GOING R GUE
Some planets are stuck following the same orbital paths their entire lives. Others break free to wander alone through the vast, empty darkness of interstellar space. And there’s a lot more of them than you might think
Imagine a world where the sun never rises. A planet that doesn’t even have a sun. A place with no pastel-painting-sunsets and no dawn choruses, just a constant veil of faint stars twinkling in a perpetual, indelible inky night.
This unfamiliar scenario would be the reality for any life calling a starless planet home, such as one that’s somehow become untethered from its star, rendering it free to wander through the Universe.
When we think about planets, we usually picture the eight worlds of our Solar System silently orbiting the central star that’s pulling on invisible gravitational strings to keep them close. Yet in recent years astronomers have uncovered an increasingly large population of a very different kind of planet. Worlds that no longer orbit a star at all, worlds that wander the void between stars. Free-floating, rogue planets.
“They are planets that originally orbited a star, but then something happened and they were kicked out,” says Dr Alexander Scholz, an astronomer from the University of St Andrews, who studies these strange, orphaned worlds.
Early in their lives, solar systems are particularly chaotic places. Two sibling planets could gravitationally duel for supremacy, flinging the losing planet out of the system entirely. A planet’s trajectory could also be set onto a similar exit route simply by interacting with the disc of material that it formed from in the first place.
Or perhaps the planet’s banishment came later. A passing star could wrench a planet out of place, or the death of the planet’s star could tip the delicate gravitational balance and destabilise that planet.
Computer simulations have shown that somewhere in the region of 20 to 30 per cent of gas planets could get ejected from their home solar systems to end their lives wandering free. “There are likely to be billions of rogue planets in the Milky Way,” says Scholz. There may even be trillions.
ALONE IN THE DARK
Despite their large number, finding these worldsgone-walkabout is no mean feat. For one thing, there’s no light from a star to illuminate them. Worse, many of the usual methods for finding planets beyond our Solar System rely on spotting the effect they have on their host stars.
Without a host star, many rogue planets are found through their gravitational influence instead, thanks to a technique called microlensing. “Microlensing occurs when the light of a distant star is magnified by the gravitational field of an intervening object, [such as a rogue planet],” says Dr Przemek Mróz, a free-floating planet researcher at the University of Warsaw, Poland.
“Microlensing’s advantage is that it doesn’t depend on the brightness of the object acting as the lens, so it allows us to detect dark objects that don’t emit any light, such as planets.” From the amount of magnification they see, astronomers can estimate the mass of the otherwise invisible lensing object passing in front of it.
Just because an object has the same mass as a planet doesn’t necessarily mean it is a planet, though. “Some astronomers prefer the term ‘planemo’,” says Scholz, referring to a shortened form of planetary-mass object (PMO).
The issue is that the line between a planet and a star is a blurry one. For example, in December 2023, astronomers using the James Webb Space Telescope (JWST) found a free-floating object that was between three and four times the mass of Jupiter, the Solar System’s largest planet. This object wasn’t thought to be a planet though, but rather a brown dwarf, an astronomical object that sometimes goes by another name: failed star.
Brown dwarfs form in the same way as stars, through the gravitational collapse of interstellar gas clouds. The difference is they never gain enough mass to ignite nuclear fusion in their cores – the hallmark of all true stars.
The problem is brown dwarfs aren’t planets either, even though they form from the dusty discs around newborn stars. Given that astronomers have found planets in other solar systems with masses equivalent to more than ten Jupiters, there’s a significant overlap between massive planets and small brown dwarfs.
SCRUTINISE AND ANALYSE
So how do astronomers tell which category a freefloating object, or planemo, belongs to? Thankfully the JWST is making life considerably easier for those who study these so-called rogue planets. Astronomers have already used it to directly image free-floating objects in dense star-forming regions.
Crucially, these rogue planets are still young and hot, meaning they’re bright enough to be seen on their own. Much older ones that have cooled during their long voyages through empty space can’t be scrutinised in the same way.
“Brown dwarfs and planets have different chemistries,” Scholz says. By using JWST’s powerful
instruments to prise apart each object’s light into a colourful spectrum, astronomers can tell what they’re made of. In these spectra, the familiar colours of the rainbow are littered with conspicuous dark lines, known as absorption lines. The different chemical elements present in the object swallow some of the light, meaning it never leaves the object to travel to us. The dark lines are gaps in the spectrum where those colours should have been.
These spectra look a lot like colourful barcodes and, in effect, that’s what they are. Scanning them tells us what the free-floating object is made of and whether it’s likely to be a brown dwarf or a true rogue planet. Ingredients similar to the gas planets of our Solar System – such as water ice, methane and ammonia – rule out the possibility of the object being a brown dwarf.
Astronomers have had to be patient for abilities such as these. “I’ve waited 15 years for JWST to be ready,” says Scholz. Now that it’s finally in space, JWST is already throwing up surprises.
Last year, a team led by Samuel Pearson, a research fellow at the European Space Agency, found a host of free-floating worlds in the Orion Nebula, ten per cent of which appear to exist in pairs. They’ve been dubbed JuMBOs, short for Jupiter Mass Binary Objects. It’s not unusual for stars to exist in such binary pairs, but many of these objects have chemical inventories that scream planet.
“It was completely unexpected,” Scholz says. “We expect planets to be ejected in isolation.” If these duos are in fact jettisoned planets, it’s unclear how they managed to escape from their solar systems still entwined with one another. If indeed that’s what happened.
“It’s a real conundrum,” says Scholz. It could well be a unique quirk of the Orion Nebula, the densest star-forming region in our local Universe. Scholz has already found hints that such binary pairs don’t exist in less-crowded regions of the Universe.
Perhaps it’s only possible where multiple young stars jostle around in such close proximity to one another. The exception and not the rule.
GROWING OUR ROGUES’ GALLERY
Our ability to spot rogue planets may soon undergo a significant change. In 2027, NASA hopes to launch the Nancy Grace Roman Space Telescope (Roman, for short).
“Due to its location in space, Roman is expected to provide data of exquisite quality, much better than those that groundbased observatories can obtain,” says Mróz.
Named after a former NASA Chief of Astronomy, Roman will be a wide-field infrared telescope. One prediction suggests that Roman’s extreme sensitivity will allow astronomers to use microlensing events to spot 400 rogue planets the same size as Earth. It may even be possible to spot worlds the size of Mars, the second smallest planet in the Solar System.
According to Mróz, it will also help us to measure them more accurately.
“Simultaneous observations of microlensing events from Roman and ground-based telescopes will enable us to measure precise masses of free-floating planets,” he says.
What Roman finds could boost the total estimate of free-floating worlds in the Milky Way from the billions into the trillions.
Once in space, Roman will be able to team up with a telescope already up there: the European Space Agency’s Euclid, which launched in 2023. Euclid’s main goal is to probe the dark matter and dark energy thought to dominate the Universe, but a recent study concluded that a joint survey using both Roman and Euclid would find over 100 rogue planets in the first year alone.
“A RECENT STUDY CONCLUDED THAT A JOINT SURVEY USING BOTH ROMAN AND EUCLID WOULD FIND OVER 100 ROGUE PLANETS IN THE FIRST YEAR ALONE"
With all these orphaned worlds swarming through interstellar space, what are the chances of there being life on some of them? After all, it would be a huge chunk of cosmic real estate to dismiss out of hand.
“Life, as we know it, requires an external source of energy,” says Mróz. “For us on Earth, the Sun provides much of the energy living organisms need.”
With non-solar energy adding just one per cent of the total, life on rogue planets would likely require an alternative source of energy.
Young rogue planets would still be hot from their formation. What’s more, if they were ejected from their solar system very early, they wouldn’t have been exposed to the fierce ultraviolet light that usually sees young stars strip atmospheres away from their fledgling planets.
With much of this insulating blanket still intact, perhaps it would be enough to maintain a snug temperature in the frigid wastelands of interstellar space.
Another option would be a rogue gas giant that manages to hold on to a large Earth-sized moon during its ejection, similar to the binary worlds spotted in the Orion Nebula. We know from our own Solar System that the mighty gravity of Jupiter and Saturn constantly flexes their moons, injecting energy into them through a process known as tidal heating.
On moons such as Jupiter’s Europa and Saturn’s Enceladus, this is enough to maintain a sub-surface ocean of liquid water despite being far from the Sun.
“Nature is very rich and we know living organisms on Earth are present in even the most extreme conditions,” says Mróz, alluding to the extremophiles that eke out an existence in the driest, saltiest, coldest and most acidic places on Earth.
“It’s conceivable that life on free-floating planets might exist, but such planets would need to be shielded by thick atmospheres or thick layers of ice to maintain liquid water in their centres,” he adds.
One 2023 study, led by Giulia Roccetti from the European Southern Observatory, concluded that with the right distance from the planet, and the optimal atmospheric pressure, an ocean of liquid water could persist on the moon of a freefloating planet for over a billion years.
There have already been tantalising hints that such moons exist. As far back as 2013, astronomers announced a microlensing event known as MOA-2011-BLG-262 involving two foreground objects magnifying the background light. All astronomers could do, however, is measure the relative masses of the two objects. It could have been a star and a planet, or a rogue planet bigger than Jupiter with a moon smaller than Earth in tow.
That’s the agony and the ecstasy of microlensing: it’s a powerful tool, but once the all-important alignment is broken, the object can never be studied again. All astronomers can do is keep on looking in the hope of one day finally revealing the secrets of some of the Universe’s strangest planets.
Colin is an award-winning astronomy author, science writer and speaker, as well as a fellow of the Royal Astronomical Society.