Language
Country/Area
No result found
K2-18b: A planet with new hope for life, why is it so mysterious?
K2-18b, also known as EPIC 201912552 b, is an exoplanet located 124 light-years (38 parsecs) away, orbiting the red dwarf star K2-18. The planet has a radius about 2.6 times that of Earth and has a 33-day orbit, which places it squarely in the habitable zone of its star, receiving a similar amount of sunlight as Earth. First discovered by the Kepler Space Telescope, deeper observations were subsequently carried out by the James Webb Space Telescope (JWST) to study the planet's atmosphere. In 2019, a research report pointed out the presence of water vapor in the atmosphere of K2-18b, which attracted widespread attention from the academic community. By 2023, JWST detected carbon dioxide and methane in the planet's atmosphere.
Various interpretations of the JWST data suggest that this could be a water ocean planet with a hydrogen-rich atmosphere or a gas-rich mini-Neptune.
Main star K2-18
K2-18 is an M-type dwarf star of spectral type M3V located in the constellation Leo, approximately 38.025±0.079 parsecs (124.02±0.26 light years) from the Sun. The star is relatively small and cool, with a surface temperature of about 3457 K (3184 °C; 5763 °F), and a radius only 45% that of the Sun. The star is about 2.4 billion years old and shows moderate stellar activity, but it is unclear whether there are starspots that would create false signals. There is also a planet named K2-18c in orbit around K2-18, which may interact with K2-18b through tidal influences.
Physical properties of K2-18b
K2-18b has a radius of 2.610±0.087 times the Earth's radius, a mass of 8.63±1.35 times the Earth's mass, and orbits its host star in 33 days. The planet is at such a distance from its host star that it can be observed as it passes in front of the star. K2-18b may be tidally locked to its host star, but given its orbital eccentricity, it may also exhibit rotation-revolution resonance like Mercury. The planet's density is about 2.67+0.52−0.47 g/cm3, which is between Earth and Neptune, suggesting it has a hydrogen-rich envelope.
The planet could be rocky, have a thick envelope, or have a composition similar to Neptune.
Ocean of Possibility
In the short term, it is unclear whether a separate liquid ocean exists in the atmosphere. If a liquid ocean exists, it would likely be covered by a layer of high-pressure ice above the rocky core, which could destabilize the planet's climate by preventing material from flowing through it. The presence of any ocean on K2-18b requires further observations and evidence to confirm.
Atmosphere and Climate
Observations from the Hubble Space Telescope show that the atmosphere of K2-18b is composed mostly of hydrogen. The presence of water vapor seems possible, however the concentration observed by JWST was less than 0.1%; this may be because JWST observed a dry stratosphere. According to JWST observations, methane and carbon dioxide each make up about 1% of the atmosphere. The mass of the atmosphere accounts for only 6.2% of the total mass of the planet.
The composition of the atmosphere is probably similar to that of Neptune and Uranus.
Is it habitable?
K2-18b is affected by the radiation from its star, so its surface temperature is estimated to be between 250 K (-23 °C) and 300 K (27 °C). Under such conditions, the habitability of the planet depends on the properties of its atmosphere and the albedo of its cloud layer.
Research History
K2-18b was discovered in 2015 by the Kepler Space Telescope and later confirmed using the Spitzer Space Telescope and Doppler velocity techniques. Subsequently, observations by JWST provided us with further information, showing that K2-18b may be an "ocean planet" filled with liquid water.
The mystery of K2-18b lies in its potential habitability and unique environment. Can we find evidence of extraterrestrial life on this planet?
