An international team of astronomers has conducted a groundbreaking study of a planetary system consisting of three exoplanets orbiting the star TOI-396. This research, published on November 22 on the pre-print server arXiv, offers the first mass measurements of the planets and provides new insights into the structure and properties of this fascinating alien system.
TOI-396, also known as HR 858 A, is located approximately 103 light-years away from Earth. It is a relatively bright star, classified as a spectral type F6 V, which means it is around 26% larger and 20% more massive than our Sun. The star is estimated to be about 2 billion years old and has an effective temperature of 6,354 Kelvin. TOI-396 is part of a binary star system, with its companion being a faint M dwarf designated HR 858 B.
The planetary system around TOI-396 was discovered in 2019 through observations made by NASA’s Transiting Exoplanet Survey Satellite (TESS). The system comprises three exoplanets—TOI-396 b, TOI-396 c, and TOI-396 d—all of which are approximately twice the size of Earth. These planets orbit their host star at very close distances, within 0.1 AU (astronomical units), with orbital periods of 3.6, 6.0, and 11.2 days, respectively.
Building on the TESS data, a group of astronomers, led by Andrea Bonfanti from the Austrian Academy of Sciences, conducted follow-up radial velocity observations of TOI-396 using the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph. These observations, combined with high-precision photometric data from TESS, allowed the team to refine the properties of the planets in the system.
The primary objective of the study was to measure the masses of the three exoplanets, refine their radii, and determine whether any of the planets exhibit a mean motion resonance (MMR), a gravitational interaction that can influence the orbital dynamics of planetary systems. Through their observations, the researchers were able to obtain precise mass estimates for two of the planets: TOI-396 b and TOI-396 d. TOI-396 b was found to have a mass of 3.55 Earth masses, while TOI-396 d was found to have a mass of 7.1 Earth masses. This resulted in a mean density of about 2.44 g/cm³ for TOI-396 b and 4.9 g/cm³ for TOI-396 d.
For TOI-396 c, the team was only able to determine an upper mass limit of approximately 3.8 Earth masses, which corresponds to a maximum density of 2.9 g/cm³. The data reveals intriguing characteristics about the planetary system: TOI-396 b, the innermost planet, is less dense than TOI-396 d, the outermost planet, while TOI-396 c, the middle planet, is the least dense of the three. This configuration of planetary densities is unusual and raises questions about the formation and evolution of the system.
In addition to measuring the masses and densities, the astronomers also calculated the equilibrium temperatures of the planets based on their proximity to the host star. These temperatures, which represent the average temperature the planets would have if they were perfect blackbodies, were found to be 1,552 K for TOI-396 b, 1,309 K for TOI-396 c, and 1,061 K for TOI-396 d. These high temperatures suggest that all three planets are likely to be very hot, with TOI-396 b being the hottest and TOI-396 d the coolest.
The researchers also performed a transit timing variation (TTV) analysis, which is used to detect small deviations in the timing of a planet’s transits caused by gravitational interactions between planets in the system. The results of this analysis indicated that TOI-396 b and TOI-396 c may experience TTVs with a super-period of about five years. The semi-amplitude of these variations is expected to be around two hours for TOI-396 b and five hours for TOI-396 c. These findings suggest that the planets are gravitationally interacting with each other, potentially influencing their orbits over time.
The authors of the study emphasized that the TOI-396 system is an exceptional candidate for studying planetary atmospheres. The three planets, all sub-Neptune-sized, are ideal targets for atmospheric characterization, both in transmission (when a planet passes in front of its star) and emission (when the planet’s own thermal radiation is detected). These properties make the system an excellent laboratory for investigating the formation, evolution, and potential habitability of planetary systems.
The study of TOI-396 and its planets provides valuable insights into the diversity of exoplanetary systems and the complex processes that govern their formation and evolution. It also highlights the importance of follow-up observations and advanced techniques, such as radial velocity measurements and TTV analysis, in refining our understanding of these distant worlds. With the continued advancement of observational technology, the TOI-396 system will likely remain a key focus for future research in exoplanet science, offering opportunities to explore the atmospheres, compositions, and dynamics of exoplanets that may one day help answer the age-old question of whether life exists elsewhere in the universe.