Today the number number of confirmed exoplanets stands at 5,197 in 3,888 planetary systems, with another 8,992 candidates awaiting confirmation. The majority have been particularly massive planets, ranging from gas giants the size of Jupiter to Neptune, which have radii about 2.5 times that of Earth. Another statistically significant population has been the rocky planets which are about 1.4 Earth radii (“Super-Earths”). This presents a mystery to astronomers, especially when it comes to exoplanets discovered by the venerable Kepler space telescope.
Among the more than 2,600 planets, Kepler discovered, there is an apparent rarity of exoplanets with a radius around 1.8 times that of Earth – which they call the “radius valley”. A second mystery, known as “peas in a pod”, refers to neighboring planets of similar size found in hundreds of harmoniously orbiting planetary systems. In a study conducted 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 accounts for the interaction of forces acting on nascent planets which could explain these two mysteries .
The research was led by André Izidoro, a Welch postdoctoral fellow with the NASA-funded CLEVER Planets project. He was joined by fellow CLEVER Planets researchers Rajdeep Dasgupta and Andrea Isella, Hilke Schlichting from the University of California, Los Angeles (UCLA), and Christian Zimmermann and Bertram Bitsch from the Max Planck Institute for Astronomy (MPIA). As they describe in their research paper, which recently appeared in the Astrophysical Journal Lettersthe team used a supercomputer to run a planetary migration model that simulated the first 50 million years of planetary system development.
In their model, protoplanetary disks of gas and dust also interact with migrating planets, bringing 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 that we are the first to explain the valley of the radius using a model of the formation and dynamic evolution of planets which takes into account in a coherent way the multiple constraints of observations. We are also able to show that a model of planet formation incorporating 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 disturbance of TRAPPIST-1’s seven-planet system. In an article published on November 21, 2021, in natural astronomy, they used a many-body simulation to show how this “pea-in-a-pod” system might have retained its harmonious orbital structure despite collisions caused by planetary migration. This allowed them to impose constraints on the upper limit of collisions and the mass of the objects involved.
Their results indicate that the collisions in the TRAPPIST-1 system were comparable to the impact that created the Earth-Moon system. Said Izidoro:
“The migration of young planets to their host stars creates overpopulation and frequently results in cataclysmic collisions that strip planets of their hydrogen-rich atmospheres. This means that giant impacts, like the one that formed our moon, are likely a generic result of planet formation.
This latest research suggests that planets come in two variants, consisting of dry, rocky planets that are 50% larger than Earth (super-Earths) and planets rich in water ice about 2.5 times larger. the size of the Earth (mini-Neptunes). Additionally, they suggest that a fraction of planets twice the size of Earth will retain their primordial hydrogen-rich atmosphere. and be rich in water. According to Izidoro, these results are consistent with new observations that suggest super-Earths and mini-Neptunes are not exclusively dry, rocky planets.
These discoveries present opportunities for exoplanet researchers, who will rely on the James Webb Space Telescope to make detailed observations of exoplanet systems. With its advanced suite of optics, infrared imaging, coronagraphs and spectrometers, Webb and other next-generation telescopes will characterize exoplanet atmospheres and surfaces like never before.
This article was originally published on Universe today by Matt Williams. Read the original article here.
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