A sub-neptunian “dynamically hot” orbiting around the star BD+00 444 confirmed with HARPS-N observations.

An intriguing result, obtained thanks to the observational campaign carried out with NASA’s Kepler satellite, is that the most common transiting exoplanets are small planets with a radius smaller than 4 Earth radii and a short orbital period (less than 100 days). In particular, the most frequent planets belong to the sub-Neptune and super-Earth classes, two categories absent in the Solar System. Another difference concerns the eccentricity of the orbits: planets with radii smaller than 6 Earth radii are often observed with eccentric orbits. On average, eccentricities are higher for planets that appear to lack companions, indicating that “dynamically hot” systems are more frequent when they host fewer planets. The term “dynamically hot” refers to planets with eccentric orbits and/or orbits inclined relative to the star’s equatorial plane, typically due to gravitational interactions.

In 2021, NASA’s TESS mission identified an exoplanet orbiting the star BD+00 444. As part of the GAPS program (Global Architecture of Planetary Systems), which aims to characterize known exoplanetary systems, BD+00 444 was observed by the Telescopio Nazionale Galileo with the HARPS-N spectrograph for about a year and a half to confirm the planet’s existence and characterize its properties.

The analysis of the observations was led by astrophysicist L. Naponiello of the Astrophysical Observatory of Turin and is described in the article: “The GAPS programme at TNG: LXIV. An inner eccentric sub-Neptune and an outer sub-Neptune-mass candidate around BD+00 444 (TOI-2443)“, recently published in Astronomy & Astrophysics. The team led by the researcher confirmed the existence of the exoplanet, classifying it as a terrestrial-type planet with a rocky core and an atmosphere rich in hydrogen and helium. The planet has a radius of 2.36 Earth radii, a mass of 4.8 Earth masses, and a particularly eccentric orbit (e = 0.3). It is, in fact, the second known planet with the most eccentric orbit among those with a mass smaller than 20 Earth masses. The team also suggested the existence of a second planet in the system, a sub-Neptune with an orbital period of 96.6 days.

The study also involved astronomers S. Colombo (INAF – Astronomical Observatory of Palermo) and M. C. d’Arpa (University of Palermo).

The cover figure (click here to view it in full) shows a mass-radius diagram of known exoplanets, with the colors of the symbols indicating the planets’ equilibrium temperature. The curves represent theoretical compositions describing the measured planetary properties.