Imagine living on a planet floating all by itself in the darkness of space, without a star to orbit: no sunlight, no romantic sunsets, no seasons. Such lonely worlds, called ‘rogue planets’, were once thought rare, but they have become a hot topic in astronomy as new observations suggest our galaxy may be teeming with them.
For a long time, only a handful of these planets were known, as they are notoriously difficult to detect.
However, advances in technology over the past few decades have helped astronomers uncover these solo travellers in record numbers, revealing a hidden population of rogue planets that far exceeds previous estimates.
Astronomers have two main theories, and both are backed by observations. The first theory is a peaceful story of space gas and dust collapsing under the effect of gravity, similar to how stars are born.
Only in some cases the clump of gas and dust is not massive enough to ignite the nuclear fusion that makes stars “shine”, thus forming a planet-sized object.
The second theory explains the origin of rogue planets as a violent event. In young planetary systems, newly formed planets often jostle for position, gravitationally scattering one another.
Some interactions can be so powerful that they can cause the complete ejection of a planet out of the system into the void of interstellar space.
Rogue planets are extremely hard to detect. Without a star illuminating them, they give off almost no visible light, becoming virtually invisible to traditional telescopes.
They do emit a faint infrared glow however, if they’re young and still warm from their formation. Telescopes such as JWST (James Webb Space Telescope) and the Very Large Telescope (VLT) use infrared imaging to target these young rogues that still glow with heat.
As for those planets that do not emit any light at all, the only way is to wait for a cosmic alignment. Astronomers use a technique called Gravitational Microlensing to turn gravity into a natural magnifying glass.
This occurs because gravity bends light, acting much like a lens in a pair of spectacles. As a rogue planet passes in front of a distant star, its mass briefly warps and amplifies the starlight, revealing the hidden world.
Research in the 2020s has greatly expanded our view. By surveying the Orion Nebula region alone, JWST discovered hundreds of free-floating planets, including a bizarre phenomenon known as Jupiter-Mass Binary Objects, or “ JuMBOs”.
These are pairs of gas giants orbiting each other while drifting through space. The discovery of JuMBOs is particularly significant as they challenge our current modelling, making it hard for astronomers to explain how two planets could be ejected together without being pulled apart.
Most strikingly, a 2023 microlensing study suggested that free-floating planets may outnumber stars in the Milky Way by as much as 20 to one.
This implies that for every star in our night sky, there may be twenty dark worlds wandering nearby, amounting to hundreds of billions of planets across the galaxy.
We all know that there would not be life on Earth without the Sun. Does it mean that rogues planets are entirely dead?
Not necessarily. Astronomers have theorised that a rogue planet with a thick atmosphere or a heavy layer of surface ice could trap enough internal geothermal heat to maintain liquid water beneath the surface.
These ‘Steppenwolf planets’, as some scientists call them, could potentially host life in dark, underground oceans, thriving for billions of years without ever feeling the warmth of a star.
The growing census of rogue planets proves that planetary formation is often a chaotic and unstable process, one that is not yet fully understood.
As it becomes clear that interstellar space is populated by these rogue objects, our fundamental perspective on galactic demographics must shift.
Luckily, upcoming missions such as NASA’s Nancy Grace Roman Space Telescope are designed to detect thousands of rogues as small as Mars.
Astronomers can expect to finally map this invisible population.
