The Pulsar Habitable Zone
A common assumption when dealing with habitable zones is that they must necessarily exist around main-sequence stars. Particularly dwarf or sun-like stars. But this is not necessarily the case. Other kinds of habitable zones might exist around other objects that under the right conditions could allow life to arise. One such alternative zone is hypothesized to be possible around pulsars. This all goes back to the idea of super-habitability that super-earth planet may be better at hosting life than our world is. However, that doesn’t just mean that there is a better earth out there. It also says that there’s more breathing room for super-earths that aren’t in ideal conditions to harbor life. At first glance, it’s hard to think of anything less suitable than a pulsar. Such an object would create an environment where any planets orbiting it would be subjected to extremely high levels of X-ray radiation.
But it doesn’t stop there. Pulsars emit a wind of charged particles that would erode a planet’s atmosphere very rapidly, and in the process produce gamma rays. All of this would be deadly to life on an earth-sized planet in such a system. However, that may not be the case for a super-earth. In a 2017 paper by A. Patruno and M. Kama, Neutron Star Planets: Atmospheric processes and habitability, they describe a way for a super-earth with a very thick atmosphere, many times more abundant than earth’s, to whether the environment of a pulsar and hold that atmosphere for hundreds of millions, or even billions of years; long enough for some sort of life to arise, at least for a time. And the fact that the planet’s atmosphere is absorbing so much radiation, that could be enough to keep the surface warm enough for liquid water. But the atmospheric pressure on such a world would be astonishing. Again, this would be an atmosphere thousands of times thicker than earth at the start. But in the deep ocean of our own planet, under immense pressure, life still exists. However, the problems facing any potential life on a super-earth orbiting a pulsar don’t just stop at the harsh conditions and the generally shorter period of habitability.
There are also several other factors stacked against this kind of habitable zone. One would be that while planets around pulsars are known to exist, they seem rather scarce. There is an intuitive reason for this. A pulsar is the remnant of a supernova. During that event, any planets that were previously present in the system would have been destroyed or otherwise ejected. As a result, pulsar planets would need to either be captured by the pulsar after the supernova or form from the materials created in the supernova.
While captured planets would be a wildcard, this would mean that any planets that formed in pulsar systems would likely be productive in heavier elements. But weak in lighter elements such as hydrogen meaning it might be difficult to gather the thick atmosphere needed for life. Given that pulsar planets are probably scarce, and atmospheres around them rare, this type of habitable zone is probably not going to be one we encounter too often. However, it is possible, though marginally so, and just maybe, given that this is a vast universe with a correspondingly massive number of pulsars, there exists a pulsar planet with life, though probably only microbial and barely eking out a living in the total blackness of an immensely thick atmosphere, at least for a while.
Are Pulsars really NATURAL?
Knowing and articulating just where you are is the most essential part of navigation, without it you’re lost. We solve this problem in the modern world with various GPS networks, we now use. However, despite the technology of the past, such as compasses, not knowing exactly where you are has been disastrous in history. In 1707, a naval disaster befell the British Royal Navy who lost several capital ships since it was challenging for navigators to calculate longitude correctly. In short, they weren’t where they thought they were and crashed into rocks during lousy weather. This situation led to the development of the marine chronometer, pioneered by John Harrison.
The same is a problem for space too, though on galactic scales rather than the scale of an ocean. In 1972, Carl Sagan, Frank Drake, and Linda Sagan were faced with just such a problem when devising the Pioneer plaques. How do you tell an alien civilization where you are? Setting aside the merits of giving aliens a road map to earth, they used the relative positions of 14 pulsars with the center of the galaxy. It’s a good bet that a space-faring alien civilization would understand the principles of navigation, and also understand how uniquely odd pulsars are and how useful they are for determining where you are.
Pulsars can be like clocks. Specifically, millisecond pulsars emitting x-rays are particularly useful and have an accuracy comparable to an atomic clock. The precision of some pulsars in their emissions are so regular and accurate that they don’t, on their face, seem natural. Because of this pulsars were initially thought to be of human origin. They looked so artificial. They looked like some interference. However, then it was shown that they weren’t and they were distant. Then aliens were seriously considered. We know now that isn’t the case either, or do we. In a 2017 paper by Clement Vidal, Pulsar Positioning System: A quest for evidence of extraterrestrial engineering, the question of the origin of pulsars has been reopened. At least for a specific type. Vidal details that pulsars were eliminated as being of potential alien origin for five reasons:
- The first was because of the truly massive amount of energy a pulsar radiates. However, Vidal shoots this down by pointing out that we have no idea what the energy usage of an advanced alien civilization would be. After all, we do talk about Kardashev scales and building Dyson spheres, all of which implies high energy use. We also don’t know what the rules are there. If pulsars aren’t natural outright, it will take a lot to create one. However, if they are partially natural. But civilization is tweaking their output for navigation use, energy and resource expenditure equations change.
- The second contributing argument to Extraterrestrial Intelligence, ETI, being ruled out with pulsars was that they are not unique. The debate was made years ago that it would be unlikely for two separate alien civilizations to be broadcasting at similar frequencies towards earth at the same time. Vidal shoots this down too. Noting that aliens coming to the surface and seeing two cellphones transmitting on the same rate would be silly to conclude that it was natural. He goes further and points out that nothing says the pulsars are meant as messages to us. Instead, they could be navigational beacons.
- The third argument was that the signals from pulsars did not emanate from a planet. Vidal points out that there is a bias here, and he’s right again, that an artificial extraterrestrial signal does not necessarily need to come from a planet, or even anywhere close to a star system. It’s probably more likely that it would, but in the end, we don’t know where they might come from.
- The fourth argument is that pulsars are not narrowband. Not by a long shot. SETI tends to favor narrowband signals as candidates for detection because it doesn’t make sense, or didn’t use to make sense, to beam out a communication signal broadband because you’d waste much energy for no reason. Vidal counters that we emit in a broadband way. And if the pulsar were natural with one area of the spectrum was being tweaked for communications, the rest of the emissions could be the remaining natural transmissions of the pulsar. Fair enough.
- This final one is the strongest and this is where I get skeptical. Entirely natural models for pulsars exist we have a reasonably good understanding of them and how they form. In other words, an alien origin explanation just isn’t needed when you have natural models that fit the bill. Vidal counters this with three answers. (a) To consider pulsar formation as an effect of extraterrestrial intelligence; you don’t need to throw those models out. They could be entirely natural and simply are being manipulated by the ETI. Specifically, millisecond pulsars, or could be present in nature, but could also be created by artificially triggering a supernova. (b) there isn’t a unified model that explains all pulsars, they are a vast field with lots of variations. (c) Lastly, Vidal notes that it’s conspicuous that some pulsars pulse so accurately as to be on a level with atomic clocks, and that they are distributed well enough to be used in navigation. That looks a little odd to be sure. But the author also notes, rightly, that not all pulsars are suitable for navigation, mainly only the millisecond ones.
This was an interesting paper and serves to show the usefulness of reexamining old ideas. I’m skeptical about this one. Given the weight of pulsar research, but it does open up a new possibility for SETI and should be looked at. However, what I found most interesting is that even if all pulsars are entirely natural. They may well be the one thing in our galaxy that all spacefaring civilizations present are using for navigation. We’re looking at them too and used them on the Pioneer plaques. So in a sense, we may already be a user of the galactic positioning system without realizing it.
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