Exoplanet More Than Twice the Size of Earth Is 'Most Promising Candidate so Far' to Harbor Life, Scientists Say

Investigations into the exoplanet K2-18b have revealed that the distant world could harbor the right conditions for life to exist.

Last year, the planet—which is located around 124 light-years away in the so-called "habitable zone" around its host star—made headlines when scientists identified water vapor in its hydrogen-rich atmosphere.

However, just because water vapor is present in the planet's atmosphere and it lies in the habitable zone—the region around a host star where temperatures can be conducive for water to exist in liquid form—doesn't necessarily mean that it will be a hospitable place for life.

But now, a team led by Nikku Madhusudhan from the University of Cambridge in the U.K. has analyzed the mass, radius and atmospheric data from K2-18b, finding that it could be "potentially habitable," according to a study published in The Astrophysical Journal Letters.

"There is a reasonable chance that the planet hosts a large ocean underneath the atmosphere at pressures and temperatures similar to those in the Earth's oceans," Madhusudhan told Newsweek.

This finding is particularly intriguing given that the planet is 2.6 times the size of Earth, and 8.6 times its mass, according to the scientists. Some have previously suggested that the planet is more akin to a "mini-Neptune" than Earth, encased in a layer of hydrogen above a layer of high-pressure water, and a solid inner core of rock and iron.

If the hydrogen layer is too thick, the temperature and pressure at the surface of the water below would be too high for life as we know it to survive. But even though it is large in size, the team found that the hydrogen later may not be too thick.

"This suggests that habitable conditions can be present in exoplanets that are significantly larger than previously assumed, and that our search for life elsewhere should not necessarily be restricted to Earth-size exoplanets," Madhusudhan said.

"We also found that the planet has an atmosphere rich in molecular hydrogen with a significant amount of water vapor, confirming previous studies. However, we found that the concentrations of other gases such as methane and ammonia were significantly lower than expected at these temperatures. It is unclear if biological processes could be responsible for such depletion," he said.

Scientists already know of several other exoplanets that orbit their stars in the so-called habitable zone.

"However, for none of those exoplanets has water vapor been found in the atmosphere nor has there been any evidence that liquid water could actually exist on their surfaces," Madhusudhan said. "This makes K2-18b the most promising candidate so far [for potentially harboring life.]"

The team came to their findings after using atmospheric observations to estimate properties of the atmosphere, such as the composition and pressure-temperature structure. They then used these atmospheric properties as starting points to infer the internal structure of the planet.

"We considered an internal structure model that is composed of a hydrogen envelope, a water layer, and an inner core made of rock and iron, typical of planetary interiors," Madhusudhan said. "We explored a wide range of possible interior compositions that could explain all the data together, including the measured mass and radius of the planet."

exoplanet, K2-18b
Artist's illustration of K2-18b Amanda Smith

"At the end, we are left with a range of conditions in the planetary interior that could explain the combined data. In particular, at the boundary between the hydrogen envelope and the water layer below, the pressures and temperatures are such that liquid water can exist in many of the cases. Of these cases, there are some where the conditions are similar to those in Earth's oceans," he said.

According to the researchers, one of the main implications of the research is that exoplanets significantly larger than Earth, could host conditions that are conducive to life.

"As such, our search for life elsewhere should not necessarily be restricted to Earth-size exoplanets," Madhusudhan said. "We should be open to finding life in more diverse environments and in more diverse forms than our terrestrial experience."

Planets like K2-18b could become the targets of future observation missions with facilities such as the upcoming James Webb Space Telescope, which will be capable of providing more detailed insights into these promising worlds.