Astronomers have already discovered several stars with many planets in space, but TOI-178 is something special. The star, which is around 200 light-years away, is orbited by six planets, five of which move in a harmonious rhythm: Their orbital times show an orbital resonance and thus form one of the longest known resonance chains. At the same time, however, these exoplanets have very different, disordered densities. This makes this planetary system unique and is difficult to explain with common models.
Physical resonances arise when vibrations influence each other and thereby reinforce each other. What can end destructively when the earthquake affects buildings, creates harmonious sounds in music. For example, the frequencies of harmonic chords are in whole-number relationships to one another. But resonance effects are also known in astronomy. They occur when planets or other celestial bodies in a system influence each other through their gravity. The result is an orbital resonance in which the orbital times of their orbits form simple breaks. In our solar system, for example, the orbits of Neptune and Pluto are coupled in a 3: 2 resonance and the three Jupiter moons Io, Europa and Ganymede are also in an orbital resonance.
Five planets in orbit resonance
But now astronomers are reporting on a planetary system that drives these resonances to extremes, so to speak. Because five of the six planets around the 200 light-years distant star TOI-178 move in resonance with each other, as Adrien Leleu from the University of Bern and the University of Geneva and his colleagues discovered. Even before their study, it was known from images taken by NASA’s TESS space telescope that at least two planets orbit TOI-178. Leleu and his team investigated this planetary system in more detail with the help of the Cheops space telescope of the European Space Agency ESA and with the earth-based telescopes of the European Southern Observatory in Chile. On the basis of the light curves during the transits of the planets in front of their star and also the subtle tumbling caused by their gravity in their star, the researchers were finally able to detect even six planets around TOI-178.
The analyzes also showed that five of these planets move in resonance with one another. Their orbital times form one of the longest resonance chains known to date from planetary systems. Only the innermost planet is not involved in this harmonious dance of the planets. The other five follow a rhythm in which their orbital times are in the ratio 18: 9: 6: 4: 3: While the second planet, seen from the star, makes 18 orbits, its outer neighbor completes nine orbits. Its neighbor, in turn, makes six orbits in the same time, and so on. This unbroken chain of resonance also provides indications of the system’s past: “The paths in this system are very well ordered, which tells us that this system has developed very gently since it was born,” explains co-author Yann Alibert from the University of Bern . Had the system been significantly disrupted, for example by a collision, this fragile configuration of the railways would not have survived.
Wild mix of densities
In view of this apparently long-lived and undisturbed order, the results of the size and mass analyzes of the six planets were all the more surprising. With one to three times the size of the earth, their mass varies between 1.5 and 30 earth masses. Contrary to expectations, the densities of the planets seem to be wildly mixed up. “It is the first time that we have observed something like this,” says ESA project scientist Kate Isaak. “In the few systems that we know of with such a harmony, the density of the planets steadily decreases with the distance to the star.” In our solar system, for example, the denser rock planets are closer to the central star and the gas planets with lower density further out. “In the TOI-178 system, on the other hand, 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 that is very similar to Neptune,” says Isaak.
(Video: ESO / L. Calcada)
Such a chaotic sequence could arise if planets have changed positions and orbits in the course of their history due to collisions or other gravitational turbulence. But the unbroken chain of resonance from five of the six planets of TOI-178 speaks against such a scenario. “This contrast between the rhythmic harmony of the orbital movement and the disordered densities certainly calls into question our understanding of the formation and development of planetary systems,” says Leleu. He and his colleagues now hope to learn more about how this unusual planetary system came about through further observations.
Source: Adrien Leleu (University of Bern, University of Geneva) et al., Astronomy & Astrophysics, doi: 10.1051 / 0004-6361 / 202039767