Key Takeaways:

  1. Water vapor has been discovered in the atmosphere of a super-Earth exoplanet, K2-18 b, which orbits within the habitable zone of its star.
  2. The planet has a temperature similar to Earth, making it a promising candidate for further studies.
  3. K2-18 b is twice the diameter of Earth and eight times as massive, placing it at the upper limit of what is classified as a super-Earth.
  4. The planet could potentially be a water world with a global ocean covering its surface.
  5. The abundance of water on K2-18 b is estimated to be somewhere between 0.01 percent and 50 percent.

Astronomers have made an exciting discovery using NASA’s Hubble Space Telescope: water vapor in the atmosphere of a super-Earth exoplanet, K2-18 b. This exoplanet orbits within the habitable zone of its star, which means that liquid water could potentially exist on its rocky surface, possibly even forming a global ocean.

This discovery is significant because it’s the first time water vapor has been detected in the atmosphere of such a planet. What makes K2-18 b particularly interesting is that it likely has a temperature similar to Earth. This makes it one of the most promising candidates for follow-up studies with next-generation space telescopes.

K2-18 b orbits a red dwarf star in the constellation Leo, approximately 110 light-years away. Despite red dwarfs being known for their powerful flares, this particular star appears to be surprisingly calm. This is good news for K2-18 b, as its close proximity to its star (twice as close as Mercury is to the Sun) would otherwise pose a threat.

The researchers suggest that K2-18 b has a temperature between about –73 °C and 47 °C. For comparison, temperatures on Earth can range from below –84 °C in regions like Antarctica to above 49 °C in regions like Africa, Australia, and the Southwestern United States.

Although K2-18 b shares some similarities with Earth — such as water, habitable temperatures, and a rocky surface — it’s important to note that it’s not exactly Earth-like. For one, K2-18 b is roughly twice the diameter of Earth and about eight times as massive. This places it near the upper limit of what we call a super-Earth — planets between about one and 10 Earth masses.

The density of K2-18 b further confirms its status as a rocky planet. With a density about twice that of Neptune, K2-18 b’s composition is most similar to Mars or the Moon. Given that the planet is believed to have a solid surface and an extended atmosphere with at least some water vapor, researchers suggest that K2-18 b could potentially be a water world with a global ocean covering its entire surface.

However, this remains uncertain due to limitations in Hubble’s ability to probe the atmospheres of distant exoplanets in great detail. Although researchers were able to detect the presence of water vapor in the atmosphere of K2-18 b using a sophisticated algorithm, they couldn’t determine exactly how much water vapor there really is.

As such, they provided a broad-range estimate for the abundance of water — somewhere between 0.01 percent and 50 percent. Despite these uncertainties, this discovery marks an important step forward in our understanding of potentially habitable exoplanets.

The research was published on September 11 in Nature Astronomy.

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