Astronomers spot larger planets around other stars more often than smaller ones. Whether they measure the pull of an exoplanet on its host star, observe how much starlight an exoplanet blocks, or take a picture of an exoplanet itself, exoplanet observation methods bias towards planets 2 times the mass of Earth, that’s 12 septillion kilograms or larger. But astronomers know that small planets are out there–it’s just that they are harder to find, requiring more precise instruments the smaller the planet.
Astronomers refer to planets smaller than the Earth as sub-Earths or minor planets. Current telescopes aren’t good at finding these small planets, so astronomers rely on simulations to determine how they will behave. One team of astronomers studied the conditions for hypothetical planetary systems with only minor planets. They claimed that understanding where minor planets could crop up in abundance would give scientists a better sense of how common this kind of planet is.
To get a representative sampling of reasonable conditions for planetary systems to form, the astronomers utilized a simulation code that produces models of exoplanets that are similar to actual observations. Using this code, the team ran 33 sets of 1,000 simulations, each set with different starting parameters. They simulated systems with stars between ½ to 5 times the Sun’s mass because most Milky Way stars are in that size range. They ran all but the last 2 sets of simulations for 1 billion years of in-simulation time.
The first set was their point of comparison. It demonstrated that given the same conditions as our solar system, where we know planets smaller than Earth form, the code produced systems with minor planets. In the next 8 sets they varied the host star’s mass, how spread out the mass in the starting disc of matter was, and the ratio of the systems’ gas and dust. After that, the astronomers ran 4 sets where they changed how long the minor planets could accumulate new material, ranging from 320,000 years to 32 million years. The team did another 16 sets that varied how much dust the system had to start, ranging from exactly as much mass as is in the Earth to 10,000 times Earth’s mass worth of dust.
The astronomers’ last 4 sets of simulations varied by how massive the host star was, from 1.5 to 5 times the Sun’s mass. They ran the 2 largest sets over shorter timescales than the rest because larger stars burn through their fuel faster than smaller stars and have a shorter lifespan. When a star’s life ends, it expands, sometimes quite dramatically. The scientists used these sets to find scenarios where stars engulfed their minor planets as they expanded and scenarios where they survived.
The team noted that computing power limited the scope of their simulations, as they could not run calculations with more than 1,000 objects at a time in a given system. They also did not allow ice and rocks to accumulate at the edges of a star system, like they do in real star systems. They stated that these factors limited the accuracy of their models for the planet formation process and long-term system dynamics, respectively.
Overall, the team found that minor planets should be quite abundant in the universe. Under the parameters they studied, they found that systems consisting entirely of planets between 1 and 1 10-millionth of Earth’s mass “are readily formed.” They suggested that estimates of how often planets form around stars may substantially underestimate their actual frequency.
The astronomers identified that the most crucial factor in determining how large minor planets could get was the amount of dust available for them to initially form. However, they also found systems with only minor planets would stop forming if the initial dust available exceeded 100 times the Earth’s mass. Their last conclusion dealt with the outermost minor planets in a given system, 10 times the distance from the Earth to the Sun or greater. They found that these planets rarely grew larger than a small moon, but they survived their star’s inevitable expansion, potentially persisting for billions of years after expansion.
