Rogue worlds and the boundaries of planethood

What is a planet? The answer has never been simple. 

In our Solar System, the accepted definition has changed several times throughout history. The first formal scientific definition of a planet came in 2006, leading to the famous reclassification of Pluto as a dwarf planet. The International Astronomical Union determined that a planet had to meet three criteria: it must orbit the Sun, be massive enough to be round, and have cleared the neighborhood around its orbit. 

That definition was enough to clear things up in our own region of the Cosmos — though not without some controversy. But even in 2006, astronomers were discovering exoplanets (planets outside our Solar System) that didn’t quite fit the IAU’s definition. 

Rogue planets stand out in particular. These are free-floating worlds, not gravitationally bound to any star. Alone in space, they don’t meet the criteria for planethood that the IAU established in our Solar System. Some even sit on the blurry boundary between planets and failed stars. As we discover and study more rogue worlds, we have to expand our picture of what a planet can be.

Rogue worlds two ways

How does a planet find itself drifting alone through space? There are two main ways that we know of. 

Some rogue worlds start off the same way the planets in our Solar System did, coalescing from the swirling disk of gas and dust that surrounds a young star. Then, through gravitational interactions with other large bodies, a planet can be flung out of its star’s orbit, left to wander interstellar space. Astronomers think this tends to happen more to Earth-sized planets, but can also happen to giant ones.

Other rogue worlds are thought to form in much the same way as stars: When material in a cloud of dust and gas becomes dense enough, gravity causes it to collapse into a single object. If the resulting object is massive enough, it begins to fuse hydrogen into helium in its core, becoming a star. If it’s not quite massive enough to fuse hydrogen like a star, it’s considered a brown dwarf, or sometimes a rogue planet — the distinction is fuzzy. 

Some scientists say that it comes down to how the object formed: If it started out in orbit around a star and then got ejected, it’s a rogue planet, but if it formed through cloud collapse, it’s a brown dwarf. Even if two free-floating objects are identical, their formation could determine their categorization. But this definition isn’t universally accepted in the astronomy community. 

Finding a needle in a great cosmic void

The confusion and disagreement around what counts as a rogue planet could potentially be resolved by studying more of them. But finding them can be tricky. 

The usual techniques we use for spotting exoplanets don’t always translate well to free-floating worlds. The transit photometry method, for example, looks for the dip in a star’s brightness caused when an orbiting exoplanet passes in front of it, from our perspective. This method only works when the planet orbits a star, though — not the case with rogue planets. 

Other techniques have proved more useful for finding free-floating worlds. Large sky surveys using infrared light were the first to detect very faint, cool objects drifting alone in space. Rogue planets that are just a few million years old (babies, in cosmic terms) can also glow strongly from the heat created by their formation, emitting dim light that can be directly detectable by sensitive cameras on large telescopes. 

One technique for spotting smaller, more unambiguous rogue planets is gravitational microlensing. 

This effect was actually predicted by Einstein's General Theory of Relativity long before it was observed. Einstein recognized that mass warps the fabric of spacetime. (You don’t need to get your head around why — just know that a large object can noticeably bend light as it passes through space.) When a massive object aligns between us and a more distant light source like a star or galaxy, the effect is that the background light appears to brighten. By looking at dense stellar fields and watching for brief moments of increased brightness, astronomers have been able to find possibly hundreds of rogue planets — and some candidates are as small as Earth, or even smaller. 

New tools, new discoveries

Observations by ground- and space-based telescopes are adding to our understanding of rogue planets every year. In 2025, for example, the European Southern Observatory’s Very Large Telescope found evidence of a free-floating world 620 light-years away that appears to be consuming gas and dust from its surroundings, growing at a rate of six billion tons per second. This is unlike anything ever before seen, and gives us new insight into how rogue planets might grow after they have formed. 

The James Webb Space Telescope (JWST) is also advancing our understanding of rogue worlds thanks to its extraordinarily sensitive instruments. When astronomers pointed JWST at the star-forming region NGC 1333 – about 960 light-years away in the Perseus molecular cloud – they discovered six free-floating planets that appear to have formed independently of stars. 

None of the planets found in NGC 1333 have masses below about five Jupiters, despite JWST having the sensitivity to detect objects that small. This could mean that there is a minimum size of body that can form through cloud collapse.

One surprise was that JWST found signs of a dusty disk around one of these rogue planets, suggesting that satellite bodies might be forming around it. This could mean that entire planetary systems, like Solar Systems in miniature, could form independently of stars. JWST’s powerful imaging capabilities will no doubt enhance what we know about these strange worlds, and, in doing so, teach us more about the dynamics of the Universe. 

NASA’s Nancy Grace Roman Space Telescope also promises to expand the study of rogue planets. Aiming to launch in May 2027, Roman will be able to conduct surveys of huge areas of space over long periods of time. This makes it an ideal tool for detecting changes in brightness caused by phenomena like gravitational microlensing. The mission team has predicted that Roman will detect hundreds of rogue planets with masses as small as Mars. 

Ground and space-based observatories like these have the potential to teach us an enormous amount about rogue worlds. And as we come to better understand these wandering objects, we may continue to shift the boundaries of what we consider a planet.