Enigmatic Star System Has 5 Planets Locked in Perfect Harmony - Gizmodo

Five of six exoplanets in orbit around the star TOI-178 are in an 18:9:6:4:3 orbital resonance with each other, according to new research published today in Astronomy & Astrophysics.

When Leleu, a dynamicist (an expert in celestial mechanics) and his colleagues first observed the TOI-178 system, they thought they saw two planets orbiting around the host star in the same orbit, but this result was inconclusive.

All these instruments allowed the team to detect the six exoplanets and characterize their orbits, which they did using the transit method (looking at the dimming of the host star when a planet passes in front) and by measuring the wobble of the host star.

All six exoplanets are in close proximity to the central star, with the nearest planet taking around two days to make a complete orbit and the most distant orbiting in around 20 days.

None are inside the habitable zone, the Goldilocks region around a star where liquid water (and thus life) would be possible.

Five of the six exoplanets are locked in perfect resonance, such that some planets come into alignment every few orbits.

“In TOI-178, it’s only true for the two inner planets that are rocky, but then the third planet from the star has a very low density, then planet 4 and 5 are more dense, and then planet 6 is once again more fluffy,” he said.

Interestingly, TOI-178 could host other, more distant planets, but they just haven’t been detected.