CM – The upside-down paths of a multi-planetary system


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October 27, 2021

from the University of Geneva

When planets form, they usually continue their orbital evolution in the equatorial plane of their star. However, an international team led by astronomers from the University of Geneva (UNIGE), Switzerland, has discovered that the exoplanets of a star in the constellation Pisces orbit in planes perpendicular to one another, with the innermost planet being the only one still in the equatorial plane. Why so? This radically different configuration of our solar system could be due to the influence of a still unknown distant companion of the star. This study, which can be read in the journal Astronomy & Astrophysics, was made possible by the extreme precision of ESPRESSO and CHEOPS, two instruments whose development was led by Switzerland.

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Theories about the origin of planetary systems predict that planets form and develop in the equatorial plane of their star, unless they are disturbed by special events. This is not the case in the solar system, where our planets are near the solar equatorial plane. In this case, the planets will be aligned with their star. However, a 2019 study showed that two of the three planets around the star HD3167 do not align with it. HD3167c and HD3167d, two mini-Neptunes that orbit in 8.5 and 29.8 days, actually pass the star’s poles, almost 90 degrees from its equatorial plane.

By re-observing this system with more efficient instruments, it was possible to a team led by astronomers from UNIGE measured the orientation of the third planet’s orbital plane, the super-earth HD3167b, which orbits in less than a day (23 hours to be precise). When a planet passes through its star, the orientation of its orbit can be determined with a spectrograph, which makes it possible to measure the movement of the star regions covered by the planet and thus to infer its trajectory. The smaller the planet, the more difficult this movement is to see. With ESPRESSO on one of the four 8.2 m telescopes of the VLT in Chile, the researchers were therefore able to determine the orbit of HD3167b, which is randomly aligned with the star and perpendicular to the orbit plane of its two siblings. « We needed a maximum of light and a very precise spectrograph to be able to measure the signal of such a small planet, » says Vincent Bourrier, researcher in the Department of Astronomy of the Faculty of Natural Sciences at UNIGE. « Two requirements that the precision of ESPRESSO meets in combination with the collecting power of the VLT. »

This result would not have been possible without a precise knowledge of the time in which the HD3167b passed its star, what with that of the literature was not possible with an accuracy of 20 minutes of predicted time – an eternity for a 97 minute transit. The researchers therefore turned to the CHEOPS satellite consortium, whose main task is to measure transits with very high precision. “With CHEOPS we were able to determine the transit time with an accuracy of more than a minute. This is a good example of the synergies between different instruments, here CHEOPS and ESPRESSO, and the teams that operate them, ”says Christophe Lovis, a researcher in the Astronomy Department at UNIGE and member of the two consortia.

This new measurements seem to confirm the 2019 prediction of the presence of a fourth body orbiting HD3167. In this scenario, HD3167b’s proximity to the star kept it under its influence, forcing the small planet to orbit in the plane in which it formed. On the contrary, the two more distant mini-Neptunes were able to free themselves from the star, only to come under the influence of this fourth body, which would gradually have misaligned their orbits. This clears the way for the researchers who are now looking for this elusive companion.

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Similar title :
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