Exoplanets smaller than Neptune are common around red dwarf (M-dwarf) stars, and many of those that transit their host stars constitute the bulk of known temperate worlds ideal for atmospheric characterization. However, whether the small exoplanets surrounding red dwarfs are potentially habitable remains unclear, partly due to our lack of understanding of their composition. In new research, University of Chicago’s Dr. Rafael Luque and Dr. Enric Pallé from the Institute of Astrophysics of the Canary Islands and the University of La Laguna analyzed the masses and radii of all known small transiting exoplanets around red dwarfs, identifying three populations: rocky, water-rich, and gas-rich.
An artist’s impression of the giant exoplanet Kepler-1704b. Image credit: Sci.News.
Exoplanets which transit red dwarf stars intercept a large fraction of the stellar disk, making them potentially suitable targets for transmission spectroscopy studies.
The habitable zones of planetary systems around red dwarfs are located close to the host star, increasing the chance of transits occurring.
Whether small planets around red dwarfs are potentially habitable remains unclear, in part due to incomplete knowledge of their composition.
“The two different ways to discover planets each give you different information,” Dr. Pallé said.
“By catching the shadow created when a planet crosses in front of its star, we can find the diameter of the planet.”
“By measuring the tiny gravitational pull that a planet exerts on a star, we can find its mass.”
“By combining the two measurements, we can get a sense of the makeup of the plane.”
“Perhaps it’s a big-but-airy planet made mostly out of gas like Jupiter, or a small, dense, rocky planet like Earth.”
In their study, Dr. Luque and Dr. Pallé looked at 43 planets with a radius smaller than 4 Earth radii in 26 planetary systems from the Transiting M-dwarf Planets catalogue.
They discovered that the planets orbiting red 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.
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. That suggests the water is not in the form of surface ocean,” Dr. Luque said.
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.
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.
“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,” the astronomers said.
The team’s paper was published in the journal Science.
_____
Rafael Luque & Enric Pallé. 2022. Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars. Science 377 (6611); doi: 1211-1214; doi: 10.1126/science.abl7164
