HARPS-N unveils an intriguing multi-planet system around HIP-99452
Thanks to 141 high-precision radial-velocity measurements obtained with
HARPS-N within the GAPS (Global Architecture of Planetary Systems)
consortium,
a team of researchers led by Istituto Nazionale di Astrofisica (INAF) has
discovered and characterized a particularly interesting exoplanetary system
around the bright (V = 7.3 mag) and nearby (d = 20 pc) K-dwarf star
HIP-99452
(TOI-5789).
The team focused on this star following the discovery of a transiting
sub-Neptune-sized planet (with a radius smaller than Neptune's) on an orbit
of
approximately 13 days by NASA's Transiting Exoplanet Survey Satellite
(TESS).
By analyzing both the TESS photometry and the HARPS-N radial velocities, they were able to confirm the planetary nature of the transiting candidate named TOI-5789c, and determine its radius (~2.9 REarth), mass ( ~5 MEarth), and bulk density (1.2 g cm-3). The relatively low bulk density implies that TOI-5789c must possess a significant atmosphere, which might be hydrogen-dominated or water-rich, depending on the planet formation location: the planet may have formed within or beyond the water condensation front, at ~1-3 AU in the primordial protoplanetary disk, before it started migrating towards its host star.
Future atmospheric characterization with the James Webb Space Telescope (JWST) and the Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) mission will help clarify the origin and nature of TOI-5789c and of sub-Neptunes in general. Whether sub-Neptune planets are predominantly water-poor or water-rich remains an open question with major implications for planet-formation models.
"JWST observations in the near future may reveal the planet's true composition, which cannot be uniquely determined from bulk density measurement alone. TOI-5789c is currently one of the most favorable small planets for atmospheric characterization via transmission spectroscopy with JWST, thanks to the ideal combination of planetary parameters and host-star brightness," says Dr. Aldo S. Bonomo, researcher at INAF and first author of the study.
Careful analyses of the HARPS-N radial velocities also led to the discovery of three additional low-mass companions: one inner planet with a 2.8-day period (TOI-5789b), and two outer planets with orbital periods of 30 and 63 days (TOI-5789d and TOI-5789e), respectively.
"The more HARPS-N radial velocities we collected, the more planetary signals emerged. We had to more than double the number of measurements originally planned in order to properly characterize the architecture of this system," explains L. Naponiello, researcher at INAF and second author of the paper.
Interestingly, the planets in the TOI-5789 system appear to have relatively high mutual inclinations, as only planet c is observed to transit, unlike the majority of compact and coplanar multi-planet systems discovered by the Kepler Space Telescope. This unusual architecture makes the system even more intriguing, as it may have experienced a phase of dynamical instability, possibly leading to the ejection or collision of one or more planets before settling into its current stable configuration.
Transits of TOI-5789c observed by the TESS satellite and phase-folded with the planet ephemeris. See the paper for more details.
HARPS-N radial-velocity signals of the four planets TOI-5789b (top left panel), c (top right panel), d (bottom left panel), and e (bottom right panel), phase-folded with the planet ephemerides. See the paper for more details.
Mass-radius diagram of the known small (Rp ≤ 4 REarth ) planets, color-coded by planet equilibrium temperatures (red and blue circles indicate the hottest and coldest planets, respectively). The solid lines show different planet composition curves, as indicated in the legend. The gray dark circles indicate Venus (V), the Earth (E), Uranus (U), and Neptune (N). TOI-5789 c is indicated with a square. See the paper for more details.
Link to the paper: https://www.aanda.org/10.1051/0004-6361/202557662