Exomoons may be habitable
Lots of variety, anything
from ice worlds to lava moons.
Interactions between moons:
strong tides, easy travel,
bounced material.
Extra night cycle.
Link to: ATLAS – ICE
for image ideas
Link to: ATLAS – MOONS
for image ideas

Looking up, you see a cluster of moons dotting the sky. Floating straight above you is the massive orb of your closest moon. You can easily make out its crackled terrain and the white, glittering plumes its volcanoes are spewing into its tenuous atmosphere. Closer to the horizon, starlight bounces off the pink, icy crust of the next furthest moon in your neighbourhood. In the distance looms the silhouette of your central planet. 

Many habitable worlds aren’t planets at all but moons orbiting planets that may themselves be inhospitable. In our own solar system, there are about 200 moons circling gas giants like Jupiter and Saturn and small, rocky worlds like Earth and Mars. Even large asteroids can have their own moons, such as Ida and Dactyl. Given the abundance of moons in our own neighbourhood, it makes sense to assume that exoplanets may also host an array of moons, called “exomoons.”

If exomoons are anything like the moons in our Solar System, there is going to be a lot of variety. There may be icy water worlds such as Jupiter’s moons Europa and Ganymede, which harbour massive oceans of liquid water beneath their icy crusts. There may be lava worlds such as Io, which is covered by active volcanoes. Both Saturn’s moon Enceladus and Neptune’s moon Triton sport geysers that spew massive ice plumes into their tenuous atmospheres. Saturn’s moon Titan, the solar system’s largest moon, is enveloped by a thick atmosphere and its surface is covered by frigid hydrocarbon seas.

Not all moons are large worlds, though. While moons like Titan and Ganymede are even larger than our planet Mercury, the majority of moons are too small and not sufficiently massive to even form a sphere. Moons like Mars’ Deimos and Phobos more closely resemble flying potatoes than an orb. Small moons such as these consist mostly of ice and rock and are likely captured asteroids.

What could it be like to live in a moon system? Moons orbit much closer to each other than planets do, so there would be much more interaction between these worlds. For example, the moons would cause strong tides and even volcanic eruptions are they get close to each other. Ejecta from meteor impacts and volatiles from volcanic eruptions could fairly easily travel between worlds. And if there were space-faring civilizations inhabiting a moon system, it would be fairly easy to travel between the moons; it would only be a short hop to the next-door neighbour compared to a voyage to the next nearest planet.

The day-and-night cycle would also be different on a moon than on a planet. There would be the light/dark cycle that is due to the rotation of the moon, and then there would be an additional night period caused by the planet itself – both when the planet blocks the star’s light, and then again when the planet casts its shadow across the moon. 

So, have we actually discovered any exomoons? Maybe. Detecting exomoons is very challenging because they are very small and have little mass. Still, planetary scientists think that they have found two promising candidates. Kepler-1625b-i and Kepler-1708b-i are both huge compared to the moons in our Solar System. Both are Neptune-sized moons that orbit planets that are five to ten times the size of Jupiter. These two exomoons are unlike any other moon in our solar system. In the future, we may find as many weird, exotic exomoons as we find exoplanets.

Animation of exomoon orbiting its planet

This animation shows how the exomoon candidate Kepler-1625b-i orbits its planet. Kepler-1625b-i is the first exomoon candidate discovered so far and, if confirmed, the first moon to be found outside the Solar System.
Watch the animation

Astronomers find exomoon candidate orbiting Kepler 1708b

Astronomers found a signal that could indicate an exomoon, about 2.6 times larger than Earth, orbiting a Jupiter-sized exoplanet called Kepler-1708b.
Find out more

Hunting for Exomoons: A Survey of 70 Cool Gas Giants

In this video, intrepid exomoon hunter David Kipping describes his search for moons orbiting exoplanets. Watch to the end, where he describes the latest find, the new exomoon candidate Kepler-1708 b-i
Watch the video

Link to: ATLAS – BUGS
for image ideas
for image ideas


Scientists are excited about exomoons, because they may provide the conditions for life to develop. Icy ocean worlds such as Europa, Ganymede, and Enceladus are prime targets for astrobiologists who study life beyond Earth in our own Solar System, and similar moons may orbit the many exoplanets we have discovered so far. 

However, orbiting close to a giant planet may be as hazardous as orbiting close to a star. The gravitational push and pull from a gas giant could cause massive volcanic eruptions on the much smaller moon, similar to Jupiter’s moon Io. Very large planets also emit strong radiation, which may be deadly to life if the moon doesn’t have its own magnetosphere to protect it. 


For any civilization to develop on a moon system, it would only be a short voyage to its neighbouring world. How may this influence evolution? Could the same species survive on different moons? How may they differ?

Since moons tend to be tidally locked (meaning that one side always faces the planet, like our own Moon), inhabitants of a moon system would only be able to see one hemisphere of a neighbouring moon. What secrets may lurk on the moons’ far sides?

A small “potato-shaped” moon may not be ideal for developing life, but it may have other uses. It could provide resources like water and rare elements. It may be a parking lot for your spaceship. Or, it could spell trouble if it gets nudged out of its orbit…