There are more than 20 moons in our solar system – only the deep planets Mercury and Venus do not have their own satellites. In this regard, it is no wonder that science fiction authors have been dreaming of moons around exoplanets for decades. In their stories, these exomonds are often habitable worlds, orbiting super-Earths or large gaseous planets, and home to intelligent inhabitants.
But even among astronomers, it’s almost certain that many exoplanets have satellites: “Given the large number of moons in our solar system, it’s clear to assume that there are also exomoons around some of the exoplanets — we just have to find them,” says David. Kipping from Columbia University in New York. But that is exactly what is difficult.
Two exomonds have been discovered so far
Although there are more than 5,000 known exoplanets to date, only two almost certain exoplanets have been discovered. In 2018, Kipping and his team tracked the first moon around a gas giant about 8,000 light-years away. Exomond, nicknamed Kepler-1625b-i, is not a rocky moon like Earth’s, but it is much larger and richer in gas – it could be much closer to the planet Neptune. “It’s probably not appropriate for life as we know it,” Kipping says.
Kipping and his team discovered a second exomond in a planetary system about 5,500 light-years away. There, too, the Moon orbits a gaseous planet the size of Jupiter – also much larger and more massive than the known moons from our solar system. According to astronomers’ estimates, the exomond Kepler-1708b-i is two and a half times the size of Earth and resembles a gas-rich mini-Neptune.
How do you search for exomond?
But why are these first known Exomonds such giants? Perhaps this is because of the way astronomers search for these moons. Because exoplanets and their moons are too small, and far away, from their stars to be seen directly, astronomers have to resort to indirect methods. However, these work best when the moon is very large or very close to its star.
In the first method, astronomers look for the small vibrations caused by the moon’s gravity shaking its host planet and star. These fluctuations in the star cause slight shifts in its light spectrum, which – in theory – can be detected using special instruments on powerful telescopes. But so far, many exoplanets have been discovered with the help of this “radial velocity”, but they have never been discovered. Because its signal is too weak to be clearly identified with current instruments.
On the other hand, the transit method offers better opportunities: using it, astronomers look for the weak dimming of starlight that causes a planet orbiting its star to pass directly around its star. This transit is reflected in the uniform decrease in the light curve. If an exoplanet has a moon and is conveniently positioned in front of the star as it passes, the outer moon creates a second, smaller depression next to the planet’s main depression. Such a “moon dale” appeared in 2018 in Exomond Kepler-1625b-i.
Even if the outer moon is not standing next to its planet in transit, it can abandon itself: as the moon’s gravity pulls on the planet, its motion slows down or accelerates somewhat. This actually makes the regular depressions in the light curve a bit irregular. This effect also occurs on Earth. “If an extraterrestrial civilization observed the transit of the Earth and the Moon before the Sun, they would see similar anomalies in the times of the Earth transit,” Kipping explains. These anomalies helped him and his team track down a second exomond, Kepler-1708b-i.
How convenient are Exomonds for life?
In science fiction, exomonds are often habitable, water-rich worlds. But what does it actually look like? In fact, the odds are better with an exoplanet than with an exoplanet, astronomers determined. Because the Moon can have a temperate climate and liquid water even if its planet orbits outside the habitable zone. “Exomoons are more complex than exoplanets – but that also means more chances of being life-friendly,” explains Rory Barnes of the University of Washington.
This is possible because the influence of the planet also affects the climate on its moon. b So light reflected from a planet can provide additional light for its moon. Conversely, the exomond can also be protected and cooled by planet Earth. This is the case, for example, when he regularly wanders in the shadow of the planet.
When the moon and planet pair are relatively close to each other, the planet’s tidal forces can completely replace the sun’s heat. Because gravitational forces, which change depending on the position of the moon, stumble through the interior of the moon and heat it up. An example is given by Jupiter’s moon Europa in our solar system: it has a liquid ocean under its icy crust because Jupiter’s tidal forces are heating its interior.
So the chance of a true counterpart to Pandora or Endor somewhere out there is not that small. The trick is just to find the life-friendly Exomonds.