JON CULSHAW

Jon visits a system that contains the first exomoon ever to discovered

It’s a phrase evocative of The Last of the Mohicans: ‘the first of the exomoons’. But that’s what is believed to have been discovered around a star similar to our Sun. Four thousand lightyears away in the constellation of Cygnus lies Kepler-1625, a yellow star with a radius 80 per cent larger than the Sun but only eight per cent more mass. You’d need a large telescope to observe it from Earth with its apparent magnitude of +14.0.

On this visit I’m steering the Perihelion to Kepler-1625b, a gas giant within the star’s habitable zone. This world is about the size of Jupiter and completes a single orbit in 287 days at a distance of 0.81 AU.

This gas giant’s moon is a giant in its own right – on a scale similar to Neptune – and it’s likely not to have originally formed here, but instead been captured into its present orbit.

It’s amazing to see this system through only my eyes: the sight of these planetary bodies together in the habitable zone of their parent star is one of great richness. I’m impatient to observe more closely at the earliest opportunity.

Adjusting my ship’s gravitational balance to withstand the fairly hefty forces on this Neptune-sized Moon means I can follow a steady course over the surface at a relatively low altitude. The sky has a familiar blue hue in the moon’s rather thick outer atmosphere and there are the most intriguing cloud formations that trace curved paths similar to DNA strands.

Close up, these formations are reminiscent of Earth’s cumulus clouds. Their colours are deliciously odd too: brilliant, reflective white with patches of deep, vivid green running through. Is it possible these exomoon clouds could harbour some form of green algal life? Their luminescent silver accented with green shades makes for a bizarre view, like a holly leaf in November.

The vision that dominates the sky, however, is mighty Kepler-1625b. With contrasting raspberry-shaded bands laced with green streaks of its own, it’s like observing Jupiter in another flavour.

This exomoon has moons of its own, as if to give the scene a Russian doll aspect. Asteroid satellites pepper the sky, each with a unique shape but all sharing the same crescent phase. Binoculars show up their features and impact marks. Many may have formed here; many will have been captured from elsewhere. I can count 20 or more such moon satellites, all scattered as if they’ve been hoarded by a great satellite collector.

Back on Earth, future observations of graceful Cygnus will hold greater fascination with the knowledge that the quirky and stunning features of Kepler1625b’s family exist within it.