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