How do planets form?

When a star first forms, it is surrounded by a disk of swirling gas and dust. Over billions of years, this gas and dust gradually clumps together to form larger and larger objects, eventually becoming a “mature” system of large planets in stable orbits. This process, in which small bodies collide to form planets, is called ‘accretion’. In the final stages of planet formation, collisions can occur between large, growing planets themselves! These giant impacts have a variety of outcomes, creating new planets with different properties or even obliterating the colliding planets altogether. Learning more about these different scenarios lets us peek into the origin of our solar system and the planets within it. Our own Moon, for example, is widely thought to have been formed from the debris of such a collision! Understanding giant impacts also helps us make sense of telescope observations of planets around other stars, which can give us more context in our search for life in the cosmos.

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How do we learn about planetary formation?

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

Building models, step by step

If you want to build a computer model of a planet-scale collision, where do you even start? First, scientists look to physics equations regarding gravity to understand how planets orbit around stars during planet formation. Computer-aided calculations of these ‘orbital dynamics’ equations help guide impact modelers in understanding the types of collisions, for example impact velocities, that planets are likely to face as they grow to full size. Telescope observations are also key in focusing these orbital dynamics studies.

From orbital dynamics calculations, experts find that giant impacts during solar system formation are mostly at glancing angles and somewhat gentle velocities, but there are exceptions to every rule! Using the equations that govern the behavior of rocks and fluids under extreme conditions, ‘hydrocode’ models (computer simulations) are the primary tool used to understand giant impacts. From these simulations, scientists find that the relatively gentle collision velocities that we have come to expect produces an interesting result: a swirling merger of the two planets with a disk of debris surrounding them (see Video 1). But don’t be misled, these ‘gentle’ styles of accretion can still occur at hypersonic speeds! At these large scales, gravitational forces strongly control the outcome. These collisions take hours to evolve!

Some collisions can be just “glancing blows” (see Video 2) where the two bodies escape the collision mostly unscathed, also called a ‘hit-and-run’ collision – avoiding accretion altogether. More rare, destructive outcomes (Video 3) are also possible, but this depends intimately on the details of the growing planetary system. Machine learning and statistics will increasingly be used to find new ways to study the relationships between the initial conditions (impact velocities, angles, etc.) and the results by finding patterns in simulation data.

Our Moon – a popular collision remnant

Since the Apollo days, the Moon has generally been thought to have resulted from a collision much like that in Video 1, an off-axis accretionary 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 from Apollo samples returned from the Moon. However, the number of potential collision scenarios that seem to fit the data has grown over time and continues to be a rich area of research. In the most popular proposed scenario, Theia glanced the proto-Earth generating a large amount of heated vapor and debris, generating a disk around the Earth from which the Moon would have formed. Recent improved simulations of the event, however, indicate that the Moon may have formed directly from debris, instead of accumulating from a gassy dusty disk, challenging modern scientific thought. As models evolve, so does our understanding of Earth and Moon formation and potentially how we choose to explore the Moon – a testament to the need to continuously improve computer simulations.

“Armed with high-fidelity computer simulations, new experiments, and potentially new samples returned from the Moon and Mars, we really can begin to unearth the stories nature has to share with us.” says Dr. Travis Gabriel, USGS Physicist and first author of the recent manuscript in Annual Reviews entitled The Role of Giant Impacts in Planet Formation. “Our recent findings show that these giant impacts are such extreme events that they really push the limits of our computer simulation tools.”

There are many paths to get to the same place

Sometimes in nature, you can see something and understand how it formed. When looking at a flower in your garden, you can picture its lifecycle from seed to bloom. There’s only one way to get that flower. However, decades of research show that there are many ways to form a planet. For example, the figure below shows the many different pathways that can form an iron-rich planet like Mercury: through a catastrophic collision, a hit-and-run collision, and through a chain of hit-and-run collisions. In the Annual Reviews manuscript, the authors discuss how data from future missions, advancements in computer simulations, and machine learning can help narrow down what formation mechanisms are responsible for the planets, Moons, and asteroids that we observe today.

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What’s in store for the future?

There’s so much more to learn, which means there’s much more work to be done! One promising area of future research is applying machine learning to physics simulation data to help refine models, produce new models, and generally help us use our computer and scientific resources more efficiently to understand the world around us. There is also always a need for new data to constrain the models and refined computer software. This will require new space missions and lots of time spent coding and testing hydrocodes. Improving our understanding of how planets form will require cooperation between many different areas of expertise: engineers to build the latest instruments for the Moon, Mars, and beyond; physicists, computer scientists, geologists, chemists, and astronomers. Whatever your interest may be, there are many different career pathways to contribute to the study of our universe.