Hycean Planets: Exploring New Horizons in the Search for Extraterrestrial Life

On July 20, 1969, Apollo 11 landed on the moon and humans walked on an extraterrestrial surface for the first time in history. These astronauts saw a vast, barren land devoid of life; this moment marked the beginning of an era of space exploration that continues to this day. The moon landing also signified the start of the search for extraterrestrial life. The most vital component in the search for life is the presence of liquid water.

In 2021, a new theoretical class of exoplanets—planets outside of our solar system—was introduced by researchers from the University of Cambridge: hycean planets. Hycean is a combination of the words “hydrogen” and “oxygen,” signifying the two main identifying components of a hycean planet: a thick hydrogen-dominated atmosphere and vast liquid oceans. Hycean planets would most likely be between the sizes of Earth and Neptune; this size range allows for chemical makeups that match that of hycean planets and is also the most common size range for exoplanets. The presence of liquid water on hycean planets would make them ideal candidates for containing extraterrestrial life. 

Critical to the search for extraterrestrial life is NASA’s Kepler Mission, launched in 2009. Kepler is a telescope used to examine a portion of the Milky Way in order to discover stars and exoplanets. A majority of the approximately 5,500 exoplanets we have discovered were discovered by Kepler. In order to find these exoplanets, Kepler analyzed the dimming of stars that occurs when planets pass between the stars and the telescope. Unfortunately, because exoplanets are so far away, our current technology is unable to confirm whether or not any of these exoplanets are hycean planets. However, there are several potential candidates, selected based on their size and whether or not they are in the habitable zone. The habitable zone, also known as the Goldilocks zone, is the region around a star where it is not too hot or too cold, making it possible to support liquid water. Habitable zones are found by taking into account the planet’s atmospheric makeup, its distance from its central star—the star that it orbits around and receives heat from—and the heat released by the central star. The most well-known hycean planet candidate is K2-18 b, which was discovered by the Kepler Mission and lies about 120 light years away from Earth. 

A vital piece of technology in the search for hycean planets is the James Webb Telescope, which is able to identify molecules in a planet’s atmosphere by analyzing the light that passes through the atmosphere. Light has unique interactions with different molecules when it passes through them, creating patterns that can be used to identify the molecules. For example, nitrogen and hydrogen molecules allow different wavelengths of light to pass through them. In September, the James Webb Space Telescope found an abundant amount of carbon dioxide and methane and an absence of ammonia on K2-18 b. Carbon dioxide and methane are both commonly produced byproducts of life processes on Earth such as respiration, suggesting the presence of potential lifeforms on exoplanets. The absence of ammonia also suggests the presence of water since ammonia is highly soluble and almost all atmospheric ammonia would be dissolved by water, thus becoming undetectable in the atmosphere. Furthermore, trace amounts of dimethyl sulfide were found. On Earth, almost all dimethyl sulfide is made by living organisms, primarily marine phytoplankton. This information all supports K2-18 b being a hycean planet. 

Despite this, some recent studies modeling hycean atmospheres, conducted by a variety of physicists have revealed that many of the current candidates have a high chance of not being hycean planets after all because they likely suffer from a runaway greenhouse effect. A runaway greenhouse effect occurs when a planet contains a sufficient amount of greenhouse gasses that trap enough heat in the atmosphere to boil water. It is impossible for planets that suffer from a runaway greenhouse effect to have liquid water because it would all boil and evaporate, which would make them incapable of hosting lifeforms.

Many current hycean planet candidates suffer from a runaway greenhouse effect because their atmospheres are very different from that of Earth. Based on new models of hycean planet atmospheres, the buildup of water vapor and other greenhouse gasses such as carbon dioxide and methane would top the threshold required to cause a runaway greenhouse effect. As a result, the inner edge of the habitable zone for hycean planets would have to be pushed further back. This means that the inner edge of the habitable zone for hycean planets would become further away from their central star than initially predicted. Being further away from the star means the planet would receive less heat, thus counteracting the runaway greenhouse effect. This would mean that most of our current hycean planet candidates would no longer be in the habitable zone, as their inner edge would be vastly pushed back, making it a much smaller region. 

To illustrate, a hycean planet placed in Earth’s orbit with an atmospheric pressure of 10 to 20 times that of Earth’s—which is normal due to the high volume of hydrogen gas—would experience a runaway greenhouse effect since it’s too close to the sun in relation to its high volume of greenhouse gasses. Initial predictions placed Earth’s orbit within the habitable zone, but this new model pushes the inner edge of the habitable zone way further out, making it impossible for this hypothetical planet to support water.

Though most of the current candidates have been ruled out as hycean planets, the future is still bright for these possible locations of extraterrestrial life. Only around 5,500 exoplanets out of the potential billions in our galaxy have been discovered. There are also still a few current hycean planet candidates left in the habitable zone, including the promising K2-18 b. Additionally, because hycean planets fall into the category of the most common size of exoplanets, candidates are relatively easy to come across. This means that there may be many more potential candidates that do fall into the habitable zone. Furthermore, space exploration technology like the James Webb Telescope is only improving. These factors place a unique importance on the discovery of hycean planets and space exploration going forward. If a hycean planet is discovered, it will become the first extraterrestrial surface with sustainable bodies of liquid water. Since it is one of the key ingredients to life, sustainable liquid water being discovered would signify a huge leap in the discovery of extraterrestrial life. Finding extraterrestrial life could be far closer on the horizon than many of us would imagine.