INGREDIENTS OF OUR PLANET -70 PERCENT OF WATER
A new study suggests that many more planets may have large amounts of water than previously thought—as much as half water and half rock. The catch? All that water is probably embedded in the rock, rather than flowing as oceans or rivers on the surface.
“It was a surprise to see evidence for so many water worlds orbiting the most common type of star in the galaxy,” said Rafael Luque, first author on the new paper and a postdoctoral researcher at the University of Chicago. “It has enormous consequences for the search for habitable planets.”
Planetary population patterns
Thanks to better telescope instruments, scientists are finding signs of more and more planets in distant solar systems. A larger sample size helps scientists identify demographic patterns—similar to how looking at the population of an entire town can reveal trends that are hard to see at an 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 an M-dwarf. These stars are the most common stars we see around us in the galaxy, and scientists have catalogued dozens of planets around them so far.
But because stars are so much brighter than their planets, we cannot see the actual planets themselves. Instead, scientists detect faint signs of the planets’ effects on their stars—the shadow created when a planet crosses in front of its star, or the tiny tug on a star’s motion as a planet orbits. That means many questions remain about what these planets actually look like.
“The two different ways to discover planets each give you different information,” said Pallé. By catching the shadow created when a planet crosses in front of its star, scientists can find the diameter of the planet. By measuring the tiny gravitational pull that a planet exerts on a star, scientists can find its mass.
By combining the two measurements, scientists can get a sense of the makeup of the planet. Perhaps it’s a big-but-airy planet made mostly out of gas like Jupiter, or a small, dense, rocky planet like Earth.
These analyses had been done for individual planets, but much more rarely for the entire known population of such planets in the Milky Way galaxy. As the scientists looked at the numbers—43 planets in all—they saw a surprising picture emerging.
“I was shocked when I saw this analysis—I and a lot of people in the field assumed these were all dry, rocky planets.”
The densities of a large percentage of the planets suggested that they were too light for their size to be made up of pure rock. Instead, these planets are probably something like half rock and half water, or another lighter molecule. Imagine the difference between picking up a bowling ball and a soccer ball: they’re roughly the same size, but one is made up of much lighter material.
Searching for water worlds
It may be tempting to imagine these planets like 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 gaseous phase, which would enlarge their radius. “But we don’t see that in the samples,” explained Luque. “That suggests the water is not in the form of surface ocean.”
Instead, the water could exist mixed into the rock or in pockets below the surface. Those 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 a lot of people in the field assumed these were all dry, rocky planets,” said UChicago exoplanet scientist Jacob Bean, whose group Luque has joined to conduct further analyses.
The finding matches a theory of exoplanet formation that had fallen out of favor in the past few years, which suggested that many planets form farther out in their solar systems and migrate inward over time. Imagine clumps of rock and ice forming together in the cold conditions far from a star, and then being pulled slowly inward by the star’s gravity.
Though the evidence is compelling, Bean said he and the other scientists would still like to see “smoking gun proof” that one of these planets is a water world. That’s something the scientists are hoping to do with JWST, NASA’s newly launched space telescope that is the successor to Hubble.
The majority of the research was performed as Luque’s Ph.D. thesis at the Institute of Astrophysics of the Canary Islands.Some scientists think that some of the molecules important to life may be produced outside the Earth. Instead, they suggest that these ingredients came from meteorites or comets.
When did life on Earth begin?
Earth is about 4.5 billion years old. Scientists think that by 4.3 billion years ago, Earth may have developed conditions suitable to support life. The oldest known fossils, however, are only 3.7 billion years old. During that 600 million-year window, life may have emerged repeatedly, only to be snuffed out by catastrophic collisions with asteroids and comets.
The details of those early events are not well preserved in Earth’s oldest rocks. Some hints come from the oldest zircons, highly durable minerals that formed in magma. Scientists have found traces of a form of carbon—an important element in living organisms—in one such 4.1 billion-year-old zircon. However, it does not provide enough evidence to prove life’s existence at that early date.
Where did life on Earth begin?
Two possibilities are in volcanically active hydrothermal environments on land and at sea.
Some microorganisms thrive in the scalding, highly acidic hot springs environments like those found today in Iceland, Norway and Yellowstone National Park. The same goes for deep-sea hydrothermal vents. These chimney-like vents form where seawater comes into contact with magma on the ocean floor, resulting in streams of superheated plumes. The microorganisms that live near such plumes have led some scientists to suggest them as the birthplaces of Earth’s first life forms.The recipe consists of a steady energy source, organic compounds and water.
Sunlight provides the energy source at the surface, which drives photosynthesis. On the ocean floor, geothermal energy supplies the chemical nutrients that organisms need to live.
Also crucial are the elements important to life. For us, these are carbon, hydrogen, oxygen, nitrogen, and phosphorus. But there are several scientific mysteries about how these elements wound up together on Earth. For example, scientists would not expect a planet that formed so close to the sun to naturally incorporate carbon and nitrogen. These elements become solid only under very cold temperatures, such as exist in the outer solar system, not nearer to the sun where Earth is. Also, carbon, like gold, is rare at the Earth’s surface. That’s because carbon chemically bonds more often with iron than rock. Gold also bonds more often with metal, so most of it ends up in the Earth’s core. So, how did the small amounts found at the surface get there? Could a similar process also have unfolded on other planets?
The last ingredient is water. Water now covers about 70% of Earth’s surface, but how much sat on the surface 4 billion years ago? Like carbon and nitrogen, water is much more likely to become a part of solid objects that formed at a greater distance from the sun.
To explain its presence on Earth, one theory proposes that a class of meteorites called carbonaceous chondrites formed far enough from the sun to have served as a water-delivery system
