HARPS-N@TNG reveals a very low-mass sub-Neptune in a 2-planet system

The HCT collaboration (HARPS-N Collaboration Time) discovered and characterized two planets orbiting TOI-1453 and spanning the radius valley.

Using data coming from the high resolution spectrograph HARPS-N (High Accuracy Radial velocity Planet Searcher for the Northern hemisphere) installed at the Telescopio Nazionale Galileo and the NASA’s TESS (Transiting Exoplanet Survey Satellite), the team was able to identify the two exoplanets orbiting TOI-1453. The inner planet, TOI-1453 b, is a small super-Earth of about 1.2 Earth radii orbiting its star in 4.3 days. The outer planet, TOI-1453 c, is a sub-Neptune with 2.2 Earth radii which orbits its star in 6.6 days. The planets hence span the radius gap separating super-Earths from sub-Neptunes, namely the radius valley. The origin of this valley, as well as the compositional nature of sub- Neptunes, are still heavily debated. The planetary system TOI-1453 could provide important clues. The two planets are in a configuration close to the 3:2 mean-motion resonance, which is indicative of orbital migration and provides valuable insights into the formation history of this planetary system. Additionally, the old star TOI-1453 is part of a binary system (a pair of stars orbiting each other). Currently, few small planets have been characterized in binary systems, and the stellar companion to TOI-1453 might have a significant impact on the radius valley.

«But the most surprising discovery came thanks to HARPS-N observations» claims Manu Stalport, lead author of the study. The HARPS-N (High Accuracy Radial velocity Planet Searcher for the Northern hemisphere) instrument mounted at TNG offers indeed a precious opportunity to measure the mass of these planets. Thanks to the instrument’s exquisite precision, the researchers found that the sub-Neptune TOI-1453 c has a surprisingly small mass of only 2.95 Earth masses. «To date, TOI-1453 c stands amongst the rare sub-Neptunes detected below 3 Earth masses.», emphasizes Manu Stalport.

TOI-1453 might be key to investigate and understand the composition of sub-Neptunes. It represents a valuable target for follow-up atmospheric studies with the NASA’s JWST satellite.

Link to the paper: https://doi.org/10.1051/0004-6361/202452969

Mass-radius diagram of small exoplanets with precision on mass and radius better than 25% and 8%, respectively. TOI-1453 b (orange) and TOI-1453 c (green) are superimposed. The sub-Neptune TOI-1453 c stands amongst the least dense exoplanets lighter than 4 Earth masses. The curves indicate theoretical models of planet composition. The plain curves correspond to atmosphere-free planets with di erent interior compositions. The dotted curves represent Earth-like interior compositions surrounded by a thick H/He atmosphere with various mass fractions. The composition of TOI-1453 c is degenerate. It is compatible with either a thick atmosphere, or a thin gaseous envelope surrounding a water-rich interior.

Artistic image of TOI-1453 (Credits: Martin Farnir)