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These stars are in addition to the 5,539 exoplanets officially detected to date.

A waltz of six planets detected around a star

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The planetary system around the star HD 110067. (Artistic illustration)

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There are at least six exoplanets orbiting the star HD 110067 located in the Berenice Hair constellation, shows the work of astrophysicist Rafael Luque of the University of Chicago in the United States and Swiss colleagues published in Nature magazine (New window) (in English).

Located approximately 100 light years from Earth, all these planets have a radius between that of Earth and that of Neptune.

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HD 110067 is an orange dwarf star whose brightness vacillates between that of yellow stars, like the Sun, and that of red dwarfs, like its neighbor Proxima Centauri (around which is the closest exoplanet to the Earth discovered to date).

These orange dwarfs are much older stars [than the Sun] around which exoplanets have had time to #x27;evolve and stabilize, explains astrophysicist Olivier Hernandez, director of the Montreal Planetarium.

HD 110067 thus represents an object of study of great interest for astronomers, since it shows us the virgin configuration of a planetary system which has evolved without ever undergoing major upheavals.

The first two exoplanets in the system were detected in 2020 and 2022 using NASA's TESS telescope. The data collected by TESS revealed irregularities and several drops in brightness in front of the star, which suggested the presence of other exoplanets.

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ESA's CHEOPS telescope. (Artistic illustration)

For these reasons, astrophysicist Rafael Luque and his colleagues at the University of Chicago decided to use the telescope CHEOPS of the European Space Agency to in turn examine the HD 110067 system.

The data collected by CHEOPS quickly made it possible to detect a third planet, but also to find the key to unlocking the entire system, notes Rafael Luque. Result: there are at least six planets around the star.

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Due to the precision of the resonance orbits of the six planets, the orbits of each planet are closely related. (Artistic illustration)

If scientists found the other three so quickly, it is because they take resonant orbits, in which the planets exert regular forces on top of each other.

When we are in the presence of several planets which are sufficiently close together and which revolve around " a star, we witness a kind of dynamic anchoring between them. A bit like it happens with the Moon around the Earth. This means that the period, for example, of rotation of the Moon around the Earth and its period of rotation on itself are synchronized.

A quote from Olivier Hernandez, Montreal Planetarium

The rotation periods of the planets around the star are generally in resonance proportions of 3 at 2, that is to say while one makes three turns, the other makes two turns, adds the astrophysicist.

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So, by studying the three innermost planets, the authors of this work were able to calculate the orbits of the six planets, ranging from about 9 days for the innermost planet to about 21 days for the outermost planet. external.

For example, when the planet closest to the star completes three complete revolutions around it, the second completes one exactly two in the same time.

When the nearest planet completes six orbits, the farthest planet only completes 27 ;a.

Astronomers estimate that only about 1% of all planetary systems remain in resonance during their evolution. The discovery of this type of orbital systems therefore represents, according to them, a golden opportunity to study the formation and evolution of a planetary system.

The authors of the work believe that the six exoplanets of HD 110067 would not be small rocky super-Earths, but rather gaseous mini-Neptunes.

According to them, the system has remained virtually unchanged since its birth, at least 4 billion years ago, and offers a chance of learn more about sub-Neptunes and how systems in this configuration can form, notes Rafael Luque.

None of these planets are in the habitable zone around the star, which would accommodate liquid water, a necessary condition for life.

< p class="StyledBodyHtmlParagraph-sc-48221190-4 hnvfyV">HD 110067 is the brightest star detected to date that hosts more than four transiting exoplanets. It may contain other planets that could exist within or beyond the temperate zone, although such observations have not been made until now.

The authors calculated the masses of three of the planets in the system, work which shows that they are of low densities, suggesting the presence of dense atmospheres dominated by hydrogen.

Because these planets are all smaller than Neptune and likely have extensive atmospheres, they are ideal candidates for studying the composition of their atmospheres using the James Webb Space Telescope and future telescopes. Ariel and Plato from the ESA.

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To date, among the exoplanets located near their star, we have detected very few hot Neptunes, compared to hot Jupiters on one side and super-Earths on the other, notes Olivier Hernandez.

A plausible explanation is that they would lose their atmosphere quite quickly, becoming chthonic planets, which are in some way planetary remnants of a gas planet.

The interest of this type of study is to try to understand if we can find hot Neptunes in these regions .

Do hot Neptunes turn into super-Earths just because they are too close to their star and their atmosphere completely disappears? Or do they tend to migrate outside the planetary system and become cold like those in our solar system?

A quote from Olivier Hernandez, Montreal Planetarium

HD 110067 is the second planetary system in orbital resonance revealed by CHEOPS. The first is called TOI 178 and is located about 200 light years from Earth.

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This artist's impression represents the view of the TOI-178 system from the most distant planet.

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