Some planets lose their air.
They stay close to a bright star. The star's heat takes the air away. Now only a rocky core stays. This helps us learn about space. Do you like looking at the stars?
Some planets are very large. They are made of gas.
These planets stay close to a star. The star is very hot. The heat pulls the gas away.
Soon, the gas is all gone. Only a rocky center stays. This center is very heavy.
It might look like our Earth. We call these special worlds Chthonian. They are a big mystery in space.
Some planets might be the leftovers of giant worlds. We call these Chthonian planets.
These planets start as gas giants. Gas giants are very large worlds made of gas. They often orbit very close to a star. The star is very hot. This heat causes hydrodynamic escape. This is a way that a planet loses its outer layers. The star pulls the gas away. The hydrogen and helium gas drifts off into space.
What is left is a heavy core. This core is made of rock or metal. It might look like our Earth.
Scientists think some planets are Chthonian. One example is CoRoT-7b. It may be a rocky core left behind. Some people think it was always a rocky planet. Another world is TOI-849 b. It is very heavy and close to its star. A gas giant named HD 209458 b is losing its gas now. It might become a Chthonian planet one day. It will take billions of years to change.
A Chthonian planet is a very special kind of world. These planets are not fully proven to exist yet. Scientists call them hypothetical objects. They might be the leftovers of much larger worlds. A giant planet might lose its outer layers over time. What stays behind is a heavy, rocky core. This core could look a lot like our Earth.
How does a giant planet lose its layers? It happens through a process called hydrodynamic escape. This starts when a planet orbits very close to a star. The star is very hot and bright. This heat causes the gas to strip away. The hydrogen and helium gas drifts off into space. This leaves only the dense center behind.
People first used this name in 2003. A group led by Guillaume Hébrard came up with the term. The word comes from "Chthon," which means "earth." In Greek stories, it refers to gods from the underground. These researchers studied how gas giants evaporate. They wanted to see if cores could remain.
There are many interesting examples to study. CoRoT-7b might be the first Chthonian planet found. Some scientists argue it was always a rocky planet. Another world is TOI-849 b, found in 2020. It is very heavy and sits in the Neptunian desert. We also see HD 209458 b losing its gas now. It will take billions of years to change.
We can use these worlds to understand space. Some planets like Kepler-52b might be very dense. These worlds could be 30 to 100 times the mass of Earth. They might even be denser than iron planets. This shows how stars can change their planets. A big gas giant can turn into a small rock. It is a slow and amazing change.
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.
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