Could Aliens Know We Are Here?

While an alien could not see us without coming here, why pick here as opposed to all the other stars? We see exoplanets and speculate on whether they could hold life, but how many exoplanets could see our planet, if they held life with technology like ours or a little better? When I wrote the first edition of my ebook “Planetary Formation and Biogenesis” I listed a few techniques to find planets. Then, the most had been found through detecting the wobble of stars through the frequency changes of their line spectra to which a Doppler shift was added. The wobble is caused by the gravity of the planets. Earth would be very difficult to see that way because it is too small. This works best with very large planets very close to stars.

While there are several methods for discovering planets that work occasionally, one is particularly productive, and that is to measure the light intensity coming from the star. If a planet crosses our line of sight, the light dims. Maybe not a lot, but it dims. If you have seen an eclipse of the sun you will get the idea, but if you have seen a transit of Venus or of Mercury you will know the effect is not strong. This is very geometry specific because you have to be able to draw a straight line between your eye, the planet and part of the star and the further the planet is from the star, the smaller the necessary angle. To give an idea of the problem, our planetary system was created more or less on the equatorial plane of the accretion disk that formed the sun, so we should at least see transits of our inner planets, right? Well, not exactly because the various orbits do not lie on one plane. My phrase “more or less” indicates the problem – we have to be exactly edge-on to the plane unless the planet is really close to the star, when geometry lends a hand because the star is so big that something small crossing in front can be seen from wider angles.

Nevertheless, the Kepler telescope has seen many such exoplanets. Interestingly, the Kepler telescope, besides finding a number of stars with multiple planets close to the star has also found a number of stars with only one planet at a good distance from the star. That does not mean there are no other planets; it may mean nothing more than that one is accidentally the only one whose orbital plane lies on our line of sight. The others may, like Venus, be on slightly different planes. When I wrote that ebook, it was obvious that suitable stars were not that common, and since we were looking at stars one at a time over an extended period, not many planets would be discovered. The Kepler telescope changed that because when it came into operation, it could view hundreds of thousands of stars simultaneously.

All of which raises the interesting question, how many aliens, if they had good astronomical techniques, could see us by this method, assuming they looked at our sun? Should we try to remain hidden or not? Can we, if we so desired?

In a recent paper from Nature (594, pp505 – 507 2021) it appears that 1,715 stars within 100 parsecs of the sun (i.e. our “nearest neighbours”) would have been in a position to spot us over the last 5,000 years, while an additional 319 stars will have the opportunity over the next 5,000 years. Stars might look as if they are fixed in position, but actually they are speedily moving, and not all in the same direction. 

Among this set of stars are seven known to have exoplanets, including Ross 128, which could have seen us in the past but no longer, and Teegarden’s star and Trappist-1, which will start to have the opportunity in 29 years and 1642 years respectively. Most of these are Red Dwarfs, and if you accept my analysis in my ebook, then they will not have technological life. The reason is the planets with composition suitable to generate biogenesis will be too close to the star so will be far too hot, and yet probably receive insufficient higher frequency light to drive anything like photosynthesis.

Currently, an Earth transit could be seen from 1402 stars, and this includes 128 G-type stars, like our sun. There are 73 K stars, which may also be suitable to house life. There are also 63 F-type stars. These stars are larger than the sun, from 1.07 to 1.4 times the size, and are much hotter than the sun. Accordingly, they turn out more UV, which might be problematical for life, although the smaller ones may be suitable and the Earth-equivalent planet will be a lot further from the star. However, they are also shorter-lived, so the bigger ones may not have had time. About 2/3 of these stars are in a more restricted transit zone, and could, from geometry, observe an Earth transit for ten hours. So there are a number of stars from which we cannot hide. Ten hours would give a dedicated astronomer with the correct equipment plenty of time to work out we have oxygen and an ozone layer, and that means life must be here.

Another option is to record our radio waves. We have been sending them out for about 100 years, and about 75 of our 1402 stars identified above are within that distance that could give visual confirmation via observing a transit. We cannot hide. However, that does not mean any of those stars could do anything about it. Even if planets around them have life, that does not mean it is technological, and even if it were, that does not mean they can travel through interstellar space. After all, we cannot. Nevertheless, it is an interesting matter to speculate about.

Oumuamua (1I) and Vega

Oumuamua is a small asteroidal object somewhere between 100 – 1000 meters long and is considerably longer than it is broad. Basically, it looks like a slab of rock, and is currently passing through the solar system on its way to wherever. It is our first observation of an interstellar object hence the bracketed formal name: 1 for first, I for interstellar. How do we know it came from interstellar space? Its orbit has been mapped, and its eccentricity determined. The eccentricity of a circular orbit is zero; an eccentricity greater than zero but less than one means the object is in an elliptical orbit, and the larger the eccentricity, the bigger the difference between closest and furthest approach to the sun. Oumuamua was found to have an eccentricity of 1.1995, which means, being greater than 1, it is on a hyperbolic orbit. It started somewhere where the sun’s gravity is irrelevant, and it will continue on and permanently leave the sun’s gravitational field. We shall never see it again, so the observation of it could qualify it for entry in “The Journal of Irreproducible Results”.

Its velocity in interstellar space (i.e.without the sun’s gravitational effects) was 26.3 km/s. We have no means of knowing where it came from, although if is trajectory is extrapolated backwards, it came from the direction of Vega. Of course it did not come from Vega, because when it passed through the space that Vega now occupies, Vega was somewhere else. Given there is no sign of ice on Oumuamua, which would form something like a cometary tail, it presumably came from the rocky zone closer to its system’s star, and this presumably has given rise to the web speculation that Oumuamua was some sort of alien space ship. Sorry, but no, it is not, and it does not need motors to enter interstellar space.

The way a body like Oumuamua could be thrown into interstellar space goes like this. There has to be a collision between two rocky bodies that are big enough to form fragments of the required size and the collision has to be violent enough to give the fragment a good velocity. That will also make a lot of dust. The fragments would be assumed to then go into elliptical orbits, but if there are both rocky planets and giants, the body could be ejected in the same way the Voyager space craft have left our solar system, namely through gravity assists. If the object is on the right trajectory it could get a gravity assist from an earth-like rocky planet, then another one from a giant that could give it enough impetus to leave the system. This presumably happened a long time ago, so we have no idea where the object came from.

Notwithstanding that, Oumuamua brought Vega to my attention, and it is, at least for me, an interesting star. That, of course, is because I have published a theory of planetary formation that is at odds with the generally accepted one. Vega has about twice the mass of the sun, and because it is bigger, it burns faster, and will have a life of about a billion years. It is roughly half-way through that, so it won’t have had time for planets to evolve intelligent life. The concentration of elements heavier than helium in Vega is about a third that of the sun. Vega also has an abnormally fast rate of rotation, so much so that it is about 88% of what would be required to start the star breaking up. This is significant because one of the oddities of our solar system is that the bulk of the angular momentum resides in the planets, while by far the bulk of the mass lies in the star. The implication might be that the lower level of heavier elements meant that Vega did not form cores fast enough and hence it does not have the giant planets of sufficient size to have taken up sufficient angular momentum. The situation could be like an ice skater who spins very fast, but slows the rotation by extending her arms. If the arms are very short, the spin cannot be slowed as much.

The infra-red emissions from Vega are consistent with a dust disk from about 70 – 100 A.U. out to 330 A.U. from the star (an A.U. is the distance from the sun to the Earth). This is assumed to have arisen from recent collisions of objects comparable to those in the Kuiper Belt here. There is apparently another dusty zone at 8 A.U., which would have to have originated from collisions between rocky objects. So far there is no evidence of planets around Vega, but equally there is no evidence there are none. We view Vega almost aligned with its axis of rotation, so most of the usual techniques for finding planets will not work. The transiting technique of the Kepler program requires us to be aligned with the ecliptic (which should be aligned with the equator) and the Doppler technique has similar limitations, although it has more tolerance for deviation. The Doppler technique detects the gravitational wobble of the star and if you could detect such a wobble directly, you could see it from along the polar axis. Unfortunately, we can’t, at least not yet, and worse, detecting such wobbles works best with very large planets around small stars. Here, if you follow my theory and accept the low metallicity, we expect small planets around a very large star. Direct observation has so far only worked for the first few million years of the star, where giant planets are radiating yellow to white light from their surface temperature that is so hot because of the gravitational accretion energy. These cool down reasonably quickly.

What grabbed my attention about Vega was the 8 A.U. dust zone. That can only be generated by a number of collisions because such dust zones have to be replenished. That is because solar radiation slows dust down, and it gradually falls into the star. So to have a good number of frequent collisions, you need a very large number of objects that could collide, which effectively requires a belt of boulders. So why have they not collided and formed a planet, when the standard theory of planetary formation says planets are formed by the collision of boulders to form planetesimals, and these collide to form embryos, which collide to form planets. In my ebook, “Planetary Formation and Biogenesis” I provide an answer, which is basically that to form rocky planets, the collisions have to happen in the accretion disk, and they happen very fast, and they happen because water vapour in the disk helps set cement. Once the accretion disk is removed, further accretion is impossible, other than from objects colliding with a big enough object for gravity to hold all the debris. Accordingly, collisions of boulder-sized objects or asteroids will make dust, and that would create a dust belt that would not last all that long. The equivalent of the Kuiper Belt around Vega appears to be between 3 – 6 times further out. In my theory, if the planet accreted in the same as the sun, it would be approximately 8 times further out. However, lower dust content may make it harder to radiate energy, hence accretion may be slower. If this second belt scales accordingly, it could correspond to our asteroid belt.  We know occasional collisions did occur in our asteroid belt because we see families of smaller fragments whose trajectories extrapolate back to a singe event. So maybe dust belts are tolerably common for short periods in the life of a star. It would not be a great coincidence we see one around Vega; there are a huge number of stars, we see a very large number of accretion disks, so dust belts should turn up sooner or later.

Finally, why does the star spin faster? Again, in my theory, the planets accrete from the solid and take their angular momentum, but then they also take angular momentum from the disk gas through a mechanism similar to the classical Magnus force. Vega has less dust to make planets, hence less angular momentum is taken that way, and because the planets should be smaller there is less gravity to take angular momentum from the gas, and more gas anyway. So the star retains a higher fraction of its angular momentum. All of this does not prove that my theory is right, but it is comforting that it at least has some sort of plausible support. If interested further, check out http://www.amazon.com/dp/B007T0QE6I.