A problem with mapping out the history of our Solar System is that it is very hard not to lose a planet. When the last wisps of our protoplanetary disc evaporated in the heat from the young Sun, the planets were released from the gas drag. As we have seen, their motion did not stop here. Pounded by a sea of leftover planetesimals, the orbits of the gas giants begin to move and cross. The resulting gravitational mayhem shuffled Uranus and Neptune outwards and hurled rocks around the Solar System.
While the Moon’s pockmarked surface is evidence of this wild planetary adolescence, the exact details are surprisingly difficult to pin down. The issue is that Jupiter is a huge bully. Attempts to model the movement of the planets during this epoch find that our mammoth planet’s huge gravity will frequently slingshot one of its neighbours into outer space. This leaves a planetary system with one less gas giant. But what if this really happened?
Modelling virtual systems that begin with five gas giants turns out to be more successful at reproducing our own Solar System than with just the big four. The extra planet forms just past Saturn with a similar size to the two icy worlds, Uranus and Neptune. During the ensuing chaos of planet rearrangement, this extra world passes slightly too close to Jupiter, which aggressively boots it from the Solar System. No longer bound to our Sun, we can never know for sure if we once had this fifth gas giant. The lost world will have sailed away into deep space. That is, the planet will have gone rogue.
The two most successful methods for hunting down planets are the radial velocity technique, which searches for the telltale wobble in the star’s position, and the transit technique, which spots a dip in the starlight. The drawback is that both of these depend on the planet having a star. A rogue planet is an orphan world that does not orbit any star. There is therefore no regular, periodic effect to act as a beacon for these stray waifs of our Galaxy. This leaves two options for detection: gravitational microlensing and direct imaging.
For reasons that at first appear baffling, astronomers like building telescopes on the tops of volcanoes. This is actually because the dry and still air at the mountain tops of Hawaii provides the best view of the northern hemisphere sky anywhere in the world. The fact that these peaks are volcanic is a small disadvantage compared with the unrivalled vistas they allow of the Universe.
The Haleakala (‘House of the Sun’) volcano takes up the majority of the Hawaiian island of Maui. It is on this summit that the Pan-STARRS 1.8m (6ft) telescope sits. Standing for PANoramic Survey Telescope And Rapid Response System, the instrument aims to image the entire visible sky several times a month. This wide area results in a lower resolution than for more focused observations, but it is perfect for identifying moving objects. Quick changes in our sky could reveal an asteroid or comet that might pose a danger to Earth. The huge database of images that Pan-STARRS collects is equivalent to 60,000 smartphone photographs each night. It was in this mass of information that an oddball was seen.
While primarily designed to spot asteroids, the wealth of data Pan-STARRS produces is a valuable resource for many projects. One of these was a direct image search for very low mass stars known as brown dwarfs. These dim objects are not massive enough to burn hydrogen in their cores, but do emit a faint red heat signature. Then an object was imaged that was redder than any other brown dwarf in the data.
This redder than red source was 80 light years from Earth and designated PSO J318.5-22. The ‘PSO’ stands for ‘Pan-STARRS1 Object’, while the subsequent digits provide its sky coordinates. Comparing the dim red beacon of PSO J318.5-22’s light to known stars and planets, it seemed far more similar to a young planet than other known brown dwarfs. If that was the case, where had this world come from?
With no star close enough to claim the planet, PSO J318.5-22 appeared to be free floating alone in space. However, near the rogue world was a collection of young stars known as β Pictoris. β Pictoris sits in the constellation of Pictor, the Painter. The group is close to PSO J318.5-22, moving at a similar velocity, and is also young. Moreover, at least two of its stellar members are known to host gas giant planets.
Comparison with the star ages in β Pictoris makes PSO J318.5-22 around 12 million years old. With the protoplanetary gas disc liable to evaporate after 10 million years, the world is a newly formed adolescent in planet terms. It weighs in at close to 6 Jupiters, putting it well below the mass of even a brown dwarf star.
A likely history for PSO J318.5-22 is that the planet formed around a star within the β Pictoris group. Ejection could happen near the end of a star’s life. After the star swells to a red giant, a planet may be expelled from the system as the outer layers of the star are blown away. Alternatively, the remaining stellar remnant may lose too much mass for its gravity to keep the planet in orbit. However, both PSO J318.5-22 and the β Pictoris stars are young. It is therefore more likely that ejection happened as a result of this planet being scattered by a second planet or neighbouring star. Kicked from the group, the planet went rogue. Plausible though this sounds, it is hard to prove this theory. Do we know for sure that rogue planets have always been scattered?
The planet HD 106906b is not rogue. It orbits a star about 300 light years from Earth in the constellation of Crux, the Cross. What is difficult to explain is the incredibly large distance between planet and star.
HD 106906b is a young, super-sized gas giant with 11 times the mass of Jupiter. Like PSO J318.5-22, it was spotted via its heat glow using direct imaging. Being so small and dim, direct imaging of planets is often impossible due to the overpowering light from the star obliterating the planet’s glow. For rogue or very distant planets, the absence of starlight makes this process considerably easier. Both HD 106906b and PSO J318.5-22 are also massive and young, and their bodies still smoulder hot from their formation. HD 106906b is close to PSO J318.5-22 in age; a teenager at 13 million years old.
HD 106906b’s distance from its host star makes Neptune look like a hot Jupiter. It is a whopping 650au from the star, comparable to Sedna in our Solar System. Our furthest large planet, Neptune, sits at just 30au. At such distances, the protoplanetary disc would have been too thin to form a gas giant by either core accretion or disc instability. So how did this planet get to its current location?
A logical answer to this question could be provided if HD 106906b is ‘almost rogue’. The planet may have been scattered by a larger planet to a very distant orbit, but just managed to stay orbiting its star. But in this particular case, the situation is complicated by two other pieces of evidence.
First, HD 106906b is the sole planet spotted orbiting the star. There is no sign of another planet that would have scattered HD 106906b on to a far-out orbit. Such a planet kicker would need to be comparable in mass to HD 106906b’s significant 11 Jupiter masses, so should be detectable. There is also no binary sibling to the star that could have disrupted the planet’s orbit.
Second, the star is surrounded by a sizeable debris disc. These rocky remains of planet formation were what drew observers to examine the system, not expecting to find a planet so far out. In 2014, the disc was observed to extend from roughly 20–120au; extending to just beyond the prime planet-forming real estate. A scattered planet would therefore have to be kicked right through the debris, disrupting the disc.
This led to a new idea being put on the table: could the planet actually be a stellar sibling, forming as a star by collapsing directly from the gas? If HD 106906b and its host star were a binary, there is a spectacularly large difference in mass. HD 106906b is only 1 per cent of the mass of HD 106906, whereas binary systems typically have ratios greater than 10 per cent. Was this really plausible?
A year after HD 106906b’s discovery, fresh evidence changed the picture once again. New observations of the debris disc revealed that it extended much further than previously thought, running from 50au to 500au. More importantly, it was not the initially supposed undisturbed field of rocks. Instead, the outer part of the disc appeared highly asymmetric, with a needle of rubble extending outwards. Could this be the evidence that HD 106906b had indeed been scattered and ploughed through the debris?
The answer remains ambiguous. Due to the tiny size (relative to a star) of HD 106906b, a scattering event is the most physically simple. Yet, there still remains no evidence of the planet that did the scattering. Either a massive world is managing to hide from the observations, or a chance event saw a passing star give the planet a kick. The disruption in the debris disc also appears to be mainly confined to its outer parts. This could imply that a planet has not ploughed through the whole disc. Instead, the pull from HD 106906b forming at its current position could have dragged on the outer parts of the debris, leaving the more distant inner region less affected. A similar possibility is that HD 106906b was actually scattered from around another star. Ejected from its original system, the planet might have become a rogue world, but then passed close enough to HD 106906 to be snaffled and drawn into a distant orbit.
The degeneracy of the scattering and formation models could be resolved if it were definitely impossible to form a planet-sized object by a star-like direct gas collapse. Surprisingly, it turns out that this can happen.
The difficulty with forming planet-sized objects via gas collapse is that of mass. More mass equates to stronger gravity. Therefore, to produce a strong enough gravitational force to collapse inwards, the object must contain enough mass to overwhelm the outward pushing pressure of the gas. For an object as small as a planet, this means that the gas must be incredibly dense. Such densities could occur in a disc instability within a protoplanetary disc, but were not thought to ever happen in the more diffuse gas clouds that birth stars. However, that assumption proved not to be entirely right.
When a cluster of stars forms, the energy pouring from the new stars blows a hot bubble in the surrounding gas cloud. As the bubble expands, it pushes gas outwards to pile up at the bubble’s rim to form a dense shell of material. These regions create the contrasting images of the nebulae, with the dark features marking the cold gas that has been compressed into shells. It was close to these shells that broken-off teardrop fragments of dense gas were seen.
The Rosette Nebula is a star nursery about 4,600 light years away. It is home to many of these tiny fragments, which gained the name globulettes. Formed in the outward shove of the expanding bubble, the globulettes are very high density but have masses of less than 13 Jupiters. If their cores collapsed, a planet-sized object would be born that was not attached to any star. It was a second possible method for a planet to be rogue.
There is no surefire way to differentiate between a free-floating planet ejected from around a star, versus one born as an orphan from a globulette. The best we can do is scrutinise the neighbourhood of rogues for stars that may have given up their parenting duties. So between these two methods for forming planets without stars, just how common are rogue worlds?
Although directly imaging a planet free from the overwhelming starlight is easier than for regularly orbiting worlds, the dim heat signature is still difficult to find. To attempt a survey of rogue worlds, we need to turn to the second method of detection: gravitational microlensing.
As we saw in Chapter 9 when hunting for planets around dim binary stars, gravitational microlensing does not require the light of a host star. Instead, it detects the planet as its mass bends light from a passing background star, briefly brightening its luminosity like a lens. Because it involves the chance alignment between the planet and background star, planet microlensing events are only visible for a few days and requires lucky timing to catch on. However, this is exactly what microlensing surveys are designed to do.
Two large surveys working to catch microlensing events are OGLE and MOA. It was the OGLE survey that discovered the planet orbiting within the binary star system. The second survey, MOA, stands for Microlensing Observations in Astrophysics, and is a collaborative project between New Zealand and Japan to hunt for dimly lit objects from dark matter to exoplanets in the southern hemisphere. Like OGLE, MOA searches for microlensing events near the galactic bulge, where the high density of stars increases the probability of a chance alignment.
The fleeting nature of microlensing events makes it essential to take observations as soon as a lensing alignment is detected. To catch these opportunities, OGLE and MOA have alert systems that spot the sudden brightening of stars and allow swift follow-up observations. In 2011, the two surveys jointly presented 10 rogue planet finds, all with a similar size to Jupiter. By estimating what fraction of the dark planets the surveys probably found, the teams put an approximate number on the rogue worlds that wander our Galaxy. Their conclusion was that there could be a staggering 400 billion rogue planets, a figure almost twice that of the number of stars.
Regardless of how they formed, the Jupiter-sized rogue worlds are not planets that we could picture standing upon. Since ejection of planets is easier if the worlds are smaller, it is nearly definite that the Galaxy is also littered with terrestrial planets without suns. These smaller bodies are currently too low mass to be detected with the present techniques. Yet even if a rogue planet is a gas giant, there is the possibility that it might harbour rocky moons.
The huge mass of the gas giant planets makes them big attractors for smaller worlds to orbit. Jupiter has at least 67 moons, with the four largest – Io, Europa, Ganymede and Callisto – ranging from two-thirds the mass of our Moon to double its mass. This makes them sizeable worlds in their own right. 1 If the rogue gas giants were ejected from their star system, it is likely that their moons would followed.
Moons may not be restricted to planets that had already acquired their satellites before ejection. The Chamaeleon complex is a star-forming region consisting of three clouds. These are descriptively named Chamaeleon I, II and III. As their names imply, the clouds reside in the constellation Chamaeleon, the Lizard, in the southern sky. Chamaeleon I has a few hundred stars, and among these an unusual disc-shaped region was observed.
Cha 110913-773444 takes its name from its coordinates within the Chamaeleon clouds. It is a free-floating object with a mass of 8 Jupiters, making it a planet-sized rogue world. Surrounding the rogue planet is a flat, dusty disc, just like the protoplanetary discs that surround young stars. If the dust in this disc later makes orbiting worlds of its own, then the rogue planet acquires one or more moons.
Although a prospective moon provides a rocky surface, would these worlds be nothing more than heat-less deserts? Without the warmth of a star, is the moon doomed to be a perpetually dark, pockmarked rock?
This is where the moons around our own gas giants offer a sliver of hope. The outer neighbourhood of our Solar System is too cold to support liquid water on the surface of an Earth-like world. Yet there is evidence that several moons may harbour oceans under their icy surfaces. The most likely candidates for these secret seas are Jupiter’s moons, Europa and Ganymede, and Saturn’s moon, Enceladus. So far from the Sun, these icy orbs do not receive enough radiation to keep their oceans liquid. Instead, it is the presence of their parent gas giant that provides the heat.
Because these moons are not the lone satellite children of their gas giants, their orbits around the planet are not perfectly circular. Europa and Ganymede tug on one another and on Jupiter’s innermost large moon, Io, while Enceladus is pulled by its sister moon, Dione. These different pulls result in the moons maintaining slightly elliptical orbits that cause them to feel a changing gravitational pull from the planet as they circle. The varying force flexes the moon like a rubber ball, creating heat from the friction of its interior continuously having to distort. This is the tidal heating mentioned in Chapter 7 as a way of driving volcanic activity on 55 Cancri e. It is a heat source so effective that Europa is considered to be the best bet for life in our own Solar System, outside the Earth.
While the presence of a sea certainly does not guarantee seafood, life is found on Earth wherever there is water. A subsurface ocean on a moon of a rogue planet therefore might be habitable to someone or something. But what if there is no gas giant? Could an Earth-sized rogue be a habitable shelter in the freezing bleakness of deep space?
There is no doubt that if the Sun went dark, we would be in trouble. 2 While the Earth gets a small amount of heat from radioactive materials and the residue warmth from its collisional formation, this is thousands of times smaller than what we receive from the Sun. Alone, this energy would not be sufficient to stop our seas and oceans from freezing solid. To stand the slightest chance of life, a rogue planet needs to stay warm.
However, this comparison with the fate of the Earth if we lost the Sun may be unfair. A rocky planet sent rogue from a protoplanetary disc would not look like the Earth today. If it were ejected during the late stages of planet formation, rogue Earth would still have an early atmosphere made from protoplanetary disc material. Rather than our current mix of nitrogen, carbon dioxide and oxygen, this would be a blanket predominantly of hydrogen.
In the normal course of terrestrial planet formation, the primitive atmosphere of light hydrogen atoms is stripped by the ultraviolet rays from the young sun. Yet if the planet is ejected before its hydrogen has been lost, the coldness of outer space would make it easier to retain. As the hydrogen air cools and becomes denser, it becomes extremely bad at radiating heat and forms an effective blanket over the planet. Even at the remarkably low temperatures of outer space, the hydrogen would remain a gas and not condense on the planet’s surface. The small amount of energy from the planet’s radioactive rocks would therefore be trapped by the atmosphere, giving a surface temperature potentially warm enough to support liquid water.
The only catch is that the planet would need to have accrued a thick layer of hydrogen while within the protoplanetary disc, at least 10–100 times thicker than our current air. This amount is not impossible for the Earth, which could have grabbed enough hydrogen to produce an atmosphere 1,000 times greater than at present.
Of course, if rogue Earth were booted from the solar system sometime after the gas disc had evaporated, it might have already lost its primitive atmosphere. This eventuality could even still occur if our Solar System were disrupted by a passing star. The probability of such an event before the Sun moves towards its red giant phase in 3.5 billion years is about 0.002 per cent. These are not the odds to keep you up at night, but they are significantly higher than winning the National Lottery. 3 The ejection of our planet might save it from being roasted in the Sun’s brightening luminosity, but would anything on Earth survive a trip to deep space? With our current atmosphere, the liquid water on the rogue Earth’s surface would freeze solid. This would doom human life, but what about life in a subterranean ocean like the oceans of the gas giant moons?
Without a gas giant to flex the rock and provide tidal heating, rogue Earth would have to keep an underground sea liquid with only its radiogenic heat. The prospect does not look good. Completely removed from the Sun’s energy, our planet’s inner sources would lead to a frosty surface temperature of just -235°C (-391°F). This is cold enough to produce a layer of ice around 15km (9mi) thick. Beneath this frozen lid, liquid water could potentially exist. Unfortunately, the Earth’s total water supply is only sufficient for a global ice layer of around 4km (2.5mi) deep. All of our planet’s water would therefore end up solid, with none remaining to pool as a hidden sea.
The best we could hope for is for rogue Earth’s geological activity to produce hot pockets of liquid water around erupting vents on the frozen ocean floor. Life could develop in these mini pools, but it would be isolated from the rest of the planet and be exterminated if the hot pockets disappeared.
Rogue Earth is not a prospect that fills anyone with hope for a holiday destination. But does this mean that all terrestrial rogue planets are dead if they lack a primitive atmosphere? It turns out that there are four ways to improve rogue Earth’s prospects: increase its water content, increase its mass, adjust its atmosphere or take the Moon along for the ride.
All these options are plausible. Terrestrial planets around other stars may have accumulated more water by forming near the ice line, or undergoing a heavier bombardment of icy meteorites. One of the prospective compositions for 55 Cancri e in Chapter 7 was indeed a world drowning in oceans. If there is enough water to create a 15km (9mi) ice layer and have liquid to spare, the rogue planet will have a subterranean sea.
Alternatively, a more massive rogue planet will have commandeered a larger supply of radioactive elements and residue formation heat. The extra internal warmth would thin the icy lid required to support liquid water. A 3.5 Earth-mass planet with a similar water fraction to ours could have an ice layer a few kilometres thick and enough water for a hidden sea below.
Even assuming Earth mass and water content, rogue Earth might stay warmer if its atmosphere adjusts. Volcanic activity on the Earth ejects carbon dioxide into the air, which is then removed in a chemical reaction with silicate rock. When the temperature cools, chemical reactions slow and the atmospheric carbon dioxide increases. Normally, this causes the planet to warm as a result of the raised levels of carbon dioxide efficiently trapping heat in a greenhouse effect. 4 On rogue Earth, the carbon dioxide would freeze on the planet’s surface, providing an extra layer of insulation that would reduce the required thickness of the ice.
A final interesting option might occur if the Moon went rogue with the Earth. Ejection from the Solar System would give the Earth and Moon a substantial shake-up. If they remained bound together, the Moon would probably find itself on an eccentric orbit. As the Moon approached rogue Earth on its new elliptical path, both bodies would feel a varying gravitational tug. In the case of a moon around a gas giant (or a planet around a star), the changing pull of the small satellite makes little difference to the huge planet. However, the Earth is a rocky world and much closer in size to the Moon than Jupiter is to its own satellites. Both rogue Earth and the Moon would therefore flex in the changing gravitational pulls and become tidally heated.
Without additional moons to provide a countering tug, the Moon would eventually be pulled back on to a circular orbit. During this time, tidal heating of the rogue Earth could boost its energy up to 100 times above the Earth’s current internal energy supply. This would last (in dwindling amounts) for around 150 million years. Generally speaking, this makes moons good news for rogue planets.
A hidden liquid ocean on a rogue planet would endure as long as the planet’s internal heat would last. Over time, the planet’s heat would leak away to leave a cold interior. The timescale for this turns out to be comparable to the Sun’s lifetime before becoming a red giant and destroying our habitat. While this is a slightly morbid comparison, the similar time limits for Earth and rogue Earth suggest that life in hidden oceans on a rogue world has a chance of developing.
Life on a rogue world would evolve very differently from the Sun-loving surface creatures of the Earth. However, it would be wrong to claim that it would be entirely unknown. Deep in the Earth’s oceans, fissures in the sea bed give rise to hydrothermal vents where sea water touches magma. These boiling-hot springs are teeming with life, despite being too deep to receive any sunlight. This may even have been where the first life on Earth began. If so, life developing on a rogue world might begin in a way that is very similar to that on Earth.
Creatures living around the deep-sea vents are known as chemoautotrophs, and they utilise the strong changes in temperature around the vents to power their biology. This process is not as efficient as photosynthesis, but a planet with no Sun might evolve organisms to better take advantage of this technique.
The theory that a rogue planet could truly support life opens the door to a couple of intriguing speculations. First, a rogue world could potentially be our nearest source of extraterrestrial life, should such a freely roaming planet pass close to our Solar System. Second, an inhabited rogue world ejected from its star system could form a delivery service for spreading organisms between planetary systems. This could be a way of life expanding through the Galaxy without requiring a separate genesis around each star system, or an advanced civilisation capable of interstellar travel.
The idea that life on Earth may have begun via microorganisms carried through outer space is known as panspermia. While not the most widely believed scenario for our planet, the prospect of rogue worlds as planetary starships makes an exciting possibility.
However, even if a complex life could develop in the dark vents of a rogue planet’s seas, it would be nothing like the view from your window. To discover more recognisable inhabitation, would it not be easier to hunt for aliens on another Earth?
Notes
1. Whether a moon could form an Earth-like environment for life is discussed in Chapter 16.
2. Although not for about eight minutes, since that’s how long it takes the Sun’s light to reach us.
3. So if you buy a lottery ticket, you should probably buy a spacesuit.
4. This is Earth’s carbon-silicate thermostat, which is discussed much more in the next chapter.