An international team of astronomers has discovered an incredible star system. Six planets orbit star TOI-178 and the outer five orbit in perfect harmony, a phenomenon known as resonance. Other planetary systems have been seen to be in resonance but this is the first time the planets involved are so wildly different from one another.
The study, published in Astronomy & Astrophysics, reports that the five external planets are in a 18:9:6:4:3 chain of resonance. A resonance of 2:1 would mean that for every orbit of the outer planet the inner one does two.
In the case of TOI-178, this means that for every three orbits by the most external planet, the next does four, the one after does six, the one after that does nine, and the final one (the second from the star) does 18.
Using combined data from the European Space Agency (ESA) CHEOPS mission as well as NASA’s planet-hunter TESS and the ESPRESSO instrument on ESO's Very Large Telescope allowed not only the precise characterization of the orbital properties of the planets but also determined that they are all different. Something that goes against the expectations astronomers had for these types of systems.
“It is the first time we observe something like this,” co-author and ESA project scientist Kate Isaak said in a statement. “In the few systems we know with such a harmony, the density of planets steadily decreases as we move away from the star. In the TOI-178 system, a dense, terrestrial planet like Earth appears to be right next to a very fluffy planet with half the density of Neptune followed by one very similar to Neptune.”
“The system, therefore, turned out to be one that challenges our understanding of the formation and evolution of planetary systems,” added lead author Adrien Leleu of the University of Bern, the University of Geneva, and the National Center of Competence in Research Planets.
The incredible findings also had a sprinkling of drama and danger. Preliminary calculations based on the first set of observations by the CHEOPS space telescope suggested the existence of what was later found to be the second outermost planet, so the team scheduled more observations with the ESA's satellite. That's when things went sideways.
CHEOPS was threatened with damage or possibly even being destroyed by a piece of space debris. ESA had to stop all observations and shift CHEOPS' orbit to get it out of the way.
“But to our great relief, this manoeuver was done very efficiently and the satellite could resume observations just in time to capture the mysterious planet passing by,” explained Nathan Hara, co-author and astrophysicist from the University of Geneva. “A few days later, the data clearly indicated the presence of the additional planet and thus confirmed that there were indeed six planets in the TOI-178 system.”