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According to wikipedia, the Earth is about 4.543 billion years old. If I understand correctly, this number is based on radiometric age-dating of meteorites. So basically, the logic is this:

  1. We've got this material
  2. We can show that it's about 4.543 billion years old
  3. We haven't found anything older.
  4. Therefore this is probably about the age of the Earth

However inference 4 seems a bit problematic. How do we know that the Earth isn't significantly older? Could it be 5 billion years old? Could it be 8? Discounting the big bang theory, could it be 13 or even 15 billion years old, using only what we know about the science of the Earth?

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    $\begingroup$ The thing you're missing is that science isn't based on logic, it's based on empiricism. And that is a very significant thing. $\endgroup$
    – Davor
    Jul 13, 2018 at 22:00
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    $\begingroup$ @Davor -- Science is so much more than merely empirical. -- Hop on over to Philosophy Stack Exchange and search on "Philosophy-of-Science" tagged questions. You'll be amazed. Truly. $\endgroup$
    – user23715
    Jul 14, 2018 at 3:18
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    $\begingroup$ @Davor, I think that's an interesting comment. However... $\endgroup$
    – goblin GONE
    Jul 14, 2018 at 4:55
  • $\begingroup$ ...I also agree with @user23715 that how science achieve its goals is pretty complicated, and I wouldn't really dismiss logic quite so readily. A further comment: the question is specifically about how certain we can be that the Earth isn't a great deal older than the currently-accepted number. It's not about how science works, it's about "can we be sure of this?" So in some sense at least, how science works is irrelevant to this question. $\endgroup$
    – goblin GONE
    Jul 14, 2018 at 4:58
  • $\begingroup$ @user23715 - I'm very familiar with various philosophical arguments, don't worry. I also give them very little credence because they have about as much evidence supporting their claims as young earth creationists. $\endgroup$
    – Davor
    Jul 14, 2018 at 19:31

5 Answers 5

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The age of the solar system is 4.6 billion years. We know that because almost all meteorites are 4.6 billion years old¹. Therefore, that puts a very solid upper boundary to the age of the Earth.

According to solar system models, it took between 10 million and 100 million years for the Earth to form in the early Solar System. That result is consistent with the independently estimated age of the Earth of 4.54 billion years old. The Earth could maybe be 10 million years older, but it certainly couldn't be older than the solar system.

None are older. We've never found a meteorite from outside the solar system. I expect some scientists would have given an arm and a leg for the opportunity to sample some rocks from interstellar Oumuamua but it was not possible.

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    $\begingroup$ I'd bet you're right, but the logic, as you've presented it, is not sound. The age of meteorites would put a minimum limit on the age of the solar system, not a maximum limit. Could you go into more detail on that? $\endgroup$
    – jpaugh
    Jul 13, 2018 at 19:07
  • $\begingroup$ @jpaugh I believe the logic is that out of many many thousands of samples, all of them less than 4.6 billion years, the probability of the real range of possible ages being significantly different is low. Read the bit about the german tank problem what-if.xkcd.com/65 $\endgroup$
    – wedstrom
    Jul 13, 2018 at 20:05
  • $\begingroup$ @jpaugh The uncertainty in the age of the solar system is rather small. The age of the solar system puts an upper limit to the age of the Earth. $\endgroup$
    – gerrit ♦
    Jul 13, 2018 at 20:19
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    $\begingroup$ It might be worth noting in your answer that the Moon Rocks put a 'very solid lower boundary to the age of the Earth' as well. -- This helps limit possible models for the Earth's evolution while also working as a check against presumed processes of plate tectonism in the very early Earth. $\endgroup$
    – user23715
    Jul 14, 2018 at 3:17
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The ages are not the only piece of information that we have. Gerrit covered most of the age issue in his answer.

To summarise: We know the age of the solar system (4.5-4.6) and the oldest material we have on earth is 4.4.

To add to that, we know that the composition of the Earth is very similar to what we would expect from having formed in the solar system. If the Earth was here from before that, why would it have the same composition? Therefore, we have to conclude the the Earth formed together with the solar system.

Of course, there is some chance that Earth is older and just happens to be in the composition similar to the rest of the solar system, but we have no reason to believe this is the case.

This is similar to pink elephants. How do you prove that they do not exist?

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From zircon dating we know that the age of the earth is close to the age of the asteroids in our solar system (i.e. there is not a huge discrepancy between their ages). Without evidence indicating otherwise, scientist are left with no other conclusion but that the asteroids and the earth are of nearly the same age.

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    $\begingroup$ No. The zircons (< 4.4) we date on earth are younger than meteorites (4.5) $\endgroup$
    – Gimelist
    Jul 14, 2018 at 6:33
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    $\begingroup$ It takes longer for something large to cool to the temperatures crystals can form at. $\endgroup$
    – John
    Jul 14, 2018 at 14:45
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    $\begingroup$ My apologies. I meant to say the earth and the asteroids were close to the same age, which I thought my last sentence clarified, with apparently it did not. I have reworded it to make it less ambiguous. $\endgroup$
    – BillDOe
    Jul 14, 2018 at 19:20
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The chances of this actually being accurate are remote to the extreme, but Earth could in theory be a captured planet older than the solar system, and the resurfacing from the giant impact removed all the evidence. First, captured rogue planets with stable orbits in the inner solar system are virtually impossible. Second, the timing would need to be nearly perfect where the capture happened when the solar system was very young. So, I don't like this idea. It's so unlikely as to be preposterous. But it's theoretically possible that the Earth is older than the solar system, if it was an older rogue planet that somehow got captured, perhaps by gravity assist around baby-Jupiter and stabilized over time by collisions.

The question asked if it was possible, and I see no reason why this wouldn't be possible. So unlikely it's not worth considering - yes, very much yes. But not, to my knowledge, impossible.

Now as to 15 billion years old? Older than the Universe. Lets just say no and leave it at that, unless you want to debate that the estimates to the age of the Universe are wrong, and that's Cosmology, not Earth Science.

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Claire Patterson et al settled the Age of the Earth in 1956. In this paper, which can be obtained for free from Colorado.edu, we read:

"The following assumptions are made concerning meteorites: they were formed at the same time; they existed as isolated and closed systems; they originally contained lead of the same isotopic composition; they contain uranium which has the same isotopic composition as that of the earth." (p.1)

...and...

"If the earth is a late agglomeration without differentiation of meteoritic material then it can have any age less than meteoric material. Rather than arguing that such a process would be accompanied by chemical differentiation (and a change of the U/Pb ratio), it seems reasonable to believe instead that such a late agglomeration process would be less probable than one where both meteorites and the earth were formed at the same time. It is a fact that extreme chemical differentiation occurred during the process which led to the mechanical isolation of the mass of material of which the earth is made, and since changes in this mass were accompanied by chemical differentiation, the Pb/Pb isochron age properly refers to the time since the earth attained its present mass." (p.7)

The present age of the earth, then, is calculated upon an astronomical model which asserts that the formation of the solar system happened in the way we believe it did, which results in our assertion that measurements taken on meteorites apply to the earth as well.

Contrasted against meteorites, zircons are a nice earth-bound reference because unlike everything else in the earth's crust, we do not believe that they give falsely young readings due to the crust's constant re-melting and cooling (zircons are hard to re-melt).

If the earth really were older than the meteorites and zircons are telling us, then its long early history was probably inhospitable to life due to our observation that no zircons have been found that are older: it would have been crazy hot during that time.

It's worth noting that radiometric methods are the only methods that give an affirmative earth age greater than 0.05 million years: the ice core layers are only dated beyond that age according to curve fitting, not layer counting.

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