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There are billions of planets out there that don't orbit stars. The sheer abundance of these planets has led some scientists to wonder if life could emerge without a star.

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When we look at life on Earth, we see rich, diverse ecosystems powered by our host star, the Sun, so when we think about life elsewhere in the universe, we usually imagine something pretty similar, an Earth-like planet orbiting at an Earth-like distance from a Sun-like star, but we now know that there are billions of planets in our galaxy that look nothing like this.  In fact, there are billions of rogue planets that aren't orbiting a star at all.  It's thought they formed inside star systems like regular planets but were then somehow ejected from their original orbit and flung into deep space.  The sheer abundance of these planets has led scientists to wonder if life could emerge without a star, and though obviously we don't have conclusive evidence, there's actually good reason to think that it could.

It's hard to imagine anything thriving with no star because our Sun is so vital to life on Earth, but it turns out that a star's light and heat might not be dealbreakers.  For instance, while the life we're most familiar with is powered by sunlight, there are plenty of things that survive without it.  In fact, for at least a little while, no life on Earth used sunlight as an energy source.  The molecular tools to perform photosynthesis arose after the first microbes and that's part of why multiple hypotheses on how life first emerged involve some pretty dark places, like today, we know the large number of micro-organisms that live deep underground, where they survive off chemical reactions in the surrounding rocks, so one hypothesis is that life first emerged in subterranean pockets of water or life could have started around hydrothermal vents, places on the sea floor where volcanic activity produces jets of steam.

A variety of organisms live around these vents, so it's not hard to imagine life beginning there before it found its way to the surface.  What all these origin stories have in common, though, is liquid water.  That's because water is vital to all life on this planet and if we assume that life, period, needs liquid water, then its existence on a rogue planet is much less likely as water can only be liquid at a very narrow range of temperatures and pressures.

Of course, it's not guaranteed that water is needed for life.  As we've said in a previous video on this topic, there's an awful lot of chemistry out there, but even if life can live without water, it probably still needs some heat.  Deep space is just too cold to envision any interesting biochemistry going on, water or no water, and without host stars to warm them, most rogue planets are probably deep space cold.  We're talking just a few degrees above absolute zero, except there are a surprising number of ways they could be heated up just enough to support life.  

They might warm themselves from the inside, for instance.  That's something we see with a lot of planets, including Earth.  In our case, about 10% of the core's heat is left over from the collisions that formed Earth, while the rest is from radioactive decay and it's been suggested that similar processes could produce enough heat inside a rogue planet to warm a subsurface ocean of water for billions of years, plenty of time for life to emerge and evolve.  

Even with this kind of core heat, though, a world like this would probably need a surface layer of ice several kilometers thick to act as insulation, much like, I don't know, we see on Jupiter's moon, Titan, or there's another potential way to insulate a rogue planet: a super thick atmosphere.  A hydrogen-rich one, 10-100 times thicker than ours would do the trick, and it turns out rogue planets may be better suited to retaining these atmospheres than ones in so-called habitable zones around stars because stellar radiation can blast that sort of atmosphere away.  

It's also possible a rogue planet could get a temperature boost from a mechanism called tidal heating.  Essentially, gravity warms up two orbiting bodies for the same reason it causes tides.  The differences in gravity felt by different parts of the worlds makes them squash and stretch, generating large amounts of friction and again, there seems to be a somewhat local example of a sub-surface ocean that's heated this way, Saturn's moon Enceladus.

So there's no reason to think that this couldn't happen on a rogue planet with its own moon.  Now, I know what you're thinking.  If these planets are off roaming the galaxy, they're probably doing it solo, but it seems like rogue planets can have moons.  In fact, simulations suggest that close to 50% of moons could stick by their planets when they go rogue, but before we get too excited about the possibility of life on these wandering worlds, it's worth noting that it's hard to imagine anything more complex than micro-organisms on rogue planets.

That's because these heating mechanisms give nowhere near as much energy as direct starlight, like what we get from our Sun.  As far as we know, the Sun is what gave life on Earth the ability to evolve the diversity and complexity we see today.  Still, it's fun to imagine what strange forms of life could be living in pure darkness in a vast, sub-terranean ocean.  Plus, it's definitely possible that there are some ambitious creatures out there, eking out a life for themselves on a rogue planet, and who doesn't want to carve out their own little piece of the universe?  

Luckily for us here on Earth, that's not quite as tough as it is on a starless world.  Like you can learn more about how to start and run your own business by watching the Crash Course: Business Entrepreneurship learning playlist, hosted by Anna Akana.  Like SciShow Space, it's produced by Complexly and it shows that anyone can be an entrepreneur.  In fact, the first video can help you figure out if you want to be or already are one.  So whether you're considering taking your hobby to the next level or trying to rebuild after a major setback, which is a totally normal part of the process, this 17 episode course can help you figure out how to take an idea and grow it into a thriving business.  If that sounds  like something you'd be interested in, check it out.  The link for the playlist is in the description.