Water is the only thing that all life on Earth needs, and the cycle from rain to river to sea to rain is an essential part of what keeps our planet’s climate stable and hospitable. When scientists talk about where to look for signs of life throughout the galaxy, planets with water are always at the top of the list.
A new study suggests that many more planets may have large amounts of water than previously thought – up to half water and half rock. The problem? All of this water is probably embedded in rock rather than flowing as oceans or rivers on the surface.
“It was a surprise to see evidence that so many water worlds orbit the most common type of star in the galaxy,” said Rafael Luque, first author of the new paper and postdoctoral researcher at the University of Chicago. “This has enormous implications for the search for habitable planets. »
Planetary population models
Thanks to better telescope instruments, scientists are discovering more and more planets in distant solar systems. A larger sample size helps scientists identify demographic patterns—in the same way that a survey of the population of an entire city can reveal trends that are hard to see at the individual level.
Together with co-author Enric Pallé of the Institute of Astrophysics of the Canary Islands and the University of La Laguna, Luque decided to take a population-level look at a 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 named dozens of planets around them.
But because stars are so much brighter than their planets, we can’t see the planets themselves. Instead, scientists detect faint signs of the impact of planets on their stars—the shadow created when a planet passes in front of its star, or the slight jerk in a star’s motion as a planet orbits. This means that there are still many questions about what these planets actually look like.
“The two different ways of discovering planets each give you different information,” Pallé said. By capturing the shadow created when a planet passes in front of its star, scientists can find the planet’s diameter. By measuring the small gravitational force a planet exerts on a star, scientists can find its mass.
By combining the two measurements, the researchers can get an idea of the planet’s composition. Maybe it’s a large but airy planet made up mostly of gas like Jupiter, or a small, dense and rocky planet like Earth.
These analyzes had been performed for individual planets, but much more rarely for the entire known population of these planets in the Milky Way. When scientists looked at the numbers — 43 planets in all — they saw a startling picture emerge.
The density of a large percentage of the planets suggested 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. Imagine the difference between picking up a bowling ball and a football: they are about the same size, but one is made of a much lighter material.
In search of aquatic worlds
It might be tempting to imagine these planets as something out of Kevin Costner Water the world: completely covered by deep oceans. However, these planets are so close to their suns that any water on the surface would exist in a supercritical gas phase, which would enlarge their radius. “But we can’t see it in the tests,” Luque explained. “This suggests that the water is not in the form of a surface ocean. »
Instead, water could exist mixed with rock or in pockets beneath the surface. These conditions would be similar to Jupiter’s moon Europa, which is thought to contain liquid water underground.
“I was shocked when I saw this analysis — I and a lot of people in the field assumed they were dry, rocky planets,” said UChicago exoplanet researcher Jacob Bean, who the Luque group joined to conduct further analysis.
The discovery matches a theory of exoplanet formation that had fallen out of favor in recent years, which suggested that many planets form further out in their solar system and migrate inward over time. Imagine clumps of rock and ice forming together in cold conditions far from a star, then slowly being pulled inward by the star’s gravity.
Although the evidence is compelling, Bean said he and other scientists would still like to see “irrefutable evidence” that one of these planets is an aquatic world. That’s something scientists hope to do with JWST, NASA’s recently launched space telescope successor to Hubble.
Source of the story:
Materials provided by University of Chicago. Originally written by Louise Lerner. Note: Content can be edited for style and length.