Exomoons

As our catalogue of exoplanets grows, the hunt is on for moons around distant worlds

Reported by Robert Lea

© Tobias Roetsch

Astronomers may have finallydiscovered a moon orbiting around a planet outside our Solar System. The development could have important implications for our understanding of how planetary systems evolve, as well as indicating how typical the planets of our Solar System are. Since the first discovery of planets around stars other than the Sun was made in the early 1990s, our catalogue of planets outside the Solar System has burgeoned, now containing over 4,000 confirmed worlds. One of the most important aspects of our investigation of these extrasolar planets – or exoplanets – is the assessment of how similar or diverse they are in comparison to the planets of the Solar System.

It’s unsurprising given how Earth’s own Moon has dominated our imagination and played a vital role in both astronomy and humanity’s exploration of space that the hunt for moons around these extrasolar worlds – or exomoons – is a subject of intense interest.

This is compounded by the fact that moons are abundant in the Solar System, with an average of 20 moons for each planet. And we have no reason to believe that our planetary system is unique in this respect, making the elusive nature of exomoons confusing and frustrating.

To understand why the discovery of exomoons may be important, it’s worth considering just how influential some researchers believe our natural satellite was to the development of life on Earth. This is coupled with the fact that many scientists theorise that if life exists elsewhere in the Solar System, aside from Mars, the most likely places to find it would be on the moons of gas giants Jupiter and Saturn, with the icy moon Enceladus being a particularly enticing suspect. This means that exomoons could be a target for habitability investigations in their own right, especially when they orbit gas giants and could potentially be the size of Earth, or even larger.

Determining if an exoplanet is orbited by a moon could be important for investigations of that planet’s habitability potential in another way. For rocky terrestrial exoplanets in the habitable zones of their stars – the area that has the right temperature to allow for the existence of liquid water – an undetected moon may add an additional source of thermal or reflected light that hinders the determination of characteristics belonging to the planet itself.

Observations of our Solar System, in which moons are plentiful, suggest that such objects should be extremely common throughout other systems. However, until recently these exomoons have proven to be particularly elusive. This could be because our detection methods are not sensitive enough as of yet to spot these moons outside the Solar System, or because moons are less common in other planetary systems for some reason. Determining which of these possibilities is correct is an important and missing aspect of our ongoing investigations of extrasolar planetary systems.

TYPES OF EXOMOON

How we can categorise the alien moons we expect to find

SNOWBALL EXOMOON

A snowball exomoon is one where the entire moon is permanently frozen, and less than one-tenth is habitable. These worlds are likely to have been born far from their stars, or were perhaps moved there from a position nearer the star.

HABITABLE EXOMOON

Habitable exomoons are defined as those where at least a tenth of the surface could support liquid water. These will need to be in the star’s habitable zone, as well as the right distance from the planet, but could be great bets for finding life elsewhere.

TRANSIENT EXOMOON

A transient exomoon is said to be sort of habitable, but its habitability changes dramatically over time. The idea that a moon may shift in and out of habitability will need careful consideration when we look for and study exomoons.

HOT EXOMOON

Researchers Duncan Forgan and Vergil Yotov came up with a system for classifying exomoons in 2014. One class was a hot exomoon, defined as one with an average surface temperature of more than 100 degrees Celsius (210 degrees Fahrenheit), with less than one-tenth being habitable.