New theory explains why super-Earths have the same size
What's the story
Scientists have come up with a new theory that can explain the formation of "super-Earths"- a class of exoplanets bigger than the Earth. Super-Earths are the most common type of exoplanet in the Milky Way. The latest study helps us better understand why super-Earths within a single planetary system often end up looking similar in terms of size.
Context
Why does this story matter?
Super-Earths are among the most sought-after class of exoplanets and are unlike any other planets in our solar system. They can measure anywhere between twice the size of Earth and up to 10 times its mass. The possibility that these alien planets can possess an atmosphere makes them a suitable candidate in the hunt for life beyond our planet.
Beginning
How do planetary systems begin their lifecycle?
Planetary systems start as large spinning disks of gas and dust that amalgamate over the period of a few million years or so. Most of the gas in this disk accretes to form the star—or the parent body—at the center of the system, while solid material gradually coalesces into planets, asteroids, comets, and moons.
Solar system
Our solar system has two distinct types of planets
Talking about our solar system, there are two distinct types of planets: inner small rocky planets closest to the Sun and outer larger water- and hydrogen-rich gas giants. A 2021 study explains that planet formation in our solar system occurred in two distinct rings in the protoplanetary disk: inner one where small rocky planets formed and outer one for more massive icy planets.
Observations
Recent observations demonstrate that most super-Earths are rocky
According to a former model developed by scientists, super-Earths formed in the "icy part" of the protoplanetary disc and migrated near the central star. This model could explain the masses and orbits of super-Earths but assumed that all are water-rich. However, recent observations suggest something different: most super-Earths are found to be rocky, like our Earth, even if they contained a hydrogen atmosphere.
Information
Super-Earths are often found orbiting close to their stars
Some super-Earths have a gas giant-like appearance, which is because of the hydrogen atmosphere. They are often found orbiting close to their host stars, and it is believed that they drifted to their current location from more distant orbits.
Findings
Super-Earths were found to share similar sizes and other features
Here's where the story gets interesting. Over the past five years scientists, including those from Caltech, the University of Notre Dame, and UCLA, studied these exoplanets and discovered something unusual. While there is a wide variety of super-Earths, all of the super-Earths within a single planetary system tend to be similar in terms of orbital spacing, size, mass, and other key features.
Explanation
Scientists turned to a 2020 study to gain insights
To better understand this process, scientists turned to a study published in 2020 that hypothesized the formation of Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto. The theory suggests that for a specific size range of dust grains, the force dragging the grains toward Jupiter and the force carrying those grains in an outward flow of gas canceled each other perfectly.
2020 theory
The theory explains how similarly sized planets might form
This balance in forces created a ring of material that served as building blocks for the subsequent formation of the Jovian moons. Further, the theory propounded that bodies would grow in the ring until they become large enough to exit the ring, due to gas-driven migration. After that, the growth stops and this explains why the process produces bodies of similar sizes.
Current study
Planetary formation was hypothesized to occur in a similar manner
Now, the new study suggests that the mechanism governing planetary formation around stars is predominantly the same. In this case, large proportions of solid rocky material occur at a narrow region in the protoplanetary disk called the silicate sublimation line where silicate vapors condense to form solid rocky pebbles, which give rise to the formation of similarly sized planets.
Basis
The study is based on an assumption
This latest theory is based on the assumption that the solid material is dispersed throughout the protoplanetary disk. If there was a lot of mass in the ring, then planets would grow until they migrated away, resulting in a network of similar super-Earths. And if the ring contained little mass, it would produce a system that looks much like our solar system's terrestrial planets.