A new study suggests that many more planets may have large amounts of water than previously thought up to half water and half rock. Catch? All this water is probably embedded in the rock, rather than flowing like oceans or rivers on the surface. “It was surprising to see evidence for so many water worlds orbiting the most common type of star in the galaxy,” said Rafael Luque, first author of the new paper and a postdoctoral researcher at the University of Chicago. “It has huge implications for the search for habitable planets.
Thanks to better telescopes, scientists are finding signs of more and more planets in distant solar systems. A larger sample size helps researchers identify demographic patterns—just as looking at the population of an entire city can reveal trends that are hard to see at the individual level. Luque, along with co-author Enrico Pall of the Institute of Astrophysics of the Canary Islands and the University of La Laguna, decided to look at the population level of the group of planets seen around a type of star called an M-dwarf. These stars are the most common stars we see around us in the galaxy, and scientists have so far cataloged dozens of planets around them.
But because stars are much brighter than their planets, we can’t see the planets themselves. Instead, scientists detect faint signs of the planets’ influence on their stars—the shadow cast when a planet crosses in front of its star, or a tiny tug on the star’s motion as the planet orbits. This means that many questions remain about what these planets really look like.
Two different ways to discover planets
“Each of these two different ways of discovering planets gives you different information,” Palle said. By capturing the shadow cast when a planet crosses in front of its star, scientists can determine the planet’s diameter. By measuring the tiny gravitational force a planet exerts on a star, scientists can determine its mass.
By combining these two measurements, scientists can get an idea of ​​the planet’s composition. Maybe it’s a large but airy planet made mostly of gas like Jupiter, or a small, dense, rocky planet like Earth. These analyzes have been performed for individual planets, but more rarely for the entire known population of such planets in the Milky Way galaxy. When scientists looked at the numbers – 43 planets in total – they saw a surprising picture emerge. The densities of a large percentage of the planets indicated that they were too light for their size to be pure rock. Instead, these planets are probably something like half rock and half water or some other lighter molecule. Think of the difference between picking up a bowling ball and a soccer ball: they’re roughly the same size, but one is made of a much lighter material.
It might be tempting to imagine these planets as something out of Kevin Costner’s Water World: completely covered in deep oceans. However, these planets are so close to their suns that any water on the surface would exist in a supercritical gas phase, increasing their radius. “But we don’t see that in the samples,” Luque explained. “This suggests that the water is not in the form of a surface ocean.”
The existence of water
Instead, the water could exist mixed into the rock or in pockets beneath the surface. These conditions would be similar to Jupiter’s moon Europa, which is thought to have liquid water underground. “I was shocked when I saw this analysis — I and many people in the field assumed these were dry, rocky planets,” said UChicago exoplanet scientist Jacob Bean, whose group Luque joined to conduct the further analyses. The finding fits with the theory of exoplanet formation that has fallen out of favor in the past few years, which suggested that many planets form further out in their solar systems and migrate inward over time. Imagine that clumps of rock and ice form together in cold conditions far from the star and then are slowly pulled inward by the star’s gravity.
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