similar in both collisions, the thermodynamics are very different. The VMF is different in collisions between rocky planets and rocky impactors compared to impacts between icy planets and impactors.

This figure from the study shows some simulation results. The top two rows show a collision between a rocky planet and an impactor. The bottom two show a collision between an icy planet and an impactor. The authors found that the impact dynamics are similar in both cases, but the thermodynamics are different. Image Credit: Nakajima et al. 2022.

The mass of the planets and the impactors affects the moon-forming disks, too. Overall, the researchers found that rocky planets larger than six Earth masses and icy planets larger than one Earth mass, both produce disks with VMFs of 1.0. Impacts between these larger bodies produce more heat which vapourizes more material in the disk. The resulting high VMFs likely disqualifies them from forming moons.

This figure from the study shows the disk VMF in rocky collisions. The planet mass is on the left vertical axis. The bottom horizontal axis is the impactor-to-total mass ratio. Brown colours represent higher VMFs, and blue colours represent lower VMFs. Image Credit: Nakajima et al. 2022.

“We found that if the planet is too massive, these impacts produce completely vapour disks because impacts between massive planets are generally more energetic than those between small planets,” Nakajima said.

For a moon to form, the vapour in a debris disk must cool and condense into liquid moonlets. But when the VMF is too high, vapour drag is too strong, and potential moon-forming material falls into the planet before it can cool and condense. This equates to smaller moons. But when the VMF is smaller, the drag is weaker and more material is available for moons to form.

This figure from the study shows the VMF produced by collisions between icy planets and impactors. Image Credit: Nakajima et al. 2022.

As a result, we conclude that a completely vapour disk is not capable of forming fractionally large moons,” Nakajima said. “Planetary masses need to be smaller than those thresholds we identified in order to produce such moons.”

This graphic from the study illustrates the findings. The vertical axis shows planetary mass and the horizontal axis shows mantle composition. Rocky planets smaller than 6?M? and icy planets smaller than 1?M? can form fractionally large moons as indicated by the orange shading. The researchers’ simulations are consistent with planet–moon systems in the Solar System. Image Credit: Nakajima et al. 2022.

Moon formation is a complex process with many variables. Simulations like the ones in this study involve some assumptions, but the assumptions are based on evidence and prior research. The simulation results are in line with what we see in our own Solar System which gives them weight. So what do these results tell us?

Life on Earth relies on myriad factors, some of which we only partially understand. The Moon plays a huge role, but so do other factors.

Earth’s magnetic shield protects life from the Sun’s harsh radiation. Without the shield Earth would be barren and sterile like most worlds. We have our planet’s differentiated core to thank for that.

Plate tectonics may be required for life, too. They help regulate a planet’s temperature over billions of years. That type of stability favours life.

But we can’t see plate tectonics on other worlds, at least not yet. Astronomers only measured the first exoplanet magnetosphere in 2021. And we’re only now groping our way to detecting exomoons.

But we can gauge an exoplanet’s mass and density, and hence its composition. With that data, and with simulations like the ones in this study, we can start to understand which planets may have natural satellites that enable life.

That could be a big step when it comes to assessing exoplanet habitability.

“The exoplanet search has typically been focused on planets larger than six earth masses,” Nakajima said. “We are proposing that instead we should look at smaller planets because they are probably better candidates to host fractionally large moons.”


Evan Gough

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