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According to most articles I've read online, the estimated age of Earth is 4.5 billion years.

Question: How old is the Earth?

So I actually want to confirm that this is the case. Maybe there are other famous theories saying it is 3 billion years old or 6, therefore my question.

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Yes, the age of the Earth is about 4.5 billion years (4,500 million years). Your linked articles describes well how it was formed and how we know about it. The uncertainty is less than 1% and depends partly on the radiometric dating methods and partly on the definitions. Sometimes the age is said to be 4.567 Ga, that might be a little too exact number to claim, but at least it's easy to remember.

I haven't heard any different age constraints that are scientifically founded, but before we had access to the methods and data we have today, there was a broader range of estimates.

Early estimates were based on the supposed increased salinity of the oceans, but those estimates fail to recognize that salt is also removed from the seawater in subduction zones and to form evaporites.

Noteworthy is Lord Kelvin's very sensible estimate of 20–40 million years, published first in 1863. The estimate was based on the heat lost since formation and Earth's capacity to withhold its energy. Unfortunately, Lord Kelvin worked too early to know that $E=mc^2$, so energy is also generated within Earth and that is the main source of heat. He also didn't understand the Sun's source of energy and convection and therefore miscalculated the solar age constraint and Earth's heat flux.

Radioactive decay is not only the source of heat but also the process we use to measure the age by radiometric methods. These methods became known decades later and quickly provided age constraints sin the right magnitude.

Earth scientists have assumed the deep time age of Earth since the antiques. It must have been frustrating for the early geologists to not be able to put a number of the Earth's age. As a rudiment of that time, we still use their relative terminology to describe the development.

Our present estimate is based on a range of agreeing models and data. The Earth science community has, in general, moved on to new and more challenging questions.

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    $\begingroup$ Note: early British geologists (around Kelvin's era) were very well aware of the Earth being at least billions of years old. It were the physicists who denied it, due to a lack of a known process that would keep the Sun (nuclear fusion) and the Earth (nuclear fission) hot over such vast time scales. Of course, if you get a century or so earlier than that, you'd find plenty who were fine with the Earth being just 6000 years old, with all the surface features created by the biblical flooding. The British gained huge amounts of knowledge about the Earth while mapping their vast empire :) $\endgroup$
    – Luaan
    Commented Jul 13, 2017 at 7:09
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    $\begingroup$ Minor nitpick: The Earth stays warm due to radioactive decay (Mostly Potassium-40 beta decay and Uranium-238 alpha decay, from memory), plus a bit of core crystallization. There is/was a theory that enough uranium was concentrated at the Earth's core to form a natural fission reactor, but I'm not sure that it's taken very seriously. $\endgroup$
    – Andrew Jon Dodds
    Commented Jul 13, 2017 at 7:45
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    $\begingroup$ @Luaan I agree, the deep time of geology have been known for a long time. The Islamic scholars around year 1000, e.g. Ibn-Sinaa had a good idea about the stratigraphy and an age reaching far beyond history. There are also antique Chinese records that show an awareness of the true magnitude of Earth's age, e.g. from Mengxi Weng. As for competing estimates, as OP is interested in, I think they are mostly of dogmatic rather than scientific origin. $\endgroup$
    – user2821
    Commented Jul 13, 2017 at 9:39
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    $\begingroup$ It's interesting to line up two early estimates, based on ocean salinity increase and heat loss, that were reasonable based on uniformitarian assumptions, but ended up being way off due to important processes and events we weren't aware of. Makes you wonder what important processes and events we aren't aware of now, and how that could affect future estimates. $\endgroup$
    – LarsH
    Commented Jul 13, 2017 at 14:38
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    $\begingroup$ @16807 Thanks for the edit and great article suggestion! It's very true and especially concerns the age of the Sun, that was supposed to be a constraint for Earth's age as well. Convection invalidated the whole assumption of a cooling ball, but they also needed a source of energy. $\endgroup$
    – user2821
    Commented Jul 14, 2017 at 1:07

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