Tag Archives: Solar System

The Outer Edge

For the planetary chauvinists in the space movement, the prospects are not bright. In our Solar System only two planets (Mars and Venus) are suitable for human colonization, though colonizing Venus would require terraforming that planet (which would take at least some 200 years). On the other hand suitable planets in other planetary systems are minimally several light years away from us, and any journey to those exo-planets will take a period comparable to terraforming Venus. And in case we arrive at a yet uninhabitable planet, we would need to terraform it anyway.

Well, this analysis assumes that our understanding of our Solar System is complete. Unfortunately, our understanding Solar System is far from complete. As far as we know, there are four terrestrial and four giant planets in our Solar System. (Regardless whether you consider Pluto as a planet or not – we do not chose a side in that controversy -, Pluto is unsuitable for colonization.) The giant planets are unsuitable for colonization, and can’t be terraformed and Mercury is too close to the Sun.

Nevertheless there are still people looking for new planets within our Solar System. The question is whether such planets might exist? Depends on what you are looking for. If you consider Pluto and similar objects as planets, then you could expect the discovery of many planets during the next few decades. However, if you are looking for something more substantial, it will be much harder.

The point is that massive objects are exerting gravity upon each other: the orbits of objects in our Solar System are the result of complex interaction between the several planets and the Sun. Scientists consider the Solar System as a chaotic system, i.e. a small change in the configuration of the Solar System, will have enormous consequences for the orbits of all Solar System objects.

At this moment planetary scientists are able to explain the orbits of the planets at the hand of all known planets. (The so-called anomaly of the orbit of Uranus has been solved, after the mass of Neptune was determined more accurately.) Consequently the gravitational effect of any trans-Neptunian planet on the known planets should be negligible. Either because such planet has a small mass (as in case of Pluto) or because such planet is located at large distance from the known planets.

But if you are a planetary chauvinist looking for planet suitable for colonization, you want this distance to be as small as possible. Hence what is the minimal distance at which a planet might be located?

Many astronomers have proposed trans-Neptunian planets in the last two decades. Computer simulations performed by Rodney Gomes showed that a Mars-sized planet could exist at 53 AU, and a Neptune-sized planet at 1500 AU. An Earth sized planet could exist in between. Calculations done by Lorenzo Iorio show that a planet twice the mass of Earth, should have a minimal distance from the Sun of 496 to 570 AU. An Earth-sized planet might consequently exist somewhere between 53 and 500 AU from the Sun.

(Recent empirical evidence seems to rule out Saturn-sized planets up to 10,000 AU from the Sun, and Jupiter-sized ones up to 26,000 AU.)

Even in the optimistic case that we find a Earth-sized planet within 500 AU from the Sun, the planetary chauvinist has to face an important issue. At such a distance away from the Sun, the planet is most likely to be covered with ice of frozen water, nitrogen and similar substances. In other to terraform such planet, one would need to heat it up.

At first glans Solar power seems to be unsuitable for this task. However, by using a vast amount of space mirrors it might be possible to concentrate enough Solar power to heat up a planet. The important question is, of course, how much we want to spend on Solar mirrors to make a far-away planet inhabitable for humans.

Interstellar space and resources

In reference to our recent post on generation ships, a commenter addressed an important issue: the amount of resources in interstellar space. Most people will probably know that the Solar System contains enormous amounts of resources. But it’s a common perception that space between stellar systems is very empty.

Emptiness is space is, of course, a very relative concept. A cubic meter of average interstellar space does contain much less matter than the best human made matter. This is mainly due to the very large distances between material objects in outer space.

But an important question is where does interstellar space start, or more accurately what are the boundaries of our Solar System? Unfortunately this is not a question with a definite answer. Basically there are two “popular” ideas of what constitutes the boundary of our Solar System: the heliopause and the Oort cloud.

When last summer it was announced that the Voyager 1 had “left” our Solar System, scientist referred to the fact that this space probe that crossed the heliopause. But what is the heliopause? The heliopause is the outer-boundary of the heliosphere, which turns us to the question what is the heliosphere?

Consider this analogy: if you let water flow from your tap into the sink, you will notice there is a kind of “gap” in the layer of water at the bottom of the sink. A similar thing happens with the Sun. The Sun emits constantly all kind of stuff, the so-called Solar winds. We can compare the Sun with your tap, and the Solar winds with the water flowing out of the tap. But just in case of your tap, the Solar wind will ultimately encounter a “thick” medium, and this medium is called the heliosphere. The inner-boundary, where the Solar wind is slowed down, is known as the termination shock. But after a certain distance this thick medium will end, which creates an outer-boundary for the heliosphere: the heliopause.

Because the Solar winds are effectively terminated at the heliopause, many scientists see this mark as the boundary of our Solar System.

The heliopause is located at approximately 119 astronomical units or AU (1 AU is equal to the average distance between the Sun and the Earth) from the Sun. In comparison the orbit of Pluto is located between 30 and 49 AU. Pluto is now considered to be a member of the so-called Kuiper belt, a collection of Trans-Neptunian Objects orbiting the Sun at between 30 and 50 AU form the Sun. Hence the heliopause seems to be a fair choice for the boundary of the Solar system.

But the gravitational field of the Sun has a reach far beyond the heliopause. The orbit of Sedna around the Sun has an estimated perihelion of 76 AU and a perihelion of 943 AU, and hence is large part of Sedna’s orbit is well beyond the heliopause. In fact the Sun’s gravity is in no way affected by the existence of the heliopause. Actually if there were no other stars in the universe, the Sun’s gravity would reach to infinity, though it would still decrease according to an inverse square law.

Astronomers hypothesize that there is a cloud around the Sun, at around 50,000 to 100,000 AU distance, which referred to as the Oort cloud. The Oort cloud is believed to house trillions of objects large than 1 kilometer. All these objects are orbiting the Sun, but what would be the largest possible distance of an orbit around the Sun?

Since the nearest star is roughly the size of the Sun, and is located at about four light years away, the point at which both stars exert an equal gravity is located at about 2 light-years or 125,000 AU. If other star systems have also Oort clouds, those clouds might overlap each other. Consequently a group of Space Settlers which is moving outwards in small increments might enter another star system even without notice.

But even beyond those Oort clouds interstellar space is far from empty. Astronomers speculate that the number of rogue planets, i.e. planets who do not orbit any start but directly around the center of the galaxy, is twice to 100,000 times greater than the numbers of stars in our galaxy. Though the closest known rogue planet is located at about 100 light-years away, there is no particular reason to assume such planet could not be found closer to our Solar System. Only because rogue planets do not radiate much light, they are hard to detect.

For interstellar colonists such a rogue planet might be an attractive place to settle. Enough resources for hundred or even thousands of years, and while the rogue planet continues its way through the galaxy the might came close the another star system. At that point they could make a jump to that star system, if they would wish so.

Though interstellar space is far from empty, we should nevertheless realize that interstellar travel and colonization is an endeavor for the far future and it will probably not happen in this century, maybe not even during the next thousand years. Instead current plans for space colonization should focus on our own Solar System.

UFOs from our own Solar System?


A few years ago I found an interesting article by Gregory Lee Matloff: The Reenchantment of the Solar System: A Proposed Search for Local ET’s. In this very speculative article Matloff argues for the possibility that intelligent extraterrestrials may have colonised our Solar System already. This hypothesis may have serious consequences for the humanization of Space. First we will discuss how plausible Matloff’s hypothesis really is, then we will discuss the possible consequences.

Dr. Matloff argues that extraterrestrial space ships may have crossed interstellar space and may have arrived in our Solar System in a distant past. These local ET’s might live in artificial worlds that masquerade as natural objects like comets. Subsequently he describes two types of civilisations that might be interested in migrating to other star systems than their. This might be a nice subject of a science fiction novel, however his description is nothing but speculation (I would say mostly garbage).


Whether Matloff’s typology makes any sense, is quite irrelevant as long as we can imagine any group of intelligent extraterrestrials with whatsoever motivation to leave their own star system and to cross interstellar space for a few thousand years. The main question is whether such migration is feasible from a technological perspective. Therefore Matloff discusses several suggested propulsion systems. Some of them are simply impossible, such as hyperdrive. Others are not feasible (yet), such as the Bussard ramjet or antimatter rockets. Since we do not know whether any of these system will work in practice we cannot assume that extraterrestrials has developed them.

However, there proposed propulsion systems which are feasible now. Matloff mentions fusion pulse propulsion and Solar sails. They are feasible because they can be build with present day technology, so we can reasonably sure that more advanced extraterrestrials would be able to handle these systems. So if there is civilisation with at least our level technology, they would be able to leave their stellar system and to cross interstellar space.

The next variable we have to consider is whether they can sustain themselves during the voyage, which will take several thousands of years. In theory a civilisation slightly more advanced than ours should be able to build a generation ship. The main problem on such ship would be an adequate energy supply, all other stuff could be recycled if enough energy is available. Nuclear fusion seems to me as the most likely source of energy for such ship, since a relatively small amount of hydrogen will be required annually and could be collected en route.

Another possibility would be sleeper ships, in which the crew is in suspended animation. Or our ET’s could live for several centuries. And if they can travel at near light speed, time dilatation will enormously reduce travel time for the crew. From our perspective their trip would take several thousands of year, but for the immigrants only a few decades.

Therefore it would be possible for a group of extraterrestrial nomads to cross interstellar space. And there so no reason why it couldn’t have happened. As yet we have no evidence for the presence of any extraterrestrial intelligence in our Solar System. But the absence is no evidence of absence, after all it is easier to proof something does exists than to proof that something does not exist (the best way to prove a negative, is to prove something cannot exist).

Detecting ETs in our Solar System

So if ET’s are in our own Solar System, where do we have to look? Matloff’s answer: in the Oort cloud and the Kuiper belt, because these are the outer layers of the Solar System. Any “invader” has to pass them, and since these outer regions contains millions of minor Solar System objects, it would be a nice place to settle down.

If intelligent extraterrestrials has settled the Kuiper belt or Oort cloud, how can we detect them? Especially if they, as has been suggested by Matloff, have disguised their habitat as natural objects. If this would be the case, then it would be a hard job to find them with optical telescopes. However, Matloff has pointed out that these habitats should have an internal temperature several hundred degrees centigrade higher than their environment. (The temperature in the Outskirts of the Solar System is in order of -200 degrees centigrade.) Therefore these world should emit more infrared radiation than we should expect from a natural object in that part of the Solar System.

Fermi Paradox

By scanning the skies for objects with unusual high infrared emissions, we might detect potential alien hide outs. But if intelligent ET’s has already colonised our Solar System, we might wonder why they have not visited us already? This is the so-called Fermi-paradox. One of the traditional replies to this questions, is that aliens would have to travel several light years just to visit us, which would take several hundreds of years for them to complete. Such investment for satisfying a curious interest would not justifiable.

But the Fermi paradox becomes much more relevant, if we assume that intelligent ET’s might live in our Solar System. However, there a few simply explanations. The first one is the most simple one: the simply do not know about our existence. This might seem quite strange to most people, but it is a very reasonable option. Many people would be tempted to think that aliens living in our Solar System cannot possibly miss our planet. However, this would be a kind of what I would call psychological geocentrism. The planet Uranus is larger than our planet and it is much closer to us than Pluto, one of the innermost Kuiper belt objects, and we were not able to discover this particular planet until the invention of the telescope.

For this reason, we cannot assume that intelligent beings who have immigrated from other stellar systems might have discovered Earth. It is quite reasonable that they have never been in the inner Solar System. Why? That is because it cost more energy to through something to the Sun, than out of the Solar System. So unless they have a strong reason to explore the inner Solar System, they would stay in the outskirts.

If they should have discovered Earth, some thousands of years ago if not longer, then they might have visited our planet in a distant past. However if it was some fifty thousand years ago, they might have concluded that our planet contained our planet contained some primitive life. Recall that fifty thousand years ago, there were only a few ten thousand humans on Earth, and their effect on the biosphere was negligible. In this case the aliens might have concluded that our planet was not quite interesting and they have ignored us since.


If either of both scenarios would be true, this might solve another problem. Reported sightings of UFOs have only been made since the 1940s. So if Earth would regularly be visited by aliens, why are there no reports of those visits from before the twentieth century? If you live at more than a few astronomical units away from our planet, it is a hard job to detect whether there is intelligent life. At least that was the case until the early 1900s, when started to use Radio waves for communication.

Since man-made Radio waves leave our planet in all directions. Some of these waves might have been detected by intelligent ETs living in Outskirts of our Solar system. Unlike aliens living at distant star systems, these aliens might travel to Earth within a few years. Before we started to emit artificial radio waves, those aliens had simply no reason to visit us.

However, when the learned that there was a sudden outburst in radio waves from the center of the Solar System, this might have caused some worry among them. Whether it would be a natural phenomenon or the sign of intelligent life, this is a potential danger for their existence. So these aliens might have decided to start a mission to check out this sudden spectral emission.

Of course, this hypothesis is no prove that UFOs are indeed alien spacecraft. Most reported sightings of UFOs are misidentified natural phenomena such as birds, planets, aircraft etcetera or outright lies. There are only a very few genuine reports of unidentified flying objects. The idea that intelligent life might live in the outskirts of the Solar system, is only a possible explanation for some UFO sightings.

Consequences for Space colonies

If there are intelligent extraterrestrials in our own Solar System, this would have serious consequences for any Space settlement. These aliens might be hostile, and they might have the means to destroy space habitats. It’s impossible to know their attitude to us, until we have discovered them, which might be too late. Therefore Space colonists should be prepared to deal with this possibility.

If these aliens exist, they are likely technologically more advanced than us. Therefore governments of Space communities should develop adequate strategies for dealing with potential hostile intelligent ETs. Although we hear from time to time, that governments are designing plans to deal with the threat of an alien invasion, we seriously doubt the soundness of such plans. The problem is that such plan might be based on false assumptions, and we cannot be sure whether their developers know what they are dealing with. Our impression is that all talk of such plans is just propaganda for the general public, in order to prevent panic.

However it’s also possible, that those aliens will choose to be as quiet as possible in order to prevent their discovery. They might choose to leave the inner Solar System to us, just for keeping the peace. This would depend on the state of their military and their moral dispositions.