A sextuplet of planets found orbiting a bright star in a synchronous way
In the northern constellation of Coma Berenices stands a star named HD110067. Although it is ten times too faint to be visible to the naked eye from the still dark Calar Alto skies, it is an orange dwarf star close thus bright enough to be visible with common binoculars, as it lies only about 100 light-years away from the Sun, albeit it is 20% smaller than our star.
A paper published today (Nov. 29th, 2023) in the prestigious Nature journal reveals that this star is surrounded by a very peculiar “solar system”, detected thanks to a careful re-analysis of its multiple micro-eclipses as seen from Earth: each of the 6 planets discovered shows a simple ratio between its orbital period and the one of its neighbor, following a delicate chain of orbits in resonance
Artistic representation of a planetary system. Credits: Thibaut Roger /NCCR Planets.
Back in 2020, the Transiting Exoplanet Survey Satellite (TESS) from NASA detected a few small, periodic dips in the HD110067 brightness which implied that one or more planets were passing in front of the star. The existence of two planets, HD110067b and c, was then suggested, although their orbital periods (the time a planet needs to orbit fully its star) could not be clearly determined until 2022, when TESS reobserved the field in Coma Berenices. Yet, the properties of the system remained puzzling.
Rafael Luque, a Spanish researcher working at University of Chicago, decided to use CHEOPS, another space-based telescope, from ESA, to better study the system. Indeed, with the CHEOPS data, a third (actually the largest) exoplanet was detected. In addition, all three bodies were found to have synchronized revolutions, their orbital periods being equal each other within a x1.5 factor. This is known as a 3:2 resonance: the inner planet has the shortest period of 9.1 days, the second 13.7 and the third, 20.5 days, meaning that the inner one does three revolutions while the outer one orbits only twice the star.
Intrigued by this synchronous “dance of the three planets” around HD110067, the international team reanalyzed the data and, fine tuning their models based on the space-based observations, Luque et al. found that actually, three more planets were needed to explain the full characteristics of the light dips observed by TESS and CHEOPS. In detail, one more 3:2 resonance, with a period of 30.8 days for HD110067e; and two 4:3 resonances for HD110067f and g, the outermost couple of bodies orbiting the star, with periods of 41 and 55 days, resp.
Also, from the depth of these micro-eclipses, the sizes of the 6 planets were all found to be around 2 to 3 Earth radii, although high-quality spectra of the system were needed to determine the planetary masses, thus their densities.
“Cheops gave us this resonant configuration that allowed us to predict all the other periods. Without that detection from Cheops, it would have been impossible” Luque says. Planetary systems were usually born in resonance, but they are commonly perturbed e.g. by another star or a giant planet passing by, which usually and definitely change the initially “ordered” chain of orbits. Multi-planet systems that remain in resonance over more than several hundreds of millions of years are quite uncommon.
“We think only about one percent of all systems stay in resonance,” confirms Rafael Luque who adds that “HD110067 shows us the pristine configuration of a planetary system that has survived untouched.”
This very special case may thus be key to understand better the mechanisms of birth and evolution of planetary systems, in particular considering that more than half of the Sun-like stars host planets in close orbits having sizes between that of the Earth and Neptune.
HD110067 is also the brightest known star hosting more than four planets, which makes it much easier to follow-up with other facilities from ground. In particular, the monitoring of the host star with different telescopes and instruments (spectrographs) was required to determine the changes in its radial velocity due to the planetary wobbling and to disentangle it from stellar activity.
Combined observations with CARMENES and HARPS-N, another high-resolution spectrograph on a telescope similar in size to the 3.5 m CAHA, finally provided mass estimates of the planets: all are around 4 to 8 Earth masses, implying they are low-density (not rocky) planets, like reduced versions of gaseous objects like Neptune. Also, their proximity to their star imply they have pretty hot equilibrium temperature, ranging from about 170ºC for the most distant HD110067g, to 530ºC for the innermost HD110067b, the real surface temperature being likely hotter in case of (dense) atmospheres around those planets.
The sextuplet of sub-Neptunes orbiting in perfect resonance around the nearby, bright HD110067, having likely extended atmospheres of hydrogen, makes them ideal candidates for studying the composition of their atmospheres using current (James Webb) and future (Ariel and Plato) space telescopes. Meanwhile, this remarkable planetary system will continue to be monitored from Calar Alto with the CARMENES spectrograph, to refine the mass estimates of the six known planets and perhaps, to discover new, long-period ones in more external orbits, possibly located in the habitable zone around the orange star HD110067.
Animation & music by Dr. Hugh Osborn (University of Bern)
REFERENCE
Luque et al. “A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067”, Nature, November 2023, DOI 10.1038/s41586-023-06692-3
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