If water worlds exist, what do they really look like?

Our solar system has two kinds of planets: rocky ones such as Mercury, Earth, or Mars, and gas giants like Neptune, Jupiter, or Uranus. However, beyond our solar system, there are planetary types that do not exist in our system. One particularly fascinating type is the water world.

It's difficult to even envision what a water world would be like. We're not completely certain they exist, despite numerous studies revealing compelling clues to their existence. If they do exist, these planets (also known as Hycean worlds) are larger than Earth. In reality, they are sometimes referred to as mini-Neptunes, mainly because they would be closer in size to Neptune rather than Earth. Besides that, there is limited knowledge about them.

Astronomer Nikku Madhusudhan from Cambridge closely observed a potential Hycean world named K2-18b using the James Webb Space Telescope in 2023. However, even then, he could not ascertain much about the interior of these planets. Consequently, he turned to modeling. Specifically, they developed various models for mapping potential planetary structures.

Earth and water

One of the most captivating aspects about these planets is their potential to support life. They contain a significant amount of water, which works in their favor. However, they are so different from Earth that water alone might not be sufficient.

Madhusudan and co-author Frances Rigby exclusively investigated the conditions that enable habitable temperatures and pressures at the ocean's surface, where water meets the atmosphere. With these limitations, they modeled the potential oceans and internal structure of these planets and identified several different possibilities.

To begin with, the ocean depth of these planets could vary significantly, ranging from an average of 10-20 km deep to an immense 1000 km deep. For comparison, Earth's average ocean depth is 3.7 km. This divergence may pose a significant challenge as the pressure at these depths could be completely unsuitable for life.

Another issue is the likely presence of ice. Even if the temperature is suitable for hosting liquid water on the surface, the water becomes colder and colder as you descend. Eventually, it could form a layer of ice between the liquid water and the planet's crust. Here on Earth, the weathering of rocks on the seafloor generates important nutrients. The existence of an ice layer could trap these nutrients and make them unavailable to potential life forms.

Nonetheless, if the conditions are right, the nutrients could permeate through the ice via convection, as highlighted by the researchers.

Examining water world candidates

The models provide extensive information about the potential structure of these planets, but practical observations are needed to confirm it. Currently, researchers are focusing on three specific potential water worlds: TOI-270 d, TOI 1468 c, and TOI-732 c (TOI refers to planets observed by the TESS space telescope).

All three candidates orbit around small red stars — stars that are not as warm as the Sun but sometimes have strong bursts of energy. However, these specific stars the planets orbit are relatively peaceful. This makes them very promising planets that could be habitable — if we can confirm they are actually covered with water.

However, the study highlights the potential habitability of these planets under different conditions, challenging the traditional view of habitable zones. Rigby and Madhusudhan's research also emphasizes the potential of upcoming observations, particularly from the James Webb Space Telescope (JWST), to further narrow down the features of Hycean worlds. They stress that the distinct atmospheric signs and interior compositions of these planets make them excellent targets for the search for signs of life outside our solar system. By examining the internal structures and atmospheric compositions of five potential Hycean worlds, the study establishes limits on their habitability and creates new opportunities for understanding how planets form and the conditions that support life.

Our quest for life beyond Earth just became more intriguing — and there are numerous promising places to explore in the Universe.