Who believes the earth was once Pandora?

[Wanderlust]

@Wanderlust
To assume that simply because Pandora's landmasses are (apparently) mostly covered by forests as it was the case on Earth 100 millions ago (and who knows when it will be the case again) that Pandora's state of evolution must be therefore like Earth's 100 million years ago would be like forecasting tomorrow's weather to be exactly the same as one year ago when we had very similar clouds in the sky. In both cases, 99,9% of the variables would be left aside in the analysis.

I never did that (or tried to, you may see something in my writing that i am not seeing), i was only showing that there was impressive megafauna in the past. You alluded that we never had anything impressive.

The luminosity of a star like the sun increases by 10% per billion years as a star gets older. This is caused by the buildup of fused hydrogen (ie helium) in the core of the sun causing it to burn more vigorously. The habitable zone continuously moves out as time moves on. A few billion years ago Venus was in the habitable zone and a few billion years form now Mars will be. A planet may only have a billion year window of habitability. This does not forgo local atmospheric conditions that may affect the planet and allow it to extend habitability. Eventually a point will be reached where a biosphere cannot be sustained in its original format.

['upxare]

I never did that (or tried to, you may see something in my writing that i am not seeing), i was only showing that there was impressive megafauna in the past. You alluded that we never had anything impressive.

The luminosity of a star like the sun increases by 10% per billion years as a star gets older. This is caused by the buildup of fused hydrogen (ie helium) in the core of the sun causing it to burn more vigorously. The habitable zone continuously moves out as time moves on. A few billion years ago Venus was in the habitable zone and a few billion years form now Mars will be. A planet may only have a billion year window of habitability. This does not forgo local atmospheric conditions that may affect the planet and allow it to extend habitability. Eventually a point will be reached where a biosphere cannot be sustained in its original format.

If the habitable zone around a star would move outwards so quickly (I know it's a pretty relative term), water on Earth wouldn't have been possible over a period of 4,2 billion years, as in this time span, the luminosity of Sun would have increased by 50%. Together with the variable athmospheric conditions, Earth's (and other planets) orbit around the sun will be slightly pushed outwards, as the sun constantly looses mass, which is transported away by solar winds- 30% during the next 5 billion years, which will have effects to it's gravitational pull for sure. Alone these few variables would leave enough room to imply that the possible habitational life span of a planet may be much longer than a billion years.
And please don't get me wrong, I never wanted to say that Earth hasn't had an 'impressive' era. I was purely referring to reasons and concepts of advanced evolution in comparison to fictional Pandora in accordance with the thread's topic.

[Wanderlust]

Alone these few variables would leave enough room to imply that the possible habitational life span of a planet may be much longer than a billion years.

I agree with this. Increased greenhouse gasses may have helped with a liquid ocean for the time the sun was dimmer than now. The atmosphere was also quite different before life came around and changed it, perhaps reducing greenhouse capability. Regardless in 1-2 billion years the increasing luminosity of the sun will become too hot.

The variables can send it in all sorts of directions.

The total number of particles carried away from the Sun by the solar wind is about 1.3×10³⁶ per second.^[19] Thus, the total mass loss each year is about (2–3)×10⁻¹⁴solar masses,^[20] or 6.7 billiontons per hour. This is equivalent to losing a mass equal to the Earth every 150 million years.^[21] However, only about 0.01% of the Sun's total mass has been lost through the solar wind.^[22] Other stars have much stronger stellar winds that result in significantly higher mass loss rates.

Solar wind - Wikipedia, the free encyclopedia

The mass loss is 66 earths over the lifespan of the sun. I don't know how much this affects orbit but the sun is 332,864 Earth masses.

['upxare]

Solar wind - Wikipedia, the free encyclopedia

The mass loss is 66 earths over the lifespan of the sun. I don't know how much this affects orbit but the sun is 332,864 Earth masses.

Well, there may be some inconsistencies on Wikipedia as this source indicates other data. (Section: Earth's fate)

"However, by the time it is an asymptotic giant branch star, the Sun will have lost roughly 30% of its present mass due to a stellar wind, so the orbits of the planets will move outward."

Sun - Wikipedia, the free encyclopedia

See also: Schröder, K.-P.; Smith, R.C. (2008). "Distant future of the Sun and Earth revisited"

Nevertheless, Earth still has 1-2bil years to go before life gets boiled away. Considering how long this world has seen life forms on it's landmasses, it's still a long way to go. And any planet orbiting AC-B has even more time to breed life as it's host star is smaller than sun, allowing it to burn it's hydrogen at a slower pace and therefore over a longer time span.

[Wanderlust]

I think i was talking about main sequence, not how much mass is lost during the dying phase which is considerable. I know that the earth will experience a significant orbital expansion when in the giant phase possibly allowing for it to survive. thanks for the paper.

Certainly, with the 10 per cent increase of solar luminosity over the next 1 Gyr (see previous section), it is clear that Earth will come
to leave the HZ already in about a billion years time, since the inner (hot side) boundary will then cross 1 au. By the time the Sun comes to leave the main sequence, around an age of 10 Gyr (Table 1), our simple model predicts that the HZ will have moved out to the range 1.29 to 1.86 au. The Sun will have lost very little mass by that time, so the Earth’s orbital radius will still be about 1 au – left far behind by the HZ, which will instead be enveloping the orbit of Mars.

['upxare]

I think i was talking about main sequence, not how much mass is lost during the dying phase which is considerable. I know that the earth will experience a significant orbital expansion when in the giant phase possibly allowing for it to survive. thanks for the paper.

As your quote implies, it's just a model - actually one of many existing models predicting the Earth's orbit in a billion + years from now. Fact is, as we both have concluded, Earth still has about a billion years to harbor life on it. And to refer to the thread's topic - the lifespan for life on a planet orbiting Alpha Centauri B is longer than the on on Earth, and AC is an older system, which are strong hints that the ecosystem on fictive Pandora is more advanced than ours on Earth.

[Raidrik]

Please....for the love of.....no, just no. First off, despite statements by some; the prehistoric world was not all Rainforest. Almost all tropical 'rainforest' plants are flowering plants [angiosperms], which only came in prevalence in the Paleocene less then 60 million years ago.
Dinosaurs in the Mesozoic, and therapsids in the Paleozoic; did have rain forests, but they looked nothing like those in the modern world, or Avatar.


Only real resemblance prehistoric earth would have to Pandora would be the top carnivores could kill you in a single bite (or in a Tyrannosaurid, Carcharadontosaurid; or some Megalosaurid's case, 1/2 a bite)


PS: when referring to well known periods of time, such as the Jurassic and Cretaceous; the correct term is 'period', not era. An Era is a group of periods with similar life forms and are usually started and ended by mass extinction. The Mesozoic Era for example (Triassic, Jurassic, Cretaceous) was started by the Permian-Triassic Extinction (the Great Dying); and ended by the Cretaceous - Tertiary/Paleocene extinction 65.5 mya

It's a common slip o the tongue, just informing everyone of the correct terms

[Ximphron]

And during Pangea most land was a big ol' desert.

[Raidrik]

And during Pangea most land was a big ol' desert.

during the 'Great Dieing'*, that statement is actually very accurate =)
(Props, you're the first I've seen to get that right)

The rest of the term that Pangea was still formed you did have rainforest, grasslands; and forests.



*Mass extinction, 90+% of all life died out*

[_Omaticaya_]

And all the big species we're extinct... Not enough food for the big lads :-/