Orphaned Worlds: The Planets Without a Sun

Polluted White Dwarf (Artist’s Concept) NASA/JPL-Caltech

Aimless Worlds

I have seen the dark universe yawning, Where the black planets roll without aim; Where they roll in their horror unheeded, without knowledge or lustre or name.

-Nemesis; H.P. Lovecraft

It is certainly evocative imagery: a dark world aimlessly adrift in the void. An orphan planet, never given a name and unseen, unknown by anyone. A planet randomly wandering the space between stars without a solar system to call home.

But is this mere poetic imagery, or could there be something akin to this feature of cosmic horror in the real universe?

Yes, there are starless worlds that roam the cosmos, dark, homeless, and nameless. In the field of astronomy, the most commonly accepted term for these is “rogue planets.”

How do these black planets end up rolling without aim through the universe?

Before we answer that question, we have to discuss three types of planets possible in the universe.

Types of Planets

The first of the three types of planets are dwarf planets, like Pluto. Cosmic objects which may be roughly spherical, but are not big enough to have much of an atmosphere, and barely qualify as planets.

Like Pluto, these cold objects form at the rims of solar systems, made mostly of ice and some rocky elements. Because of their distance from the star, it wouldn’t take much of a gravitational nudge to launch them into interstellar space to wander and roam for future eternities.

The second of these are rocky planets like earth or mars. As far as we know, such planets tend to form closer to their parent stars, but in the early formation of a solar system, when the star is still condensing out of its cloud of gas, it has less mass to hold its planets in line. Things are a bit chaotic during this period, and a rocky planet might escape the forming solar system and be launched out into the universe to wander a cold and uncaring cosmos.

The third kind of planet is a gaseous planet like Jupiter or Saturn. These planets are mostly made of gas which is packed together by gravity so that it stays in a dense and solid ball rather than drifting apart in clouds of vapor.

These objects form in much the same way that stars do. They condense out of nebulae and gas clouds in the universe, slowly packing together like snowballs until they are a semisolid sphere.

When such a sphere of gas is big enough, the massive gravity will condense it so tightly that the molecules within it start to fuse, and it becomes a star. But sometimes the gas sphere never quite gets massive enough to begin internal fusion, and it becomes a gaseous planet instead.

Such planets may form from gas clouds and never reach star status, so that they wander forever.

In fact, there are such objects that get so large that they almost become stars. These objects, known as brown dwarfs, emit some light and heat from the energy within them, generated by intense internal pressure, trying to start nuclear fusion, but not quite enough to actually achieve star status. Because the light and heat they emit is so slight, they are very dark and difficult objects to spot.

The Search for Orphaned Worlds

Hot-Lava World Illustration NASA/JPL-Caltech

So how do we know these rogue planets exist if they are so dark and drift far between the stars? There are two ways in which we can detect these black planets. The first of these is a thing called “gravitational lensing.”

Way back in the early 20th century, a man named Albert Einstein turned the world of physics on its head by explaining that gravity isn’t some mysterious, invisible force that reaches out, grabs things, and drags them down. Instead, gravity is the bending of space and time around massive bodies. In fact, Einstein said, this bending of gravity would even affect the path of light such that light would bend in the direction of a massive object as it passed by.

People weren’t entirely willing to accept this strange new idea, and shortly after Einstein published his findings, there was a mad dash around the globe to record a total solar eclipse to see if the starlight passing the sun could be observed to bend.

In fact, it could. Einstein was right. As something with mass passes through the universe, it bends and distorts the starlight coming past it.

So as astronomers record the distant regions of the universe, sometimes the starlight they see bends around something, and they know that an invisible cosmic object has passed by.

The second way in which these dark planets may be spotted is through infrared. Unlike visible types of light, infrared light tends to be emitted by heat sources, whether or not those sources are hot enough to create visible light.

But wouldn’t a planet drifting between the stars be utterly frozen and cold?

Not necessarily. If the planet is large enough to have an atmosphere it has one distinct advantage being far between the stars: it is not receiving a bombardment of ultraviolet rays which would strip its atmosphere of hydrogen. If it has enough hydrogen in its atmosphere, it can retain the resonant heat of its formation and still show up on ultraviolet surveys of the cosmos.

So, one might think, there are a few cold, dead worlds without a star. These must be a few anomalies, right?

Actually, no. When aiming cosmic survey equipment at the scorpios constellation, close to 200 rogue planets have been found drifting in the space just beyond. Lovecraft’s black planets rolling without aim may form a significant amount of the mass of the galaxy.

Could a Rogue Planet Sustain Life?

When speaking of new planets, the first question on everyone’s mind is, “could they have life?”

The only life about which we know anything is the life here on earth. As far as we know, life is very dependent on liquid water and sunlight. After all, things survive by ingesting other living things, or by creating energy through photosynthesis. Without sunlight there would be no liquid water, and no light to photosynthesize.

However, there is the slightest possibility that these planets still have a heat source. Recall that I said they may have some residual warmth trapped inside a hydrogen atmosphere. And we’ve been able to keep microorganisms alive in a heavily hydrogenated environment.

Further, if the planet has a rotating liquid core like earth does, that core generates heat which would be released through geothermal vents. On earth we have life forms which gather and thrive around geothermal vents.

Or, if the rogue planet has one or more moons, it is experiencing the constant tidal force of the moon’s gravity, squeezing and pulling it to generate heat through friction.

In all of these ways, a rogue planet may have enough heat to form and retain some form of life.