The earth formed between 4.5 and 4.6 billion years ago, at the same time as the other planets in the solar system were forming. How do we know this? By measuring the age of the earth’s oldest rocks and minerals, the earth as a whole, the solar system as a whole, and the Moon. All those ages converge on an age of the earth, and the solar system, of between 4.5 and 4.6 billion years. All these ages have been measured by analyzing radiometric isotopes such as U-Pb and Sm-Nd in samples of rocks and minerals – except for the age of the sun, which is based on theoretical models of how stars form and evolve, derived from the laws of physics.

Here are the lines of evidence that, combined, tell us how old the earth is:

The oldest rocks on earth: The oldest rock so far found on earth has an age of 4.0 billion years. We would not expect to find any rocks from the very beginning of earth history, because the earth appears to have gone through a largely molten stage soon after it formed. In addition, the earth is a dynamic planet that erodes, buries, metamorphoses, and recycles its rocks in ways described in the rock cycle. Few, if any, rocks from the earth’s original solid crust would be expected to still exist.

The oldest minerals on earth: There are some mineral grains in ancient sandstone from the continent of Australia that have radiometric ages going back to 4.3 billion years old. This indicates that when the sand was being deposited (which is though to be about 3.5 billion years ago by other age measurements), some rocks in the mountains undergoing erosion into sand were 4.3 billion years old.

The age of the earth’s crust and mantle, theoretically combined. The earth’s mantle and crust can be considered a “bulk rock,” distinct from the partly molten core, which is made mostly of iron. Adding up the isotope ratios of uranium and lead from the earth’s crust and mantle as a whole gives isotope ratios that converge at an age of between 4.5 and 4.6 billion years ago. This age is based on some assumptions about how the crust formed from melted mantle rocks and evolved separately from the mantle over the course of earth history, so it is an indirect age measurement rather than a direct one like the age of a single rock or mineral.

The evidence that the earth formed as part of the solar system. The ratios of isotopes and chemical elements of all components of the solar system that we have been able to measure – Sun, several planets, earth’s Moon, asteroids, and comets – show the same characteristics, the same “fingerprint,” which indicates they all formed from the same batch of chemical elements. This batch of chemical elements is different from the ratios of isotopes and chemical elements that other star systems are made of, which shows that these components of the solar system share a common origin.

The ages of the oldest meteorites, taken as representing the first stage in the formation of the planets. Most meteorites are pieces of asteroids that have crashed to earth. The oldest meteorites have ages between 4.6 and 4.5 billion years. The asteroids, and these oldest meteorites, are thought to be pieces of planetesimals. Planetesimals are small, solid bodies that formed early in solar system history, most of which combined together to become the planets. Asteroids are the remains of planetesimals that did not condense into a larger planet, and thus have not changed much since that early stage of solar system history. One type of meteorite, known as chondrites, is thought to represent the first type of solid that condensed during the initial formation of the solar system, and chondrites have ages close to 4.6 billion years. Other types of meteorites appear to represent stages of planetesimal formation, with ages tapering down to about 4.55 billion years.

The age of the oldest Moon rocks. The Apollo astronauts brought back samples of moon rocks thought to have formed soon after the Moon originated, as it solidified from a largely molten state. The oldest Moon rocks are about 4.4 billion years old.

The theoretical age of the Sun. Stars are, in a sense, a lot less complicated than planets such as earth, because earth is made of solids, liquids, and gases, whereas the Sun is essentially just a big ball of gas. This makes it possible for astrophysicists to write mathematical models, based on the laws of physics, which duplicate how stars like the Sun form and evolve, and ultimately meet their own natural fate as either a supernova or else a white dwarf, depending on how big the star is. The Sun, according to these calculations, is not a big enough star to explode as a supernova once it has burned out; instead, it will shed its outer parts and leave behind a dense, hot, white dwarf star that no longer produces any new heat. Based on these calculations, and the current composition and temperature of the Sun, the Sun is now between 4.5 and 5.5 billion years old. This seems a bit older than the age of the solar system, but it is an approximate age based on theoretical calculations, not a precisely measured age. It overlaps with the 4.6 billion year age of the solar system derived from direct measurements of meteorite ages.

Ancient Hindu View on Earth Age

Annual rotations of the stars in the Universe were noted by many civilizations. Universe was considered ageless, without beginning and end. Time therefore was loop of cycles. This took roots in Hindu faith about 1000 BCE and reflected in ancient Hindu scripts: Vedas (see: https://en.wikipedia.org/wiki/Vedas)

The Vedic ages are named Satya, Treta, Dvapara, and Kali; each one declines in moral excellence due to an increasing deficiency of Dharma. The Hindu Maha Yuga (or Great Yuga, i.e., world age) lasts for 4,320,000 years (10 Kali Yugas) and is divided into unequal time periods. At the top of the cycle, is the Golden Age or Satya (truth) Yuga, a period of 1,728,000 years (four Kali yugas) where the human life span is 100,000 years; Then comes the Treta, 1,296,000 years (three Kali yugas) where the human life span is 10,000 years; Next is the Dvapara Yuga, 864,000 years (two Kali yugas) where the human life span is 1000 years; Last comes the Kali Yuga, 432,000 years where the human life span is 100 years.  One thousand Maha yugas — 4,320,000,000 years by human count — constitute one day, or Kalpa, for Brahma which begins with Sristi, the emanation of a universe out of the Divine; and terminates with Pralaya, re-absorption back into the Divine.

Brahma lives for 100 Brahma years of 360 Brahma days and nights and concludes with universal dissolution of all three worlds of being (earth, heaven and the space between), even the highest worlds. Re-absorption prevails for another 100 Brahma years, after which the entire cycle (311,040,000,000,000 human years) begins again.

What is interesting from the above information is that one day of Brahma equal to 4.3 billion years, very close to the age of Earth based on meteorites.