A research team with Bulgarian participation has discovered one of the most interesting planetary systems to date

A new study by a research team, including Bulgarian participation, has revealed the existence of three exoplanets in one of the most intriguing multi-planetary systems discovered so far. The planetary system is located 1,100 light years from Earth, around the star TOI-4504 in the Kiel constellation, according to the team from the Faculty of Physics at Sofia University "St. Kliment Ohridski."

"NASA's TESS space telescope is one of the best exoplanet hunters available to humanity. An exoplanet is any planet that orbits a star outside the Solar System. The TESS telescope detects exoplanets using the transit method. A transit occurs when a planet passes in front of its star's disk, similar to the transits of Mercury and Venus in front of the Sun. The transit of an exoplanet causes a decrease in the observed brightness of the star, allowing astronomers to estimate the size of the exoplanet and its orbital period—the time it takes to orbit the star. However, if there are other exoplanets in the system, their gravitational influence can affect the timing of the transits. By assessing these changes, scientists can discover new planets, even if they don't transit their star," explained the team from the Faculty of Physics.

The discovery has been published in the prestigious Astrophysical Journal Letters and involves key contributions from Bulgarian astronomer Dr. Trifon Trifonov. Dr. Trifonov is a scientist at the Max Planck Institute for Astronomy in Heidelberg, Germany, and a researcher in the Department of Astronomy at the Faculty of Physics at Sofia University "St. Kliment Ohridski."

Dr. Trifonov leads the Bulgarian team searching for exoplanets as part of the EXO-RESTART project, supported by contract KP-06-DV/5 from December 15, 2021, with the Science Research Fund under the National Scientific Programme "Excellence in Research and People for the Development of European Science" 2021.

Working with an international team using data from the TESS space telescope and observations from the FEROS spectrograph, located on the 2.2-meter telescope of the Max Planck Institute in Chile, the study shows that the TOI-4504 system contains three exoplanets. The closest, TOI-4504b, is about two and a half times larger than Earth, while the other two, TOI-4504c and TOI-4504d, are classified as "warm Jupiters." With masses 3.7 and 1.4 times that of Jupiter, respectively, and orbital periods of 82.4 days for the first and 40.5 days for the second, they are giant gas planets located relatively close to their star. Notably, TOI-4504c has one of the largest cyclical transit timing variations recorded—about four days! This is the largest variation observed in any exoplanetary system to date. The effect is caused by the gravitational pull of TOI-4504d on TOI-4504c, leading to earlier or later transits across its star.

"We were extremely surprised to find such large variations in the passage times of the exoplanet TOI-4504c. For this observing period, the amplitude of the passage variations is about 4 days. The previous record holder had twice the amplitude, while the typical passage variation in the majority of multi-planet systems is on the order of a few minutes," says the study's lead author Michaela Vitkova, a PhD student at the Institute of Astronomy of the Czech Academy of Sciences.

"The study of TOI-4504 is important for analyzing the evolution of similar exoplanetary systems. Exoplanets TOI-4504c and TOI-4504d are in a 2:1 orbital resonance, indicating that they were likely in a stable resonant configuration from the time of their formation in the protoplanetary disk and subsequent migration to their current orbits," explained Dr. Trifonov, who conducted a significant part of the data analysis.

The team hopes to discover more exoplanets with similar or even larger transit timing variations in the future, which will further enhance astronomers' understanding of the formation of different planetary systems.

Dr. Rafael Brahm from Adolfo Ibáñez University in Chile, a co-author of the study and coordinator of the international consortium WINE, which focuses on "warm Jupiter" planet searches, highlighted the importance of global teamwork among scientists for the successful achievement of this goal.

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