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Astronomers have used two different exoplanet detection 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 sun-like star called HD110067, which is located in the Coma Berenices constellation in the northern sky. Larger than Earth but smaller than Neptune, the planets belong to a poorly understood class called sub-Neptunes that are commonly found orbiting Sun-like stars in the Milky Way. And the planets, labeled a b a g, rotate around the star in a celestial dance known as orbital resonance.
There are discernible patterns as planets complete their orbits and exert gravitational forces on each other, according to a study published Wednesday in the journal Nature. For every six orbits completed by planet b, the planet closest to the star, the outermost planet g completes one.
While planet c makes three revolutions around the star, planet d makes two, and when planet e completes four orbits, planet f makes three.
This harmonic rhythm creates a resonant chain, with the six planets aligning every few orbits.
What makes this planetary family an unusual find is that little has changed since the system formed more than a billion years ago, and the revelation could shed light on the evolution of planets and the origin of the predominant sub-Neptunes in our galaxy.
Detecting a mystery
Researchers first became aware of the star system in 2020 when NASA’s Transiting Exoplanet Survey Satellite, or TESS, detected dips in the brightness of HD110067. A dip in starlight often suggests the presence of a planet passing between its host star and an observing satellite as the planet travels along its orbital path. Detecting these luminosity drops, known as the transit method, is one of the main strategies used by scientists to identify exoplanets through ground-based and space-based telescopes.
Astronomers determined the orbital periods of two planets around the star from that 2020 data. 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 author of the study Rafael Luque and some of his colleagues decided to take another look at the star using a different satellite: the European Space Agency’s ExOPlanet characterizing satellite, or Cheops. While TESS is used to observe fractions of the night sky in short observations, Cheops observes one star at a time.
“We went looking for signals among all the potential periods those planets could have,” said Luque, a postdoctoral researcher in the department of astronomy and astrophysics at the University of Chicago.
The data collected by Cheops helped the team solve the “police story” started by TESS, he said. Khufu was able to determine the presence of a third planet in the system, which was crucial in confirming the orbital periods of the other two planets, as well as their rhythmic resonance.
When the team compared the rest of the unexplained TESS data with the Khufu observations, they discovered the other three planets orbiting the star. Subsequent observations with ground-based telescopes confirmed the presence of the planets.
The dedicated time Cheops spent observing the star helped astronomers clear up the mixed signals in the TESS data to determine how many planets were crossing in front of the star and the resonance of their orbits.
“Khufu gave us this resonant configuration that allowed us to predict all other periods. Without that Cheops detection it would have been impossible,” said Luque.
The closest planet takes just over nine Earth days to complete an orbit around the star, and the most distant takes about 55 days. All planets have faster revolutions around their star than Mercury, which takes 88 days to complete one revolution around the sun.
Given how close they are to HD110067, the planets likely have dizzying 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 is planetary rhythm important?
The formation of planetary systems, like our own solar system, can be a violent process. While astronomers believe that planets tend to initially form in resonance around stars, the gravitational influence of massive planets, a brush with a passing star, or a collision with another celestial body can upset the harmonic balance.
Most planetary systems are not in resonance, and those with multiple planets that have retained their initial rhythmic orbits are rare, which is why astronomers want to study HD110067 and its planets in detail as a “rare fossil,” Luque said.
“We believe 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 intact.”
The discovery is 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 scientific team puts it: Cheops is making outstanding discoveries seem ordinary. Of only three known six-planet resonant systems, this is now the second found by Cheops, and in just three years of operations,” Maximilian Günther, ESA’s 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.
While sub-Neptunes are common in the Milky Way, 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 determine more about their origin, 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. While the TRAPPIST-1 system also has a resonant chain, the Weakness 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.
Initial detections of the planets’ masses suggest that some of them have swollen, hydrogen-rich atmospheres, making them ideal study 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 sub-Neptune planets in the HD110067 system appear to have low masses, suggesting that they may be rich in gas or water. Future observations of these planetary atmospheres, for example with the James Webb Space Telescope, could determine whether the planets have rocky or water-rich interior structures,” said study co-author Jo Ann Egger, a doctoral student in astrophysics at the University of Bern in Switzerland. in a sentence.
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