How We Define Habitable Planets Could Change, Marking 'Paradigm Shift' in Search for Life

Two separate teams of scientists have looked at the question of what makes a planet outside the solar system—or an exoplanet—habitable, arriving at answers that could redefine how and where we search for life in the Universe.

University of Cambridge Institute of Astronomy professor Nikku Madhusudhan considered Hycean planets, a term he coined to define a water-rich planet with a surface covered entirely in ocean and an atmosphere that is made of mostly molecular hydrogen. Meanwhile, Penn State University Astronomy and Astrophysics Ph.D. graduate student Noah Tuchow questioned how the regions of habitability around stars themselves can change over time.

Both studies could break the mold of what we consider a habitable planet and where within star systems other than our solar system astronomers look for such worlds.

Thus far, this search has focused on Earth-like terrestrial worlds in the zone around a star that has the right temperature to allow liquid water to exist. The fact that this means a region that is neither too hot nor too cold, but just right, has led to it being dubbed the Goldilocks zone by astronomers.

Considering Hycean worlds expands the possible range of habitable planets because they can hang on to liquid water outside what is considered the traditional Goldilocks zone.

"Hycean planets can be both significantly larger and significantly hotter or cooler than the planets that have been considered for habitability in the past," Madhusudhan told Newsweek.

Madhusudhan, who is the lead author of a study discussing Hycean worlds in The Astrophysical Journal, said that there are more of these planets beyond the limits of the solar system than there are rocky worlds like Earth, and that their atmospheres are ideal for analysis.

As Hycean planets are large and abundant in the exoplanet population, it is easy to detect them and to study their atmosphere, said Madhusudhan. "All these properties allow for a much broader range of planets that can be potentially habitable and, therefore, be searched for signatures of life, majorly boosting our chances of finding life elsewhere.

"This work could be a paradigm shift in our search for life."

Exoplanet K2 18b
An artist's impression of K2 18b. The exoplanet and its water rich atmosphere will be be prime target for the James Webb Space Telescope. , M. Kornmesser/ESA

Madhusudhan and his team have already identified 11 Hycean world candidates orbiting nearby stars, with the conditions of a Hycean world first spotted by the astronomers in the water-rich atmosphere of the exoplanet K2-18b in 2020.

The exoplanet which is identified as a Super-Earth—a planet like ours but much larger—orbits a red dwarf 124 light-years from Earth, and the characterization of its atmosphere by Madhusudhan and his team pointed them to this exciting new class of exoplanets.

"Hycean planets greatly expand and accelerate our search for life on exoplanets beyond just focusing on an Earth-twin," Madhusudhan said.

Olivier Demangeon, a researcher at Portugal's Institute of Astrophysics and Space Science who was not involved with the research, told Newsweek: "The new class of potentially habitable planet that the study presents could quite quickly affect the search for life. There are still several open questions regarding the real habitability of these planets and they are not as promising as terrestrial rocky planets in this regard.

"However, their larger atmosphere is much easier to detect and study. It significantly increases our capacity to detect biosignatures if they are present."

Approaching the Goldilocks Zone From a Different Angle

Tuchow and his co-author Jason Wright, a professor of astronomy and astrophysics at Penn State, took a different approach to the definition of habitable worlds.

In a paper published in Research Notes of the American Astronomical Society, they pointed to the fact that star systems are dynamic and change over time. This means that Goldilocks zones can change too and planets may not remain habitable throughout their entire existence.

"The habitable zone only gives a snapshot of the current day. This is a problem because the brightnesses and temperatures of stars evolve over time, causing their habitable zones to change with them," Tuchow told Newsweek. "Just finding a planet in the habitable zone doesn't tell you whether it has been habitable for billions of years or if it just entered the habitable zone recently."

Tuchow and Wright say this means there are two types of planets in habitable zones, planets that form there and stay there, and belatedly habitable planets that form outside Goldilocks zones and find themselves within them due to changes in their parent star.

"These belatedly habitable planets have often been overlooked, to the extent that they didn't even have a name prior to this study, but they may comprise many if not most of the planets that will be found in the habitable zone," Tuchow said. "When we search for life on planets around other stars, we need to reevaluate whether these belatedly habitable planets would make good targets."

Astronomers should not only focus on whether a planet is in the current day habitable zone, Tuchow said, but they should also take the host star's evolution into account and consider how long a planet has remained in the habitable zone and whether it has remained habitable for its entire lifetime.

Demangeon, who was not involved in the study, said: "The fact the researchers have found a quite convincing name for these worlds might help to disseminate the idea. However, as they also wrote the real question is: does being in the belated habitable zone affect the capacity of a planet to have the conditions to host life?"

The next major step in the redefinition of habitable worlds and the regions around stars they exist will likely come with the launch of the James Webb Space Telescope (JWST), which will have the observational power to properly assess the atmospheres of distant exoplanets.

Demangeon thinks that Hycean worlds will be a prime target for ground-based telescopes too, not just the space-based JWST. "I am expecting that in the near future several of these planets will be targeted by space and ground-based facilities for atmospheric characterization," he said.

Madhusudhan already intends to train the space telescope on K2-18b, and search its atmosphere for signs of life. The researcher says that the debate regarding the habitability of Hycean worlds could be settled in as little as 20 hours of observation with the JWST.

"We know that planetary systems are extremely diverse in the universe, so there is no reason to think that life will be limited to Earth-like environments," Madhusudhan said. "The discovery of a biosignature in a Hycean planet will naturally be transformational, not only signaling the existence of life elsewhere but also its presence in environments very different from our own."

exoplanet, hycean
A stock image shows an illustration of an exoplanet. Hycean planets could hold on to liquid water outside the traditional habitable zone around stars. Getty Images

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