Small bodies in the solar system

Small bodies in the solar system The fine-tuning of planet Earth to life, as we know it is tightly linked to the formation of our Solar System. We know of more than 5000 extrasolar planetary systems within our galaxy, with many more to be discovered with future space missions like PLATO. How do these planetary systems form? How do they evolve? The solar system itself provides unique stepping stones to help answering these questions. During the formation of the solar system from a planet-forming disk orbiting a new born star, not all material ended up in gaseous or rocky planets. The leftover planetesimals form the asteroid belt between Mars and Jupiter, the Kuiper belt at a distance of 30 to 50 au from the Sun, and the Oort cloud at a distance of 2000 to 200.000 au around our solar system. The asteroid belt is made of large rocks, the Kuiper belt and the Oort cloud contain cold icy objects that may get gravitationally kicked out, and dive towards the Sun, hence appearing as comets in our night sky.

The interaction of such an icy object with solar radiation and the solar wind gives rise to various phenomena. Irradiation heats up the object and, when crossing the orbit of Jupiter, the received energy is enough to make volatiles sublimate and atmospheric outgassing of the object starts. It results a comet with its bright nucleus and two tails behind it, one made of dust, the other made of ionised neutrals or plasma.

In principle, the asteroids are inert and show little interaction with the solar wind, except for space weathering, i.e., the chemical changing of the surface due to the impact of the solar wind particles. Some asteroids have been observed to display cometary behaviour, where (multiple) gas jets are ejected from the surface. The boundary between comets and asteroids may, hence, not be well defined.

Next to these big groups of small bodies, there are some other regions in our solar system where these tracers of the pre-planet-formation phase of our solar system still exist. Jupiter has collected asteroids (the Trojans) which gather around the L4 and L5 Lagrange points. Mars has two small moons, Phobos and Deimos, which might be captured asteroids. Investigating small objects in the solar system with space missions will enable us to study these witnesses of planet formation in situ. Specific challenges include their very low gravity, hence, landing will require special technological techniques. Open questions that are still to be answered include their chemical compositions, which type of comets could be the water source for the Earth’s oceans, or do Phobos and Deimos originate from a Mars impact or are they captured asteroids. Two space missions to small bodies in our solar system are presently under development: ESA/JAXA’s Comet Interceptor is a three-spacecraft mission to a dynamically new comet (i.e. on its first approach of the Sun). NASA’s Psyche mission was launched on 13October 2023 and will study the metallic asteroid Psyche. The Alpbach Summer School 2025 will develop more mission concepts to explore the small bodies in our solar system.