Exoplanet Guide

Scienceline’s Guide to the Exoplanets: Shadow Satellite

Kepler-1625b: In search of a Neptune-sized moon

November 2, 2020

Shadow Satellite
Discovered by: Kepler
Discovered: 2016
Distance from Earth: ~8000 light-years
Mass: ~11 Jupiter masses
Radius: ~0.6 Jupiter radii
Surface climate: Unpleasantly warm
Habitability for humans: No.

Today, we go to the neighborhood of a yellow star approximately 2500 parsecs in the direction of the constellation Cygnus.

Once, this star was rather like the Sun; even today, it’s similar in mass. But Kepler-1625 is over eight billion years old, twice as old as the Sun. It’s well on its way into the sunset years of its life, steadily depleting hydrogen to fuse into helium. Before very long (on a cosmic timescale) it will blister out into a red giant. It’s already expanding outwards: even though it’s approximately the same mass as the Sun, its radius is 70% larger.

It’s almost fitting that, for a star that’s inflated in size, a planet that orbits it at roughly around the distance of Earth is similarly large and blown-up. Kepler-1625b, the only confirmed planet in this star system, is a gas giant.

Although it’s physically no bigger than Jupiter, Kepler-1625b is quite a bit heavier. Estimates suggest its mass floats around 11 Jupiter masses. That mass and that density suggests the planet may actually be a brown dwarf; a few more Jupiter masses, and the planet starts the most rudimentary internal fusion. Even if it isn’t a brown dwarf, such a massive and dense world’s internal heating would likely torch its surface gases to a temperature hotter than Venus.

But, again, there are quite a few known planets that would be hot gas giants like this. That’s not why it’s in this space.

Kepler-1625b may also have a moon. A very large moon, in fact.

Let’s back up. There are no confirmed moons around exoplanets, what astronomers call exomoons. There are, however, quite a few planets that have unconfirmed traces, and Kepler-1625b is one of them.

In 2017, researchers combing through Kepler observational data found tantalizing traces of something else passing in front of the planet, blocking out some of its reflected light. That dip in light was consistent with what a moon around the planet would cause. They proposed that it wasn’t just any moon; they proposed that orbiting Kepler-1625b was a world the size of Neptune.

If this world actually exists, then it would be far larger than any moon in our Solar System. In fact, it would be a gas giant in itself. A hot Neptune orbiting a hot Jupiter. It’s hardly impossible for moons to be larger than planets: after all, Jupiter’s Ganymede and Saturn’s Titan are both larger than Mercury.

Of course, it’s not clear whether this second world actually exists; other astronomers believe that the dip was just signal noise, and later observations haven’t quite seen it again. But the possibility remains over astronomers’ heads.

There’s an even more intriguing possibility: some astronomers believe that Kepler-1625b and its Neptune-sized companion are not planet and moon, but instead a double planet. Instead of the smaller world orbiting the larger world, both orbit a point outside that world, in space. That would make it like Pluto and Charon in our solar system — only many times larger.

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