Planets Without Permission

tl;dr: Trillions of planets drift through space unattached to any star. They’re cold, dark, and probably more numerous than stars themselves.

Most planets orbit stars. That’s the deal – gravity tethers them, starlight warms them, and we call it a “solar system” because we lack imagination.

But some planets said “no thanks” and left.

Or got kicked out.

Or never had a star to begin with.

We call them rogue planets, which sounds cooler than it is. Nobody’s impressed when you’re drifting alone through interstellar night at -220°C.

The Eviction Notice

Two ways to become a rogue planet:

Option 1: Get thrown out. Your home solar system decides it doesn’t like you anymore. Maybe a gas giant migrates inward and flings you into deep space. Maybe another star wanders too close during the chaotic toddler years of planet formation. Either way, you’re fired from the solar system. Simulations show this happens all the time – for every orderly planetary system we see, several orphans got yeeted into the void.

Option 2: Never move in. Some rogues formed alone, condensing straight out of gas clouds without ever getting a star. They’re basically failed stars – too small to ignite fusion, but they collapsed under their own gravity anyway. Star-formation-lite. Diet star.

Both pathways work. We don’t know which is more common yet, but microlensing surveys suggest billions to trillions of rogues in the Milky Way alone. One study claims 20 rogues for every star. If accurate, most planets in the galaxy are homeless.

How to Find Something Invisible

Rogue planets don’t emit visible light (unless they’re still warm from formation) and they don’t reflect starlight (no star to reflect). So how do you find them?

Gravitational microlensing: When a rogue passes in front of a distant star, its gravity bends the star’s light like a lens. The background star briefly brightens. Blink and you miss it – these events last hours or days for Earth-mass planets. You have to monitor millions of stars continuously and hope a rogue wanders through your field of view.

In 2020, astronomers detected OGLE-2016-BLG-1928, an Earth-mass rogue. First confirmed Earth-sized planet drifting alone. The event happened once, will never repeat, and we’ll never see the planet directly. But we know it’s out there.

Direct imaging: Young, massive rogues (5-13 Jupiter masses) are still warm from formation – around 1000-2000 K – and glow faintly in infrared. Point a sufficiently large telescope at a young star cluster and scan for faint red dots that move independently of stars.

In 2021, astronomers found ~70 free-floating planets in the Upper Scorpius region. Just… dozens of Jupiter-sized worlds, each drifting alone, each probably 3-10 million years old, still radiating their birth heat into emptiness.

Radio detection: In 2018, astronomers detected SIMP J0136 via its auroras. This ~13 Jupiter-mass rogue has a magnetic field 200 times stronger than Jupiter’s, creating massive auroral displays that emit radio waves. No star required – it’s generating its own light show, powered by internal magnetic dynamo, audience of zero.

What It’s Like

A rogue planet’s surface temperature without a star: whatever heat remains from formation, plus geothermal energy from the core, minus billions of years of radiating into the cosmic microwave background.

For a young gas giant, maybe 1000 K and cooling. For an old Earth-mass rogue, the surface drops below 50 K. Your atmosphere freezes and falls as snow. Your oceans become ice kilometers thick. Darkness, forever.

But.

Some rogues might have subsurface oceans. If you’re massive enough (~3.5 Earth masses), geothermal heat from radioactive decay could keep water liquid beneath an insulating ice crust. No sunlight, but maybe chemosynthetic life near hydrothermal vents. Europa, but planet-sized and untethered.

Or if a rogue gas giant kept its moons during ejection (simulations say they often do), tidal heating could warm those moons for billions of years. The planet’s gravity squeezes its satellites, generating friction, melting ice, creating hidden oceans. Life without stars – just gravity, friction, chemistry.

Is this likely? No.

Is it impossible? Also no.

The Numbers Don’t Make Sense

Recent surveys suggest there could be 20 rogue planets for every star in our galaxy.

Let’s pause on that.

Our galaxy has ~200-400 billion stars. If the ratio holds, that’s ~4-8 trillion rogue planets. Most smaller than Jupiter, probably many Earth-sized, all drifting silently through interstellar space, invisible unless you know exactly where to look.

The Nancy Grace Roman Space Telescope (launching late 2020s) will find hundreds via microlensing. Maybe thousands. We’ll finally know if these estimates are real or if we’ve wildly overshot.

Either way, the galaxy is fuller than it appears. Stars are the noisy minority. Between them: darkness, yes, but also planets. So many planets. Wandering, cooling, carrying whatever moons or atmospheres they managed to keep, each following its own lonely orbit around the galactic center.

What This Means

Rogue planets teach us that planet formation is violent. Systems don’t assemble peacefully – they eject debris, scatter siblings, sometimes keep one or two planets and toss the rest into the void.

They also complicate the definition of “planet.” If you’re planetary-mass but orbit no star, are you still a planet? (Yes, apparently.) If you formed like a star but are too light to fuse hydrogen, what are you? (A sub-brown dwarf, which is not a helpful name.)

We’ve found binary rogues – pairs of Jupiter-mass objects orbiting each other in empty space, no star involved. JuMBOs, they’re called: Jupiter-Mass Binary Objects. How do two planets end up in a mutual orbit with nothing else around? Did they form together? Get ejected as a pair? We don’t know yet.

Somewhere in this galaxy, there’s a planet roughly Earth’s size drifting through interstellar space. Its surface is frozen solid. Its oceans are ice. Its atmosphere, if it still has one, is a thin shell of frozen nitrogen on the ground.

But a few kilometers down, where geothermal heat seeps upward, pockets of liquid water might persist. And in those pockets, perhaps, something survives.

Or not.

Either way, it’s out there.

Probably trillions of them are.

Dark, cold, and unimaginably alone – just doing laps around the galaxy, asking no one’s permission.