Watery exoplanets could be more common than we thought

When hunting for exoplanets that could potentially host life, one big factor that scientists consider in habitability is the presence of water. The presence of liquid water is required for almost all life as we know it, so when looking for other worlds which might host life, looking for water is a good place to start. Now, a new study suggests that exoplanets with water may be far more common than we previously thought, with many planets potentially being made up of as much as half water, half rock.

Researchers from the University of Chicago looked at a group of known exoplanets orbiting around M-dwarf stars, which are the most common type of stars in our galaxy. Dozens of these exoplanets have been discovered through two different methods: the transit method, where a planet passes in front of a star and causes a dip in its brightness, and the radial velocity method, where a planet’s gravity causes a very slight change to a star’s movements. The researchers combined information from both methods to learn more about the planets in question.

The surface of Jupiter's moon Europa, where a liquid ocean is thought to lurk beneath an icy crust. The surface of Jupiter’s moon Europa, where a liquid ocean is thought to lurk beneath an icy crust. NASA/JPL-Caltech/SETI Institute

“The two different ways to discover planets each give you different information,” explained co-author Enric Pallé of the Institute of Astrophysics of the Canary Islands and the University of La Laguna in a statement. The transit method tells you about the planet’s diameter and the radial velocity method tells you about its mass. When both size and mass are considered together, you can see the density of a planet — whether it is light and puffy or small and dense.

When looking at a set of 43 exoplanets, the researchers were surprised to discover how many of them had lower densities which meant they couldn’t be made purely of rock. Many of them seemed to be around half rock, and half water.

“I was shocked when I saw this analysis — I and a lot of people in the field assumed these were all dry, rocky planets,” said exoplanet scientist Jacob Bean of the University of Chicago, who will be conducting future research into this topic.

However, despite the evidence indicating more watery worlds than we thought, this doesn’t mean that all these planets have liquid water on their surfaces. With many of the planets orbiting close to their stars, it is more likely that their water would be beneath the surface — like the subsurface oceans which Jupiter’s moons Europa, Callisto, Ganymede, and Io are thought to host.

This finding could even have implications for theories about how exoplanets form, as it could be evidence that planets form further away from their stars where temperatures are lower and ice can form more easily, before migrating inwards over time.

“It was a surprise to see evidence for so many water worlds orbiting the most common type of star in the galaxy,” said lead researcher Rafael Luque. “It has enormous consequences for the search for habitable planets.”

The research is published in the journal Science.

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