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Astronomers have used two different exoplanet-detecting satellites to solve a cosmic mystery and reveal a rare family of six planets located about 100 light-years from Earth. The discovery could help scientists unlock the secrets of planet formation.
The six exoplanets orbit a bright star similar to the Sun called HD110067, which is located in the constellation Coma Berenices in the northern sky. Larger than Earth but smaller than Neptune, the planets belong to a little-understood class called sub-Neptunes, which are commonly found around Sun-like stars in the Milky Way. And the planets, named b through g, orbit the star in a celestial dance known as orbital resonance.
Patterns are emerging as the planets complete their orbits and exert gravity on each other, according to a study published Wednesday in the journal Nature. For every six orbits that planet b, the planet closest to the star, completes, the outermost planet g completes one.
While planet c makes three orbits around the star, planet d makes two, and when planet e makes four orbits, planet f makes three.
This harmonic rhythm creates a resonant chain, with all six planets aligning every few orbits.
What makes this family of planets an unusual find is that little has changed since the system formed more than 1 billion years ago, and the revelation could shed light on the evolution of planets and the origins of our ruling sub-Neptunes own galaxy.
Uncovering a mystery
Researchers first noticed the star system in 2020 when NASA’s Transiting Exoplanet Survey Satellite, or TESS, spotted dips in HD110067’s brightness. A dip in starlight often indicates the presence of a planet passing between its host star and an observing satellite as the planet moves along its orbit. Detecting these dips in brightness, known as the transit method, is one of the main strategies scientists use to identify exoplanets through ground- and space-based telescopes.
Based on that data from 2020, astronomers have determined the orbital periods of two planets around the star. Two years later, TESS observed the star again, and the evidence suggested different orbital periods for those planets.
When the data sets didn’t add up, astronomer and lead study author Rafael Luque and some of his colleagues decided to look at the star again using another satellite: the European Space Agency’s signature ExOPlanet Satellite, or Cheops. While TESS is used to observe parts of the night sky for short observations, Cheops observes one star at a time.
“We started looking for signals from all the potential periods that these planets could have,” said Luque, a postdoctoral researcher at the University of Chicago’s department of astronomy and astrophysics.
The data collected by Cheops helped the team solve the “detective story” started by TESS, he said. Cheops was able to identify the presence of a third planet in the system, which was crucial for confirming the orbital periods of the other two planets, as well as their rhythmic resonance.
As the team linked the rest of the unexplained TESS data to the Cheops observations, they discovered the other three planets orbiting the star. Follow-up observations with ground-based telescopes confirmed the presence of the planets.
The dedicated time Cheops spent observing the star helped astronomers smooth out the mixed signals from the TESS data to determine how many planets passed in front of the star and the resonance of their orbits.
“Cheops gave us this resonant configuration that allowed us to predict all other periods. Without that detection by Cheops that would have been impossible,” says Luque.
The closest planet takes just over nine Earth days to complete an orbit around the star, and the most distant planet takes about 55 days. All planets orbit their star faster than Mercury, which takes 88 days to complete one orbit around the sun.
Given how close they are to HD110067, the planets likely have sweltering average temperatures similar to those of Mercury and Venus, ranging between 332 degrees Fahrenheit and 980 degrees Fahrenheit (167 degrees Celsius and 527 degrees Celsius).
Why planetary rhythm matters
The formation of planetary systems, such as our own solar system, can be a violent process. While astronomers believe that planets initially form in resonance around stars, the gravitational influence of massive planets, a collision with a passing star, or a collision with another celestial body can disrupt the harmonic balance.
Most planetary systems are not in resonance, and multi-planet systems that have retained their initial rhythmic orbits are rare. That’s why astronomers want to study HD110067 and its planets in detail as a “rare fossil,” Luque said.
“We think that only about one percent of all systems remain in resonance,” Luque said in a statement. “It shows us the pristine configuration of a planetary system that has survived untouched.”
The discovery marks the second time Cheops has helped reveal a planetary system with orbital resonance. The first, known as TOI-178, was announced in 2021.
“As our science team puts it: Cheops makes extraordinary discoveries sound normal. Of only three known six-planet resonance systems, this is now the second found by Cheops, in just three years,” Maximilian Günther, ESA Cheops project scientist, said in a statement.
A perfect observation target
The system can also be used to study how sub-Neptunes form, the study authors said.
Although sub-Neptunes are common in the Milky Way Galaxy, they do not exist in our own Solar System. And there is little agreement among astronomers about how these planets form and what they are made of. So an entire system made up of sub-Neptunes could help scientists learn more about their origins, Luque said.
Many exoplanets have been found orbiting dwarf stars that are much cooler and smaller than our Sun, such as the famous TRAPPIST-1 system and its seven planets, announced in 2017. Although the TRAPPIST-1 system also has a resonant chain, is the faintness of the host star makes observations difficult.
But HD110067, which has 80% the mass of our Sun, is the brightest known star with more than four planets in orbit, so observing the system is much easier.
The first detections of the planets’ masses suggest that some of them have puffy, hydrogen-rich atmospheres, making them ideal research targets for the James Webb Space Telescope. As starlight filters through the planets’ atmospheres, Webb can be used to determine the composition of each world.
“The planets south of Neptune of the HD110067 system appear to be low-mass, suggesting they may be gas or water rich. Future observations, for example with the James Webb Space Telescope, of these planetary atmospheres could determine whether the planets have rocky or watery inner structures,” said co-author Jo Ann Egger, a doctoral student in astrophysics at the University of Bern in Switzerland. in a statement.
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