Think about every space movie you've seen and name the worst imaginable way that a character has been killed off (and try to forget about the baby alien popping out of that guy's chest). Got it? OK. The second-worst way to die in space surely must be floating away from the mother ship -- just being alone out there, with no control over direction or fate (although it would be a really quick death).
Now imagine that there may be millions of planets untethered to the mother ship in real life. They're called nomad planets (aka free-floating, interstellar or orphan planets), and astronomers and sci-fi fans have been pondering their existence for a while now.
Existing in the netherworld between solar systems, these planets may arise after being ejected from their orbits, or they may form after an interstellar cloud collapses. In either case, they wind up roaming the Milky Way. Could Earth get the boot, too? Could a rogue planet come crashing through our solar system? The discovery of nomad planets has forced us to re-examine some very basic concepts, ranging from how planets are formed to how they hold their places in orbit.
At the same time, these wanderers have made us realize that we're not as alone as we thought on the planetary level. In fact, we might have far more company than is provided by just exoplanets. Yep, nomad planets may number in the quadrillions, according to one estimate from Dr. Louis Strigari, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology.
Since this is such a new concept, the scientific community is still hard at work confirming the existence of planets that aren't bound to a star. In this article, we'll explore how nomad planets are made, how they may go rogue, where they're headed and, finally, if they could sustain life.
There's a lot we still don't know about nomad planets, but that's part of what makes them so exciting.
First, the Scary Stuff About Nomad Planets
Much of the world first learned of nomad planets in mid-2011, courtesy of a study authored by Takahiro Sumi, an astrophysicist at Osaka University in Japan. Sumi's team relied on gravitational microlensing, which is the same technique used to discover exoplanets across the galaxy. The technique relies on the light of a background source like a star being distorted as an object passes between the background star and Earth; in fact, that light is temporarily magnified. Sumi and his colleagues detected 10 planets with roughly Jupiter-sized mass that didn't seem to follow a recognizable orbit [source: Sumi]. The news lit up space junkies' Facebook walls and Twitter feeds faster than a "Next Generation" movie announcement.
Like we said, the theory is that these chronic meanderers may develop from the same clouds of gas and dust that stars are born from or solar systems may eject them from orbit. On a related note, solar systems could attract and trap nearby nomad planets into wide orbits. It's entirely possible that, at some time, our solar system had more planets than we currently know of. Perhaps shedding a few extraneous planets is what made it possible for Earth to find its Goldilocks position in relation to our sun.
As for the fear Earth could collide with a rogue planet, yes, there's a small chance that it could happen, but our galaxy is chock-full of smaller, more common and equally lethal threats (translation: asteroids) to worry about. Our chances of getting wiped out by a comet or an asteroid a few miles wide are much greater, although still relatively slim in the grand scheme of things [source: Mosher].
Sumi's original study estimated that there could be two or three nomad planets for every star in our galaxy. Another study less than a year later by the Kavli Institute at Stanford ratcheted the estimate way up, saying that there could be up to 100,000 nomad planets for every star in the Milky Way.
The key word in that last sentence is "could." The Kavli estimate was determined by factoring in things like the following [source: Freeberg]:
- the known gravitational sway of the Milky Way
- the amount of matter that's available to create nomad planets
- the way that matter would arrange itself into nomad planets, which could be as small as Pluto or as large as Jupiter
If correct, that theory calls into question how planets are formed and what role nomad planets have in our galaxy. For now, let's work on what we do know and explore the possibilities of what might be.
Where Do Nomad Planets Come From?
Let's get some background on what could prove to be very common planets. So far, much of what scientists have assumed about nomad planets is derived from what we've already learned from studying our own solar system and younger solar systems in our galaxy.
As far as we know, they can range in a variety of sizes and makeups. Remember, the first nomad planets to appear in Sumi's findings were Jupiter-class planets. So it's not unreasonable to assume that smaller planets with less mass could go rogue, too. In fact, the smaller ones may be better candidates for being pushed out of a young planetary system; the larger ones may be "born" rogue; that is, arise from star-forming clusters.
The idea behind ejection is that nomad planets are subject to gravitational pulls from a variety of sources, ranging from other stars to nearby planets. To give an example of how this might work, scientists have observed Jupiter-sized planets in other solar systems that maintain orbits very close to their sun (think Mercury or Venus close). Now imagine if our own Jupiter started slowly shortening its orbit and moving closer to the sun. Its mass is more than 300 times greater than Earth's, giving it a substantial gravitational field.
As a planet the size of Jupiter moves inward, it would disrupt the path of many planets, moons and dwarf planets in its vicinity by drawing smaller planets toward it. The on-and-off tug of a passing large planet could pull smaller planets out of their orbital paths. As their orbits changed, they could get slung from the solar system and into the void. It's a lot like how we use the gravitational pulls of planets and moons to slingshot satellites farther out into space, only this time we're talking about a planet, rather than a spacecraft, being given a gravity assist. That's just one example of how a planet can get thrown off course.
Another proposal holds that a planet could be ejected from its solar system if its star turns into a supernova. Of course, some planets would be destroyed in the violent explosion, but those orbiting farther out would get pushed outward into space, untethered to a home star.
Once outside of a solar system, a nomad planet can fall under a variety of gravitational influences. Computer models have shown that if a planet is tossed out early in a star cluster's life, that planet is more likely to be drawn into a wide, loose orbit around another neighboring star. The bigger the star, the more likely it is to recapture a nomad planet. Computer models also show that it's highly possible for nomad planets to be captured by the gravitational pull of black holes [source: Perets].
Other possibilities include planets traveling within the larger orbit of the Milky Way galaxy. This means that they are still on the galactic dance floor and moving with the crowd, but they just aren't traveling within a group. And finally, it's completely possible that two nomad planets could meet and form their own binary system -- just a couple of lonely planets romantically twirling around each other in the emptiness of space. Suddenly nomad planets sound a lot less menacing, right?
What Could Conditions Be Like on a Nomad Planet?
We won't know for sure until we get to study a nomad planet, but there are some basic assumptions to be made based on what we already know from observing planets, dwarf planets and moons within our own solar system and beyond. So let's explore some of the possible attributes of nomad planets.
Could they have day and night? No. As we already know, our daylight is generated by our position in relation to the sun. Without that nearby sun, there is no daylight as we know it. That being said, you can presumably cross photosynthesis off the list, too.
Could they have an atmosphere? Yes, it's entirely possible for a nomad planet to have an atmosphere. In order to maintain an atmosphere, planets must have sufficient gravity to hold onto the gasses and temperatures low enough that gasses aren't broken down and allowed to escape into space. As you look toward the outer reaches of our solar system, even tiny Pluto maintains a hold on its atmosphere. So yes, a nomad planet could maintain an atmosphere, but that's not to say it would be breathable by Earth's standards.
What climate could they have? Let's just say you'll probably want to bring a heavy jacket if you're visiting the surface of most nomad planets. Most of Earth's surface temperatures are dictated by sunlight. Without that sun, things will get pretty chilly pretty quick. But that's not to say every nomad planet will be an infertile chunk of icy death. Most of the Earth's subterranean temperatures come from the forces of radioactivity, friction and pressure working at the planet's center. While their surfaces might be cold, some nomad planets could generate warmer, life-sustaining temperatures toward their centers, given enough mass. If they have a thick enough atmosphere, they even stand a chance of having some warmth at their surfaces [source: Freeberg].
Could they have water? It's a possibility that water -- or more likely ice -- could exist on some nomad planets. Using unmanned robots and satellites, scientists have detected ice on Mars and evidence of ancient liquid water there, too. Farther out in the solar system, Europa (one of Jupiter's moons) is believed to have a surface consisting of ice, covering an ocean of water. If we're finding water on other planets and moons in our own solar system, the likelihood of it existing on some nomad is high as well. Multiply that by up to 100,000 nomad planets per star in our galaxy, and the possibilities grow rapidly.
Could Life Exist on Nomad Planets?
Life ... now that's the game-changing question. Is the interstellar space in our galaxy actually full of life? Possibly. From what we know of other planets, we have to assume that nomad planets will share some similarities. And from what we've discovered so far, we know that most planets couldn't support life as we know it. But the law of averages on a galactic scale says that life can exist. We're living proof of that.
Even Louis Strigari, the leader of the team behind the 100,000 nomad-planet-per-star-estimate, said this to Stanford News: "If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist." In this case, some rare planets could harbor life forms that have adapted to live in the most extreme conditions in our galaxy.
Likewise, some nomad planets might be carrying the remnants -- or building blocks, depending on which way you choose to view it -- of life in their previous solar systems. Given enough random encounters with asteroids and other debris, they could be spreading that material across the galaxy.
Finally, it's entirely plausible that humans could inhabit a nomad planet one day in the future. Proxima Centauri, the closest star to our sun, is a considerable 4.22 light-years away from Earth. Nomad planets could become the space bases of human space travel. Once we break out of our solar system, we could use hospitable nomad planets to island-hop our way across to another star system. Then again, with so many planets out there, we might not even need to go all the way to another star to explore new ones. That big, empty blackness between the sun and the stars is suddenly full of potential -- and questions to be answered.
Are you good at spotting stars in the night sky? Test your knowledge of constellations with this quiz at HowStuffWorks.
Author's Note: How Nomad Planets Work
"Wait. What?!" That's response I received from every single person when I said that I was working on an article about nomad planets. This is a subject that elicits equal amounts fear and fascination. And the more I've learned about this very new subject, the more captivated I've become. The possibility of visiting planets just outside of our own solar system is nothing short of captivating.
- Freeberg, Andy. "Researchers say galaxy may swarm with 'nomad planets.'" Stanford University News. Feb. 23, 2012. (April 3, 2012) http://news.stanford.edu/news/2012/february/slac-nomad-planets-022312.html
- Mosher, Dave. "'Nomad' Planets More Common Than Thought, May Orbit Black Holes." National Geographic Daily News. Feb. 24, 2012. (April 4, 2012) http://news.nationalgeographic.com/news/2012/02/120224-rogue-nomad-planets-stars-black-holes-space-science/
- Perets, Hagai. "On the origin of planets at very wide orbits from the re-capture of free floating planets" Cornell University Library. Feb. 10, 2012. (April 4, 2012) http://arxiv.org/abs/1202.2362
- Sumi, Takahiro. "Unbound or distant planetary mass population detected by gravitational microlensing." Nature. May 18, 2011. (April 3, 2012) http://www.nature.com/nature/journal/v473/n7347/full/nature10092.html