We should look for signs of life on small planets with large moons

A rocky planet with a large moon may have good potential for the emergence of life, given that the moon controls aspects important to life on Earth, including day length, ocean tides and a stable climate.

There is no perfect way to do anything, including finding exoplanets. Each method of searching for planets has a certain error. We found most exoplanets using the transit method, which favors large planets. Large planets closer to their stars block more light, which is why we detect large planets passing in front of their stars faster than small ones.

This is a problem because some studies say that planets suitable for life are most likely to be small ones – like Earth. It's all about the satellites and the instability of the flows.

Consider Earth's Moon. While there is no consensus on all aspects of the Moon and its role, there is evidence that it helps make life on Earth possible and helps life sustain itself for so long. As befits natural satellites, it is huge. Of the approximately 300 (or more) satellites in our solar system, the Moon is the fifth largest. But that’s not all about its relationship with our planet.

The diameter of the Moon is about a quarter of the diameter of the Earth, and its mass is about 1.2% of the Earth's mass. The solar system's four natural satellites, larger than the Moon, orbit the gas giants Jupiter and Saturn. These moons are tiny compared to their planets.

This means that the Moon influences the Earth differently than other moons influence their planets.

The Moon stabilizes the tilt of Earth's orbit, which helps maintain a stable climate and allows life to flourish and organisms to adapt. It creates ebbs and flows that may have played a role in the formation of nucleic acids and life. The Moon may even help Earth maintain a protective magnetosphere. One way or another, the Earth would be completely different without its huge Moon.

A new study published in The Planetary Science Journal shows that we should look for small planets if we want to find habitable worlds, because small planets are more likely to host large moons. The study is called “Limited role of flow instability in the formation of satellites and exosatellites“Lead author is Miki Nakajima, assistant professor of earth and environmental sciences at the University of Rochester.

“Relatively small Earth-sized planets are more difficult to observe and have not been the main target of satellite hunts,” says lead author Nakajima. “However, our predictions are that these planets are the best candidates for satellites.”

The leading theory of the formation of the Moon is the giant impact hypothesis. It says that when the Earth was very young, about 4.5 billion years ago, a Mars-sized protoplanet called Theia crashed into the Earth. The collision created a rotating torus of molten rock that orbited the Earth. Some of the material fell back to Earth, and the rest turned into the Moon. There is still debate about this, but this theory is the leading one.

This is where the question of flow instability arises.

This study casts doubt on the role of flow instability in the formation of the Moon. Some scientists believe that the formation of planets occurs in the same way as the formation of the Moon. However, while flow instability is important for the formation of planets, it may not be so important for the formation of large satellites, such as Earth's Moon, which help make planets habitable.

In their study, Nakajima and her colleagues used simulations to study the role of flow instability in satellite formation. Stream instability describes the effect of drag on the accretion of matter in a protoplanetary disk, which leads to the formation of planets. Inside the disk, resistance quickly causes solid particles to spontaneously concentrate into clumps. These clumps can then collapse and form planetesimals.

The question is, does flow instability play the same role in the formation of planetary satellites? In this case, we are not talking about a protoplanetary disk, but about a disk of debris formed as a result of the collision.

“Here we study for the first time the influence of flux instability in the lunar-forming disk and find that this instability can rapidly form satellites ~100 km in size,” the authors write in their paper. “However, these satellites are not large enough to avoid strong drag, and they still fall quickly to Earth.”

“These satellites can continue to grow when the disk cools enough and the mass fraction of vapor in the disk becomes small,” the researchers write in their paper. “However, by this time, a significant portion of the disk's mass will have been lost, and the remaining disk will only be able to form a small satellite.”

This figure from the study shows four snapshots taken from the simulation. At t = 2.87, flow instability begins to form clumps. Gravity turns on at t = 3.18, and at t = 3.39 and 3.55, satellites begin to form under the influence of gravitational instability.

To form a large satellite like Earth's Moon, the collision must be less energetic than the collision between much more massive planets. If Theia were more massive, the heat from the collision would result in a completely vaporized disk. Only a much smaller satellite could form in such a disk.

This graphic from the study shows how long moons can remain in the disk before crashing into their planet. The two lines show collisions with an icy planet and a rocky planet. The x-axis shows the mass of the planet, and the y-axis shows the time in days. Since satellites cannot remain in the disk for long, this indicates that "flux instability likely plays a limited role in collision-formed disks"as the authors explain. — This graphic from the study shows how long moons can remain in the disk before crashing into their planet. The two lines show collisions with an icy planet and a rocky planet. The x-axis shows the mass of the planet, and the y-axis shows the time in days. Since satellites cannot remain in the disk for long, this indicates that “flux instability likely plays a limited role in disks formed by collisions,” as the authors explain.

The researchers believe that flow instability may not favor the formation of large satellites in evaporation-rich disks. Fractionally large satellites, such as Earth's Moon, which may be necessary for life, can only form in disks with low vapor content. More massive planets experience more energetic collisions, resulting in steam-rich disks. Small planets have steam-poor disks where large satellites can form.

This graph from the study illustrates the researchers' hypothesis. It shows how steam-rich disks produce only small satellites through energetic impacts. Stream instability plays a small role in the formation of satellites in impact-induced disks because they are rich in vapor.

So if we want to find habitable planets, look for small worlds where large moons are more likely to form.

“We found a limited role for flux instability in impact-induced disk satellite formation, whereas it plays a key role in planet formation,” the authors conclude.