A team of astronomers has announced the discovery of a remarkable six-planet system around the star TOI-178, where five planets engage in a precise orbital resonance. The findings are detailed in Astronomy & Astrophysics.
Initially detected as hosting two planets in similar orbits, further observations revealed six planets total, with five locked in a rhythmic orbital dance. Unlike previous resonant systems, this one's planets are strikingly diverse in size and composition.
The planets range from 1 to 3 times Earth's size, with masses between 1.5 and 30 times that of Earth. Inner ones are rocky Super-Earths, while outer ones are gaseous Mini-Neptunes, akin to scaled-down versions of our solar system's gas giants.
All except the innermost planet follow an 18:9:6:4:3 resonance chain. This means the second planet completes 18 orbits while the third does 9, the fourth 6, the fifth 4, and the outermost 3—aligning them periodically around their star.
This echoes a simpler 4:2:1 resonance among Jupiter's moons: Io laps Ganymede four times, and Europa twice per Ganymede orbit. Such delicate resonances suggest the TOI-178 system has remained largely undisturbed since formation, as disruptions like giant impacts would shatter this harmony.
Leveraging data from ESA's CHEOPS satellite and Chile's Very Large Telescope (VLT), researchers found these exoplanets orbit much closer and faster than Earth around the Sun.
The innermost planet zips around TOI-178 in just days, while the outermost takes weeks. None reside in the habitable zone where liquid water could exist.
Future observations with ESO's Extremely Large Telescope (ELT), set for 2025, promise direct imaging and atmospheric analysis of these worlds.
