Water is the one thing all life on Earth needs, and the cycle from rain to river to ocean 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 published in Science 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 like oceans or rivers on the surface.
“It was a surprise to see evidence of so many water worlds orbiting 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 huge 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 examining the population of an entire city can reveal trends that are hard to see at the individual level.
Luque, along with co-author Enric Pallé of the Institute of Astrophysics of the Canary Islands and the University of La Laguna, decided to take a population-level look at a group of planets that are seen around a type of star called M- dwarf. These stars are the most common stars we see around us in the galaxy, and scientists have listed dozens of planets around them so far.
But because stars are so much brighter than their planets, we can’t see the planets themselves. Instead, scientists detect faint signs of planets’ effects on their stars – the shadow created when a planet passes in front of its star, or the little tug on a star’s motion as a planet orbits. This means that many questions remain 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 diameter of the planet. By measuring the tiny gravitational pull a planet exerts on a star, scientists can find its mass.
By combining the two measurements, scientists can get an idea of the composition of the planet. 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 done for individual planets, but much more rarely for the entire known population of these planets in the Milky Way galaxy. As scientists looked at the numbers – 43 planets in all – they saw a startling picture emerge.
The densities 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 soccer ball: they’re about the same size, but one is made of a much lighter material.
In search of aquatic worlds
It can be tempting to imagine these planets as something out of Kevin Costner’s Waterworld: entirely 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, which would enlarge 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.”
Instead, water could exist mixed with rock or in pockets below 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 — me and a lot of people in the field assumed they were dry, rocky planets,” said UChicago exoplanet scientist Jacob Bean, whose the Luque group joined to conduct further analyses.
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 being slowly pulled inward by the star’s gravity.
Although the evidence is compelling, Bean said he and other scientists would still like to see “irrefutable proof” that one of these planets is a water world. That’s something scientists hope to do with JWST, NASA’s recently launched space telescope successor to Hubble.
Up to six billion Earth-like planets in our galaxy, new estimates show
Rafael Luque, Density, Not Radius, Separates Small Rocky, Water-Rich Planets Orbiting M Dwarf Stars, Science (2022). DOI: 10.1126/science.abl7164. www.science.org/doi/10.1126/science.abl7164
Johanna Teske, Three types of planets around red dwarfs, Science (2022). DOI: 10.1126/science.add7175. www.science.org/doi/10.1126/science.add7175
Provided by the University of Chicago
Quote: Startling Discovery Suggests “Water Worlds” Are More Common Than We Thought (2022, September 8) Retrieved September 8, 2022, from https://phys.org/news/2022-09-worlds-common-Thought .html
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