When we think about the necessary ingredients for life, we typically think of liquid water, oxygen, and energy from a star to provide heat for photosynthesis and overall temperature regulation. For humans, the Sun is a very important star. Mostly because it's OUR star, but also because without it, we wouldn't be here.... or would we?
Most planets that we think of have stars. With the help of those stars, we've been able to discover over 4,000 exoplanets! However, there are still many more planets lurking in our galaxy that we have not found yet... those that would be difficult to detect from Earth. These planets are called rogue planets. Rogue planets are planets that travel throughout interstellar space without orbiting a host star. Kinda sad to think about, I know.
With upcoming space telescopes, however, we may be able to detect many rogue planets within our own galaxy, the Milky Way. But the real question here is... can rogue planets support life? I mean, the first answer that probably pops up into your head is no, they can't possibly support life, they don't have a star! And I would be lying if I said that I didn't have the same initial thoughts as well. However, we might be thinking about rogue planets the wrong way...
Think about how a planet might become a rogue planet. The planet would have to somehow be ripped from its solar system and flung into the cold depths of space. Even though this is terrifying to think about, the planet in question could still hold on to a liquid ocean and even LIFE... beneath an icy crust of course. Scientists that have looked at planet formation models which suggest that small planets are actually regularly flung from their solar systems due to close encounters with nearby gas giants. This is because the gravitational field of the gas giants creates a sort of slingshot, which quickly sends those smaller planets into unstable orbits causing them to leave their stars behind. I could equate this to an analogy of some sorts but it's already sad enough...
In fact, there are actually more wandering, rogue planets than there are stars in the universe. Yes, you read that correctly. This fact shows that rogue planets are actually fairly common throughout the universe, we just have't really found ways of detecting them yet.
Prior to these planets being literally flung away from their host stars, they could have supported the conditions to harbor life. In other words, they could have been like Earth, with an unfortunate circumstance. They could have had oceans, continents, and everything in between. There is a new model which actually suggests that life on such a planet could ACTUALLY have a chance of survival! This may sound like pure sci-fi, but it's not! Isn't that crazy? The universe truly is a strange place with MANY mysteries.
Some have theorized: "What if you "turned off" the Sun?" Geophysicist Dorian Abbot at the University of Chicago co-authored a paper on the subject, which was submitted to the Astrophysical Journal Letters. He ran this study along with a fellow University of Chicago astrophysicist Eric Switzer. They wanted to see if a planet could hold a liquid ocean even while away from its host star. They assumed that the planet was between 0.1 and 10 times the mass of the Earth with a similar amount of water and rocks. One would assume that once a planet with liquid water on its surface is flung away from its star that the oceans would start to freeze. However, there would actually be leftover heat from the planet's formation along with decaying radioactive elements in the rock could actually keep the ocean warm beneath a layer of ice. This is as long as the planet is able to keep the ice from freezing all the way to the core, of course. Otherwise, there is no hope for any life to be left.
Abbot and Switzer calculated that a planet about 3.5 times Earth's mass would be warm enough to maintain its liquid oceans beneath a layer of ice only a few kilometers thick. The crazy part is that the ocean on such a planet could last up to 5 BILLION years! That's the amount of time that the Sun has left in its lifetime! For a planet without a star, this is a long time scale, much longer than we would assume that it could last with liquid water while being away from its heat source.
Similar to the conditions described on these rogue planets, Jupiter's moon Europa yields the possibility of life existing under its icy crust, but further studies need to be conducted. Unlike a rogue planet, however, Europa's heat comes from tides raised by Jupiter.
Since all planets are not alike, Abbot and Switzer also imagined a world covered in volcanoes that filled the atmosphere with carbon dioxide (a bit like Venus in the past). On a rogue planet like this, the gas would almost immediately freeze and fall as snow. This would cover the world with a layer of dry ice. In this case, a planet as small as 0.3 times the mass of the Earth could potentially harbor a liquid ocean underneath this layer.
Life on such a planet does not only have to consist of those life forms which survived the traumatic event and adapted later on, but also those that evolved later around hydrothermal vents on ocean floors. The two scientists declined to hypothesize what such life would even look like, but they unanimously concluded that it would almost certainly be microscopic (but it is still life nonetheless). This point makes sense because it would be very surprising to see such a planet be able to sustain life forms on the macroscopic scale with such little energy from an outside source.
These free-floating planets could have also been part of the reason that the "seeds of life" were brought to Earth, but who knows. We simply don't have the technology yet to observe these planets, let alone find out if they could even house life. This topic raises a lot of questions, but it truly is something to consider as we continue the search for extraterrestrial life. Scientists may have to consider that these worlds really may have something for us to discover. We haven't even talked about the fact that bigger rogue planets may even take their moons with them when they are "kicked out" of their solar system. These planets could even keep their atmospheres if they weren't stripped away from their star when they were orbiting it. There are so many possibilities, but in my opinion, it is very important that we don't rule out any of them, no matter how "far fetched" they seem. After all, the universe is full of surprises.
This topic is one that I think we should think about more. I know that the technology is not fully there yet, but we could still somehow simulate conditions here on Earth, with MANY different possibilities and outcomes. Many rogue planets seem to be Earth sized (the ones we have detected), so why couldn't they have life? I feel like I'm dragging this on a bit, but I hope that everyone reading this has learned something. I really had fun looking into this topic, because it seems so new, like too many people aren't talking about it, so I just wanted to bring it to people's attention.
Here's to hoping that we find something fascinating in the future. There is life out there somewhere, I just know it.
Space is full of surprises....
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