I hope you all had a good Christmas period. We have been having a heat wave, with temperatures way above normal, and a fairly high humidity as well. Even my cat Horatio can’t get up the energy to pester me for early meals. Anyway, something about astronomy, astrophysics, and even science fiction to start the year. During the break, I entered a debate regarding evidence, which eventually led me to Tabby’s star.
There has been odd behaviour in the star KIC 8462852, sometimes called Boyajian’s star, but more commonly called Tabby’s star, after Tabetha Boyajian, who led the team that discovered the strange behaviour. (Fancy having a star named after you.) The reason it is of interest is it has variable flux, with massive dimmings (up to 22% of total flux that occur with 750 day period) and a number of minor ones (approximately 2% that, because there is a number of them, have not as yet been assigned periods). The star is an F type star, about 1.43 times the size of our sun, and it has a surface temperature of about 6750 oK.
So what is going on? What is causing the light to dim? There are two possibilities: the star itself has a variable output, or something crosses between us and the star, and thus dims it. That, of course, is what happens when a planet crosses in front of the star, and that is what the Kepler telescope looks for. However, a planet crossing does not usually manage such a dimming as this because the planet is compact. For example, during the transit of Venus, you would not notice it on Earth without specially looking for it. To get a 22% reduction in light intensity there has to be something with a very large cross-section getting in the road.
Could the star do it by itself? There are variable stars, but they do not usually behave like this. Some multiple stars do, thus when one star goes behind the other, its light gets cut out, but so far there is no evidence of a companion for Tabby’s star. If the star is variable because it changes output, it usually does so rather slowly, and in ways that an astronomer would recognise. There are exceptions. Extreme magnetic activity or a huge swarm of sunspots might do it but it is difficult to envision this happening with a 750-day period.
Suppose something is getting in the road. For a 750-day period, assuming there is only one major body, it would be about 1.8 AU from its star. (An AU is the distance of Earth from the sun.) That makes it somewhat further from its star than Mars is from ours. One proposal is that if the star is far younger than we think, there may be the remains of an accretion disk, that is, a large mass of dust and small stones that is gravitationally coming together. That raises the objection, why not others at other distances? Also, if the standard theory of planetary formation were correct, this would make the star extremely young, because such an accumulations should create planets. Of course that theory could be wrong, as I believe it is. There have been other proposals such as a swarm of comets, and even the debris from a planetary collision. That is usually strongly rejected, but the logic is interesting. It is asserted the probability of seeing such an event is extremely small. So? Kepler has looked at something like 100,000 stars and found this one event, which makes it rare. Once you have a sample of only one, I do not think a probability argument makes any sense at all since no matter how rare the event, if it happens, it is possible to see it.
Another proposal is a large ringed planet, with Trojans. If that is the case, you will see the large event, and a minor event with about 1/6 the periodicity of the main event before and after it. This at least has the merit of being testable. However, the rings would have to be huge, and in one plane normal to the path of the planet.
One of the more bizarre proposals was that the star is surrounded by parts of a megastructure (a Dyson swarm) constructed by an alien civilization to gather energy from the star. Even in my science fiction, I would not suggest that. It took our planet 4.5 billion years to get a technological society, but we are a very long way from being able to construct such a megastructure, yet others are talking about just possibly this star could still be in its formative years. The other point is, why would any alien want to do that? The proposal was that societies might build them to capture their energy needs, but is that plausible? There are other potential shortages besides energy, including materials that you would have to devote to constructing such a monstrous structure. One problem is the periodicity. If you wanted to capture energy, would you not put it a bit closer to the star? If you put it at half the distance, you only need ¼ the materials to get the same energy.
Then there is the question of the absolute size. To get a 22% dimming, and assuming whatever it is totally eclipses the star, the area has to be a dead minimum of 362 billion square miles. In most cases, it has to be seriously bigger. That is a little under 8,000 times the area of the earth. Given that it would have to have a certain amount of thickness for mechanical strength, the mass of this beast would be a serious fraction of the mass of a rocky planet. Where would aliens get the materials? Destroy a planet?
My guess as to what it is? The mechanism for forming rocky planets outlined in my ebook “Planetary Formation and Biogenesis” was that when the star is accreting, the temperatures in the inner part of the disk get quite high, and where Mercury formed, the rocks and iron got sufficiently hot that the silicates stayed in a sticky molten state long enough for the planet to form. Further out it was hot enough to melt the silicates, but because the distances increase, at that point all that formed were a large number of boulders and lumps of iron encased in rock. As the disk started to run out of material, it would cool down. The boulders would collide and make a lot of dust, some of which acted as a cement. That would permit rocks to come together, and water vapour would set the cement, thus sticking them together. The planet Venus was in a rather delicate position because while the rock density was higher there that at Earth’s position, the temperatures from the star were hotter, and it was more difficult to set the cement. Accordingly, Venus was more difficult to get started. One possibility was that it might not get started, and hence it was predicted that some stars might have a boulder belt around them. These might come together gravitationally, but they would not stick.
Weird though it might seem, Tabby’s star more or less fits what might be expected from that theory. Because of the size of the star, if the initial accretion disk had the same characteristics proportionately to our star, the Earth equivalent would be about 2.75 AU from the star, which puts the “blocking object” more or less where the Venus equivalent should be. If it is as I predicted, there should be effects on the colour of the light, because blue light scatters more than red light if it goes through dust. I am waiting to see what happens. If it does turn out to be a gravitationally focused mass of boulders and dust, remember you heard about it here. Then ask yourself, if the standard theory of planetary formation is actually correct, why has this mass not formed a planet? Then the question is, is this evidence for my theory, or is it something else that is misleading me?