Surprise finding suggests 'water worlds' are more common than we thought

All life on Earth depends on water, and the cycle of rain, rivers, oceans, and rain again is crucial to maintaining a stable and habitable climate. Planets with water are always at the top of the list when scientists discuss where to look for life throughout the galaxy.

According to a recent study, there may be many more planets than previously believed that contain significant amounts of water, possibly even half of them being rock and half water. The problem? Instead of flowing as seas or rivers on the surface, all that water is likely entrenched in the rock.

Rafael Luque, a postdoctoral researcher at the University of Chicago and the paper's first author, stated that it was unexpected to find evidence for so many water worlds orbiting the most prevalent form of star in the galaxy. It has significant ramifications for the search for terraforming planets.

Scientists are discovering evidence of an increasing number of planets in distant solar systems as a result of improved telescope instruments. Similar to how examining a town's population as a whole might show tendencies that are difficult to observe at an individual level, a bigger sample size aids scientists in identifying demographic patterns.

Luque and co-author Enric Pallé of the University of La Laguna and the Institute of Astrophysics of the Canary Islands made the decision to examine a population-level look at a collection of planets that are detected orbiting an M-dwarf star. Numerous planets have been discovered so far around these stars, which are among the most frequent stars in our galaxy.

However, we are unable to see the planets themselves because stars are so much brighter than their planets. However, there are subtle evidence of the planets' influence on their stars that may be seen by scientists, such as the shadow cast as a planet passes in front of its star or the slight tug on the star's motion as it orbits. That means there are still a lot of unanswered concerns regarding the appearance of these planets.

According to Pallé, "the two different methods of discovering planets each provide you different information." Scientists can measure a planet's diameter by observing the shadow that is cast as it passes in front of its star. Its mass can be determined by measuring the minuscule gravitational attraction that a planet has on a star.

Combining the two readings allows researchers to understand the composition of the planet. It might be a large, airy planet like Jupiter that is primarily formed of gas, or it might be a small, dense, rocky planet like Earth.

Individual planets had undergone these examinations, but the total known population of such planets in the Milky Way galaxy had been done much less frequently. The scientists noticed a startling pattern emerge as they studied the data—43 planets in total.

Many of the planets' densities indicated that they were too light to be entirely composed of rock because of their size. Instead, these planets are most likely made up of a mixture of water or another lighter molecule and rock. Try picking up a soccer ball and a bowling ball; they are both about the same size, but one is made of much lighter material.

It could be tempting to picture these worlds as being fully submerged beneath vast oceans, like something from Kevin Costner's Waterworld. These planets are so close to their suns, though, that any surface water would be in a supercritical gaseous phase, increasing the radius of the planets. However, Luque said, "We don't see that in the samples." That implies that the water is not an ocean's surface.

Instead, the water can be present in pockets below the surface or incorporated with the rock. In those circumstances, the moon Europa of Jupiter, which is thought to have liquid water beneath, would be comparable.

"I was astonished when I saw this research," said UChicago exoplanet scientist Jacob Bean, whose group Luque has joined to do additional analyses. "I and many people in the field assumed these were all dry, rocky worlds," he said.

The discovery supports an exoplanet formation theory that had lost favor in recent years, which postulated that many planets form farther outside of their solar systems and move closer in over time. Imagine ice and rock clusters coalescing in the chilly surroundings distant from a star, then slowly being drawn inward by the star's gravity.

Bean stated that even while the evidence is strong, he and the other researchers are still waiting for "smoking gun proof" that one of these planets is a water world. With JWST, NASA's recently launched space telescope that will replace Hubble, the scientists hope to do that.

University of Chicago