First exomoon discovered: a world 73 light-years away that defies our definitions

We now know for certain that there are more than 7,000 planets in other solar systems, and we can say that there are probably a great many stars that have planetary systems. In short, recent discoveries – over the last 30 years – have robbed us of even the last shred of hope that we are ‘unique’ in the universe.

If we may take a brief historical detour, first Copernicus and Galileo told us that it is we – a small rocky planet – who revolve round the Sun; then, over the last 100 years, we have learnt that that whitish band that cuts across the sky on the darkest nights is our true home: the Galaxy, made up of at least 100 billion stars; and finally, that there are at least a hundred billion galaxies. Then, in 1995, the first planet orbiting another star was discovered, and so the uniqueness of our Solar System was lost. The Universe has, in a sense, taken its revenge and relegated us to the corner that is rightfully ours.

Today we also find that ‘eso’ planets have ‘eso’ moons, and we need to give this some careful thought.

It’s a fascinating discovery, which above all teaches us a great deal, starting with the fact that we have a lot to learn about the astrophysics of planetary systems. But hang on a moment: astrophysicists have done their best with what they could observe, namely our own solar system. Now that we’re exploring the vicinity of the Sun with ever more powerful telescopes, we’re discovering an astonishing variety of different cases. To give an example, it’s as if, right up until 1995 – ever since humanity first appeared on the planet – we’d been studying cars based solely on the one we had in our garage. Today, however, we can see cars of every type, make and colour. That’s quite a difference – it’s no wonder we often feel a bit lost.

Let’s now take a closer look at what the researchers are telling us. At the centre of the system is an M-type star – small and cool, with a surface temperature of around 2,500 degrees. In the image, it is the bright spot in the top left-hand corner. The star evolves very, very slowly – so slowly, in fact, that it is thought that perhaps no red dwarf has yet completed its life cycle since the beginning of the universe. It can last for many, many billions of years – somewhat in keeping with our proverb: ‘Slow and steady wins the race’. We know of several stars of this type: Proxima Centauri, the star closest to us, just 4.24 light-years away, is in fact a red dwarf.

In this system, a brown dwarf – shown in the foreground of the image – orbits the red dwarf, with a mass 37 times that of our planet Jupiter. This is actually what remains of a celestial body that failed to trigger nuclear fusion reactions at its core and become a small star. It orbits its parent star like a planet, but it is not a planet.

The discovery concerns a third, previously unknown body – a gaseous body with a mass similar to that of Jupiter – which appears to orbit the brown dwarf that acts as a planet.

This is why the newly discovered body, identified through highly sophisticated measurements of the system’s rotational speed, is called an exomoon.

“This system is rather difficult to define using Solar System terms such as ‘planet’ and ‘moon’. The exomoon is clearly massive enough to be considered a planet, but it does not orbit a star, although it does orbit an object which in turn orbits a star. As it is the third component of this system, we tend to call it a moon, even though it bears no resemblance whatsoever to the small rocky moons we have in our own system,” said Kevin Hoy of Diego Portales University, Chile, who is the lead author of the paper on this research.

In short, we are faced with an intriguing case, in which our established models – and even our definitions of planets and moons – are being called into question and need to be revised: indeed, what exactly constitutes an ‘ordinary’ star system?

Ultimately, it is a lesson in scientific humility. Every new observation reminds us that the cosmos is richer, more varied and more surprising than our usual frameworks would suggest. And perhaps this is precisely what makes astronomy so fascinating.