Chapter 1: The Discovery of TOI-178

An artist's rendition of the TOI-178 planetary system illustrates the complexities of its formation. This recent discovery has left astronomers puzzled. Within this six-planet system, the outer planets exhibit a rhythmic orbital pattern, yet their densities present a chaotic arrangement.

As astronomers delve deeper into planetary formation, they find that conventional explanations may no longer suffice. Using our solar system as a reference point fails to shed light on the vast diversity of planetary systems. In mid-2020, I reported on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) capturing high-resolution images of a young star system forming. Now, the VLT has unveiled another perplexing finding that calls into question established models of planetary formation.

The TOI-178 system, located 200 light-years away in the Sculptor constellation, has been revealed through the efforts of an international team of astronomers. They utilized various instruments and telescopes, including the European Space Agency's CHEOPS satellite, along with ground-based observatories such as ESPRESSO on the VLT, and NGTS and SPECULOOS, to uncover this system by employing transit imaging and radial velocity techniques.

"This contrast between the rhythmic harmony of the orbital motion and the disorderly densities certainly challenges our understanding of the formation and evolution of planetary systems."

~ Adrien Leleu, Study Lead

Exoplanets are typically elusive, which is why astronomers rely on imaging transits and radial velocity methods for observation. To explain briefly, imaging transits detect the dimming of a star's light as an exoplanet passes in front of it, while radial velocity measures the variations in the star's light spectrum caused by gravitational influences from the orbiting planets.

The planets in the TOI-178 system are designated TOI-178b through g. Through the aforementioned techniques, researchers gathered essential data, revealing that these six planets range from one to three times Earth's size, with masses between 1.5 and 8 times that of our planet.

The inner two planets are classified as rocky Super-Earths, while the outer four are categorized as gaseous Mini-Neptunes. All six planets are situated much closer to their star than Earth is to the Sun, with their orbital periods spanning from just a few Earth days to several months.

Further analysis of the system brought to light intriguing characteristics. The research indicates that five of the outer planets are synchronized in a rare rhythmic pattern around their central star. This resonance, represented as 18:9:6:4:3, reveals a striking order: for every three orbits completed by the outermost planet, the next completes four, followed by six and nine, with the innermost planet making 18 orbits. In our solar system, a similar resonance pattern is observed in the orbits of Jupiter's moons Io, Europa, and Ganymede, which follow a 4:2:1 pattern.

Why is this resonance noteworthy? While it mirrors phenomena observed in Jupiter's moons, the unusual resonance and the orderly orbits of the TOI-178 exoplanets suggest a more tranquil history with fewer collisions, a rarity in planetary formation, which is often characterized by chaotic interactions, as seen in our solar system.

However, the TOI-178 system presents further enigmas. The densities of the exoplanets are notably disordered; one planet is as dense as Earth while another has half the density of Neptune, followed by a third with Neptune-like density. This stands in stark contrast to our solar system, where denser, rocky planets are located closer to the central star, and lighter, gas giants are found further out.

This research sheds light on the processes of planetary formation and evolution, indicating that multiple pathways may exist beyond traditional theories. Astronomers plan to continue investigating the peculiar configurations and fragile orbital resonances of these exoplanets, a pursuit that will yield knowledge over time. For now, let us remain captivated by the universe's myriad mysteries.

Complete Research was published in the Journal of Astronomy & Astrophysics.