In a system with two known planets, astronomers noticed something new: a small object passing through a Sun-sized star. It turned out to be another extremely hot, Earth-sized planet.

The recently detected planet, called HD 63433 d, is tidally locked; This means that there is always a day side facing the star and a side always in darkness. This exoplanet, or planet outside our solar system, orbits the star HD 63433 (TOI 1726) in the HD 63433 planetary system. This burning world is the smallest exoplanet confirmed to be less than 500 million years old. It is also the closest Earth-sized planet to Earth and is approximately 400 million years old.

A team of astronomers analyzed this system using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), which detects “transits,” or blocking a small amount of starlight as planets pass in front of their orbiting stars. Two planets had previously been discovered in this planetary system, so to see what else was lurking around the star, the team took the data and lifted signals from the two known planets. This allowed them to see an additional signal, a small transition that occurs every 4.2 days. After further investigation, they were able to confirm that it was actually a third, smaller planet.

The tidally locked planet is very close to the size of Earth (about 1.1 times the diameter of our planet) and orbits a star about the size of our Sun (about 0.91 the size of the star and 0.99 the mass of the Sun). .

The star in this system is a G-type star, the same type as our Sun. But HD 63433 d orbits much closer to its star than we do, with a minimum “year” period of 4.2 days and unusually high temperatures on the dayside. Although the newly discovered planet and its star are roughly the same size as our planet and sun, HD 63433 d is quite different from our own Earth.

First, it is a very young planet in a very young system. The planetary system itself is about 10 times younger than our own, and this 400-million-year-old planet is in its infancy compared to our 4.5-billion-year-old Earth.

It is also much closer to its star than we are to ours. This planet is eight times closer to its star than Mercury is to the Sun. So close to its star, the dayside of this tidally locked planet can reach temperatures of about 2,294°F (1,257°C). This planet, which is this hot, this close to its star, and this small, probably doesn’t have a significant atmosphere.

These scorching temperatures are comparable to lava worlds such as CoRoT-7 b and Kepler-10 b, and the team behind the discovery believes that the dayside of the planet may be a “lava hemisphere.”

The planet’s small size, young age, and proximity to the star make it an interesting candidate for further study. Further research could confirm the results of this study and potentially reveal more information about the “dark side” of the planet and the state of its (possible) atmosphere. As the study notes, “young Earths are critical testing environments for constraining current theories of planet formation and evolution.”

This discovery was announced in a new study published. Astronomy Magazine, “The TESS Hunt for Young and Maturing Exoplanets (THYME) XI: An Earth-Sized Planet Orbit with a Nearby, Sun-Like Host in the 400 Myr Ursa Major Moving Group.” The study, led by co-authors Benjamin Capistrant and Melinda Soares-Furtado, was discussed in a presentation Jan. 10 at the 2024 meeting of the American Astronomical Society.

This research was conducted as part of the TESS Young and Maturing Exoplanet Hunt project, which focuses on the search for young exoplanets in moving groups, stellar associations, or open clusters.