The bright nearby star HD 110067 hosts six transiting sub-Neptunes that follow a chain of resonant orbits, according to new research led by University of Chicago astronomers.
The six planets in the HD 110067 system are all smaller than Neptune and revolve around their parent star in a very precise waltz: when the closest planet to the star makes three full revolutions around it, the second one makes exactly two during the same time; this is called a 3:2 resonance; the six planets form a resonant chain in pairs of 3:2, 3:2, 3:2, 4:3, and 4:3, resulting in the closest planet completing six orbits while the outer-most planet does one. Image credit: ESA / CC BY-SA 3.0 IGO.
HD 110067 is a bright K0-type star located 100 light-years away in the constellation of Coma Berenices.
Otherwise known as TIC 347332255, the star has a mass and radius about 80% of the Sun’s.
In 2020, NASA’s Transiting Exoplanet Survey Satellite (TESS) detected dips in the star’s brightness that indicated planets were passing in front of the star’s surface.
A preliminary analysis revealed two possible planets: one with an orbital period of 5.642 days and the other with a period that could not be determined yet.
In 2022, TESS observed the same star again. Analyzing the combined data sets ruled out the original interpretation but presented two different possible planets.
While these detections were much more certain than the originals, there was a lot about the TESS data that still did not make sense.
“That’s when we decided to use ESA’s CHaracterising ExOPlanet Satellite (Cheops),” said lead author Dr. Rafael Luque, an astronomer in the Department of Astronomy and Astrophysics at the University of Chicago.
“We went fishing for signals among all the potential periods that those planets could have.”
They confirmed a third planet in the system and realized that they had found the key to unlocking the whole system because it was now clear that the three planets were in an orbital resonance.
The outer-most planet, HD 110067d, takes 20.519 days to orbit, which is extremely close to 1.5 times the orbital period of the next planet, HD 110067c, with 13.673 days.
This in turn is almost exactly 1.5 times the orbital period of the inner planet, HD 110067b, with 9.114 days.
Predicting other orbital resonances and matching them to the remaining unexplained data allowed Dr. Luque and his colleagues to discover the other three planets in the system: HD 110067e, HD 110067f and HD 110067g.
“Cheops gave us this resonant configuration that allowed us to predict all the other periods,” Dr. Luque said.
“Without that detection from Cheops, it would have been impossible.”
“We think only about one percent of all systems stay in resonance.”
“That’s why HD 110067 is special and invites further study. It shows us the pristine configuration of a planetary system that has survived untouched.”
“As our science team puts it: Cheops is making outstanding discoveries sound ordinary,” said Cheops project scientist Maximilian Günther, of ESA.
“Out of only three known six-planet resonant systems, this is now the second one found by Cheops, and in only three years of operations.”
The HD 110067 system offers a chance to learn more about sub-Neptunes and how systems in this configuration might form.
“The fact that the planets in the HD 110067 system have been detected by the transit method is key,” said University of Bern Ph.D. student Jo Ann Egger.
“While they pass in front of the star, light also filters through the planetary atmospheres.”
“This property is allowing us to determine the chemical composition and other properties of the atmospheres.”
“Since a lot of light is required, the bright star HD 110067 and its orbiting planets are an ideal target for further studies to charachterize the planetary atmospheres.”
“The sub-Neptune planets of the HD 110067 system appear to have low masses, suggesting they may be gas- or water-rich.”
“Future observations, for example with the NASA/ESA/CSA James Webb Space Telescope, of these planetary atmospheres could determine whether the planets have rocky or water-rich interior structures.”
The discovery is reported in a paper in the journal Nature.
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R. Luque et al. 2023. A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067. Nature 623, 932-937; doi: 10.1038/s41586-023-06692-3
