Science that does not make sense

Occasionally in science we see reports that do not make sense. The first to be mentioned here relates to Oumuamua, the “interstellar asteroid” mentioned in my previous post. In a paper (arXiv:1901.08704v3 [astro-ph.EP] 30 Jan 2019) Sekanina suggests the object was the debris of a dwarf interstellar comet that disintegrated before perihelion. One fact that Sekanina thought to be important was that no intrinsically faint long-period comet with a perihelion distance less than about 0.25 AU, which means it comes as close or closer than about two-thirds the distance from the sun as Mercury, have ever been observed after perihelion. The reason is that if the comet gets that close to the star, the heat just disintegrates it. Sekanina proposed that such an interstellar comet entered our system and disintegrated, leaving “a monstrous fluffy dust aggregate released in the recent explosive event, ‘Oumuamua should be of strongly irregular shape, tumbling, not outgassing, and subjected to effects of solar radiation pressure, consistent with observation.” Convinced? My problem: just because comets cannot survive close encounters with the sun does not mean a rock emerging from near the sun started as a comet. This is an unfortunately common logic problem. A statement of the form “if A, then B” simply means what it says. It does NOT mean, there is B therefor there must have been A.

At this point it is of interest to consider what comets are comprised of. The usual explanation is they are formed by ices and dust accreting. The comets are formed in the very outer solar system (e.g.the Oort cloud) by the ices sticking together. The ices include gases such as nitrogen and carbon monoxide, which are easily lost once they get hot. Here, “hot” is still very cold. When the gases volatalise, they tend to blow off a lot of dust, and that dust is what we see as the tail, which is directed away from the star due to radiation pressure and solar wind. The problem with Sekanina’s interpretation is, the ice holds everything together. The paper conceded this when it said it was a monstrous fluffy aggregate, but for me as the ice vaporizes, it will push the dust apart. Further, even going around a star, it will still happen progressively. The dust should spread out, as a comet tail. It did not for Oumuamua.

The second report was from Bonomo, in Nature Astronomy(doi.org/10.1038/s41550-018-0648-9). They claimed the Kepler 107 system provided evidence of giant collisions, as described in my previous post, and the sort of thing that might make an Oumuamua. What the paper claims is there are two planets with radii about fifty per cent bigger than Earth, and the outer planet is twice as dense (relative density ~ 12.6 g/cm^3) than the inner one (relative density ~ 5.3 g/cm^3). The authors argue that this provides evidence for a giant collision that would have stripped off much of the silicates from the outer planet, thus leaving more of an iron core. In this context, that is what some people think is the reason for Mercury having a density almost approaching that of Earth so the authors are simply tagging on to a common theme.

So why do I think this does not make sense? Basically because the relative density of iron is 7.87 g/cm^3. Even if this planet is pure iron, it could not have a density significantly greater than 7.8. (There is an increase in density due to compressibility under gravity, but iron is not particularly compressible so any gain will be small.) Even solid lead would not do. Silicates and gold would be OK, so maybe we should start a rumour? Raise money for an interstellar expedition to get rich quick (at least from the raised money!) However, from the point of view of the composition of dust that forms planets, that is impossible so maybe investors will see through this scam. Maybe.

So what do I think has happened? In two words, experimental error. The mass has to be determined by the orbital interactions with something else. What the Kepler mehod does is determine the orbital characteristics by measuring the periodic times, i.e.the times between various occultations. The size is measured from the width of the occultation signal and the slope of the signal at the beginning and the end. All of these have possible errors, and they include the size of the star and the assumed position re the equator of the star, so the question now is, how big are these errors? I am starting to suspect, very big.

This is of interest to me since I wrote an ebook, “Planetary Formation and Biogenesis”. In this, I surveyed all the knowedge I could find up to the time of writing, and argued the standard theory was wrong. Why? It took several chapters to nail this, but the essence is that standard theory starts with a distribution of planetesimals and lets gravitational interactions lead to their joining up into planets. The basic problems I see with this are that collisions will lead to fragmentation, and the throwing into deep space, or the star, bits of planet. The second problem is nobody has any idea how such planetesimals form. I start by considering chemical interactions, and when I do that, after noting that what happens will depend on the temperatures around where it happens (what happens in chemistry is often highly temperature dependent) you get very selective zoes that differ from each other quite significantly. Our planets are in such zones (if you assume Jupiter formed at the “snow zone”) and have the required properties. Since I wrote that, I have been following the papers on the topic and nothing has been found that contradicts it, except, arguably things like the Kepler 107 “extremely dense planet”. I argue it is impossible, and therefore the results are in error.

Should anyone be interested in this ebook, see http://www.amazon.com/dp/B007T0QE6I

Our closest planetary system?

One of the interesting things about science is how things can change. When I wrote my ebook Planetary Formation and Biogenesis, finishing in 2011, it was generally accepted that the star Tau ceti had no planets, and all the star had orbiting it was a collection of rocks or lumps of ice, in short, debris that had never accreted. Now, it appears, five planets have been claimed to orbit it, and most have very low eccentricities. (The eccentricity measures the difference between closest and farthest distance from the star in an elliptical orbit. If the eccentricity is zero, the orbit is circular.) Orbits close to zero indicate that there have been no major disruptions to the planetary system, which can occur if the planets get too big. Once they get to a size where their gravitational pull acts on each other, the planets may play a sort of game of planetary billiards, often ejecting one from the system, and leaving the rest with highly elliptical orbits, and sometime planets very close to the star.

The question then is, how does my theory perform? The theory suggests that planets form due to chemical interactions, at least to begin, although once they reach a certain size, gravity is the driving force. This has a rather odd consequence in that while the planets are small, their differences of composition are marked, but once they get big enough to accrete everything, they become much more similar, until they become giants, in which case they appear more or less the same. The chemical interactions depend on temperature, and for the rocky planets, on a sequence of temperatures. The first important temperature is during stellar accretion, when temperatures become rather high in the rocky planet zone. For example, the material that led to the start of Earth had to get to at least 1538 degrees Centigrade, so that iron would melt. All the iron bearing meteorites almost certainly reached this temperature, as there is no other obvious way to melt the iron that forms them. At the same time, a number or silicates melt and phase separate. (That is forming two layers, like oil and water.) There is then a second important temperature. When the star has finished forming, which occurs when most of the available gas has reached it, there remains a much lower density gas disk, which cools.

The initial high temperatures are caused by large amounts of gas falling towards the star, and it gets hot due to friction as it loses potential energy. Accordingly, the potential energy depends on the gravitational field of the star, which is proportional to the mass of the star. The heat also depends on the rate of gas falling in, i.e. how much is falling, and very approximately that depends on the square of the mass of the star. Unfortunately, it also depends on how efficient the disk was at radiating heat, and that is unknowable. Accordingly, if all systems have the same pattern of disk cooling, then very very roughly, the same sort of planet will be at a distance proportional to the cube of the stellar mass, at least on this theory.

There are only two solitary stars within 12 light years from Earth that are sufficiently similar to our star that they might be considered to be of interest as supporting life, and only one, Tau ceti is old enough to be of interest as potentially having life. Tau ceti has a mass of approximately 0.78 times our sun’s mass, so on my theory the prediction of the location of the earth equivalent based on our system being a standard (which it may well not be, but with a sample of one, a statistical analysis is not possible) would be at approximately 0.48 AU, an AU (astronomical unit) being the distance from Earth to the sun. The planets present are at 0.105 AU, 0.195 AU, 0.374 AU, 0.552 A.U. and 1.35 AU. If the 0.552 planet is an Earth equivalent, all the others are somewhat further from the planet than expected, or alternatively, if the 0.374 AU planet is the earth equivalent, they are much closer than expected. Which it is within the theory depends on how fast the star formed or how transparent was the disk, both of which are unknowable. Alternatively, the Earth equivalent would be defined by its composition, which again is currently unknowable. The Jupiter equivalent should be at about 2.5 AU on my theory. If it were to be the 1.35 AU planet that was the Jupiter equivalent (mainly water ice) then the 0.552 planet would be the Mars equivalent, and while it would be just in the habitable zone (0.55 – 1.16 AU estimate) the core would have the chemistry of Mars, plus whatever it accreted gravitationally.

Tau ceti is thus the closest star where we have seen a planet in the habitable zone. The planet in the habitable zone is about 4.3 times as massive as earth, so it would be expected to have a stronger gravitational acceleration at its surface, but possibly not that much more because Earth’s gravity is enhanced by its reasonably massive iron core. Planets that accrete much of their mass through simple gravity probably also accrete a lot more water towards the end because water is more common than rock in the disk, apart from the initial stones and iron concentrated through melting. So, with a planet possibly in the habitable zone, but of unknown water content, and unknown nature, if we had the technology would you vote to send a probe to find out what it is like? The expense would make the basic NASA probes look like chickenfeed, and of course, we would never get an answer in our lifetimes, unless someone develops a motor capable of reaching relativistic speeds.

The Fermi paradox, raised by Enrico Fermi, posed the question, if alien life is possible on other planets, given that there are so many stars older than our sun, why haven’t we been visited (assuming we have not)? For all those who say there are better things to spend their money on, they answer that question. The sheer expense of getting started may mean that all civilizations prefer to stay in their own system. Not, of course, that that stops me and others from writing science fictional stories about them.