Round and round Earth goes, year after year, bound by gravity to our Sun.
Yet imagine I were to tell you that Earth is in the minority and there could be more planets moving alone through interstellar space like starships than there are planets orbiting stars.
Such strange worlds, couched in eternal darkness, would be fated to wander the cosmos forever.
As space voyagers, they wouldn’t be heading for a destination, but would cruise endlessly through the final frontier, rarely passing close to other stars.
And yet, some scientists suspect that despite lacking a sun, these rogue planetary starships could nevertheless harbour a crew, in the form of life on their surface.
More amazing alien worlds
How planets lose their stars
Hunting for rogue planets is the mission of astronomer William DeRocco at the University of Maryland and Johns Hopkins University in the United States.
"What got me involved in rogue planets was learning that there are roughly 20 free-floating planets – rogue worlds – in the mass range between Mars and Earth, for every star in the Galaxy," he says.
There are at least 100 billion stars in the Milky Way, which means there are a heck of a lot of rogue planets roaming the darkness between the stars.
"They may be the largest demographic of exoplanet in the Galaxy," DeRocco adds. Yet of all planets, they are the least understood.
How did these worlds come to cruise interstellar space rather than be anchored to a sun? There are two classes of rogue planet, each with a different origin.
In 2024, NASA’s James Webb Space Telescope (JWST) identified half a dozen giant ‘free-floating’ planets in the nebula NGC 1333, part of the Perseus Molecular Cloud star-forming region.
These planets had masses between five and 10 times the mass of Jupiter.
A year before that, JWST had found a treasure trove of 540 free-floating planets, including 40 bizarre binary pairs – their precise origin remains unclear – within the Trapezium star cluster at the heart of the Orion Nebula.
This class of free-floating planet seems to form like stars, condensing from gravitationally collapsing clumps of gas.
They are the low end of the star-formation process, but DeRocco isn’t that interested in them.
It’s the second class of unbound planetary body that grabs his attention. This class has masses similar to Earth and Mars, and forms like typical planets around stars.
Yet something has the power to throw them clear of their home system.
The three-body problem
We know that in their early years, planets don’t tend to stay put. There’s a lot of jostling as they interact with their star’s protoplanetary disc and begin migrating.
As they move, they enter into gravitational resonances with each other – for example, one planet might orbit twice for every three orbits of an inner planet.
This gives the less massive planet a boost, sometimes large enough to kick it out of the system.
"It’s the three-body problem – the physics one, not the book!" says DeRocco.
In this case, the three bodies would be the migrating massive planet, the less massive inner planet and the star that they both orbit.
However, planetary systems are not just three bodies. "A three-body gravitational system is chaotic," says DeRocco.
"Generally you lose one thing and the other two become more closely bound. Now imagine a million-body system – that’s pretty darn chaotic! You end up losing a lot of stuff."
DeRocco pays attention to the range of masses of the ejected planets because hidden within that range are clues as to how planetary systems form.
"The majority of rogue planets are ejected very early in their lifetime," says DeRocco.
"They retain the mass distribution of their systems, which leaves a footprint of these very complicated dynamical processes occurring during the early stages of planet formation. That’s important, because it also teaches us about the planets that get left behind."
The population of rogue worlds can tell us how many planets of a given mass are typically built around stars, which therefore gives clues regarding the details of how planets form and why they have the masses they have.
Planetary magnifying lenses
However, finding rogue worlds is tough. The primary means of discovering exoplanets – transits in front
of their parent star and radial velocity wobbles in stars caused by orbiting planets – do not apply.
Instead, we must watch and wait for chance alignments between rogue planets and stars that create gravitational ‘microlenses’.
Gravitational lensing is predicted by Albert Einstein’s general theory of relativity, which describes how mass warps space, the warp manifesting as gravity.
Astronomers use massive galaxy clusters as cosmic gravitational lenses, but on a smaller scale, even individual stars and planets can cause a degree of gravitational microlensing.
Projects such as Microlensing Observations in Astrophysics (MOA) at Mount John University Observatory in New Zealand watch for microlensing events.
They occur when an unseen foreground object aligns so perfectly with a background star that the foreground object temporarily lenses and brightens the light of the star before moving out of alignment.
Sometimes the lensing object is a rogue planet. A team led by Japan’s Takahiro Sumi of Osaka University used the statistics from MOA to predict that there are 20 rocky rogue planets and two wandering Jupiter-mass gas giants for every star.
NASA’s Nancy Grace Roman Space Telescope, which is set to launch in 2026, will perform the Galactic Bulge Time Domain Survey.
This promises to be the most sensitive microlensing survey ever undertaken, transforming the search for rogue worlds.
“Roman’s sensitivity cuts off just below a Mars-mass planet, so anything we’re going to see will be about Mars’s mass, or greater,” DeRocco says.
Life on a starship planet
Perhaps even more remarkable than the sheer number of wandering planetary starships is the possibility that some could be sufficiently habitable to support a ‘crew’.
How is that possible, if they don’t have a star to breathe warmth on them?
Planets like Earth are not born with a nitrogen–oxygen atmosphere. Earth’s primordial atmosphere, and that of other planets, was a thick mixture of hydrogen and helium.
Soon enough, this was stripped away by the solar wind, but many rogue planets might be ejected from their systems before this occurs, allowing them to retain their primordial hydrogen envelope.
"A hydrogen atmosphere would be an amazing greenhouse insulator, and a planet could stay warm for billions of years," says DeRocco.
The heat would be left over from the planet’s birth and from the decay of radioactive isotopes.
Back in 1999, CalTech’s David Stevenson proposed that this could keep some rogue planets warm enough to support liquid-water oceans and potentially life.
This was quite an insight: the first rogue planet wasn’t discovered until a year later, by Patrick Roche at the University of Oxford and Phil Lucas of the University of Hertfordshire.
Stevenson’s concept was further shown to be ahead of its time when, in 2021, Cambridge astronomer Nikku Madhusudhan proposed the existence of ‘hycean’ worlds – ocean worlds wrapped in a swathe of hydrogen.
Although the concept remains controversial, tentative biosignatures have been detected on one such candidate hycean planet called K2-18b. Habitable rogue planets would be very much like hycean worlds.
Given the possible huge numbers of rocky rogue worlds, that’s a lot of potential unexpected habitats.
The rogue planet next door?
The revelations don’t stop there. If you thought the planets orbiting Proxima Centauri, 4.2 lightyears away, are the closest exoplanets to us, think again.
Odds are there’s a rogue rocky planet even closer.
This begs the question, could we find it and take a good look? There are two problems with that.
One is that such worlds will be very faint and nearly impossible to image, even by the JWST.
The second is that even if a rogue world is closer than Proxima Centauri, that’s still a lot of space to cover and we have no idea where it could be.
Could a rogue planet help us out by paying us a visit like interstellar comets do?
Don’t get your hopes up. There are many more interstellar comets than rogue planets, and while we may see an interstellar comet once every year or so, we could go hundreds of millions – even billions – of years before a rogue planet comes close.
“This is a calculation I do over and over again in case I’m wrong, because I dearly want one to come by tomorrow!” confides DeRocco.
Alas, the maths is not in our favour, and rogue worlds may remain the most enigmatic of all planets because of it.
