Chthonian planet

A Chthonian planet is a hypothetical type of celestial object. These worlds are not yet proven to exist in our universe. Scientists believe they are the leftover cores of much larger planets. A Chthonian planet forms when a gas giant loses its outer layers. This process leaves behind a dense, rocky, or metallic core. Such a core would resemble a terrestrial planet like Earth in many ways.

The transformation happens through a process called hydrodynamic escape. This occurs when a planet orbits very close to its host star. The intense heat from the star acts upon the planet's atmosphere. This heat causes the hydrogen and helium layers to strip away. The gas is essentially evaporated into space by the stellar energy. Eventually, only the heavy, solid center of the planet remains.

Astronomers look for specific signs to identify these potential worlds. One method involves observing the density of a planet. For example, planets like Kepler-52b, Kepler-52c, and Kepler-57b show interesting data. Transit-timing variation measurements suggest these worlds have very high masses. Their masses might be between 30 and 100 times that of Earth. Because their radii are only about two Earth radii, they are extremely dense. They could even be denser than an iron planet of the same size.

The term "Chthonian" has a deep connection to ancient language. It was coined by a group of researchers led by Guillaume Hébrard in 2003. The name comes from the Greek word "Chthon," which means "earth." In Greek mythology, the term refers to deities from the infernal underground. This name fits the idea of a planet's hidden, inner core being revealed. The researchers studied the evaporation rates of "hot Jupiters" to develop this idea.

We can see the early stages of this process in other planets. HD 209458 b is a gas giant currently losing its atmosphere. It is undergoing atmospheric stripping right now. However, it will not become a Chthonian planet for billions of years. Another example is Gliese 436 b, which has already lost 10% of its atmosphere. These planets show us how the transition might look over vast amounts of time.

There is much debate regarding specific planets in our observations. CoRoT-7b is often cited as the first exoplanet that might be Chthonian. However, some researchers disagree with this classification. They argue that CoRoT-7b was always a rocky planet. This argument is based on the young age of its star system. In 2020, scientists found another candidate named TOI-849 b. This planet is more massive than Neptune and sits in the "Neptunian desert." It may very well be a true Chthonian planet.

Studying these objects helps us understand the life cycles of planetary systems. The presence or absence of certain planets tells us about stellar radiation. For instance, there is a lack of gaseous "hot-super-Earths" in certain size ranges. Specifically, there are few planets between 2.2 and 3.8 Earth-radii exposed to high stellar flux. Scientists assume that planets in this range have had their envelopes stripped by photoevaporation. This helps us map how stars shape the worlds that orbit them.