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Astronomers 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. This discovery could help scientists unlock the secrets of planet formation.
The six exoplanets orbit a bright sun-like star called HD110067 in the constellation Coma Berenices in the northern sky. Planets larger than Earth but smaller than Neptune are in a little-understood class called sub-Neptunes, which are commonly found orbiting sun-like stars in the Milky Way. And the planets, labeled b through g, orbit 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, planet g, the outermost planet, completes one orbit.
When planet c completes three orbits around the star, planet d makes two orbits, and when planet e completes four orbits, planet f makes three orbits.
This harmonious rhythm creates a chain of resonance in which the six planets align 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 resulting discovery could shed light on the evolution of planets and the origin of common sub-Neptunes in our home galaxy.
detecting a mystery
Researchers first noticed the star system in 2020 when NASA’s Transiting Exoplanet Survey Satellite, or TESS, detected dips in the brightness of HD110067. A decrease in starlight usually indicates the presence of a planet passing between its host star and an observing satellite as the planet moves along its orbital path. 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 this 2020 data, astronomers determined the orbital periods of two planets around the star. Two years later, TESS observed the star again and evidence showed different orbital periods for these planets.
When the data sets didn’t come together, astronomer and lead author of the study, Rafael Luque, and some colleagues decided to look at the star again using a different satellite (the European Space Agency’s ExOPlanet Satellite, or the satellite that characterizes Cheops). While TESS is used to observe parts of the night sky for short observations, Cheops observes one star at a time.
“We went looking for signals across all the potential periods that these planets could have,” said Luque, a postdoctoral researcher in the department of astronomy and astrophysics at the University of Chicago.
He said the data collected by Cheops helped the team unravel the “detective story” started by TESS. Cheops was able to detect the presence of a third planet in the system; this was crucial for confirming the orbital periods and rhythmic resonances of the other two planets.
While matching the rest of the unexplained TESS data with Cheops observations, the team discovered three other planets orbiting the star. Follow-up observations with ground-based telescopes confirmed the presence of planets.
The time Cheops spent observing the star helped astronomers sort through mixed signals from TESS data to determine how many planets passed in front of the star and the resonance of their orbits.
“Cheops gave us this resonance configuration that allowed us to predict all other periods. This would have been impossible without Cheops’ detection,” Luque said.
The closest planet takes just over nine Earth days to complete one orbit around the star, and the farthest takes about 55 days. All planets orbit their star faster than Mercury, which completes one orbit around the Sun in 88 days.
Given how close they are to HD110067, the planets likely have average temperatures similar to Mercury and Venus, ranging from 332 degrees Fahrenheit to 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 initially tend to form in resonance around stars, the gravitational pull of large planets, grazing a passing star, or colliding with another celestial body can disrupt the harmonic balance.
Most planetary systems are not in resonance, and those with multiple planets that maintain their initial rhythmic orbits are rare, so astronomers wanted to study HD110067 and its planets in detail as a “rare fossil,” Luque said.
“We think only 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 uncover a planetary system with orbital resonance. The first one, known as TOI-178, was announced in 2021.
“In the words of our science team: Cheops makes extraordinary discoveries seem ordinary. Of only three known six-planet resonance systems, this is the second to be found by Cheops in just three years of operation,” ESA Cheops project scientist Maximilian Günther said in a statement.
A perfect observation target
The system could also be used to study how sub-Neptune was formed, the study authors said.
Although Sub-Neptunes are common throughout the Milky Way galaxy, they do not exist in our own solar system. There is little consensus among astronomers about how these planets form and what they are made of, so a system of sub-Neptunians 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 resonance chain, the weakness of this system makes observations of the host star difficult.
However, the system is much easier to observe as HD110067, which has 80% the mass of our sun, is the brightest known star orbiting more than four planets.
Initial determinations of the mass of the planets show that some have puffy 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-Neptunian planets of the HD110067 system appear to have low masses, suggesting that they may be rich in gas or water. Future observations of the atmospheres of these planets, for example with the James Webb Space Telescope, could determine whether the planets have rocky or water-rich interior structures.” ” says study co-author Jo Ann Egger, a doctoral student in astrophysics at the University of Bern in Switzerland. In a statement.
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