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K2-18b: What unknown secrets are hidden in the hydrogen atmosphere of this extraterrestrial planet?
K2-18b is an exoplanet orbiting the red dwarf star K2-18, approximately 124 light-years from Earth. The planet is classified as sub-Neptune-sized, with a radius about 2.6 times that of Earth, and orbits within the habitable zone of its parent star with a period of 33 days. This means it receives about the same amount of starlight as Earth. The planet was originally discovered using the Kepler Space Telescope and has since been observed by the James Webb Space Telescope (JWST) to study its atmosphere.
In 2019, scientists first reported the presence of water vapor in the atmosphere of K2-18b, a discovery that attracted widespread attention.
In 2023, JWST detected carbon dioxide and methane in the atmosphere of K2-18b. Different interpretations of the data suggest that K2-18b could be a water-ocean planet with a hydrogen-rich atmosphere, or a gas-rich mini-Neptune. In any case, K2-18b is more like an imitation of Neptune or Uranus than it is of Earth’s habitable conditions. This has led to widespread discussion of its potential habitability.
K2-18 Features
K2-18 is an M dwarf star located in the constellation Leo, with a temperature of only 3457 K and a radius of only 45% of the Sun. The star is approximately 2.4 billion years old and shows moderate stellar activity. The star also hosts a planet, K2-18c, that lies within the orbit of K2-18b and may interact with K2-18b through tidal effects.
Physical properties of K2-18b
K2-18b has a radius of 2.610±0.087 R🜨 and a mass of 8.63±1.35 ME. The planet's density suggests it could have a hydrogen-rich crust, possibly a rocky core plus a thick envelope, or a Neptune-like composition. Such medium-sized planets are unexpectedly rare, forming what is known as a radius valley. The planet may have taken millions of years to form, but its tidal heating is unlikely to have accelerated this process.
Possible Ocean Existence
The question of whether K2-18b has liquid water remains unclear. Even though previous data indicated that the water was in a supercritical state, JWST's observations pointed to the possible existence of a liquid ocean. But confirming this is difficult because the existence of an ocean cannot be inferred from mass and radius alone.
Researchers said: In the supercritical state, there is no longer a boundary between liquid and gas, making it even more difficult to define the existence of the ocean.
Previous studies have suggested that there may be other components in the atmosphere of K2-18b, such as ammonia and simplified carbon molecules, but different models have different explanations for these components, and the scientific community is still debating.
Atmosphere and Climate
Based on observations from the Hubble Space Telescope, K2-18b has a hydrogen atmosphere, with some uncertainty surrounding the presence of water vapor. JWST observations show that the concentration of water vapor in the atmosphere is extremely low, less than 0.1%. Additionally, JWST observed that methane and carbon dioxide each make up about 1% of the atmosphere. The concentrations of other carbon oxides have not been determined and only upper limits are available. The atmosphere makes up at most 6.2% of the planet's mass and is probably similar in composition to that of Neptune or Uranus.
Planetary Evolution
K2-18b's upper atmosphere will be affected by high-energy radiation from its parent star, which will heat the atmosphere and form hydrogen, which may lead to the escape of the atmosphere. Such processes suggest that even if this process continues, K2-18b may not be able to completely lose its atmosphere during its lifetime.
Habitability
The planet is located in the habitable zone of its star and receives about 1368+114−107 W/m2 of stellar radiation, similar to that of Earth. However, confirming the actual habitability of K2-18b will depend on the composition of its atmosphere and the albedo of its clouds. The observations suggest that the planet's deep atmosphere could be superheated, while a layer with a higher water content could be suitable for the development of life.
Microorganisms can survive in hydrogen-rich environments, indicating that hydrogen is not a stumbling block to life.
K2-18b is undoubtedly a fascinating object of study. With the James Webb Space Telescope's return to space, in-depth exploration of this planet is expected to reveal more of its secrets. So, can this planet actually support life?
