Life is best hunted for on small exoplanets with large moons

In a new study, scientists suggest focusing the search for extraterrestrial life on relatively small exoplanets that have massive moons like our own. They believe that the latter are extremely important for the emergence of a stable biosphere.

Exoplanet and its large moon. Source:

Where to look for the other moon

Scientists continue to search for life and potentially habitable planets outside the Solar System. They usually focus on finding large super-Earths that are much larger than our planet. This is because they are relatively easy to detect.

But the new study, which scientists from the University of Rochester published in The Planetary Science Journal, talks about the need to focus on the harder task of finding planets similar to or smaller than Earth.

The reason for this is that, according to scientists, these worlds have a much higher probability of forming a large moon like our own. The debate around how the latter formed 4.5 billion years ago is still raging, although the dominant theory is that it happened as a result of our planet colliding with some large body, after which a large debris disk formed and assembled into our moon.

Meanwhile, there are at least 300 other moons in the Solar System. However, they are much smaller, especially when compared to their host planet. It seems that they were formed by a different mechanism, and it is the giant collision that provides the appearance of such a large body. 

Why a large moon is important

Many scientists don’t believe that a large moon is vital to the existence of our planet as we know it. However, they agree that its presence has greatly facilitated the development of complex biological life.

It happened due to the fact that the Moon causes tides, in the zone of which the chemical evolution of complex organic compounds is particularly intense. In addition, the moon stabilizes the tilt of the planet’s axis to the ecliptic, which makes the climate much more stable.

Why look for small planets

However, why look specifically for small planets? Is it not conceivable that super-Earths could also form large moons? The reason for this is a phenomenon called flux instability. This is what causes particles in the gas-dust disk to stick together, forming planetesimals. Some of them then combine to form planets, while the rest become moons, asteroids, or simply get lost in the depths of space. 

When scientists applied this model to the disks formed by the collision of exoplanets with large bodies, it turned out that massive moons were not always formed. In many cases, they are too small and fall quickly to the planet due to its gravity. 

In order for a massive moon like ours to form, the collision must be “soft” enough.In the case of Earth, this means that the impactor must not have a mass greater than that of Mars, otherwise the disk formed will simply scatter in space without producing a large body.

In the case of super-Earths, however, the energies can be even larger, meaning that the body with which the collision occurs must either be much smaller and, as a result, the moon will also turn out to be small. Or it could end up blowing up the disk altogether. That’s why scientists suggest looking for mostly small planets. 

According to