Second Huge Supermoon Discovered Outside Our Solar System

The largest moon in our solar system is Jupiter’s Ganymede, which has a radius of 1,636.8 miles or about four-tenths of the planet Earth, and is twice as massive as our Moon. That makes it a supermoon in our solar system ... but a puny satellite when compared to an exomoon discovered recently by astronomers that measures 2.5 times the size of Earth while orbiting a planet the size of Jupiter. Do planets and moons suffer from size anxieties?

"Astronomers have found more than 10,000 exoplanet candidates so far, but exomoons are far more challenging. They are terra incognita."

Columbia University astronomer David Kipping is a superstar in the land of supermoons – he’s discovered the only two known to orbit exoplanets. As explained in the journal Nature Astronomy, using the Keplar space telescope, he first found a Neptune-sized exomoon orbiting the Jupiter-sized planet Kepler 1625b in 2017. Proving that exomoons really are terra incognita (a great name for a band – it means ‘unknown land’), he just now located a second one orbiting planet Kepler 1708b 5,500 light-years from Earth. These should correctly be called ‘potential exomoons’ because confirming them involves peer review and scientific consensus amongst the astronomical community, according to Kipping.

“It's a stubborn signal. We threw the kitchen sink at this thing but it just won't go away.”

However, Kipping is pretty certain Kepler 1708b’s exomoon will be confirmed. His Cool Worlds Lab was studying a subset of the more than 10,000 know exoplanet candidates -- 70 cool, giant transiting exoplanet candidates. They were looking for transmit signals seen when a moon passes in front of a planet, causing it to dim. Only one survived Kipping’s “kitchen sink” testing – the gas giant Kepler-1708 b. He believes its exomoon is also a gas giant which, according to another study, may have once been a planet that was captured by Kepler-1708 b. This planet and moon swapping seem to be common the closer a planet is to its star, so Kipping’s team focused on exoplanets with far-out orbits like our Jupiter and Saturn – which have the biggest and the most satellites in our solar system.

“Together then, the formation and properties of a moon such as this certainly challenge conventional thinking, but plausible mechanisms have been previously proposed. Ultimately, the reality of supermoons such as Kepler-1708 b-i and Kepler-1625 b-i will require follow-up transit photometry, as both their nature and supporting evidence demand appropriate scepticism at this time.”

While you would think discovering a second giant supermoon would be a big deal, not all astronomers are buying it. Some think it was just an errant signal and want more before they are convinced. Others doubt the existence of exomoons at all – the same was true of exoplanets before stronger telescopes and different discovery techniques proved otherwise.

As the James Web Space Telescope chugs its way to becoming operational, it’s nice to salute the Kepler telescope, which spent a decade successfully looking for exoplanets and whose data continues to be beneficial three years after it retired.