Scientists have spent centuries investigating the many moons in our solar system. From Galileo’s first discovery of the four moons of Jupiter in the 17th century to the imaging of Pluto’s moons by the New Horizons spacecraft in 2015, the fascination with these planetary satellites is enduring. So far, the search for moons has been restricted to our closest planetary neighbours. However, NASA’s launch of the Kepler spacecraft in 2009 has revolutionised the search for planets outside of our solar system. To date, there are 2,327 of these exoplanets confirmed to exist and a further 2,244 candidates proposed. It would be natural to assume that some of these exoplanets have moons in their orbit, but good science does not tolerate guesswork. Evidence is needed to assert that such exomoons exist, and the first piece of such evidence may have been found this month.

Researchers David Kipping and Alex Teachey from Columbia University, New York claimed to have discovered the existence of a large exomoon orbiting an exoplanet called Kepler-1625b 8,000 light-years away. The exoplanet was originally discovered by the Kepler mission in 2016 through the method of transit photometry. This method involves the use of the 42 charge-coupled device (CCD) arrays (like the technology used in digital cameras) on board the spacecraft to capture light from a distant star. It looks out for dips in the star’s emitted flux of light. These dips correspond to a transit of the planet around its host star, effectively blocking some of the light emitted. Exomoons could theoretically be discovered in a similar way; a moon trailing a planet could block some of the starlight too.

“NASA’s launch of the Kepler spacecraft in 2009 has revolutionised the search for planets outside of our solar system”

The potential existence of the exomoon, called Kepler-1625b-I, was first gleaned from raw data from Kepler using this transit photometry method. Kipping and Teachey analysed the host planet’s transit and noticed some irregularities in the emitted starlight that grabbed their attention. These blips were attributed to another object: a potential exomoon. An exciting find – but more accurate data was required to confirm their hypothesis.

The Hubble Space Telescope conducted a more thorough observation. Its sensitive equipment was scheduled to point towards star Kepler-1625 in October 2017. The resulting light-curve (a plot of the emitted starlight against time) showed a significant dip after the planet transition, a full 3.5 hours after the planet had disappeared from Hubble’s field of view. Kipping described it as “a moon trailing the planet like a dog following its owner on a leash”. The next year was spent analysing the Hubble data before their trailblazing paper was published this month detailing the new exomoon.

Kipling and Teachey even have some idea of the conditions on the moon. Like its host planet, which is about the size of Jupiter and mostly composed of hydrogen and helium, the exomoon Kepler-1625b-I is predicted to be gaseous, and it is approximately the size of Neptune.

“It would be audacious to claim that from the billions of stars in our universe there are no planets with an exomoon in the habitable zone”

To understand the significance of an exomoon discovery, we need only take a look at humanity’s fascination with our own moon. The “Space Race” lead us to achieve a feat which was unimaginable centuries ago: Man walked on the moon’s surface, making it inhabited by life for a few hours. That dream of exploration has now extended to other worlds beyond our neighbourhood, be it exoplanets or exomoons. The same enduring questions have permeated science and pop culture since the dawn of modern astronomy: Are we alone in the universe? Are there others like us? Where do they live? Can we visit?

For life as we know it to develop on a world, it must lie within the so-called “habitable zone” of its host star; the sweet spot where liquid water can exist on its surface because it is not too hot or too cold. Astrobiologists agree that liquid water is an important prerequisite for life. Any potential habitable world must also have an atmosphere, meaning it must be massive enough for its gravitational pull to keep the molecules in the atmosphere from drifting off into space. These two conditions have yet to be confirmed for any moon in the solar system, but that’s just one star and one planetary system. It would be audacious to claim that from the billions of stars in our universe that there are no planets with an exomoon in the habitable zone, and Kepler has already observed planets in the habitable zones of their stars. If technology continues to improve at its current astronomical rate (an appropriate adjective in this context), we should expect to hear of a lot more exomoon discoveries in the years to come, and more exomoons means even more habitable zone hopefuls. Things could be looking up for the alien conspiracy theorists. Although, when are they not looking up?

We might have found some potentially habitable worlds, which begs the question: Could we get there if Earth were no longer become a feasible option for our survival? The answer is a definite “no” with our current technology. Elon Musk, founder of SpaceX, is working hard to get to Mars, but the truth is that a trip to an exoplanet or exomoon such as Kerpler-1625b-I is unfathomable compared to the 12.5 light-minute distance to Mars. That does not mean scientists will stop looking for these elusive worlds, and perhaps there are even similarly curious beings watching Earth transit our Sun and watching our moon trail behind us today.