Planetary Formation

Some Star Systems Create a Planet Sandwich

A recent study presented at the National Astronomy Meeting 2023 (NAM2023) examines a newly discovered planetary formation theory that challenges previous notions on how planets are formed in the disks of gas and dust surrounding young stars, also known as protoplanetary disks. Along with being presented at NAM2023, the study has also been submitted for peer-review to the journal Monthly Notices of the Royal Astronomical Society and holds the potential to help scientists better understand not only how planets form, but how life could form on them, as well.

Based on data from observations and models, the researchers determined that two large protoplanets—planets that are still forming from gathering dust throughout the disk, also known as accretion—orbiting at an indeterminate distance from each other can produce a smaller planet orbiting between them, which the researchers refer to as “sandwiched planet formation”.

The researchers state this is due to the two larger planets limiting the amount of dust that flows into the inner portions of the protoplanetary disk since they both will continue to collect it. When this happens, any planet that forms between them will be smaller than the two larger planets, which is analogous to the filling inside of a sandwich.

“This is very different to the conventional view of planet formation, where we typically expect that the planets form sequentially from the inside to the outside of the disk and get more and more massive further out,” Dr. Farzana Meru, who is an Associate Professor and Dorothy Hodgkin Fellow in the Department of Physics at the University of Warwick, said in a statement. “What is also really interesting is that there are examples that we have found from exoplanet observations that actually show this sandwiched planet architecture — where the middle planet is less massive than its neighbors; it is a reasonable proportion of the systems too.”

Artist rendition of the new “sandwiched planet formation” theory examined for this study. (Credit: University of Warwick/Mark A. Garlick; License Type: Attribution (CC BY 4.0))

Further studies are required to better understand this new formation process, but it could lead to developing plausible clarifications for the existence of Mars and Uranus, which are both smaller planets orbiting between larger planets in our own solar system. In the case of Mars, the larger Earth orbits inward while the much larger Jupiter orbits outward. In the case of Uranus, the much larger Saturn orbits inward while the similar-sized Neptune orbits outward. While the radius of Uranus is greater than Neptune, the former’s mass is smaller, and this is due to Neptune’s density being approximately 30 percent greater than Uranus.

Our solar system’s planets and their relative sizes to each other. Credit: NASA/Lunar and Planetary Institute

The current state of our solar system consists of a (mostly) neat and organized assemblage of planets, moons, asteroids, and comets. This includes the Sun at the center, followed by the inner planets, asteroid belt, outer planets, Kuiper Belt, and Oort Cloud, but things were much different approximately 4.6 billion years ago.

This was when scientists hypothesize a massive cloud of gas and dust collapsed from the shockwave of a nearby supernova and formed into a solar nebula, which consists of a spinning disk of gas and dust. As this disk rotates, gravity begins to take over and continues to pull material towards the center of the disk, eventually forming our Sun, which contains 99 percent of the entire mass of the solar system, and a protoplanetary disk is born.

Over millions of years, the terrestrial planets of the inner solar system and large gas giants of the outer solar system—along with their many moons—are formed from the accretion of the gas and dust throughout the disk. This includes Mars possibly being sandwiched by Earth and Jupiter, and Uranus possibly being sandwiched by Saturn and Neptune. The remaining gas and dust formed the asteroids and comets that make up the asteroid belt, Kuiper Belt, and Oort Cloud.

What new discoveries will scientists make about protoplanetary disks and “sandwiched planet formation” in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!

Laurence Tognetti

Laurence Tognetti is a six-year USAF Veteran who earned both a BSc and MSc from the School of Earth and Space Exploration at Arizona State University. Laurence is extremely passionate about outer space and science communication, and is the author of “Outer Solar System Moons: Your Personal 3D Journey”.

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