Observations shed more light on the properties of three-planet system TOI-396

An international team of astronomers has investigated a planetary system consisting of three alien worlds orbiting the star TOI-396. The study, published Nov. 22 on the pre-print server arXiv, provides the first mass measurements for these three planets, shedding more light on the properties of the whole system.

Located some 103 light years away, TOI-396, also known as HR 858 A, is a bright star of spectral type F6 V, about 26% larger and 20% more massive than the sun. The star, which is estimated to be two billion years old, has an effective temperature of 6,354 K and is part of a binary system—its companion is a faint M dwarf designated HR 858 B.

In 2019, three exoplanets orbiting TOI-396 were detected using NASA’s Transiting Exoplanet Survey Satellite (TESS) and they received designations TOI-396 b, TOI-396 c and TOI-396 d. All the extrasolar worlds turned out to be about two times larger than the Earth and were found to be relatively close to the host—within 0.1 AU from it. The orbital periods of these planets, starting from the innermost, were measured to be 3.6, 6.0 and 11.2 days.

Now, a group of astronomers led by Andrea Bonfanti of the Austrian Academy of Sciences in Graz, Austria, has carried out radial velocity observations of TOI-396 with the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph and analyzed photometric data from NASA’s Transiting Exoplanet Survey Satellite (TESS). This allowed them to get more insights into the properties of the system.

“To measure the masses of the three planets, refine their radii, and investigate whether planets b and c are in MMR [mean motion resonance], we carried out HARPS radial velocity observations of TOI-396 and retrieved archival high-precision transit photometry from 4 TESS sectors,” the researchers explained.

The observations found that the masses of TOI-396 b and TOI-396 d are 3.55 and 7.1 Earth masses, respectively. This yields a mean density for TOI-396 b of about 2.44 g/cm³ and 4.9 g/cm³ for TOI-396 d. When it comes to TOI-396 c, the astronomers were able to obtain only an upper limit for its mass—of approximately 3.8 Earth masses, which gives a maximum density at a level of 2.9 g/cm³.

According to the paper, the obtained results underline that TOI-396 is quite a unique system in which the mid planet is the least dense and the outermost planet is the densest one. The study also calculated equilibrium temperatures of TOI-396 b, c and d, which were found to be 1,552, 1,309 and 1,061 K, respectively.

Moreover, the astronomers performed a transit timing variation (TTV) dynamical analysis of the TOI-396 system. As a result, it turned out that TOI-396 b and TOI-396 c may exhibit TTVs with a super-period of about five years and semi-amplitudes of about two and five hours, respectively.

In concluding remarks, the authors of the paper noted that all the three planets orbiting TOI-396 have favorable metrics for atmospheric characterization in both transmission and emission among the sub-Neptune exoplanets. This makes the TOI-396 system an excellent laboratory to investigate the formation and evolution of planetary systems.

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