How Science Figured Out the Age of Earth

A few hundred thousand years after the Big Bang, the Universe cooled to the point where atoms could form.

The age of the Earth is estimated to be 4.54 ± 0.05 billion  years (4.54 × 10 years ± 1%).

This age may represent the age of the Earth’s accretion, of core formation, or of the material from which the Earth formed.

This dating is based on evidence from radiometric age-dating of meteorites material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.

Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old. The oldest such minerals analyzed to date—small crystals of zircon from the Jack Hills of Western Australia—are at least 4.404 billion years old. Calcium–aluminium-rich inclusions—the oldest known solid constituents within meteorites that are formed within the Solar System—are 4.567 billion years old, giving a lower limit for the age of the Solar System.

In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, in an accretion disk.

Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes.

Accretion of galaxies

A few hundred thousand years after the Big Bang, the Universe cooled to the point where atoms could form. As the Universe continued to expand and cool, the atoms lost enough kinetic energy, and dark matter coalesced sufficiently, to form protogalaxies. As further accretion occurred, galaxies formed.

Indirect evidence is widespread. Galaxies grow through mergers and smooth gas accretion. Accretion also occurs inside galaxies, forming stars.

Over millions of years, giant molecular clouds are prone to collapse and fragmentation.

These fragments then form small, dense cores, which in turn collapse into stars.

    Accretion of planets

In the formation of terrestrial planets or planetary cores, several stages can be considered. First, when gas and dust grains collide, they agglomerate by microphysical processes like van der Waals forces and electromagnetic forces, forming micrometer-sized particles; 

Grains eventually stick together to form mountain-size (or larger) bodies called planetesimals. Collisions and gravitational interactions between planetesimals combine to produce Moon-size planetary embryos (protoplanets) over roughly 0.1–1 million years. Finally, the planetary embryos collide to form planets over 10–100 million years

The formation of terrestrial planets differs from that of giant gas planets, also called Jovian planets.

The particles that make up the terrestrial planets are made from metal and rock that condense in the inner Solar System. However, Jovian planets begin as large, icy planetesimals, which then capture hydrogen and helium gas from the solar nebula.

Credit: space.com

https://www.scientificamerican.com/article/how-science-figured-out-the-age-of-the-earth/

https://en.m.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System

https://en.m.wikipedia.org/wiki/Accretion_(astrophysics)

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