1/n OK, so #exomoons. First, here’s the article I wrote with the latest news, and links to the press release and paper: syfy.com/syfywire/more-…
2/ It’s important to note that the evidence is pretty compelling for an exomoon orbiting Kepler-1625b, but it’s not proof. Still, let’s assume for this thread it’s there. I started thinking about it…
3/ The planet is about the same size as Jupiter, though probably more massive. The moon is about the size of - get *this* - Neptune. That's big. WAY bigger than any moon in our solar system.
4/ It may have formed along with the planet; other formation scenarios (e.g. impacts knocking stuff off the planet) have a harder time making it that big. It's currently about 3 million km from the planet. A bit of a hike.
5/ The star is old, about 9 billion years old (twice the Sun's age). It's actually very Sunlike, but "evolved": It's expanding into a red giant. Long way to go yet for that, but it's bigger and radiating more energy than the Sun.
6/ So the planet and moon are warmer than Earth (assuming how much light they absorb and other factors). They're both presumably gas giants, so it's not like we could live there anyway.
But I wonder...
But I wonder...
7/ There's a concept called the Hill Sphere. Basically, if you have a small object (say, a planet) orbiting a big one (a star), it's the distance from the planet you have to go before the star's gravity becomes more important.
8/ A moon orbiting inside the planet's Hill Sphere is safe from being perturbed out of orbit by the star's gravity. The exomoon in this case, even though it's far out, is still inside the exoplanet's HS. (The Moon is inside the Earth's too: syfy.com/syfywire/moon-….)
9/ So I wondered: What's the Hill Sphere for the exomoon? Could you put something in orbit around it and have it be stable against tugs by the planet's gravity? The answer is: yes!
10/ Now, I didn't do the full math on this to include the effect of the star; this was a simple calculation. But ignoring the star, the exomoon's HS is about 500,000 km. A decent distance. Even if it's actually smaller, still plenty of room for a smaller moon. An exomoonmoon.
11/ That means you could put a smaller moon orbiting the exomoon. But what's "smaller"? Earth is 1/4 the diameter of the exomoon! So maaaaayyyybe the exomoon could have a planet-sized moon itself. Maybe. Did I mention "maybe"?
12/ I'm speculating, but let's say there's one there, and it's maybe Earth or Mars sized. Could it be habitable? Well, probably not. Like I said, the star is more radiant than the Sun, and the distance to the exoplanet means it's likely too warm for us.
13/ ... *now*. But the star used to be more like the Sun, so temps for the planet + moons were more clement. Maybe a billion or two years ago.
But whoa. What a thought! A habitable planet-sized moon of a bigger-planet-sized moon of an even bigger Jupiter-sized giant!
But whoa. What a thought! A habitable planet-sized moon of a bigger-planet-sized moon of an even bigger Jupiter-sized giant!
14/ Again, just speculating, but I love this. I used to think Star Trek was silly for having such weird planets depicted, but as usual my imagination isn't as good as nature's.
15/ OK, enough for now. I didn't include any of this in my blog because the post was already long enough. But it's fun to play with the concepts and see if the math at least doesn't immediately negate it. That's science, folks. Part of it, at least.
16/16 And remember, we're still new at this. We may not confirm an exomoon for years, or it could happen next week. Every planet but 2 in our solar system has at least 1 moon. No reason to think they don't outnumber planets everywhere in the Universe. /fin
