It's been a good week for exoplanetary science. The announcement of the discovery of 7 beautifully transiting planets around the dim, ultracool dwarf star TRAPPIST-1 a mere 39 light years from us, not only provides a terrific target for future space and ground-based telescopes, but also reinforces the fact that the cosmos is overflowing with planets that could be suitable for life.
I'm still digesting the details of what we already know about this system, including its seemingly fragile hold on orbital stability (likely saved by tidal dissipation effects). But one of the most intriguing characteristics is the sheer smallness of the system architecture. The estimated mass ratio between the total planetary mass of the inner 6 worlds and the star's mass is also about 0.02% - very similar (strikingly similar!) to the ratio between the total mass of the Galilean moons and Jupiter in our own system. Could this suggest a similar mechanism of formation? Some models for Jupiter's moons have shown that this ratio is consistent with a scenario of formation in a constantly evolving disk of material, where moons (or perhaps planets) migrate quickly inwards and only the last generations survive as the disk dissipates.
Take a look at the absolute scale of the TRAPPIST-1 system:
The TRAPPIST-1 planetary orbits are far closer in scale to the Jovian moon system than they are to those of the planets of our solar system - by a long shot. And of course TRAPPIST-1 itself, at about 8% the mass of our Sun is only a little larger in diameter than Jupiter.