TOI-5398 is an abbreviation that may not tell us much and yet hides a record: it is the youngest "compact" multi-planetary system, in which there is the co-presence of a small planet close to the star together with a giant planetary companion with an orbital period of about 10 days. This system is only the sixth with such characteristic co-presence among the more than 500 systems hosting short-period giant planets. The data for this confirmation were published in the journal Astronomy & Astrophysics by a group led by the National Institute of Astrophysics and the University of Padua. According to the authors of the paper, this system is virtually unique, potentially a "milestone" for the study and understanding of short-period giant planets. Measurements were obtained with the HARPS-N spectrograph at INAF's Galileo National Telescope (TNG) in the Canary Islands (INAF) as part of the Global Architecture of Planetary Systems (GAPS) national collaboration. In this study, the use of space data from NASA's Transiting Exoplanet Survey Satellite (TESS), and the coordination of numerous researchers and astronomical observatories scattered around the world, was also crucial. TOI-5398 is by far the youngest of the so-called "compact" systems: 650 million years old versus 3-10 billion years for the other systems. An infant, one might say. Moreover, the largest planet in the system turns out to be the best candidate for atmospheric characterization studies by NASA's James Webb Space Telescope among all known hot giants. "Warm giants are defined as giant planets between 10 and 100 days of orbital period, not to be confused with hot giants, which possess orbital periods under 10 days." TOI-5398 consists of a hot "sub-Neptune" (TOI-5398 c) orbiting internally relative to its Saturn-like companion of mass with a short orbital period.
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