2023-11-29 18:00:12
Magazine Nature publishes this week the discovery of six exoplanets that orbit a nearby Sun-like star called HD 110067. It is located in the constellation Coma Berenices, about 100 light years away, and is visible from Earth’s northern hemisphere.
The study, led by astrophysicist Rafael Luque from the University of Chicago (USA), has been possible thanks to the observations of the Transiting Exoplanet Survey Satellite (TESS) of NASA and the CHaracterising ExOPlanets Satellite (CHEOPS) of the European Space Agency (ESA).
Brightness changes in HD 110067 and other signals detected by these and other instruments allowed us to confirm the existence of the six planets passing in front of their star with resonant orbitsa kind of synchronized ‘waltz’.
Although multiplanetary systems are common in our galaxy, those found in this tight gravitational formation known as resonance (orbits synchronized in a particular way) are observed much less frequently.
The planets closest to the star have an orbital resonance of 3/2 and those furthest away have a 4/3 orbital resonance.
In this case, the planet closest to the star makes three orbits for every two of the next planet, which is called resonance 3/2a pattern that is repeated among the four closest planets.
Regarding the other more distant planets, there are four orbits for every three of the next planet, one resonance 4/3.
Resonant orbital systems like this are extremely important because they tell astronomers about the formation and evolution of the planetary system.
A fossil planetary system
These systems tend to form in resonance, but can be easily disturbed. For example, a very massive planet in the system, a close encounter with a passing star, or any type of merger or collision can upset the delicate balance. Therefore, finding a resonant system is like observing a fossil planetary system.
HD 110067 invites keep studying itas it shows us the unaltered configuration of a planetary system that has maintained its resonance since its formation: it is likely that the planets have been practicing this same gravitational dance since the system formed, ago more than a billion years.
Planets have likely been practicing this same gravitational dance since the system formed more than a billion years ago.
Furthermore, it is about brightest known system with four or more planets. Since all of these planets are smaller than Neptune and have probably extensive atmospheres, they are ideal candidates for studying the composition of their atmospheres with the James Webb Space Telescope from NASA, ESA and the Canadian Space Agency (CSA).
Spanish participation
Various Spanish research centers have participated in this study: the Institute of Space Sciences (ICE-CSIC), the Institute of Space Studies of Catalonia (IEC), the Institute of Astrophysics of Andalusia (IAA-CSIC), the Astrobiology Center (CABINTA-CSIC) and the Institute of Astrophysics of the Canary Islands (IAC).
Juan Carlos Morales, Guillem Anglada-Escudé e Ignasi Ribasfrom the ICE-CSIC and the IEEC, contributed observations made with CARMEN ISthe exoplanet search instrument of the Calar Alto Observatory (Almería) co-developed by the IAA.
They also collaborated by programming the observations with the planner of this instrument, based on the Stars software, a solution of artificial intelligence for the planning of space mission operations and astronomical instruments developed by the ICE-CSIC, the IEEC and the Institute of Cosmos Sciences of the University of Barcelona (ICCUB).
The CARMENES instrument at the Calar Alto Observatory (Almería) has helped measure the mass of some of these ‘subneptunes’
“The high-resolution spectroscopic observations of CARMENES over a year, together with those of the spectrograph HARPS-N (in it Galileo National Telescope from the Roque de los Muchachos Observatory, Canary Islands), were used to determine the time of three of the planets in the system and set strict limits for the others, revealing that they are what we call sub-Neptune class planets,” explains Morales.
These step-nephewsplanets with radii between that of the Earth and that of Neptune, are found in close orbits in more than half of the Sun-like stars, but the details of their composition, formation and evolution are not well known.
The detective story of the discovery
The discovery of these planets is reminiscent of a detective story. The first clues came from TESS satellite from NASA, whose goal is to examine the entire sky piece by piece to find exoplanets of small period (short years). In 2020, it detected decreases in the brightness of the star HD 110067, which indicated the passage of planets in front of its surface. These small eclipses are what astronomers call transits.
Two years later, TESS observed the same star again. By adding both sets of measurements, scientists had a range of transits to study. But it was difficult to distinguish how many planets they represented, or to specify their orbits. The two sets of observations seemed to disagree with each other.
To carry out the observations, NASA’s TESS and CHEOPS satellites from the European Space Agency (ESA) were used.
“That’s when we decided to use CHEOPS”Luque remembers. This Exoplanet Characterization Satellite is the first ESA mission dedicated to studying bright, nearby stars that are already known to host exoplanets, and which includes the participation of ICE-CSIC and the IEEC.
“We went to fish for signals between all the potential periods that these planets could have,” explains the first author.
Finally, astronomers identified the two innermost planets, with orbital periods of 9 days for the closest one and 14 days for the next. A third planet, with a year of about 20.5 days, was identified with the help of data also from the European satellite.
The CHEOPS mission has been key to studying the strange system of the six exoplanets. / THAT
Then, scientists made the extraordinary observation: the orbits of the three planets matched what would be expected if they were fixed on a resonance 3/2. They had found the key to unlocking the entire system.
The researchers reviewed a well-known resonance list that could occur in this type of systems, trying to make them coincide with the rest of the transits that TESS had captured. Thus, they were able to predict that the three outer planets have orbital periods of 31, 41 and 55 days.
From the inside out, the orbital periods of the six exoplanets are 9, 14, 20.5, 31, 41 and 55 days
“CHEOPS gave us this resonant configuration that allowed us to predict all other periods. Without that detection, it would have been impossible,” Luque acknowledges.
However, TESS observations that had any chance of confirming the predicted orbits of the two outermost planets had been left out during processing as they had too much scattered light.
A new analysis of the data to correct for this excess of light revealed two hidden transits, one for each of the planets, at exactly the times expected by the predictions. Finally, all the pieces of the puzzle fit together.
The instrument was also used in traffic detection. MuSCAT2 installed in the Carlos Sánchez Telescope from the Teide Observatory (Canary Islands).
“Among the more than 5,000 exoplanets discovered orbiting stars other than our Sun, resonances are not rare, nor are systems with several planets,” says the co-author. Enric Pallé of the IAC, “what is extremely rarehowever, is to find systems in which the resonances span such a long chain of six planets, demonstrating that this system has not undergone major changes since its formation more than a billion years ago.”
“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. This, in addition to its interest in understanding how they form and evolve, may perhaps provide us with additional information about why our planetary system is the way it is,” concludes another of the authors, Pedro J. AmadoIAA researcher.
Orbital motion of the six planets relative to a single planet year c. For every 360 degree rotation around HD110067 from this planet, planet b moves 540 degrees, planet d 240, planet e 160, planet f 120, and planet g 90 degrees. / Dr. Hugh Osborn (University of Bern)
Reference:
Rafael Luque et al. “A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067.” Nature2023
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