Surprising six-exoplanet system with rhythmic motion challenges theories about how planets form

Using a combination of telescopes, including the Very Large Telescope of the European Southern Observatory (ESOs VLT), astronomers have uncovered a system of six exoplanets, five of which are locked in a rare rhythm around their central star. is. The researchers believe that the system can provide important clues about how planets, including those in the solar system, form and develop.

The first time the team observed the TOI-178, a star about 200 light-years in the constellation of Sculptor, they thought they had seen two planets in the same orbit. A closer examination, however, revealed something completely different. “Through further observations, we realized that there are no two planets orbiting approximately the same distance from the star, but rather multiple planets in a very special configuration,” says Adrien Leleu of the Université de Genève and the University of Bern, Switzerland. which led to a new study of the system that today in Astronomy & Astrophysics.

The new research has revealed that the system boasts six exoplanets and that all but the closest to the star are locked up in a rhythmic dance as they move in their orbits. In other words, they are in resonance. This means that there are patterns that repeat themselves as the planets orbit the star, with single planets aligning each pair of orbits. A similar resonance is observed in the orbits of three of Jupiter’s moons: Io, Europa and Ganymede. Io, the closest of the three to Jupiter, completes four full orbits around Jupiter for each orbit that Ganymede makes, the farthest way, and two full orbits for each orbit that Europe makes.

The five outer exoplanets of the TOI-178 system follow a much more complex chain of resonance, one of the longest ever discovered in a system of planets. While the three Jupiter moons are in a 4: 2: 1 resonance, the five outer planets in the TOI-178 system follow an 18: 9: 6: 4: 3 chain: while the second planet from the star (the first in the resonant chain) completes 18 orbits, the third planet from the star (second in the chain) completes 9 orbits, and so on. In fact, the scientists initially found only five planets in the system, but by following this resonant rhythm, they calculated where an additional planet would be in its orbit the next time they had a window to observe the system. take.

More than just a curiosity, this dance of resonant planets offers clues about the system’s past. “The orbits in this system are very well organized, which tells us that this system has developed quite gently since its birth,” explains co-author Yann Alibert of the University of Bern. If the system had been significantly disturbed earlier in its life, for example by a giant impact, this fragile configuration of orbits would not have survived.

Disorder in the rhythmic system

But even if the ordering of the orbits is neat and well-ordered, the density of the planets is ‘much more disorderly’, says Nathan Hara of the Université de Genève, Switzerland, who was also involved in the study. “It appears that there is a planet as dense as Earth, right next to a very soft planet with half the density of Neptune, followed by a planet with the density of Neptune. This is not what we are used to. . ” In our solar system, for example, the planets are neatly arranged, with the rocky, denser planets closer to the central star and the soft, low-density gas planets further out.

“This contrast between the rhythmic harmony of the orbital motion and the disordered densities definitely challenges our understanding of the formation and evolution of planetary systems,” says Leleu.

Combination of techniques

To investigate the unusual architecture of the system, the data used data from the European Space Agency’s CHEOPS satellite, together with the ESPRESSO instrument on the ground on ESO’s VLT and the NGTS and SPECULOOS, both in the Paranal Observatory of ESO is located in Chile. Since exoplanets are extremely difficult to detect directly with telescopes, astronomers would rather rely on other techniques to detect them. The most important methods used are image transmissions – observation of the light emitted by the central star, which fades as an exoplanet passes in front of it when observed from Earth – and radial velocities – which observe the star’s light spectrum for small signs of wobbling that occur as the exoplanets move in their orbits. The team used both methods to observe the system: CHEOPS, NGTS and SPECULOOS for passages and ESPRESSO for radial velocities.

By combining the two techniques, astronomers were able to gather important information about the system and its planets, orbiting their central star much more centrally and much faster than the earth orbiting the sun. The fastest (the innermost planet) completes an orbit within a few days, while the slowest takes about ten times longer. The six planets have sizes ranging from about one to about three times the size of the earth, while their masses are 1.5 to 30 times the mass of the earth. Some of the planets are rocky, but larger than Earth – these planets are known as Super-Earth. Others are gas planets, like the outer planets in our solar system, but they are much smaller – they have the nickname Mini-Neptunes.

Although none of the six exoplanets found lie in the habitable zone of the star, the researchers suggest that by continuing the resonance chain, they may find additional planets that may exist in or near this zone. ESO’s Extremely Large Telescope (ELT), which will be in operation this decade, will be able to image rocky exoplanets directly in a star’s habitable zone and even characterize their atmosphere, providing an opportunity for systems such as TOI -178 even to get to know. larger details.