Today, the number of confirmed exoplanets is 5,197 in 3,888 planetary systems, with another 8,992 candidates awaiting confirmation.
Most have been particularly massive planets, ranging from gas giants the size of Jupiter and Neptune, which have radii about 2.5 times that of Earth.
Another statistically significant population has been rocky planets measuring about 1.4 Earth radii (also known as “super-Earths”).
This presents a mystery to astronomers, especially when it comes to exoplanets discovered by the venerable Kepler space telescope.
Of the more than 2,600 planets that Kepler discovered, there is an apparent rarity of exoplanets with a radius of about 1.8 times that of Earth, referred to as the “radius valley.”
An illustration depicting the paucity of exoplanets about 1.8 times the size of Earth that were observed by NASA’s Kepler spacecraft. (A. Izidoro/University of Rice)
A second mystery, known as “peas in a pod,” concerns neighboring planets of similar size found in hundreds of planetary systems with harmonious orbits.
In a study led by Rice University’s Cycles of Life-Essential Volatile Elements in Rocky Planets (CLEVER) project, an international team of astrophysicists provides a new model that explains the interplay of forces acting on newborn planets that could explain these two mysteries .
The research was led by André Izidoro, Welch’s postdoctoral fellow in Rice’s NASA-funded CLEVER Planets project. They were joined by CLEVER Planets researchers Rajdeep Dasgupta and Andrea Isella, Hilke Schlichting of the University of California, Los Angeles (UCLA), and Christian Zimmermann and Bertram Bitsch of the Max Planck Institute for Astronomy (MPIA).
As they describe in their research paper, which recently appeared in the Astrophysical Journal Letters, the team used a supercomputer to run a planetary migration model that simulated the first 50 million years of the planetary system’s development.
In their model, protoplanetary disks of gas and dust also interact with migrating planets, pulling them closer to their parent stars and locking them into resonant orbital chains.
Within a few million years, the protoplanetary disk disappears, breaking the chains and causing orbital instabilities that cause two or more planets to collide. While planetary migration models have been used to study planetary systems that retained orbital resonances, these findings represent a first for astronomers.
As Izidoro said in a statement from Rice University: “I believe we are the first to explain the radius valley using a model of planet formation and dynamical evolution that consistently accounts for multiple observational constraints.
“We can also show that a model of planet formation that incorporates giant impacts is consistent with the pea-in-a-pod feature of exoplanets.”
This work builds on previous work by Izidoro and the CLEVER Planets project. Last year, they used a migration model to calculate the maximum disruption of TRAPPIST-1’s seven-planet system.
In a paper appearing Nov. 21, 2021, in Nature Astronomy, they used N-body simulations to show how this “pea-in-a-pod” system could have preserved its harmonic orbital structure despite collisions caused by the planetary migration. This allowed them to put constraints on the upper limit of collisions and the mass of the objects involved.
Their results indicate that collisions in the TRAPPIST-1 system were comparable to the impact that created the Earth-Moon system.
Izidoro said: “The migration of young planets towards their host stars creates overcrowding and often leads to cataclysmic collisions that strip the planets of their hydrogen-rich atmospheres.
“This means that giant impacts, like the one that formed our moon, are probably a generic result of planet formation.”
This latest research suggests that planets come in two varieties, consisting of dry, rocky planets that are 50 percent larger than Earth (super-Earths) and planets that are about 2.5 times as rich in water ice the size of Earth (mini-Neptune).
They also suggest that a fraction of planets twice the size of Earth will retain their primordial hydrogen-rich atmospheres and be rich in water.
According to Izidoro, these results are consistent with new observations that suggest that super-Earths and mini-Neptunes are not exclusively dry, rocky planets.
These findings present opportunities for exoplanet researchers, who will rely on the James Webb Space Telescope to conduct detailed observations of exoplanet systems.
Using its advanced suite of optics, infrared imagers, coronagraphs and spectrometers, Webb and other next-generation telescopes will characterize the atmospheres and surfaces of exoplanets like never before.
This article was originally published by Universe Today. Read the original article.