On this Website I often talk about habitable zones and exoplanets that might be something close to earth, or even nearly identical to it.
While this is eminently possible in the universe, even probable, and we’ll no doubt soon discover candidates for such worlds, there are also planets in the universe that we will probably never go near, even if we become a spacefaring galactic empire. But it’s actually worse than that, there are worlds so different from the conditions of a main sequence star system that it seems likely that no one would ever choose to attempt to visit them, not even a highly advanced alien civilization.
To evoke sci fi, a forbidden planet is not a strong enough description for these worlds. They are more like forsaken planets, where no life can ever exist, at least as we can envision it. Before we get to them, it’s worth noting that these worlds are rare.
Even rogue planets hold at least a marginal possibility of geothermal heating maintaining subsurface liquid water and conceivably life. It gets even better within a star system, our own contains more ice shell moons that may have subsurface liquid water than actual planets that can. Life is not limited, at least it’s thought, to habitable zones. Only Earth-Analogue life is, as in surface liquid water, but even Earth shows life under ice. So the prognosis for life in the universe is good, particularly microbial. But life is life, and when you live in a star system with multiple liquid water ocean moons along with an entire wet planet like earth, then a very common, and very good solvent for life must be everywhere in the universe.
Liquid water. But even then, there are other liquids, such as hydrocarbons and even ammonia that might provide life other avenues independent of water. On the surface of Titan, it’s giant lakes of hydrocarbons at very low temperature that might support some kind of microbial biosphere. That on a world where water is really a rock, rather than a fluid and if you told such an alien about your water world, it might exclaim “lava people!” and fear your hot beverage of choice as a lethal weapon. Tea, even simply iced, is not welcome to such a creature. And it’s own hydrocarbon beverages at cryogenic temperatures are not something you’d want to try either, and definitely don’t mix the two, but I digress.
There are planets in the universe that very likely no one wants anything to do with. The first of these, and this really only a sampling because we will find more very wild and strange exoplanets under conditions we haven’t even thought of yet. The question here is about worlds and their relation to the element carbon. Now we live on a world that is teeming with organic chemistry. Carbon is the atom of life, it could be said. And one just needs to step outside and see a green lawn to see a whole lot of carbon chemistry going on. Or look in the mirror, we are carbon-based organisms. But the reason we are is because Carbon has just so many chemical possibilities that it’s not even funny. As an atom that allows for the chemical reactions of life, few others compare, though it’s been noted that silicon can do quite a bit of complex chemistry itself, and might also be an atom that could form the basis of life. But Carbon is unique in its widespread chemical properties. But those properties can turn against life, it’s abilities are so wide ranging.
The first planet type to describe here is a carbon world. It seems to be possible within protoplanetary disks for them to be oxygen poor, and carbon rich. Carbon is among the most common elements in the universe, as is oxygen, but if you start messing with the ratios between carbon and oxygen strange things happen to planets. Carbon worlds are hypothetical planets where the ratios of carbon and oxygen are essentially the opposite of earth. These worlds would be water poor, given that any free oxygen would more likely react with carbon as opposed to hydrogen which would normally form water. In the case of carbon worlds, this more along the lines of forming carbon monoxide, but even deeper than that since the high presence of carbon would alter the geology of such worlds. Our rocky planets like Venus, Earth and Mars have high oxygen geology in the grain of silicon-oxygen compounds. This starts at a carbon planet’s core.
Normally we think of the cores of the Earth, moon, Mars, Venus etc. as iron rich cores.
Metals enthusiasts will know however that when you start adding carbon into that mix, you get into steel territory so the cores of these planets may be better classed as a kind of natural, though primitive steel, or iron-steel composition. These cores might be surrounded by silicon and titanium carbide in molten form, and then we get to graphite.
Graphite in the context of our solar system is interesting. First, we find graphite deposits here on earth, and in fact graphite deposits play a strong role in the history of none other than the pencil. Famous pencil brands in both Europe and the US that were amongst the earliest were all dependent on a local enough supply of natural graphite deposits for the so-called leads, which aren’t lead, but rather graphite and additives, such as clay to adjust hardness. The earliest pencils in Europe were carpenter’s pencils four hundred years ago that just made use of pure rods of graphite mounted in wood. But earth is not the only place that has very pure graphite deposits. Iron Meteorites sometimes have them as well. The Odessa, Texas meteorite that fell in prehistoric times and blew a series of craters into the Texas landscape are noted for inclusions of graphite. I’ve done this, you can take a graphite inclusion from this meteorite and actually write with it like you would a pencil lead.
I don’t recommend this today, since this meteorite in the early 90’s when I did that sold for cents on the pound since it was a large crater forming meteorite that sprayed fragments all over the place, but today may be a bit too expensive to do such experiments with now with moderately scarce graphite inclusions. But needless to say, other than the hardness, meteorite graphite was a whole lot like earth graphite deposits. But there’s more. Also in iron meteorites, an example here would be the Canyon Diablo iron meteorite, better known by the giant Barringer crater it formed in Arizona again in prehistoric times. This meteorite occasionally takes carbon to its next level in that diamonds can be found within it.
They’re tiny and not jewelry grade, but diamond is another very high pressure expression of carbon. The carbon planets are very likely no strangers to this and are thought to have kilometers thick substrata of diamond if the pressures internally to the planet were or are high enough. This means that the volcanism on these worlds would be interesting. Earth can do this, it can bring up diamonds and metals and all sorts of interesting things up from below during volcanic events. Carbon worlds too, so you could end up with deposits of silicon carbide and diamonds in volcanic regions of these worlds. But with carbon, while water may not be on the table, liquid hydrocarbons are.
So if these worlds are low temperature, you could have a situation like Titan where hydrocarbons might fill the role of water and open up for the option of life based on hydrocarbon solvents and low temperature. They might even look something like Titan, with a carbon based smog suspended in a carbon dioxide or monoxide atmosphere. But do they really exist, these Carbon Planets. Short answer is that there are some indications that they do. In 2020 a survey looking at nearby stars found that 12 percent of the sample of those stars had high carbon ratios. If the star has a high carbon ratio, it’s planets may well have high rations as well. In particularly one exoplanet, 55 Cancri E, seems to directly have a very high ratio of carbon so might be one of these worlds. But it’s not that earth like even beyond it’s high carbon in that it’s eight times more massive than earth and very hot, and probably is so geologically different than earth that’s not funny, with a surface dominated by diamond and graphite. So maybe life of some sort on some of the carbon worlds? Well, we need to take a look at Titan’s surface first and really get a handle on environments like that. But for most of the carbon planets, it’s hard to envision how they could form life at all if they are dry even though we are carbon based life. But life surprises, but there are some environments where one might find carbon planets where life probably won’t surprise. Another example of suspected carbon worlds is the Pulsar PSR 1257+2. If you want to remain alive, or arise alive, do not do so around a pulsar.
You won’t last long. Pulsar’s can have planets if they manage to survive the death of the star that formed them, or form after. In this example, the planets may have formed during the pulsar’s disruption of a high carbon star.
Some stars produce carbon and belch it out into the universe. This leads to the possibility of a carbon planet right zone near the center of the galaxy and in dense globular star clusters. Here you have a bunch of giant stars belching out carbon populating the area with the materials needed for high carbon planets. And as that process continues, it’s possible for the numbers of those planets in those dense areas to increase over time, leading to tons of carbon planets in the far future of the universe.
But at the same time, life seems less likely on these worlds in the compact high radiation environment of galactic centers and globular clusters. All manner of close supernovas and irradiating events happen in these places. But there’s also hidden in here a very strange class of planet that at this point is very poorly understood. The reason is that in the case of pulsar planets, you can actually have planets that were formerly stars, or something close to that if you can even class this object like that. It may be in a class of its own. It’s the case of the millisecond pulsar PSR J1719-1438, there appears to have been a binary companion star that appears to be the remains of an evaporated white dwarf, a remnant of a star, where its out laters have been stripped leaving a Jupiter mass world that is crushed to the point of only being about five times the size of earth. This object is very dense, needless to say, and appears to have a makeup of mostly carbon and oxygen in a kind of crystalline form, or in other words, it’s a giant diamond.
Rather than a planet, this object has been described as a diamond star and is really something sort of new as far as objects in the universe go. But given the intense magnetic fields of pulsars, this is not a planet you’d want to visit to mind diamond, you’d be irradiated horribly going anywhere near this system. That said, oddly enough, it has been suggested that Pulsars can actually have habitable zones. This brings us to the pulsar B1257+12. This pulsar has been found to likely host multiple planets, two being super earths and one very small world. But it’s possible that they could be habitable due to the conditions.
The reason is that the super earths in this system might actually be warm enough for liquid water. But it’s a tall order, in that you’d need an atmosphere orders of magnitude thicker than earth’s the protect the planet’s surface against the intense radiation from the pulsar. But, in principle, that may be possible for super earths of high enough mass, even though the atmospheric pressures of these worlds would be comparable to the deepest parts of Earth’s oceans, but those are known to host life. But when thinking about pulsar planets, another class is worth a mention.
This involve strange matter, which is a hypothetical type of matter that we don’t entirely know exists, but isn’t prohibited by nature. Think of it like this. Normal matter is made up of protons and neutrons in atomic nuclei, those in turn are made up of quarks, of which there are six kinds. The combination of those quarks is what makes protons and neutrons, but those aren’t the only possible combinations. But those strange quarks and combinations tend to decay very quickly into more stable forms. Except when you have a neutron star involved. Neutron stars are dead stars, the ultra dense remnants of stars that went supernova. The immense gravitational pull of a neutron star basically crushes matter down into neutrons, forcing protons and electrons together. But it’s possible to get worse near the core of a neutron star where everything gets further crushed down into their component quarks, and that might lead to the formation of strange quarks not normally stable in nature.
Under these conditions, that strange matter that was formed may actually be quite stable. But there’s a problem, it’s effectively poisonous to normal matter. The idea is that this stable strange matter could convert normal matter into strange matter on contact, and it’s even been modeled that a quark star of this nature can actually expel this material and if it came into contact with anything else in the system, it would convert it very rapidly into strange matter. Almost instantaneously, even if the object is another neutron star.
This allows for the possibility of strange matter planets in neutron star systems. No one could ever visit such a world, because if you did, you’d instantly be converted into strange matter and die.
This goes for us, and very likely even the most advanced alien civilizations. Such planets and systems are the do not go zone of the galaxy, even more so than black hole systems. These planets would have very strange properties, such as densities so high that it would be virtually impossible to tear them apart, even if they passed very close to their host neutron star. And, there are candidates for this type of planet that have been found. What it would actually look like if a human were converted into strange matter is anyone’s guess, but regardless, it would not be good.
But now for the ultimate question here, and perhaps the ultimate in worlds that may not be the greatest places to visit. These are exoplanets in orbit of black holes, and there are two types. The first is a hypothetical but unconfirmed type of planet known as a blanket. If you have a black hole with sufficient size, and an accretion disk, it may be possible for planets to actually form in that accretion disk. This would be a very harsh environment indeed, loaded with radiation and energy, but it’s also can be very matter dense allowing for a kind of safe zone within the accretion disk of supermassive black holes that could in principle harbor not just a few planets, but thousands of blanets. But there is another option.
How about a habitable planet orbiting a black hole directly, as in right above it? In principle, this seems possible, but it would yield a world so astonishingly different from anything we can relate to that it’s just mind blowing. First, I’d be remiss if I didn’t mention the movie Interstellar, which tried to envision something like this. This movie was kind of a lost opportunity in that it got the representations of a black hole and a worm hole fairly close, though there have been some things pointed out since but gargantua really us about as close to what a black hole and a worm hole might look like if you were close to them. But for the planets orbiting the black hole, they didn’t get any of that quite right. One of the many reasons for this is that you can’t really have a daylight planet like earth. It’s a different situation, since the black hole isn’t a normal star emitting normal radiation like a star, but rather sucking it in like a heat sink. So to drive habitability and life, you need something more than the black hole and it’s accretion disk. And this is where it gets bizarre. There is such a source if you’re close enough to a supermassive black hole. It’s the cosmic microwave background radiation, the afterglow of the big bang.
This ghostly light is represented by radiation just a few degrees warmer than absolute zero. But subjected to the bizarre and extreme gravity environment just above the event horizon of a supermassive black hole, this light can be crushed and concentrated down to visible light wavelengths.
This could create a sun of sorts. So think of it this way. You’re standing on a black hole planet right above the event horizon. The black hole itself dominates more than half of your sky, with the accretion disk’s light also visible, but with a bright point-like star near the limb of the black hole which would be the concentrated CMB.
It would be your sun, or equivalent of it. And, under the right conditions life might arise or survive here, but only if the black hole was spinning fast, as in approaching the speed of light. Otherwise, the planet would get sucked in before it had much of a chance at anything. Here it’s worth an explanation of how black hole event horizons work on very large scales. If you have a supermassive black hole, and I mean supermassive, the actual destruction of such a planet and spagettification and those sorts of things don’t happen until after you enter the event horizon, indeed if you passed the event horizon of such a black hole you might not even know it until you tried to escape and found you could not. But you’d also need some seriously toned down radiation, such the accretion disk and anything falling into the black hole should need to be pretty depleted as not to irradiate such a planet. And then comes time dilation.
Such a world would be pretty severely time dilated, so a year on a world like this hovering just above a supermassive black hole’s event horizon would mean thousands of years on earth. But in the end, the show stopper for these kinds of planets is that there really isn’t a mechanism for spinning a black hole up to the speeds required to allow for a semi-stable orbit for a planet. And other questions are could such a planet maintain an atmosphere, probably not. But it’s fun to think about, and no doubt equally strange environments exist in the universe that no one has thought of yet.
Thanks for listening! I am futurist and science fiction author John Michael Godier currently worried about alien vacation schemes to close orbiting blanets. Visit Glorkonworld to pass the time getting suntanned by the cosmic microwave background radiation for just 49,000 galactic credits. Sounds nice until you get irradiated by an exoplanet falling into the black hole causing a bad sunburn, only to find when you return from your vacation centuries have passed on your home world. And where is Glorkon? Long gone, and no refunds allowed in normal space time.