The list of exoplanets continues to grow. More than 5,500 worlds outside our solar system are already known, but some of the findings continue to surprise astrophysicists. This is the case of an atypical planetary system made up of six planets that orbit in a synchronized way around their star: they revolve around their host star in perfect harmony, following a sort of orbital waltz that they repeat with precision.

The reason for the astonishment is that this phenomenon, which they call orbital resonance, is common in young planetary systems, but it is exceptional to find systems with such a long chain of planets that retain this type of gravitational synchronization, as their discoverers argue in an international study published this Wednesday in the journal Nature, which has a significant Spanish participation.

The research is led by astronomer Rafael Luque, who is currently doing research at the University of Chicago, and its co-authors include scientists from the Instituto de Astrofísica de Canarias (IAC), the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC). "I'm from a town in the south of Cordoba called Priego de Cordoba, one of the most beautiful in all of Andalusia. I have been working at the University of Chicago since March 2022, but my doctoral thesis was done at the Instituto de Astrofísica de Canarias under the supervision of Professor Enric Palle, one of the main co-authors of the article," Luque told this newspaper in an email.

At the heart of this planetary system is the star HD 110067, which is smaller and cooler than the Sun. It is located in the constellation Coma Berenices, a hundred light-years away, and is estimated to be between 4,000 and 12,000 million years old. Another salient conclusion is that HD110067 is the brightest known system with four or more planets.

As explained by the IAC, "collisions between planets, mergers or ruptures, the presence of giant planets such as Jupiter and even the close passage of another star, can alter the orbital balance". As a result, "99% of known multiplanetary systems are not in resonance, although they might have been at one time." Discovering a resonant system is like observing a fossil planetary system, according to the authors' comparison.

In this case, the planet closest to the star makes three orbits for every two of the next planet, which is called a 3/2 resonance, a pattern that repeats itself between the four closest planets. In the case of the farthest planets, they make four orbits for every three of the next planet, a 4/3 resonance.

More than six planets?

These six worlds are part of a group of planets called sub-Neptune because they are smaller than Neptune and, as Luque explains, it is possible that there are more worlds revolving around the same star: "The orbits of these six planets are so perfectly aligned that if there were planets even farther away than planet g (with an orbital period of 54 days) they could pass in front of the disk of their star and be detected in the future."

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How did they find out? The first observations of the system were made with NASA's TESS satellite in 2020 and 2022, and were completed with an additional study conducted with the European exoplanet hunter CHEOPS. By studying the three planets closest to the star, the authors calculated the orbits of the six worlds. They were also able to estimate the planets' masses and densities, which are relatively low, which could be explained by their hydrogen-rich atmospheres.

Several Spanish instruments have been used to unravel the details of these six exoplanetsand their transits, such as the CARMENES spectrographs at the Calar Alto Observatory (CAHA), HARPS-N at the Roque de los Muchachos Observatory (ORM) and MuSCAT2 at the Teide Observatory.

Habitable planets?

As for whether these planets could harbor life, Luque admits that they don't know yet: "If we consider the star's habitable zone for a rocky planet like Earth, the planets in this system would be too hot (too close to their star) for liquid water to be on their surface. However, these worlds are not terrestrial, but are two to three times larger than Earth and have large atmospheres (probably rich in hydrogen, helium, and water vapor). These atmospheres can act both as a blanket (as on Venus) and warm the planet's surface even more, as a sunshield and reflect most of the radiation they receive from their star and cool its surface. Therefore, even though they are close to their star, it is possible that their atmospheres allow them to have liquid water somewhere in their interior," he says.

Enric Pallé, IAC researcher and co-author of the study, agrees, pointing out that "the planets in the HD110067 system appear to have low masses, suggesting that they may be rich in gas or water".

For Luque, "this is one of the great unknowns of our time, but it can be solved in the coming years thanks to the James Webb Space Telescope, which will be able to study in great detail the atmospheric composition of the six planets in this system." Since all of these planets are smaller than Neptune and have atmospheres rich in hydrogen and helium, they are ideal candidates to be observed by telescopes such as Webb, in order to determine their chemical composition, their internal structures, and whether they are rich in water.

"The universe shows us that our Solar System does not seem to be the norm when it comes to planet formation, and once again gives us an example of the great variety of planetary systems that exist," says Pedro J. Amado, a researcher at the IAA-CSIC, in a press release. The scientist, who has also participated in this work, believes that perhaps it can provide us with additional information about why our planetary system is the way it is.

  • Astronomy