Density separates the rocky and water-rich exoplanets that orbit red dwarf stars
Density measurements of small exoplanets reveal three distinct populations – gassy, rocky and water-rich planets – that can form around a red dwarf star, according to a new study. Small planets are common around red dwarf stars (M dwarfs), and many of those that transect their host star constitute the bulk of known exoplanets ideal for atmospheric characterization via transmission spectroscopy and represent some of the best places to search for signatures of life. However, whether the small worlds surrounding M dwarfs are potentially habitable remains unclear, partly due to our lack of understanding of their composition. Moreover, studying these distant planets from Earth is challenging because of the faint red light their stars emit. In general, the radii of small exoplanets are known to have a bimodal distribution, which has been interpreted as two separate rocky and gassy populations, planets with a thin or thick hydrogen-helium atmosphere, respectively. Using radius and mass measurements from 34 newly detected planets found around closer M dwarf stars from the Transiting Exoplanet Survey Satellite (TESS), Rafael Luque and Enric Pallé show that the planets’ densities provide a cleaner distribution of planet types. Differing from previous understanding, Luque and Pallé discovered that the planets orbiting M dwarfs fall into three different density categories – rocky, gassy and watery exoplanets. According to the findings, the third population matches the density predictions from a planetary model of 50% rock and 50% water. The authors suggest that these water-rich planets likely form with ice and rock far away from the host star before migrating into a closer orbit with the host star. “Although the presence of watery small exoplanets is particularly enticing, all three types of planets around red dwarfs could present potentially habitable conditions for life,” writes Johanna Teske in a related Perspective. “Leaving aside this possibility for discovering alien life-forms, measuring the compositional diversity of planets around red dwarf stars – the most common type of star in the Milky Way – is important for piecing together the complex puzzle of small planets’ formation and evolution.”
JOURNAL
Science
ARTICLE TITLE
Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
ARTICLE PUBLICATION DATE
9-Sep-2022
Surprise finding suggests ‘water worlds’
are more common than we thought
Analysis finds evidence for many exoplanets made of water and rock around small stars
Peer-Reviewed PublicationWater is the one thing all life on Earth needs, and the cycle of 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 search 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—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.
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.
JOURNAL
Science
ARTICLE TITLE
Density, not radius, separates rocky and water-rich planets orbiting M dwarf stars
ARTICLE PUBLICATION DATE
8-Sep-2022
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