Astronomers at the University of Cambridge have presented a new category of potentially habitable planets, called Hycean worlds. This discovery could radically redefine the way humanity searches for life elsewhere in the galaxy. These celestial bodies are characterized by having global oceans of liquid water covered by an atmosphere rich in hydrogen, and can support life in much more diverse stellar conditions than previously imagined for planets similar to Earth.
The research suggests that not only are these planets more common, but they could also offer an environment more conducive to detecting signs of life with current telescope technology, in contrast to the difficulty of analyzing atmospheres of smaller rocky exoplanets.
A new frontier in the search for extraterrestrial life
For more than six decades, exploration for life beyond Earth has focused mainly on the search for worlds analogous to our own. Early efforts were aimed at Mars, the closest neighbor in the Sun’s habitable zone, where liquid water is believed to have existed in its geological past. With the detection of the first exoplanet in 1995, attention turned to identifying rocky bodies with a size and temperature similar to Earth’s, orbiting stars similar to the Sun.
This perspective, however, proved to be considerably limited in the face of new evidence. In August 2021, Nikku Madhusudhan, an astronomer at the University of Cambridge, in collaboration with Anjali Piette and Savvas Constantinou, published a study that proposed a fundamentally different approach to conceiving habitable planets, introducing the concept of Hycean worlds.
What defines a Hycean planet: oceans under hydrogen
The Hycean planets, according to the model proposed by Madhusudhan’s team, represent a distinct class of worlds. They have a mass that varies between approximately one and ten times the mass of the Earth, and a radius between 1.1 and 2.6 times the Earth’s radius. The main feature is its atmosphere, which is dominated by hydrogen and helium, similar in composition to the atmospheres of Neptune and Uranus, but with a notably thinner hydrogen layer.
Beneath this atmospheric layer lies a vast global ocean of liquid water, the depth of which is greater than that of any terrestrial ocean, covering the entire surface of the planet without the presence of continents or emerging land masses. The thickness of the hydrogen atmosphere is a crucial factor, as if it is too thin, the planet’s surface conditions resemble those of a rocky super-Earth. On the other hand, if the hydrogen layer is excessively dense, atmospheric pressure at the ocean’s surface can become so extreme that liquid water cannot persist, turning the planet into a mini-Neptune. The Hycean interval resides in this delicate balance, where hydrogen is abundant enough to generate significant greenhouse warming but still allows the underlying ocean to remain in a liquid state.
Several variations of the Hycean worlds and their particularities
Within the range that defines a Hycean planet, scientists have identified the possibility of several subtypes, each with unique characteristics that further expand the notion of planetary habitability:
- Standard Hycean planet:This type has an ocean surface temperature that broadly resembles conditions found on Earth, indicating a potentially more familiar environment for the development of life.
- Dark Hycean Planet:In these worlds, the planet is tidally locked to its star, meaning that one side is always facing the star (the day side) and the other is always in darkness (the night side). The day side is too hot for habitability, but the night side maintains a temperature low enough to support liquid water.
- Cold Hycean Planet:Despite receiving less stellar radiation than would be needed to keep an Earth-like planet warm, the powerful greenhouse effect provided by the hydrogen atmosphere ensures that liquid water remains on the surface. This demonstrates the surprising adaptability of these worlds to low-irradiation conditions.
Each of these variations expands the parameters under which a planet can be considered a candidate for habitability, challenging previous conceptions about the conditions necessary for the emergence and maintenance of life.
Expanding the possibilities of finding life in the universe
The structure of the Hycean planets has two significant implications that the popular approach to the search for habitable planets has not proportionately considered. Firstly, Hycean worlds substantially expand what can be classified as a habitable planet. The terrestrial habitable zone around a star, defined by the orbital distance at which an Earth-like planet could maintain liquid water on the surface, is relatively restricted.
In contrast, the habitable zone of the Hycean planets extends considerably closer and further from the star. This is because greenhouse heating from hydrogen allows liquid water to persist even under lower stellar irradiation. Furthermore, the deep thermal mass of the ocean on these planets acts as a regulator of temperature variations that would destabilize a terrestrial atmosphere. For red dwarf stars, which make up approximately three-quarters of all stars in the Milky Way, the Hycean habitable zone is so vast that a huge population of habitable planets can exist around stars that, by the conventional search for Earth analogues, would be ruled out as inhospitable. The ability to detect biosignatures on these worlds, given their distinct atmospheres and larger proportions, is one of the most promising avenues for future exobiology.