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I have two planets located in the habitable zone of a 1.02-mass Star(Class: G1.7V).

The first planet is located 1 AU away from the Star and the Sun is 1.08 times brighter than our sun to anyone on the surface. The second planet is 1.2 AU away from the Star and the Sun is 0.75 times as bright as our Sun, with regards to someone on the surface.

The first planet has 1.7x the mass of Earth with a radius of 1.138 times that of Earth. The second planet has the same mass as Earth with a radius of 0.997 times that of Earth.

The first planet is mostly water with the land located near the equator. Over millions of years, the land moved very little to the North and South. The second planet is mostly land(I have tried to figure out the actual percentage but have been unsuccessful).

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  • $\begingroup$ Absolutely! Mars theoretically could have been inhabited to this day if it were a bit more massive so that it could retain its internal heat and magnetic field. $\endgroup$
    – isdi0
    May 5, 2023 at 15:25
  • $\begingroup$ The first planet may be too hot. The Earth is believed to now be close to the inner edge of the habitable zone, and will cease to be habitable in a billion years or so as the sun continues to heat up. $\endgroup$
    – Mike Scott
    May 5, 2023 at 15:30
  • $\begingroup$ I propose that there is a fundamental law of physics that prevents life from arising on two adjacent planets. That's why we should be checking Mercury and Jupiter in this solar system, not Mars. $\endgroup$
    – John O
    May 5, 2023 at 15:31
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    $\begingroup$ @MikeScott I forgot to mention that the first planet is a predominantly water world. This would cause a reflection of large amounts of solar radiation cooling the planet. It has massive polar ice caps that float at the poles because the little land there is, is located in near the equator. $\endgroup$
    – Martamo
    May 5, 2023 at 15:34
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    $\begingroup$ @cconsta1 The first planet has an atm of 0.9. The O2 is 27.3%, the CO2 is 2.1%, the Argon is 0.4%, and the N2 is 70.2%. The second planet has an atm of 1.1. The O2 is 42.6%, the CO2 is 1.81%, the Argon is 0.5%, and the N2 is 55.09%. $\endgroup$
    – Martamo
    May 5, 2023 at 19:01

4 Answers 4

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To offer an objection to other fellow writers, I think that life might not even start on these planets because the atmosphere is very similar to our modern-day atmosphere here on Earth. Recall that life emerged when Earth was still Volcaneous, there was no Oxygen in the atmosphere, and instead, the atmosphere consisted of mostly hydrogen sulfide, methane, and carbon dioxide. The first cells evolved in the oceans from complex chemical reactions between carbon-based molecules near hydrothermal vents as it is believed (See here and here).

If life jump starts by other means from the one we understand here on Earth then yes, life can evolve on both these planets (which sound awesome). If by intelligence we mean animals then yes, I think both planets can support that, with the water planet having predominantly aquatic life most probably. If we are talking about technological civilization then I'd cast my vote on the second planet that has a vast landmass.

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    $\begingroup$ Thank you so much for the amazing answer. The percentages I gave you are from a later time period, a period in which life has I guess technically started. I do love the idea that someone else said of it being panspermia that kicks it off. My project partner and I have talked in length on how the technological advancements would be. He wished for his planet to be a little medieval I guess (his is the water planet) while the other is already space faring. Again, thank you. $\endgroup$
    – Martamo
    May 6, 2023 at 3:09
  • $\begingroup$ @William you are welcome! We often see the end product, here our modern-day atmosphere, and forget about the origins that were rather non-ideal to our modern eyes as Earth was poisonous, hot, full of Volcanos, etc. :) $\endgroup$
    – cconsta1
    May 6, 2023 at 8:18
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    $\begingroup$ Yeah, we take our blue skies for granted when they used to be black and orange. $\endgroup$
    – Martamo
    May 6, 2023 at 14:33
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Assuming both are habitable and can cause life, it would be almost inevitable that they would both have life at the same time, and if we assume that animal life arises on both then they would also coexist temporally

We already have many independent intelligent species dwelling on Earth right now, so it seems easy enough to suggest that two intelligences could arise on different planets

The main issue is with human-like technology. Technology and invention moves much faster than evolution: If all of life on Earth was a standard movie, our society would take up less than the last frame. It would be a truly impossible coincidence for technological development to occur twice at the exact same on two completely different planets

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  • $\begingroup$ Thank you. I have actually thought about the technological development of both worlds and have discussed it with my project partner. We have decided that one planet will be able to travel the system before the other even has an industrial revolution. I think of this more as one planet had more resources because of the large amount of land, while the other(being of 80% or maybe even 85% water) has less. $\endgroup$
    – Martamo
    May 5, 2023 at 18:27
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There's nothing that prevents two habitable planets existing within the goldilocks zone. With enough CO2, mass, and a magnetic field, Mars could have been habitable. A magnetic field would have made Venus habitable for a while.

The "turn intelligent life around the same time" part is suspect. Mankind went from realizing that those glowing things were planets to being able to visit them in less than one millionth of Earth's current lifespan. If you isolate the history to just animal life, it becomes slightly more than a millionth. Imagine closing your eyes, spinning twice, shooting a gun, and hitting a housefly five kilometers away, on purpose.

This is actually the subject of the Drake Equation, except that it is looking for life anywhere in the galaxy, so that one-in-a-million gets multiplied by all of the stars in the galaxy. There is a broad range of answers to this equation, but the current best guess is that there should be twenty or so intelligent civilizations in our galaxy. The chance of two being in the same solar system is vanishingly small.

Let's tip the balance in our favor a bit. If we presume that panspermia came by and seeded all of the planets a billion years ago, that narrows it down so that the other planet should have animal life on it when the first one figures out how to go visit.

If you theorize that both planets shared extinction events, then we can jigger the numbers so that both planets started their rise to intelligence 65 million years ago. If we expand mankind's "rise to intelligence" to a thousand years, then the chance of them rising to intelligence simultaneously drops to maybe 1 in 1000.

The thing that really kills your idea is that, even while sharing Earth, civilizations didn't experience technological rise at the same time. Around 70k years ago, Homo Sapiens were nearly killed off, but then we spread everywhere. The planet could easily currently be populated by Neanderthals or Homo Erectus, and our suspicions are that neither of them would have generated space flight by now.

So, you could hypothesize that we'd find something trainable on other planets, but the chances of us tripping over another space-faring civilization in our own solar system is virtually non-existent.

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  • $\begingroup$ I have actually thought about the technological development of both worlds and have discussed it with my project partner. We have decided that one planet will be able to travel the system before the other even has an industrial revolution. I think of this more as one planet had more resources because of the large amount of land, while the other(being of 80% or maybe even 85% water) has less. $\endgroup$
    – Martamo
    May 5, 2023 at 19:10
  • $\begingroup$ @William, there's this thing called the anthropic principal. What it comes down to is that, no matter how unlikely it is that mankind would appear on this specific rock, that's how the dice came up, so the chances are 100% that it will have happened. You get the same benefit when you're writing your universe. $\endgroup$ May 6, 2023 at 4:57
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Absolutely. There's nothing saying that those planets can develop and evolve life at the same time, even though their different orbits and conditions would mean that if the closer planet has a mass extinction event that sets evolution back a few million years, or vice versa, the other planet is most likely spared the destruction. Thus, while it's highly unlikely, it's not scientifically impossible.

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