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Astronomers discover six planets thanks to their synchronised orbits

  • The planets orbit their central star in sync, following a predictable rhythm astronomers call ‘resonance’.

  • This configuration shows that the planetary system has not suffered major changes since its formation more than a billion years ago.

  • Researchers from the Institute of Space Sciences (ICE-CSIC), the Institute of Space Studies of Catalonia (IEEC) and the Instituto de Astrofísica de Andalucía (IAA-CSIC) have participated in this discovery, which uses observations from the CARMENES instrument in Calar Alto and data from the ESA CHEOPS mission.

Tracing a link between two neighbour planets at regular time interval along their orbits, creates a pattern unique to each couple. The six planets of the HD110067 system create together a mesmerising geometric pattern due to their resonance-chain. Credits: Thibaut Roger (NCCR PlanetS).

Tracing a link between two neighbour planets at regular time interval along their orbits, creates a pattern unique to each couple. The six planets of the HD110067 system create together a mesmerising geometric pattern due to their resonance-chain. Credits: Thibaut Roger (NCCR PlanetS).  

 

Astronomers have found a peculiar family of six planets orbiting a star similar to the Sun called HD 110067. While multi-planet systems are common in our galaxy, those in a tight gravitational formation known as ‘resonance’ are observed far less often. An international team of researchers led by Rafael Luque, of the University of Chicago, have published today a paper on this discovery in the journal Nature. Several researchers from the Institute of Space Sciences (ICE-CSIC),  the Institute of Space Studies of Catalonia (IEEC) and the Instituto de Astrofísica de Andalucía (IAA-CSIC) have participated in the research.

The resonant configuration means that the orbits are synchronised in a particular manner. In this case, the planet closest to the star makes three orbits for every two of the next planet out—called a 3/2 resonance—a pattern that is repeated among the four closest planets. For the outermost planets, it is four orbits for every three of the next planet out—a 4/3 resonance.

Orbitally resonant systems like this are extremely important to find because they tell astronomers about the formation and subsequent evolution of the planetary system. Planetary systems tend to form in resonance but can be easily perturbed. For example, a very massive planet in the system, a close encounter with a passing star or any kind of merger or collision can disrupt the careful balance. Finding a resonant system, thus, is like looking at a ‘fossil’ planetary system.

HD 110067 invites further study as it shows us the unaltered configuration of a planetary system that has kept its resonance since its formation: the planets have likely been performing this same gravitational dance since the system formed billions of years ago. Moreover, this is the brightest known system with four or more planets. Since those planets are all sub-Neptune-sized with atmospheres that are likely extended, it makes them ideal candidates for studying the composition of their atmospheres using the James Webb Space Telescope of NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA).

Juan Carlos Morales, Guillem Anglada-Escudé and Ignasi Ribas, all of them ICE-CSIC and IEEC researchers, participated in the research by providing observations carried out with CARMENES, the instrument searching for exoplanets from the Calar Alto Observatory, co-developed by the Instituto de Astrofísica de Andalucía (IAA-CSIC). They also collaborated by scheduling observations with the CARMENES scheduler based on the STARS software, an artificial intelligence solution for planning operations of space missions and astronomical instruments developed by the ICE-CSIC, the IEEC and the Institute of Cosmos Sciences of the University of Barcelona (ICCUB).

“The CARMENES high-resolution spectroscopic observations spanning one year, along with those from the HARPS-N spectrograph, were used to determine the mass of three of the planets in the system and set stringent constraints to the others, revealing they are what we call sub-Neptune class planets,” explains Juan Carlos Morales, ICE-CSIC and IEEC researcher.

Following the clues to find the planets

The discovery of these planets is something of a detective story. The first hints came from NASA’s Transiting Exoplanet Survey Satellite (TESS), which aims at scanning all of the sky bit by bit to find exoplanets with short periods, that is, short years. In 2020, TESS detected dips in the brightness of the star HD 110067, which indicated planets were passing in front of its surface. These tiny eclipses are what astronomers call ‘transits’.

Two years later, TESS re-observed the same star. Adding up both sets of measurements, scientists had an assortment of transits to study. But it was difficult to distinguish how many planets they represented, or to pin down their orbits; the two sets of observations seemed to disagree.

“That’s when we decided to use CHEOPS,” explains Rafael Luque. CHEOPS is the Characterising Exoplanets Satellite, the first ESA mission devoted to studying bright, nearby stars that are already known to host exoplanets, which counts with ICE-CSIC and IEEC participation. “We went fishing for signals among all the potential periods that those planets could have,” says Luque.

Eventually, astronomers singled out the two innermost planets, with orbital periods of 9 days for the closest planet and 14 days for the next one out. A third planet, with a year about 20.5 days long, was identified with the help of the data from CHEOPS.

Then the scientists noticed something extraordinary: the three planets’ orbits matched what would be expected if they were locked in a 3/2 resonance. They had found the key to unlocking the whole system. The science team worked through a well-known list of resonances that potentially could be found in such systems, trying to match them to the remaining transits that had been picked up by TESS. The scientists could therefore predict that the outer three planets have orbital periods of 31, 41 and 55 days. “CHEOPS gave us this resonant configuration that allowed us to predict all the other periods. Without that detection from CHEOPS, it would have been impossible,” explains Luque.

However, the TESS observations that had any chance of confirming the predicted orbits of the two outer planets had been set aside during processing, as they had excessive scattered light. Reanalysis of the data to correct for the excessive light revealed two hidden transits, one from each of the planets, exactly at the times expected by the predictions. All the pieces of the puzzle had finally come together.

"The universe shows us that our Solar System does not seem to be the standard rule when it comes to the formation of planets, and once again gives us an example of the great variety of existing planetary systems. This one, in addition to its interest in understanding how they form and evolve, perhaps can provide us with additional information about why our planetary system is the way it is," concludes Pedro J. Amado, an IAA-CSIC researcher who participates in the discovery.

More information


This research is presented in a paper entitled “A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067”, by Rafael Luque et al., to appear in the journal Nature on 30 November 2023. DOI 10.1038/s41586-023-06692-3. Available at: https://www.nature.com/articles/s41586-023-06692-3

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Jorge Rivero & Alba Calejero

Juan Carlos Morales
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Juan Carlos Morales

Guillem Anglada
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Guillem Anglada

Ignasi Ribas
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Ignasi Ribas