If We Want To Find Life-Supporting Worlds, We Should Focus on Small Planets With Large Moons

There’s no perfect way of doing anything, including searching for exoplanets. Every planet-hunting method has some type of bias. We’ve found most exoplanets using the transit method, which is biased toward larger planets. Larger planets closer to their stars block more light, meaning we detect large planets transiting in front of their stars more readily than we detect small ones.

That’s a problem because some research says that life-supporting planets are more likely to be small, like Earth. It’s all because of moons and streaming instability.

Consider Earth’s Moon. While there’s no consensus on every aspect of the Moon and its role, there’s evidence that it helps make life on Earth possible and has helped life sustain itself for so long. As natural satellites go, it’s massive. Of the approximately 300 (and counting) moons in our Solar System, the Moon is the fifth largest. But that doesn’t tell the tale of its relationship with our planet.

The Moon’s diameter is about one-quarter of Earth’s diameter, and its mass is about 1.2% of Earth’s. The four natural satellites in the Solar System that are larger than the Moon orbit the gas giants Jupiter and Saturn. Those moons are tiny compared to their planets.

This means that the Moon has different effects on Earth than other moons do on their planets.

The Moon stabilizes Earth’s orbital tilt, which helps keep the climate stable and allows life to flourish and organisms to adapt. It creates tides, which may have played a role in the formation of nucleic acids and life. The Moon may even help Earth maintain its protective magnetosphere. One way or another, Earth would be a very different place without its huge Moon.

New research published in The Planetary Science Journal shows that we should look for small planets if we want to find life-supporting worlds because small planets are more likely to host larger moons. The research is titled “The Limited Role of the Streaming Instability during Moon and Exomoon Formation.” The lead author is Miki Nakajima, an Assistant Professor of Earth and Environmental Sciences at Rochester University.

“Relatively small planets similar to the size of Earth are more difficult to observe and they have not been the major focus of the hunt for moons,” said lead author Nakajima. “However, we predict these planets are actually better candidates to host moons.”

The leading theory for the Moon’s formation is the Giant Impact Hypothesis. It states that when the Earth was very young, about 4.5 billion years ago, a Mars-sized protoplanet named Theia slammed into Earth. The collision created a rotating torus of molten rock that orbited the Earth. Some fell back down to Earth, and the rest coalesced into the Moon. There’s still a lot of debate over this, but it is the leading theory.

Here’s where streaming instability comes in.

This research questions the role of streaming instability in moon formation. Some scientists think that planet formation is the same as moon formation. However, while streaming instability is important for planet formation, it may not be for the formation of large moons like Earth’s, which help make planets habitable.

In their research, Nakajima and her colleagues used simulations to examine the role of streaming instability in moon formation. Streaming instability describes the effect that drag has on the accretion of matter in a protoplanetary disk that leads to planetesimals. Inside a disk, drag rapidly drives solid particles to concentrate spontaneously into clumps. These clumps can then collapse and form planetesimals.

The question is, does streaming instability play the same role in the formation of moons around planets? In this case, the disk isn’t a protoplanetary disk but a disk of debris resulting from a collision.

“Here, we investigate the effect of the streaming instability in the Moon-forming disk for the first time and find that this instability can quickly form ~100 km-sized moonlets,” the authors write in their paper. “However, these moonlets are not large enough to avoid strong drag, and they still fall onto Earth quickly.”

“These moonlets could grow further once the disk cools enough and the vapor mass fraction of the disk becomes small,” the researchers write in their article. “However, by this time a significant amount of the disk mass is lost, and the remaining disk could make only a small moon.”

This figure from the research shows four snapshots from the simulations. At t = 2.87, streaming instability starts to form clumps. Gravity is turned on at t = 3.18, and by t = 3.39 and 3.55, moonlets start to form by gravitational instability. Image Credit: Nakajima et al. 2024.

For a large moon like Earth’s to form, the collision has to be less energetic than one between much more massive planets. If Theia had been more massive, the heat from the impact would’ve created a completely vapourized disk. Only a much smaller moon could’ve formed in such a disk.

This figure from the research shows how long moonlets can reside in a disk before crashing into their planet. The two lines show the cases of an icy planet collision and a rocky planet collision. The x-axis shows the planet’s mass, and the y-axis shows time in days. Since the moonlets can’t stay in the disk for long, it indicates that “streaming instability likely plays a limited role in impact-induced moon-forming disks,” as the authors explain. Image Credit: Nakajima et al. 2024.

The researchers think that streaming instability may not help large moons form in vapour-rich disks. Fractionally large moons like Earth’s Moon, which may be necessary for life, might only form in vapour-poor disks. More massive planets have more energetic collisions, which creates vapour-rich disks. Smaller planets have vapour-poor disks where larger moons can form.

This graphic from the research illustrates the researchers’ hypothesis. It shows how only small moons form in vapour-rich disks from energetic impacts. Streaming instability plays a small role in forming moons in impact-induced disks because they’re vapour-rich. Image Credit: Nakajima et al. 2024.

So, if we want to find life-supporting planets, look for small worlds where larger moons are more likely to form.

“We find a limited role of streaming instability in satellite formation in an impact-induced disk, whereas it plays a key role during planet formation,” the authors conclude.

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