Ok so I was reading about the Fermi paradox, assuming life will form and thrive where possible as soon as possible and there is a very probable chance many other earths formed would they have started to form around at the same time our earth started forming. To be clear, another question is at what point in time in the universe after the big bang was life (exactly like ours) even possible?

  • $\begingroup$ Welcome to Astronomy @Cension. I see a question mark, but I don't see a question (yet). Could you edit to make the question you are asking clear please? $\endgroup$
    – andy256
    Commented May 20, 2015 at 4:24
  • 1
    $\begingroup$ This is highly dependent upon what qualifies as 'life', 'another Earth', 'roughly', etc. The early universe was probably not very hospitable; stars were too big and too close together, lots of things were happening, and elemental diversity was quite small. Regardless, not knowing the minimum requirements for life any answer would be conjecture. $\endgroup$ Commented May 20, 2015 at 4:37
  • $\begingroup$ Edited question to reflect your points i am interested in clones of earth. $\endgroup$ Commented May 20, 2015 at 4:41
  • $\begingroup$ This is too broad. There is a dissertation to be written here. $\endgroup$
    – ProfRob
    Commented May 20, 2015 at 6:30
  • $\begingroup$ 500 years is a tiny interval of time compared with the age of the Earth. Even if there were only a 1% variation in the time it took to develop a technological society and even if all "Earths" formed at the same time (which they won't - see my answer), there would still be civilisations 45 million years more advanced than ours and could have populated the entire Galaxy. Good luck - it's a well-trodden problem. $\endgroup$
    – ProfRob
    Commented May 20, 2015 at 17:04

2 Answers 2


I think this too broad, but I'll offer the following:

The star Kepler 444 is orbited by several small, assumed to be rocky, exoplanets. Kepler 444 is estimated to be a very old star, perhaps 11 billion years old, with a metal content of about one third that of the Sun.

Whilst the planets around Kepler 444 are small, they are too hot to be "earth like", but there appears to be no reason why planets at larger orbital radii (that are much harder to detect by the transit technique) should not be there.

Thus the answer appears to be demonstrably, at least 11 billion years ago.

However, the limit cannot be much longer than this, since a certain time must elapse between the formation of the first stars in the Galaxy to the enrichment of the interstellar medium with metals. These metals (all elements heavier than He are referred to as such) are required to build a "rocky" planet. While Kepler 444 demonstrates you don't need a solar metallicity, you still need some.

The fastest place this enrichment took place in general was in the Galactic bulge. A burst of star formation probably enriched the ISM in much less than a billion years.

Thus in principle I would say less than a billion years after Galaxy formation and this is probably not much more than 11 billion years ago.

Kepler 444



The second part of your question is hard. It has taken 4.5 billion years "for life like ours" to evolve. Since we don't fully understand the factors that lead to this, the only realistic answer is that it is probable that it takes another 4.5 billion years after the formation of planets for life "exactly like ours" to emerge.

  • $\begingroup$ Seven not seve, and assumed not assume? $\endgroup$
    – Tim
    Commented May 20, 2015 at 6:59
  • $\begingroup$ I fixed up the question. You might want to write another answer to its current form. $\endgroup$
    – Timothy
    Commented Dec 20, 2016 at 19:05
  • $\begingroup$ @Timothy Even better, I've rolled it back to the version I answered. $\endgroup$
    – ProfRob
    Commented Dec 20, 2016 at 19:44
  • $\begingroup$ I wrote a new question at astronomy.stackexchange.com/questions/19496/… the same way as I originally edited this one to be. $\endgroup$
    – Timothy
    Commented Dec 24, 2016 at 3:29

Already 15 million years after Big Bang! That is 13,783 million years ago. Well, that is according to the "habitable zone" criteria which is the temperature where water can exist as a liquid. The entire universe had a temperature which allowed for liquid water on all rocky planets, though rocky planets themselves were quite rare back then in the old days. But maybe the origin of life depends on something more than only temperature?

The whole thing is just a thought experiment since we have no data about alien life and no explanation of how it started on Earth. That allows for many degrees of freedom, which is the phrase physicists use when they don't know what they are talking about.

“Anything is habitable if you are clever enough,”

-Freeman Dyson (The immortal alien life optimist)

  • $\begingroup$ @RobJeffries The paper author "Loeb says rocky planets could have existed at that time, in pockets of the Universe where matter was exceptionally dense, leading to the formation of massive, short-lived stars that would have enriched these pockets in the heavier elements needed to make planets." They had to make something up. After all, they did find a new "habitable zone". $\endgroup$
    – LocalFluff
    Commented May 20, 2015 at 7:00
  • $\begingroup$ I was working on what we know rather than hypotheses and that the OP wanted to now about "clones of earth". This implies small rocky planets around vaguely normal stars. I don't understand Loeb's argument or how rocky planets are supposed to form from gas enriched by a few supernovae, or how these newly formed planets were supposed to cool in less than 15 million years. This is not the timescale even in our own solar system. $\endgroup$
    – ProfRob
    Commented May 20, 2015 at 9:48
  • $\begingroup$ Either way it took a lot longer than 15 million years for life "exactly like ours" to emerge. I'll delete my initial comment, since there obviously is a peer-reviewed paper that makes the claim that "life" could exist after 15 million years. But I doubt you'll find any mainstream support for that view. $\endgroup$
    – ProfRob
    Commented May 20, 2015 at 9:49
  • 1
    $\begingroup$ @Tim I prefer to listen to the arguments (if there are any) first. $\endgroup$
    – ProfRob
    Commented May 20, 2015 at 9:52
  • $\begingroup$ @RobJeffries I agree with you, and I think the authors would too. It is a sensationalist piece obviously created just as a show off of creativity. I take it as a satire of the habitable zone definition. But it is the earliest date imaginable, so I think it should be at least a footnote to an answer. $\endgroup$
    – LocalFluff
    Commented May 20, 2015 at 10:07

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .