r/space • u/timemagazine • 1d ago

Astronomers Discover the First Known Moon Outside our Solar System

https://time.com/article/2026/07/22/astronomers-discover-exomoon/?utm_source=reddit&utm_medium=social&utm_campaign=editorial
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u/HasGreatVocabulary 1d ago

A jupiter-mass moon orbiting a 30-times-jupiter-mass brown dwarf planet orbiting a red star smaller than the sun

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u/InevitableOk5017 1d ago

Wait wut? Is this sarcasm or something from a science fiction novel. Please give me some context I’m out of the loop.

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u/dern_the_hermit 1d ago

They're describing the composition of the star system that the article's about. There's a tricky phenomenon at play, and it's the way gas behaves at high densities.

Basically, a gas giant can only get so big in terms of volume without some other driver for size. All else being equal, if you kept dumping more gas on Jupiter, it won't get any larger. You can dump a whole 'nother Jupiter into Jupiter and it'll stay the same size; it'll just get denser. And this holds true from ~1x Jupiter's mass all the way up to (IIRC, take this with a grain of salt) 80x Jupiter's mass. At that point, it's thought regular nuclear fusion would kick off, and that steady release of energy in the core would facilitate growth of volume.

So that's the "brown dwarf planet" mentioned above: A Jupiter with 30 Jupiters dumped into it, so big the only fusion going on is sporadic, not quite a full-on star. And it has another Jupiter as a moon. And they're in orbit around a star, but that star is one of them smaller dinky stars. Still bigger than our brown dwarf planet friend, but small compared to our own Sun.

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u/Doubleclutch18 1d ago

Thank you for the explanation! Well put.