2.1: The Origin of Earth and Our Solar System

The prevailing scientific explanation for the origin of the Earth can be applied to the formation of the Sun and all the other planets. So, it’s not only Earth's story but the story of the origin of the whole solar system. Not only that, but our Sun is one star among a hundred million in our galaxy, and our galaxy is one of perhaps a hundred million in the universe. So the lessons we learn by studying our solar system can likely be applied more generally to the formation of other solar systems elsewhere, including those long ago, in galaxies far, far away - the present is the key to the past.

Using powerful telescopes, we observe many other stars, but we also observe other things, including fuzzy-looking "stars" called nebulae. A nebula is a massive cloud of gas and dust in space that is the byproduct of a previous star that suffered a supernova explosion - its leftover material. Orion's "belt" is part of a constellation of stars easily identifiable in the northern hemisphere’s winter night sky. Below the row of three stars of the belt is a smaller trio of light spots; this is Orion’s sword scabbard. Binoculars and a dark enough sky will let you examine these objects for yourself; you will discover that the middle point of light in this smaller trio is not a star. It is a nebula called Messier 42.

Figure \(\PageIndex{1}\): The Messier 42 nebula, shown in the context of the “scabbard” of the constellation Orion.

Nebulae like Messier 42 are common features of the galaxy, but not as common as stars. Nebulae appear to be short-lived features, as matter is often attracted to other matter. Particles attract one another under the influence of forces such as electrostatic attraction or "static cling". This is the same force that turns bits of dust or fabric into larger "dust bunnies" under your couch!

Figure \(\PageIndex{2}\): Three dust bunnies under the couch: a household hassle, or a glimpse into the birth of the solar system?

Electrostatic forces are effective at attracting small particles over short distances. However, if you want to make big things like planets and stars out of a nebula, gravity needs to take over at some point. Although gravity is a relatively weak force - you overcome the force of gravity every time you pick up your foot to take a step - gravity can work very efficiently over distance if the masses involved are large enough. So static cling initially organized matter into "space dust bunnies" that once large enough, were pulled together by gravity. The net result is that the countless tiny pieces of the nebula were drawn together, swirling into a denser and denser amalgamation (clump). The nebula began to spin, flattening from top to bottom and forming a rotating disk, like a fried egg with the yolk surrounded by the white.

Figure \(\PageIndex{3}\): An artist’s conception of an oblique view of a protoplanetary disk, using actual photoimagery of the HL Tauri disk, originally gathered by the European Southern Observatory.

Once a star forms in the center, astronomers call the ring of debris around it a protoplanetary disk. For our solar system, this happened 4.55 billion years ago.

Figure \(\PageIndex{4}\): This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA.

Two important processes that helped further organize the protoplanetary disk and form planets were condensation and accretion.

Condensation is a process that allows gaseous matter to stick together to make liquid or solid matter. We have evidence of condensation in the form of small spherical objects with internal layering, kind of like “space hailstones.” These are chondrules, and they represent the earliest objects formed in our solar system. (Occasionally, we are lucky enough to find chondrules that have survived until today, entombed inside certain meteorites.)

Figure \(\PageIndex{5}\): Chondrules in a meteorite, with a scale in mm.

Chondrules glommed onto other chondrules and stuck themselves together into primordial “rocks,” building up larger and larger objects. Eventually, these objects got to be big enough to have sufficient gravity to pull their mass into a round shape, forming planetesimals, the nuclei of planets. Planetesimals gobbled up nearby asteroids and smashed into other planetesimals, merging and growing through time through accretion. The kinetic force of these collisions heated the rocky and metallic material of the planetesimals, and their temperature also went up as radioactive decay heated them from within. Once warm, differentiation could happen, where denser material sank to their centers (or cores), and lighter-weight elements and compounds rose to their surface (or outer layers).

Figure \(\PageIndex{6}\): A cartoon model showing the evolution of our solar system from a pre-solar nebula.

Metallic iron meteorites, such as the Tamentit meteorite below, are used as evidence of the differentiation of planetesimals into layered bodies. Meteorites with metallic compositions represent the “core” material from these planetesimals.

Figure \(\PageIndex{7}\): Tamentit Iron Meteorite, found in 1864 in the Sahara, weighing about 500 kg (1,100 lb). From Ji-Elle, Public domain, via Wikimedia Commons.

The process of accretion continues into the present day, though at a slower pace than the earliest days of the solar system. You can observe this in the asteroid belt, where certain asteroids are simply a clump of space rocks, held together by their gravity. Consider the asteroid called Itokawa 25143, for instance which is made up of numerous uniquely-sized boulders rather than of a single solid body.

Figure \(\PageIndex{8}\): The asteroid 25143 Itokawa.

Another modern example of accretion is meteorite impacts. Every time a chunk of rock in space intersects the Earth, its mass is added to that of the planet. In that instant, the solar system gets a little bit cleaner (fewer leftover bits rattling around) and the planet gets a little more massive. A spectacular example of this occurred in 1994 with Comet Shoemaker-Levy 9 when Jupiter’s immense gravity broke the comet into chunks, then swallowed them up one after another. Astronomers on Earth watched with fascination as the comet chunks, some more than a kilometer across, slammed into Jupiter’s atmosphere at 60 km/second (~134,000 mph), creating a 23,700 ^oC fireball and enormous impact scars that were as large as the entire Earth. These scars lasted for months.

Figure \(\PageIndex{9}\): Accretion in action: scar from the July 1994 collision between a fragment of Comet Shoemaker-Levy 9 and the surface of the atmosphere of the planet Jupiter.

This incredibly dramatic event reminded Earthlings that they are not safe from accretionary impacts, just as the dinosaurs discovered some 66 million years ago. In this case, an object from the solar system was removed while adding a bit to Jupiter's mass.

In summary, Earth is part of a solar system centered on the Sun. This solar system, with its star, planets, dwarf planets, and “leftover” comets and asteroids, formed from a nebula full of elements in the form of gas and dust. Over time, these very small pieces stuck together to make larger concentrations of mass, eventually culminating in a star and its orbiting planets. Asteroids (and asteroids that fall to Earth, called meteorites) are leftovers from this process. The original nebula formed from the destruction of some previous star that exploded in a supernova - a galactic-sized example of recycling!

Key Terms

• accretion - increasing the mass of an object by adding material to it
• chondrules - small grains found in certain types of meteorites formed in the early solar nebula as molten droplets quickly cooled and crystallized
• differentiation - material settling out according to density
• nebula - a massive cloud of gas and dust in space that is the byproduct of a previous star that suffered a supernova explosion
• planetesimals - the nuclei of planets formed in the early solar system from collisions with other objects in the solar system
• protoplanetary disk - ring of debris around a star