2 newly found exoplanets reignite an outstanding question about our solar system

Astronomers have discovered two new exoplanets that are similar to other worlds found in the Milky Way, but are unlike any in our own solar system.

The two exoplanets, or planets outside of our solar system, orbit a star called TOI-1453, which is slightly cooler and smaller than our sun. Located about 250 light-years from Earth in the Draco constellation, TOI-1453 belongs to a binary stellar system — in which a pair of stars orbit each other.

Classified as a super-Earth and a sub-Neptune, these two new celestial bodies are among the most common types of exoplanets found in the Milky Way galaxy, yet are absent from our own solar system.

Astronomers detected the two new exoplanets, named TOI-1453 b and TOI-1453 c, using data from NASA's Transiting Exoplanet Survey Satellite (TESS) and the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph, which is installed on the Telescopio Nazionale Galileo at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands, Spain.

TESS hunts for exoplanets by looking for a temporary decrease in a star's brightness, also known as the transit method. When a dip in brightness is observed, that suggests a planet has passed in front of its host star (from our perspective on Earth) and briefly blocked the star's light from reaching our instruments. By measuring this effect, astronomers can estimate the size and orbital period of the possible exoplanet as well.

Complementing the TESS data, the HARPS-N instrument is a high-resolution radial-velocity spectrograph that measures the spectrum of light from a star to detect shifts caused by orbiting exoplanets. The instrument uses what's known as the Doppler method to look for a subtle "wobble" of a star caused by the gravitational pull of an orbiting planet.

"The two planets present an interesting contrast in their characteristics," Manu Stalport, first author of the study and an astrophysicist at the University of Liège, said in the statement. "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."

"Super-Earth" is used to classify exoplanets that are more massive than Earth but less massive than ice giants like Neptune and Uranus. They are believed to be primarily rocky, similar to Earth, but their increased size may lead to stronger surface gravity, influencing their atmospheres and geological processes.

Given its proximity to its host star, TOI-1453 b is likely extremely hot, with surface temperatures high enough to strip away any substantial atmosphere. (For comparison, Mercury — the planet closest to the sun in our solar system — completes its orbit in 88 days and is so hot that it also lacks a substantial atmosphere.)

"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," Stalport said in the statement. "This makes it one of the least dense sub-Neptunes ever discovered, which raises questions about its composition."

The incredibly low density of TOI-1453 c suggests the exoplanet likely has a thick, hydrogen-rich atmosphere or a composition dominated by water.

"This makes it an ideal candidate for future atmospheric studies," Stalport added. "Understanding their formation and evolution could provide clues about the development of planetary systems, including our own."

Being part of a binary system — meaning there is a second stellar companion — also makes the new exoplanet find particularly interesting, given planets that form in binary star environments are subject to more complex gravitational interactions.

Observations of TOI-1453 b and TOI-1453 c show that the exoplanets orbit their host star in a near 3:2 resonance, meaning that for every three orbits of the inner planet, the outer planet completes almost exactly two. This suggests that the exoplanets' orbits may have changed over time following interactions with neighboring cosmic objects, such as gas, planetesimals or a companion star, bringing the inner planet, TOI-1453 b, into an orbit much closer to its host star.

Astronomers hope to use additional instruments like the James Webb Space Telescope (JWST) to further study the exoplanets and peer deeper into the atmosphere of TOI-1453 c. If this sub-Neptune world has a substantial hydrogen-rich atmosphere or a water-dominated interior, it could redefine our understanding of the formation of such exoplanets — and maybe finally reveal new clues as to why we don't have one in our own solar system.

Their findings were accepted for publication on Feb. 23 in the journal Astronomy & Astrophysics.