An international collaboration with the participation of astronomers from various research centers, among which are the Institute of Astrophysics of the Canary Islands and the Institut d'Estudis Spacials de Catalunya, has just announced the discovery of a planetary system made up of six planets, all of them in a delicate balance of synchronized orbits, around a star located 100 light-years away from Earth. The details have been published today in the prestigious journal Nature.
The planets are of the “sub-Neptune” type, the name applied to objects of intermediate masses between the Earth and Neptune, and are in a configuration such that the periods of their orbits are related to each other by a ratio of integers, a phenomenon that in celestial mechanics is called resonance and that is due to mutual gravitational interaction.
Scientists think that configurations with orbital resonances are very delicate and tend to disappear over time due to various perturbations. Hence the relevance of the discovery, since it would be a star system that has managed to remain unchanged for more than 1,000 million years.
The six planets discovered, which revolve around the star HD110067 (a star with a radius of approximately 80% of the solar radius), are called b, c, d, e, f and g, and show an almost perfect coupling of their trajectories. HD110067 b, the closest to its star, takes 9.11 days to complete one orbit. The second planet (HD110067 c) does so in 13.67 days, which means that it makes 2 orbits for every 3 of the first (a resonance that, in astronomy, is represented as 3:2 and that, in our solar system, is also maintains Neptune with Pluto). This coupling is exactly the same as that presented by the orbits of the third planet (HD110067 d) in relation to the second, and of the fourth (HD110067 e) in relation to the third.
For its part, the fifth planet in order of distance from the star (HD110067 f) has an orbital period of 41.06 days, which is equivalent to a 4:3 resonance with the fourth. And this same resonance is observed between the last planet (HD110067 g) and the previous one.
The discovery of this planetary system has been very complex, and has required extensive analysis and investigation of the data recorded by various instruments.
In 2020, and based on observations made with NASA's TESS space telescope, an observatory specialized in detecting exoplanets, astronomers found what appeared to be the footprints of two planets around the star HD110067. At that time, scientists tried to estimate the orbital period of these candidates, but were only able to do so for one of them. However, a couple of years later new data obtained from the same star, and also captured by TESS, led to results inconsistent with the previous ones, since they did not confirm the period of the first object although they continued to indicate the possible presence of planets.
Astronomers then combined both sets of data and were able to corroborate the existence of the two planets closest to the star (b and c) and calculate their orbital periods. As everything seemed to point to the presence of more bodies, observations were carried out with the CHEOPS space telescope of the European Space Agency (ESA), which managed to detect a third planet (d).
When it was confirmed that the three discovered planets were in 3:2 resonant orbits, the researchers again analyzed the available data in an attempt to find signs of more objects that could have coupled orbital configurations. This work led to the detection of the fourth planet (e), also in 3:2 resonance.
But there were still signs of more planets, although the available data was insufficient for confirmation. So, based on the assumption that the other objects could also be in mutual orbital coupling, simulations were carried out testing various resonances (specifically, the 2:1, 3:2, 4:3, 5:4 and 6:5) and the results of these calculations were compared with the observations made. The conclusion was that there was only one model consistent with all the available data, and this model predicted the presence of two new planets (f and g) with orbital resonances of the 4:3 type.
However, this was a theoretical prediction that needed to be tested. With the help of the model, astronomers deduced that the first observations made by the TESS telescope should have been able to capture signals from planets f and g. So they rescued some of the data, which had initially been discarded because it had been captured in less than optimal lighting conditions, and in it they found the confirmation they were looking for.
The detections that had allowed the discovery were based on one of the two main methods of detecting exoplanets: that of transits. This system attempts to observe the passage (transit) of a planet in front of the disk of its star. Although the planet cannot be observed directly, the transit generates a small decrease in the star's brightness that can be detected by specialized instruments.
This detection method was later used again, with the participation of various ground-based telescopes, to reveal a new transit of planet f, which occurred at the precise moment predicted by the model of recently discovered resonant planets.
The second most common method for finding exoplanets is called radial velocity, and consists of detecting the minute variations in the spectrum of a star (the decomposition of its light through a prism) when the star sways due to rotation. of their planets. Specifically, the light from the star periodically shifts to blue when the star moves towards the Earth in its swing, and to red when it moves away again.
New observations of HD110067 carried out with instruments specialized in radial velocity detection and located at the Calar Alto Observatory in Almería and at the Italian Galileo telescope on the island of La Palma, confirmed the presence of planet f, and allowed us to estimate the masses of three of the planets and limit a maximum value for that of the remaining three.
For Ignasi Ribas, director of the I'Institut d'Estudis Espacials de Catalunya, researcher at the Institute of Space Sciences of the CSIC and one of the scientists who participated in the study, the way in which the discovery occurred has a great importance, since it demonstrates the value of perseverance in the face of a problem so complex that it could have been believed unsolvable with the available data.
Ribas also highlights the contribution that predictive capacity has had in the finding. “Apart from the distance,” comments the researcher, “the case would be similar to what happened with the discovery of Neptune.” On that occasion, disturbances in the orbit of Uranus made it possible not only to deduce that another planet must exist in our solar system, but also to calculate its approximate position and find it in 1846. This time, it was the resonant orbits of the first planets that have pointed out the possibility that other objects were also in a similar situation.
Although many planetary systems probably acquire coupled orbital configurations in the early stages of formation, the most common thing, Ribas comments, is that these resonances are subsequently lost due to perturbations and various gravitational interactions. Some of the causes of these perturbations would be the presence of very massive planets, the relatively close passage of other stars or even impacts.
Along these lines, Rafael Luque, astrophysicist at the University of Chicago and principal investigator of the study, estimates that only 1% of these systems would manage to remain in resonance over time.
Previously, astronomers had already detected a few multiple planetary systems with orbital resonances. One of these examples is Trappist-1, made up of seven planets, all of them terrestrial, of which three are in the so-called habitable zone (a region around the star where temperatures would allow water, in case of exist, it could be in liquid phase on the surface).
In the case of HD110067, since its planets are of the sub-Neptune type and these types of objects usually have extensive atmospheres, ESA has declared that this star system is an ideal candidate for the James Webb Space Telescope to test its capabilities. detection to analyze the composition of these planetary atmospheres. A task that, ESA highlights, in the future can also be carried out with the European Ariel and Plato telescopes.
