When searching for exoplanets that could host life, an important factor that scientists consider in habitability is the presence of water. The presence of liquid water is necessary for almost all life as we know it, so when looking for other worlds that could host life, looking for water is a good place to start. Now, a new study suggests that exoplanets with water may be much more common than previously thought, as many planets may be half water and half rock.
Researchers at the University of Chicago analyzed a group of known exoplanets orbiting M dwarf stars, which are the most common type of star in our galaxy. Dozens of these exoplanets have been discovered using two different methods: the transit method, where a planet passes in front of a star and causes its brightness to drop, and the radial velocity method, where the gravity of a planet causes a very slight change to a star’s movements. The researchers combined the information from the two methods to get more information about the planets in question.
The surface of Jupiter’s moon Europa, where a liquid ocean is believed to be hidden beneath an icy crust. NASA/JPL-Caltech/SETI Institute
“The two different ways of discovering planets 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 diameter of the planet 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 they looked 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 entirely of rock. Many of them seemed to be about half rock and half water.
“I was surprised when I saw this analysis: I and a lot of people in the field assumed they were all dry, rocky planets,” said exoplanet scientist Jacob Bean of the University of Chicago, who will conduct future research on this topic
However, despite the evidence pointing to more water worlds than we thought, this does not mean that all of these planets have liquid water on their surface. Because many of the planets orbit close to their stars, their water is more likely to be below the surface, like the subsurface oceans thought to harbor Jupiter’s moons Europa, Callisto, Ganymede, and Io.
This finding could even have implications for theories of 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 huge consequences for the search for habitable planets.”
The research is published in the journal Science.
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