The formation of our solar system was a process of destruction!

How are planets formed?

When a star first forms, it is surrounded by a swirling disk of gas and dust. Over billions of years, this gas and dust gradually clumps together to form larger and larger objects, eventually becoming a “mature” large planetary system in stable orbits. This process of small bodies colliding to form planets is called “accretion.” During the final stages of planet formation, large, growing planets can collide with each other! These giant impacts can have a variety of outcomes, creating new planets with different properties or even completely destroying the colliding planet. Learning more about these different scenarios can give us a glimpse into the origins of our solar system and its planets. For example, it’s widely believed that our own moon formed from the debris of such a collision! Understanding giant impacts also helps us understand telescope observations of planets around other stars, which can provide more context for our search for life in the universe.

How do we understand planet formation?

In science, “modeling” doesn’t mean showing off the latest fall fashions, it means running computer simulations. When trying to unlock the keys to the universe, computer models are the first step to understanding the unknown. Crashing planets together at full size in the lab is… impractical. So we turn to computer simulations to study these extreme conditions on a planetary scale.

Build the model step by step

If you want to build a computer model of a planetary collision, where do you start? First, scientists study the physics equations governing gravity to understand how planets orbit stars as they form. Computer-aided calculations of these “orbital dynamics” equations help guide impact modelers in understanding the types of collisions a planet might face as it grows to full size, such as impact velocities. Telescope observations are also key to focusing these orbital dynamics studies.

Through orbital dynamics calculations, experts have discovered that giant impacts during the formation of the Solar System mostly occurred at grazing angles and at relatively gentle speeds, but there are exceptions! Using equations that govern the behavior of rocks and fluids under extreme conditions, “Hydrocode” models (computer simulations) are the main tool for understanding giant impacts. From these simulations, scientists have discovered that the relatively gentle collision speeds we expect can produce an interesting result: two planets merge in a rotating manner, surrounded by a ring of debris (see Video 1). But don’t be misled, these “gentle” accretion methods can still occur at hypersonic speeds! At these large scales, gravity strongly controls the outcome. These collisions require Hour evolution!

Some collisions may be mere “near misses” (see Video 2), with both bodies escaping the collision almost unscathed, also known as a “hit-and-run” collision – avoiding accretion altogether. More rare destructive outcomes (Video 3) are also possible, but this is closely related to the details of how the planetary system develops. Machine learning and statistics will increasingly be used to find new ways to study the relationship between initial conditions (impact speed, angle, etc.) and outcomes by looking for patterns in simulation data.

Our Moon – a well-known collision relic

Since the Apollo era, it has been widely believed that the Moon was formed from a collision similar to the one in Video 1, an off-axis accretion event. So far, scientists have found that a Mars-sized body (named Theia) colliding with our proto-Earth seems to fit most of the data collected from orbiting spacecraft and Apollo samples returned from the Moon. However, the number of potential collision scenarios that seem to fit the data has increased over time and continues to be a rich area of ​​research. In the most popular hypothesis, Theia swept past the proto-Earth, creating a large volume of heated vapor and debris that formed a disk around the Earth, from which the Moon may have formed. However, recent improved simulations of this event suggest that the Moon may have formed directly from the debris, rather than accreting from a gaseous dust disk, challenging modern scientific thought. As models evolve, so does our understanding of the formation of the Earth and Moon, and potentially how we might choose to explore the Moon—justifying the need for continued improvement in computer simulations.

“With high-fidelity computer simulations, new experiments, and new samples brought back from the Moon and Mars, we can really start to uncover the stories that nature is sharing with us,” said Dr. Travis Gabriel, a physicist with the U.S. Geological Survey and lead author of a recent manuscript in the Annual Review titled The role of giant impacts in planet formation“Our recent results show that these giant impacts are very extreme events, and they really push the limits of our computer simulation tools.”

There are many ways to get to the same place

Sometimes in nature, you can look at something and get an idea of ​​how it formed. When you look at a flower in your garden, you can imagine its life cycle from seed to bloom. There’s only one way to get that flower. Decades of research, however, have shown that there are many different ways to form planets. For example, the graphic below shows the many different pathways for forming an iron-rich planet like Mercury: through catastrophic collisions, crash events, and series of crashes. In the Annual Reviews manuscript, the authors discuss how data from future missions, advances in computer simulations, and machine learning can help narrow down the formation mechanisms that led to the planets, moons, and asteroids we observe today.

What will happen in the future?

There is still so much to learn, which means there is more work to do! A promising area for future research is applying machine learning to physics simulation data to help improve models, generate new models, and help us use computer and scientific resources more efficiently to understand the world around us. There is also always a need for new data to constrain models and improved computer software. This will require new space missions and a lot of time coding and testing hydrology codes. Improving our understanding of how planets form will require collaboration among many different areas of expertise: engineers building the latest instruments for the Moon, Mars, and beyond; physicists, computer scientists, geologists, chemists, and astronomers. Whatever your interests, there are many different career paths to contribute to the study of our universe.