All images: ESA, NASA (Artist's Concept)
In June 2023, the European Space Agency's (ESA) mission Cheops confirmed the existence of four warm exoplanets orbiting four stars in the Milky Way galaxy. These so-called mini-Neptunes were unlike any planet in the solar system and provided a "missing link" between Earth-like and Neptune-like planets that is not yet understood.
The ESA said that mini-Neptunes are among the most common types of exoplanets known, and astronomers are starting to find more orbiting bright stars.
Mini-Neptunes are smaller, cooler and mysterious objects. They are more difficult to find than the hot Jupiter exoplanets. Hot Jupiters have surface temperatures of over 1000 degrees Celsius while mini-Neptunes have cooler surface temperatures of 300 degrees Celsius, the ESA said.
Astronomers have predicted that mini-Neptunes have an iron-rocky core with thick outer layers of lighter material.