with a nascent star at its core. After another 50 million years or so, the center of this “protostar” was hot enough that hydrogen fusion began: the Sun was born. Within the disk of debris whirling around the infant star, planet formation began. Gases condensed onto dust and ices, and the ice and dust began to accrete and grow into the precursors of planets: planetesimals. These collided with each other, growing ever larger and more complex. The end result was the diverse suite of planetary bodies seen in the solar system today; planetary systems around other stars are beginning to display even more diversity. Three major questions emerge from this story of the formation and evolution of the solar system:

• What were the initial stages, conditions, and processes of solar system formation and the nature of the interstellar matter that was incorporated?

• How did the giant planets and their satellite systems accrete, and is there evidence that they migrated to new orbital positions?

• What governed the accretion, supply of water, chemistry, and internal differentiation of the inner planets and the evolution of their atmospheres, and what roles did bombardment by large projectiles play?

Planetary Habitats

As the solar system formed, at least one planetary body experienced a remarkable event: life began, proliferated, and developed to the point that humankind now ponders its own origins. Was the origin of life a unique event or was it repeated elsewhere in the solar system or in extrasolar planetary systems? What conditions are required? The fundamental question is broader than whether or not life exists or existed on one particular planetary body like Mars, Europa, or elsewhere. Rather, the question is how life came to exist at all. Although the mechanisms by which life originated are as yet unknown, the processes likely involve the simultaneous presence of organic compounds, trace elements, water, and sources of energy. Demonstrating that other planetary environments are abodes for life will help to elucidate the origins of Earth’s life. To explore this, the following questions about past and present planetary environments that could foster life need to be addressed:

• What were the primordial sources of organic matter, and where does organic synthesis continue today?

• Did Mars or Venus host ancient aqueous environments conducive to early life, and is there evidence that life emerged?

• Beyond Earth, are there contemporary habitats elsewhere in the solar system with necessary conditions, organic matter, water, energy, and nutrients to sustain life, and do organisms live there now?

Workings of Solar Systems

The solar system displays a rich panoply of planetary environments. The known planetary systems around other stars are beginning to display an even greater range of planetary architectures. Comprehending this diversity requires a detailed understanding of the physical and chemical properties and processes that shape planetary interiors, surfaces, atmospheres, rings, and magnetospheres. Relevant interior processes include, for example, chemical differentiation, core formation, and heat transfer throughout planetary history. Impact cratering, tectonism, and volcanism are important geologic processes that have shaped planetary surfaces. Planetary atmospheres hold a record of the volatile evolution of a planet and the interactions among surfaces, weather, and climate. Equally important is to understand the intricate balance of a planet with its environment, an environment crafted and maintained by the host star that dominates the planetary system. Host stars, such as the Sun, have their own life cycle much as planets do, and the changes during that cycle play a profound role in modifying the attendant planets. A variety of critical questions arise about how planetary systems function:

• How do the giant planets serve as laboratories to understand Earth, the solar system, and extrasolar planetary systems?

• What solar system bodies endanger Earth’s biosphere, and what mechanisms shield it?

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