With our current technology we're able to travel through our Solar System (and beyond) during at least a human lifetime.

Is it physically possible that a star system has more intelligent lifeforms in different planets that belong to an habitable zone?

In other words, let's imagine there is an intelligent life form on some distant star system, with the same advancement we have in space technology; is it possible for such a life form to reach a planet where life has developed and is intelligent (not necessarily at the same technological level, but intelligent), or at least that has developed and is evolving?

And is it statistically likely?

  • $\begingroup$ With SETI covering left flank and Chinese building world largest radio covering the right flank, give or take a couple of century we will know whether human are paranoid or not? $\endgroup$
    – user6760
    Jul 30, 2015 at 12:17
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    $\begingroup$ I just don't get what this comment stands for :D $\endgroup$
    – dev_mush
    Jul 30, 2015 at 15:37
  • $\begingroup$ Me neither so let's eavesdrop on alien radio signal, the bigger the antenna the more feeble the signal it can trace as you know space is vast. $\endgroup$
    – user6760
    Jul 30, 2015 at 17:06
  • $\begingroup$ Given a large enough sample, anything the slightest bit unlikely MUST happen at least once (otherwise it wouldn't be unlikely, it would be impossible). $\endgroup$
    – slebetman
    Mar 7, 2016 at 3:48

6 Answers 6


It's possible, but it's statistically unlikely.

Intelligent life is one of the following:

  • Rare
  • Relatively short-lived (with civilizations lasting at most thousands of years)
  • Hiding from us

Otherwise we'd have some evidence of nearby aliens by now.

To get this you need two habitable planets in the same system, both which develop life around the same time on a timescale of billions of years. Otherwise it's likely that the first civilization to develop would simply colonize and take over the second planet before intelligence has a chance to develop.

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    $\begingroup$ "Otherwise we'd have some evidence of nearby aliens by now." FTFY. Of course, the question itself is concerning "nearby" aliens, but it's better to be unambiguous anyway. $\endgroup$
    – o0'.
    Jul 30, 2015 at 15:08
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    $\begingroup$ This was exactly my starting point. I was thinking "what could drive a civilization to somehow lose interest in looking for other civilizations?" And the answer, of course very funny and non-scientific, was "they might be aware of the existence of other civilizations already, so they're colonizing their star system". So my second question was "what is the likelyhood of having a civilization pretty close to home?" $\endgroup$
    – dev_mush
    Jul 30, 2015 at 15:33
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    $\begingroup$ Don't forget "natural barriers", even to our level of tech. E.g. subsurface ocean in a world in a radiation belt, or significantly different environment "why would we colonize a place with corrosive atmosphere, largely covered with solvent, all exposed to electromagnetic radiation?" The Riviera to us could be mind-bogglingly hostile to a different kind of life. $\endgroup$
    – JDługosz
    Jul 30, 2015 at 16:04
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    $\begingroup$ Reminds me of the story of ants exploring 5 bathroom tiles and deciding that non-ant intelligent life does not exist. $\endgroup$ Jul 30, 2015 at 19:09
  • $\begingroup$ @SerbanTanasa I'm sure their dreams of being the only intelligent beings were crushed once they met a human. ;) $\endgroup$
    – user64742
    Oct 14, 2016 at 17:23

It is hard to say for sure. We have the concept of a habitable zone (area around a star that allows for the presence of liquid water on the surface), but it may be that far more conditions (rotational axis stabilization by moon, active plate tectonics to recycle material lost to the oceanic abyss, presence of a Jupiter to shield us from the worst of the Late Heavy Bombardment, etc) need to be in place for life to evolve. It might well be that we are living on the only life-bearing-planet in the galaxy.

That said, if it somehow happens that two planets are auspicious to life around the same star (and there's nothing to say it can't happen), it is possible, even likely, that both planets will eventually host life, if only because of meteoric cross-contamination (some meteor impacts blast rock from one world, containing live bacteria or spores, all the way onto the second world).

Now, our ignorance is even deeper concerning the circumstance under which life becomes intelligent enough to be sentient. While there has been a historical trend on Earth towards more intelligence on average over the hundreds of millions of years ( brainless worms ---> foxes), it's far from clear that the high biological costs of intelligence are always outweighed by the advantages the extra smarts confers. We honestly don't know what fraction of planets with life on them go on to ever develop intelligent life.


Yes: Have look at Mars for example: It is in habitable zone and it can hold a life. If you add a few tweaks to such setup, you can have two civilizations inside same solar system.

  • $\begingroup$ this isn't precisely answering to my question... I was picturing in my mind this scenario where there was already an habitable planet, we were reaching it and finding life there. Mars is of course habitable, but there is no atmosphere and no life whatsoever on the red rock :D $\endgroup$
    – dev_mush
    Jul 30, 2015 at 15:36
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    $\begingroup$ @dev_mush: Mars does have a (rather thin, by our standards) atmosphere, and we currently do not know whether or not there's life there. $\endgroup$
    – jamesqf
    Jul 31, 2015 at 20:52

I think you are asking whether there could be intelligent life elsewhere in the solar system but have not noticed it yet. Gone are the tabula rosa of early SF where anything could be beneith the clouds of venus... we know a lot of what is, and we don't see any biosphere.

We need (1) life, that has not been noticed yet, and (2) intelligence that has not made itself more conspicuous through exploration and engineering.

That is possible in subsurface oceans, of which there are several candidates. Europa is the classic example. There could be life there similar to seep-sea vents here, with no reliance on sunlight.

Imagine real unambiguous intelligence, not like dolphins but like people... people as existed for tens of thousands of years before acheiving planet-shaping technology. You could have Europa-native beings living on the "ground" 200 miles below the water, with the technological level of the ancient Egyptions. They don't have the ability to travel to the roof of their sky, and even if they had bouyancy vehicles the phase changes prevent their life from surviving a high altitude trip.

They could be building magnificent architecture, writing poetry, and pondering the universe, but remain completely undetected. We don't even know the ocean and ice layer thicknesses ecxept in vague terms, and their existence does not have to leave details on the surface that we have taken a look at.

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    $\begingroup$ I'm sorry but this is not exactly what I asked. $\endgroup$
    – dev_mush
    Jul 31, 2015 at 7:38
  • $\begingroup$ The title was misleading. What does that have to do with "our current technology level"? $\endgroup$
    – JDługosz
    Jul 31, 2015 at 19:11

As part of the "Hiding from us" in the accepted answer, you could consider the possibility that the intelligent life forms are not looking in the right place. Species "A" is carbon based and needs oxygen and liquid water (ie: a warm planet), so they are looking for other habitable (oxygen-liquid water) environments for life, ignoring environments that lack these. Species "B" is silicon based and needs methane, so they ignore the too-warm oxygen-water planet and seek other silicon-methane planets. Also, what if species "B" is really small, even microscopic?

  • $\begingroup$ Interesting, but I was asking if it was statistically and physically possible to have two civilizations in the same star system $\endgroup$
    – dev_mush
    Jul 31, 2015 at 7:41
  • $\begingroup$ Statistically speaking, the answer is unknown. There are a near infinite number of star systems (really unknown how many). Of those systems, an unknown fraction have planets or places that civilizations could exist. Too much unknown to give statistics. Physically, we have multiple civilizations living simultaneously on the same planet multiple times during our human history. So, definitely possible. $\endgroup$
    – LeHill
    Sep 9, 2015 at 13:43

Can a story make the reader find it plausible that two different races evolved to intelligent civilization on two different worlds in the same star system, extant at the same time?

I think so. Planets share material, and in the resent Cosmos remake Tyson explained that rock blasted from the surface during an impact served as a lifeboat during the late heavy bombardment, so life (or however it's gotten thus far) could wait out the re-melting of the planet's surface for a while.


… interplanetary transit system that sends rocks between the planets. Such a meteorite can safely shelter microscopic cargo the seeds of life an interplanetary ark.

Most rocks are porous, full of tiny nooks and crannies, where life can stow away. We know that some microbes can survive the hostile environment of space. Take these guys, for instance.

These microbes spent a year and a half riding on the outside of the International Space Station, exposed to the extreme temperatures, vacuum, and radiation of space.

And some of them were still alive and kicking when they were brought back to Earth.

Even more astonishing are these creatures, awakened from a deathlike sleep of eight million years.

They were frozen in the Antarctic ice millions of years before our species even existed. And they're still alive.

If life can withstand the hardships of space and endure for millennia, then it could ride the natural interplanetary transit system from world to world.

It's a good bet that our microbial ancestors spent some time in space. Why do we think so? The Earth is four-and-a-half- billion-years old. For the first half of its lifetime, large asteroids were bombarding the planet every few million years.

The most violent impacts vaporized the oceans and even melted the surface rock.

Each such collision would have completely sterilized the planet for thousands of years.

But we know from fossils in the rocks that bacteria were evolving on Earth during this formative period.

So how could life have survived such a lethal series of blows? Whenever one of those big asteroids hit the Earth, the explosion would blast out a crater, launching millions of boulders into space.

Many of those rocks carried living bacteria inside. Some of the bugs would have survived in space, while all those left behind on Earth would have been fried.

A few thousand years after each impact, the Earth would have cooled down enough for water to condense into oceans.

The planet would again be habitable. Meanwhile, most of the rocks launched into space would have been orbiting the Sun.

Some of them would encounter the Earth again, reenter the atmosphere as meteorites, and deliver their precious cargo of life to re-seed the planet like Noah's ark.

What this means is that life doesn't have to start over again from scratch after each catastrophe. It can pick up where it left off.

When the solar system was young, Venus was probably more like Earth, with oceans and maybe even life.

Venus, Earth, and Mars were all exchanging rocks with each other, due to asteroid impacts.

Does life on Earth carry any traces of interplanetary voyages made in the distant past? Why is it that some microbes can survive the intense radiation and vacuum of space? These conditions don't naturally exist on Earth.

Maybe those bugs are telling us that their ancestors survived those same conditions in space, a few billion years ago. So we know that microbes can stow away in rocks and survive the voyage from planet to planet.

So, consider multiple planets in the habitable zone that were not fatally flawed and made it to the point of having a feedback system maintain a stable environment. They would all have biospheres, and the life would be related.

Suppose that major stressors such as impact events are also naturally correlated. Whatever disturbed the system such as a passing star or the sun passing through a dusty region of the galaxy, it would not be unreasonable for both planets to suffer impact events within a few million years of each other. Suppose that stresses like this are necessary for the biosphere to advance to more complex forms.

There are other stresses, such as major volcanism. It's a big galaxy, so why not? Statistically, the advancement of life follows an average time span for each step, so it's not unreasonable to suppose that both planets reached the kind of stage we are in, at the same time, within a hundred million years of each other. Perhaps the details are different in the final stages: we had an impact event wipe out the dinosaurs, but things were changing anyway due to (ultimately) plate tectonics. So maybe one planet has an impact and another has a ecosystem collapse due more to internal reasons from smaller outside stress. Maybe they had a different number of "repottings", but on average reached complexity ready to evolve intelligence at around the same time, because of the long time scale involved so individual events give way to long-term averages.

The stress that led to us was climate changes from forests to savana, etc. and eventually the ice age, and a genetic bottleneck that prevented evolving better fitness the old fashioned way. Suppose that both planets had complex advanced life and had been mature in that stage for a few million years. Some external stress, so it applies to both worlds, spurns development of intelligence. That way they'll be happening at the same time.

You can convince the knowledgeable reader to suspend disbelief that two intelligent races can emerge at the same time, within a finishing window of a few (single digit) million years. Why the final summit within a few thousand years? Perhaps ice ages, again correlated due to having external causes. At some point you have to appeal to "it's a big galaxy", but I think this is a platform that gets you most of the way there:

Development takes place, statistically, at a common rate. External stresses help synchronize things further.

Also, note that I said worlds, not planets. Perhaps they are moons of a giant planet, so they are more easily sharing material between them. Maybe the style of life on one takes spreads spores high into the air, so will naturally spread between worlds on rare cases, when those worlds are very near each other.


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