Hypothetical Planets
A collaboration with Graham Doskoch
I, along with most children in the last century, was taught in elementary school that there were two types of planets. Orbiting closest to the Sun were the rocky planets. Dense and tiny, the rocky planets boasted bizarre solid surfaces ravished by impact craters, weathering, and tectonic activity. Yielding small gravitational pulls and bathed in the warmth of the Sun, rocky planets could only cling to the heaviest of gasses, leaving them with tenuous atmospheres to defend against the onslaught of planetary debris careening through the solar system. And then there were the gas giants. Dominating the realm of the outer solar system, these planetary monsters were tens if not hundreds of times the mass of the entire Earth. Forming farther out, the gas giants were able to accumulate the two most common elements in the universe, hydrogen and helium, far away from the heat of the Sun where such elements would not be blasted into space. Encased in blankets of swirling clouds and surrounded by scores of tiny moons, the gas giants are as ominous as they are beautiful.
And then, in 2009, humanity launched the Kepler Space Telescope. The day that Kepler focused it’s keen eye to the stars of our galaxy was the day that planetary science changed forever. What was supposed to be a tool to verify our supposedly clandestine models of planetary formation instead flipped those models on their heads. Just in our tiny plot of the Milky Way, Kepler discovered rocky behemoths multiple times the mass of the Earth, gas giants racing around their host stars in tighter orbits than Mercury, and even small terrestrial worlds bearing a striking similarity to our home. As Kepler wowed the public with one astonishing discovery after another, astrophysicists scrambled to rewrite the textbooks on planetary formation and evolution. In its wake, the Kepler program taught us that planets not only exist around other stars in the universe, but likely outnumber them two to one.
Planets are some of the most diverse objects in the known universe. This is somewhat due to the loose definition of the word “planet”, but can also be accredited to the wide array of cosmic environments from which a planet can form. Though many new classes of planets have been identified in recent years due to advances in exoplanet hunting technology, still more types of planets remain illusive to detection. No one knows for sure if these hypothetical planets really exist, but studying the possible methods of their formation may teach us more about why our solar system, and our home planet, formed the way they did. This is why I have compiled a list of some of the strangest hypothetical planets theorized to exist in the cosmos. This rundown is by no means complete, but hopefully it will jar your curiosity, as it did mine, about the creation and evolution of planets in our universe. Finally, this article is done in conjunction with fellow space writer and aspiring astrophysicist Graham Doskoch, who has formulated his own article about an unlikely pair of planets bending the rules of planetary science. I implore you to check out his article, which I will link at the end.
Helium Planets
Helium is the second most common element in the universe. It seems paradoxical, then, to state that a planet made entirely of helium is unlikely. Nevertheless, the conclusion becomes inevitable when you consider how a gas planet is formed. In a young solar system, more massive planetesimals accrete mass faster due to their larger gravities; but not all materials are accreted equally. Heavier elements like iron and silicates accrete first, while gasses like hydrogen and helium are initially too light to remain bound to these planets’ weak gravitational pulls. Eventually, a planet reaches the mass required to retain helium, and a runaway growth effect is engaged. At first the planet is too small to capture lower mass hydrogen atoms, but since there is plenty of helium in the protoplanetary disk, the planet continues to grow. Soon it reaches a mass substantial enough to capture hydrogen as well, and because hydrogen is much more abundant than helium, it is gathered at a proportionately faster rate. A hydrogen-dominated gas planet is born.
Despite the inevitable formation of hydrogen dominated gas planets during planetary formation, it may still be possible for a planet comprised mostly, or even exclusively, of helium to form. Because helium is a heavier gas, it can not only remain bound to a planet of lesser mass, but also a planet with a higher temperature. Planets all throughout the galaxy have been observed to have migrated in their pasts, including in our own solar system. It is possible that an ordinary gas giant could be ushered to a new orbit yielding an equilibrium temperature that is tolerable for helium, but too hot to retain hydrogen. In this scenario, the hydrogen gas will be blown away by stellar radiation, leaving behind a helium giant. Though unique in structure, helium planets would not be much different from traditional gas giants. Besides an absence of colorful bands due to helium’s remarkable unreactivity, and a slightly higher density, helium planets and regular gas giants would be nearly indistinguishable.
Iron Planets
Another potential rarity in planetary formation is the hypothetical iron planet. In a similar manner to the helium planet, iron planets would have difficulty forming in nature due to the more abundant presence of rocky silicates, which are also solid in close proximity to the Sun. There is a thin band close to the Sun in which iron can remain solid while silicates are vaporized, making the direct formation of an iron planet possible but unlikely due to the thinly spread distribution of iron in a protoplanetary disk. However, as with the helium planet, nature may have another way to make this hypothetical world a reality.
4.5 billion years ago, Earth was nearly obliterated when a Mars sized assailant landed a grazing cosmic punch on it, destroying itself in the process. Though the impact is praised for giving us our enormous moon, it may have given us something else as well: its iron core. The lack of a large, iron core in the Moon suggests that the impactor must have separated from its iron interior, only to deposit it into the Earth, leaving the lighter, rocky material to coalesce into the Moon. This evidence points to the possibility of an impact capable of stripping a planet almost entirely of a mantle, leaving behind an exposed core predominately composed of iron. Besides yielding higher densities and proportionately smaller radii than their terrestrial cousins, iron planets would lack plate tectonics and magnetic fields due to their rapid rates of interior solidification. Life on such a planet within a star’s habitable zone would be difficult due to iron’s high reactivity with water.
Ocean Planets
Though we have no direct evidence of their existence yet, ocean planets may turn out to be one of the most common classes of planets in the universe. It was once believed that water was a rare commodity in the cosmos, and that the Earth was one of very few places that could host such a volatile molecule. We now know that H2O is actually quite common, with enough water on the solid bodies of the solar system to fill our oceans more than 30 times over. If you include the ice giants (which are primarily composed of water ice), this mass jumps to almost 20 times that of the whole Earth. Earth could have collected a larger percentage of this water had it been larger, but its growth was stunted by the inward migration of Jupiter and Saturn early in the solar system’s formation. This event deprived the inner planets of much of its building material, blasting it into deep space or sucking it into their gaping radii.
In a system without significant migrations by gas planets, immense rocky planets could govern the system’s inner regions. These terrestrial vacuum cleaners would consume a much larger fraction of the disk’s solid material than the gaseous planets, allotting them plenty of water to form oceanic shells potentially hundreds of kilometers in depth. For these worlds to declare their statuses as an ocean planets, they would need to orbit within their host stars’ habitable zones: a region tolerable for liquid water. Also, because water is a volatile molecule, such planets would require thick atmospheres and strong magnetic fields to keep their water from evaporating into space from solar radiation. This is why a planet composed entirely of water (with no iron core) could never exist. Ocean worlds would exhibit lower densities than terrestrial worlds, and would have correspondingly larger radii. Though life on such worlds would have a tough time gaining a foothold, it may still evolve if underwater sea vents are able to break through the densely packed ocean crust.
Chthonian Planets
Besides the peculiar pulsar planets discovered in 1992, almost every other exoplanet discovered in the early days of extrasolar astronomy were Hot Jupiters. A Hot Jupiter is a gas giant which is locked in a tight orbit around its host star, sometimes exhibiting atmospheric temperatures in the thousands of degrees Kelvin. Blasted by stellar radiation, hydrogen cannot remain locked to these planets for long, evaporating away into space over long enough timescales. But what happens to the planet when its hydrogen shell is gone? Most gas giants likely possess solid rocky cores at their interiors. Due to the extreme pressure of tens of thousands of kilometers of gas over millions of years, these rocky cores would be compressed to the extremes of material science, creating an object that is likely multiple times the density of iron. If the planet’s hydrogen shell is completely ripped away by its host star, this strange solid core would remain. Astrophysicists call this hypothetical object a Chthonian planet.
Multiple Chthonian candidates have been identified, but remain dubious due to the uncertainty of these object’s masses. Possibly the most likely example is COROT-7b, 500 light-years away. Current estimates of 7b’s mass and radius grant the world a density that is 1.9 times that of iron, which is impossible unless the world is indeed the stripped remnant of a gas world. Due to the nature of their formations, Chthonian planets would lack atmospheres and strong magnetic fields, and may be home to exotic, compressed materials not found elsewhere in the universe.
Carbon Planets
Possibly the most fascinating hypothetical planet that could exist in our universe is the carbon planet, which is ironic because its formation is probably the most mundane. In fact, a carbon planet would form almost identically to how the Earth formed, except in a star system rich in carbon as opposed to silicates. The resulting planet, however, would be much different. Surrounding a thick mantle of hard titanium carbide, a carbon planet’s crust would be almost exclusively graphite. This would grant the surface of the planet an extremely dark appearance, akin to pencil lead. But compressing tens of kilometers of carbon doesn’t come without consequence. Similarly to how our mantle releases pent-up pressures via volcanoes of liquid rock, carbon planets would require relief to such pressures too. With a lack of silicate matter however, a carbon planet’s “volcanoes” would instead eject the pure compressed carbon-rich contents of its own mantle: diamonds. It is likely that carbon planets would be littered with diamonds, as common as the sand here on Earth.
With a structure highly dependent on carbon, a smoggy CO2 rich atmosphere is almost certain for such worlds. Given the right surface conditions, this could allow for weather patterns similar to those found on Titan; clouds of hydrocarbons could rain down onto the surface, potentially forming rivers, lakes, and even oceans of liquid material. Though unlikely to host life due to carbon’s reactivity with oxygen and water, carbon planets would be awesome places to study and explore.
Conclusion
Isn’t our universe just amazing? Oh, and as promised, here is a link to Graham’s article, where he explores the bizarre and mysterious Kepler-36 system almost 2000 light-years away. Please check it out, and of course, thanks for reading!
