Eagle-eyed astronomers from Penn State University recently discovered this potential Goldilocks planet, officially catalogued as G 9-40b (shown below), that's estimated to be more than twice the size of our Big Blue Marble and possibly even as large as the gas giant, Neptune.
K2-18b is the only planet orbiting a star outside our Solar System known to have water and temperatures that could support life. (Molecules in the planet's atmosphere absorb certain frequencies of light, so if the starlight passes through it on its way to Earth, the light's spectrum can reveal those molecules.) They then used those possibilities to limit what conditions could exist in the planet's interior.
So, Madhusudhan and his team chose to take a closer look at K2-18b to see if, according to what we can observe, an ocean on the exoplanet would hit those inhospitable oceanic pressure levels. Last year, astronomers detected clouds of liquid water in the planet's hydrogen-rich atmosphere, a first for such a small planet.
Computer models suggest an ocean world - with liquid water below the atmosphere at pressures and temperatures similar to those found in our seas. This is evident from recent inferences of H2O in such atmospheres, including that of the habitable-zone exoplanet K2-18b. The levels of other chemicals such as methane and ammonia were lower than expected for such an atmosphere, they say.
They confirmed the atmosphere is hydrogen-rich with a significant amount of water vapour.
Dr Nikku Madhusudhan, from Cambridge's Institute of Astronomy, who led the research, mentioned: "Water vapour has been detected within the atmospheres of quite a few exoplanets, however, even when the planet is within the liveable zone, that does not essentially imply there are liveable circumstances on the floor".
It's 2.6 times the radius of Earth. If the hydrogen envelope was too thick, the temperature and pressure at the water layer would be inhospitable to life. The inside of a mini-Neptune would likely feature an inner core of rock and iron.
The scientists found that the maximum extent of the hydrogen envelope allowed by the data is around 6% of the planet's mass, though most of the solutions require much less.
"We wanted to know the thickness of the hydrogen envelope - how deep the hydrogen goes", said co-author Matthew Nixon, doctoral student at the Institute of Astronomy. "While this is a question with multiple solutions, we've shown that you don't need much hydrogen to explain all the observations together".
The minimum amount of hydrogen is about one-millionth by mass, similar to the mass fraction of the Earth's atmosphere, they said. And some of the scenarios allowed for a liquid ocean at habitable pressures.
Although the planet is too far for any spacecraft or probe to reach, space telescopes and ground-based observatories can still study its atmospheric conditions.
Researchers at the University of Cambridge introduced the latest mass, radio and atmosphere data of the planet in different models and concluded that it is very possible that K2-18b looks more like Earth than Neptune.
The Cambridge researchers hope to refine their results and observations with the aid of NASA's James Webb Space Telescope.