Picture standing under a dusky twilight sky with a dim salmon colored sun fixed on the horizon and six other planets drifting overhead, larger than our Moon. That is TRAPPIST-1, an M8V ultra cool red dwarf about 40 light years away in Aquarius with seven Earth-sized rocky planets packed inside a space smaller than Mercury's orbit. This episode explains why astronomers hunt around red dwarfs, using the streetlamp analogy for the transit method, and why a star only slightly larger than Jupiter, with 84 times its mass, ignites fusion while Jupiter does not.

We cover the fully convective interior that lets TRAPPIST-1, already 7.6 billion years old, burn for another 10 trillion years, and the Laplace resonance with 8:5, 5:3, and 3:2 orbital ratios that keeps seven crowded worlds stable like a perfectly timed playground swing. That rhythm produce ...