by Thijs Kouwenhoven

Our Moon is responsible for the tides in the Earth’s oceans and for the magnificent Solar and Lunar eclipses that we see from Earth. Our Moon does not have an atmosphere or oceans, and life is not possible there. But what about other the other moons in and beyond our Solar System?

Moons in our Solar System

A moon is natural satellite of a planet that is larger than roughly ten kilometers. Approximately 400 bodies in our Solar System are classified as moons. Some are larger than the Earth’s moon, while most are just oversized rocks. The seven largest natural satellites in our Solar System are Jupiter’s four Galilean Moons (Ganymede, Callisto, Io, Europa), Saturn’s moon Titan, Earth’s Moon, and Neptune’s moon Triton. Many of these moons have very interesting properties, such as underground oceans, volcanism, geysers (volcanoes ejecting liquid water), lakes, and atmospheres. The presence of liquid water under the surface of many of these moons led astronomers to speculate that microscopic life forms might have developed under the surface ice layers.  Habitable moons in our Solar System?  Whether or not there is life in the Universe has kept the minds astronomers and philosophers busy for many centuries. The most obvious place to look for signatures of life are the planets orbiting our Sun and other stars. However, it may be possible that moons are capable of hosting life as well. In our Solar System, the three moons Europa (Jupiter), Enceladus (Saturn) and Titan (Saturn) have recently gained a lot of attention. Although these three moons are unsuitable for intelligent life as we know it, it may well be possible that these moons are suitable for small life forms such as bacteria. The most interesting moon in our Solar System is Jupiter’s moon Europa. Since Europa receives little light from the Sun, the temperature that the surface is low and the moon is covered in ice. However, a closer look reveals that there are very few impact craters, indicating that the surface is young. The many cracks in the ice show that something is moving underneath the ice layer. Using computer models, astronomers discovered that under the 10 kilometer thick ice layer, there could be a deep ocean of liquid water. Since the formation of Europa several billions of years ago, life forms might have developed in this ocean.

Can exoplanets have moons?

Exoplanets orbiting other stars likely have moons as well. No exomoons have been discovered yet, since our telescopes are not sensitive enough to detect them. It is almost impossible to image planets, and even more difficult to see the moons orbiting them. However, exomoons can still be detected using indirect techniques. Earth- and space-based telescopes are carefully monitoring the brightness of nearby stars. When an exoplanets passes directly in front of a star, it blocks some of the light of the star and produces a small change in its brightness. The Kepler satellite was designed to discover planets using this technique, and has already found thousands of exoplanet candidates orbiting other stars. The change in brightness allows us to derive the size of the exoplanets, while its orbital period can be derived from the time difference between two subsequent dips in the brightness. From the orbital period it is possible to calculate the distance between the star and planet, and even the temperature at the planet’s surface.

In the case of a planet with a moon, both bodies move in front of the host star during an eclipse. When the planet moves in front of the star we see the brightness of the star decrease, and when the moon follows, we see and additional smaller dip. In addition, the gravitational pull of the moon can also slightly (but measurably) change the time at which the deepest eclipse occurs. By accurately timing the brightness changes of the star when a planet passes, we can detect exomoons!

Can exomoons be habitable?

When searching for habitable planets and life in the Universe, we should consider both planets and moons, since there are likely many more exomoons than exoplanets. In our Solar System, Earth is the only place at which we know life exists. Beyond Earth, the best locations to search for life are Europa (Jupiter), Titan and Enceladus (Saturn), and the planet Mars. These bodies have in common that all of them currently have underground oceans (Europa, Titan, Enceladus), or they had surface oceans in the past (Mars). Even in our Solar System, moons have underground oceans, and might be able to host life! However, we should not expect to find an alien civilization. It is more likely that we will discover micro-organisms.

There is no doubt that moons are common in the Universe. With the ever-increasing power of modern telescopes, it is only a matter of time before we detect the first exomoons! Although we cannot travel to other stars with exoplanets and exomoons, these new discoveries teach us about ourselves. We can learn more about how the Solar System formed and evolved. Did life develop on Earth because our Solar System is special? Or simply because we are lucky? Or do many planetary systems look like ours? The future will tell us…

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Thijs_KouwenhovenThijs Kouwenhoven is a research professor at the Kavli Institute for Astronomy and Astrophysics at Peking University (China). He obtained his undergraduate degree at Leiden University, and subsequently his PhD degree at the University of Amsterdam in the Netherlands. After working for several years at the University of Sheffield (UK) he moved to Peking University, where he currently works on the dynamical evolution of planetary systems, binary/multiple stars, and star clusters.

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