TOI-1453: Sub-Neptune in system of two exoplanets

Astrophysicists have once again enriched our knowledge of the cosmos with a new discovery: two small planets orbiting TOI-1453. Located at around 250 light years from Earth in the Draco constellation, this star is part of a binary system (a pair of stars orbiting each other) and is slightly cooler and smaller than our Sun. Around this star are two planets, a super-Earth and a sub-Neptune. These are types of planets that are absent from our own solar system, but paradoxically constitute the most common classes of planet in the Milky Way. This discovery sheds light on a planetary configuration that could provide valuable clues to the formation and evolution of planets.

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and the HARPS-N high-resolution spectrograph, the researchers were able to identify TOI-1453 b and TOI-1453 c, the two exoplanets orbiting TOI-1453. “The two planets present an interesting contrast in their characteristics,” explains Manu Stalport, astrophysicist at the University of Liège and first author of the publication. TOI-1453 b is a super-Earth, slightly larger than our planet, and probably rocky. It completes its orbit in just 4.3 days, making it a very close planet to its star. In contrast, TOI-1453 c is a sub-Neptune, about 2.2 times the size of Earth but with an extraordinarily low mass of just 2.9 Earth masses. This makes it one of the least dense sub-Neptunes ever discovered, which raises questions about its composition.”

Transit and radial velocity

Detecting exoplanets remains a complex task. The team relied on two key methods to confirm their discoveries. The transit method (TESS data) measures the size and orbital period as the planet passes in front of its host star, causing a slight decrease in brightness. The second method used is radial velocity measurement (HARPS-N data), which involves observing the variations in the velocity of a star under the effect of the gravity of a planet orbiting it. By studying the gravitational influence exerted by the planets on their host star, the researchers were able to measure their masses and densities.

“All these observations have revealed that TOI-1453 c is extremely light for its size, suggesting that it could have a thick hydrogen-rich atmosphere or a composition dominated by water. This makes it an ideal candidate for future atmospheric studies,” enthuses Manu Stalport. Understanding their formation and evolution could provide clues about the development of planetary systems, including our own.”

What’s more, the two planets orbit in a configuration close to a 3:2 resonance, meaning that for every three orbits of the inner planet, the outer planet completes almost exactly two. Such resonances are considered a natural consequence of orbital migration, offering clues as to how the planets move and settle into their final orbits.

This discovery opens up new research prospects. Observational instruments such as the James Webb Space Telescope (JWST) could analyse TOI-1453 c’s atmosphere to determine its main composition. If this planet has a substantial hydrogen-rich atmosphere or a water-dominated interior, it could redefine our understanding of sub-Neptunes and their formation.