Asteroids

If you have been to more than the occasional science fiction movie, you will know that a staple is to have the trusty hero being pursued, but escaping by weaving in and out of an asteroid field. Looks like good cinema, they make it exciting, but it is not very realistic. If asteroids were that common, according to computer simulations their mutual gravity would bring them together to form a planet, and very quickly. In most cases, if you were standing on an asteroid, you would be hard pressed to see another one, other than maybe as a point like the other stars. One of the first things about the asteroid belt is it is mainly empty. If we combined all the mass of the asteroids we would get roughly 4% of the mass of the Moon. Why is that? The standard theory of planetary formation cannot really answer that, so they say there were a lot there, but Jupiter’s gravity drove them out, at the same time overlooking the fact their own theory says they should form a planet through their self-gravity if there were that amny of them. If that were true, why did it leave some? It is not as if Jupiter has disappeared. In my “Planetary formation and Biogenesis”, my answer is that while the major rocky planets formed by “stone” dust being cemented together by one other agent, the asteroid belt, being colder, could only manage dust being cemented together with two other agents, and getting all three components in the same place at the same time was more difficult.

There is a further reason why I do not believe Jupiter removed most of the asteroids. The distribution currently has gaps, called the Kirkwood gaps, where there are very few asteroids, and these occur at orbital resonances with Jupiter. Such a resonance is when the target body would orbit at some specific ratio to Jupiter’s orbital period, so frequently the perturbations are the same because in a given frame of reference, they occur in the same place. Thus the first such gap occurs at 2.06 A.U. from the sun, where any asteroid would go around the sun exactly four times while Jupiter went around once. That is called a 4:1 resonance, and the main gaps occur at 3:1, 5:2, 7:3 and 2:1 resonances. Now the fact that Jupiter can clear out these narrow zones but leave all the rest more or less unchanged strongly suggests to me there were never a huge population of asteroids and we are seeing a small residue.

The next odd thing about asteroids is that while there are not very many of them, they change their characteristics as they get further from the star (with some exceptions to be mentioned soon.) The asteroids closest to the sun are basically made of silicates, that is, they are essentially giant rocks. There appear to be small compositional variations as they get further from the star, then there is a significant difference. How can we tell? Well, we can observe their brightness, and in some cases we can correlate what we see with meteorites, which we can analyse. So, further out, they get significantly duller, and fragments that we call carbonaceous chondrites land on Earth. These contain a small amount of water, and organic compounds that include a variety of amino acids, purines and pyrimidines. This has led some to speculate that our life depended on these landing on Earth in large amounts when Earth was very young. In my ebook “Planetary Formation and Biogenesis”, I disagree. The reasons are that to get enough, a huge number of such asteroids would have to impact the Earth because they are still basically rock, BUT at the same time, hardly any of the silicate based asteroids would have to arrive, because if they did, the isotopes of certain elements on Earth would have to be different. Such isotope evidence also rules these out as a source of water, as does certain ratios such as carbon to chlorine. What these asteroid fragments do show, however, is that amino acids and other similar building blocks of life are reasonably easily formed. If they can form on a lump of rock in a vacuum, why cannot they form on Earth?

The asteroid belt also has the odd weird asteroid. The first is Ceres, the largest. What is weird about it is that it is half water. The rest are essentially dry or only very slightly wet. How did that happen, and more to the point, why did it not happen more frequently? The second is Vesta, the second largest. Vesta is rocky, although it almost certainly had water at some stage because there is evidence of quartz. It has also differentiated, and while the outer parts have olivine, deeper down we get members of the pyroxene class of rocks, and deeper down still there appears to be a nickel/iron core. Now there is evidence that there may be another one or two similar asteroids, but by and large it is totally different from anything else in the asteroid belt. So how did that get there?

I rather suspect that they started somewhere else and were moved there. What would move them is if they formed and came close to a planet, and instead of colliding with it, they were flung into a highly elliptical orbit, and then would circularise themselves where they ended up. Why would they do that? In the case of Vesta, at some stage it suffered a major collision because there is a crater near the south pole that is 25 km deep, and it is from this we know about the layered nature of the asteroid. Such a collision may have resulted in it remaining in orbit roughly near its present position, and the orbit would be circularised due to the gravity of Jupiter. Under this scenario, Vesta would have formed somewhere near Earth to get the iron core. Ceres, on the other hand, probably formed closer to Jupiter.

In my previous post, I wrote that I believed the planets and other bodies grew by Monarchic growth, but that does not mean there were no other bodies growing in a region. Monarchic growth means the major object grows by accreting things at least a hundred times smaller, but of course significant growth can occur for other objects. The most obvious place to grow would be at a Lagrange point of the biggest object and the sun. That is a position where the planet’s gravitational field and the sun’s cancel, and the body is in stable or metastable orbit there. Once it gets to a certain size, however, it is dislodged, and that is what I think was the source of the Moon, its generating body probably starting at L4, the position at the same distance from the sun as Earth, but sixty degrees in front of it. There are other metastable positions, and these may have also formed around Venus or Mercury, and these would also be unstable due to different rocky planets. The reason I think this is that for Vesta to have an iron core, it had to pick up bodies with a lot of iron, and such bodies would form in the hotter part of the disk while the star was accreting. This is also the reason why Earth has an iron core and Mars has a negligible one. However, as I understand it, the isotopes from rocks on Vesta are not equivalent to those of Earth, so it may well have started life nearer to Venus or Mercury. So far we have no samples to analyse that we know came from either of these two planets, and I am not expecting any such samples anytime soon.

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A Giant Planet Around a Dwarf Star

The news here, at least, has made much of the discovery of NGTS-1b, described as a giant planet orbiting a dwarf star. It is supposed to be the biggest planet ever found around such a small star, and it is supposed to be inexplicable how such a big planet could form. One key point that presumably everyone will agree with is, a small star forms because there is less gas and dust in the cloud that will form the star than in the cloud that forms a big star. Accordingly there is less total material to form a planet. Missing from that statement is the fact that in all systems the amount of mass in the planets is trivial compared to the mass of the star. Accordingly, there is nothing at all obscure about an unexpectedly big planet if the planet was just a bit more efficient at taking material that would otherwise go into the star.

So, a quick reality check: the star is supposed to be about 60% the size of the sun, and the planet is about 80% the mass of Jupiter, but has a somewhat larger radius. Planets up to twenty times the size of Jupiter are known around stars that are not more than about three times the size of our sun, so perhaps there is more being made of this “big planet” than is reasonable.

Now, why is it inexplicable how such a large planet could form around a small star, at least in standard theory? The mechanism of formation of planets in the standard theory is that first gas pours in, forms the star, and leaves a residual disk (the planetary accretion disk), in which gas is essentially no longer moving towards the star. That is not true; the star continues to accrete, but several orders or magnitude more slowly. The argument then is that this planetary accretion disk has to contain all the material needed to form the planets, and they have to form fast enough to get as big as they end up before the star ejects all dust and gas, which can take somewhere up to 10 million years (10 My), with a mean of about 3 My. There is some evidence that some disks last at least 30 My. Now the dust collides, sticks (although why or how is always left out in the standard theory) and forms planetesimals, which are bodies of asteroid size. These collide and form bigger bodies, and so on. This is called oligarchic growth. The problem is, as the bodies get larger, the distance between them increases and collision probability falls away, not helped by the fact that the smaller the star, the slower the orbiting bodies move, the less turbulent it will be, so the rate of collisions slows dramatically. For perspective purposes, collisions in the asteroid belt are very rare, and when they occur, they usually lead to the bodies getting smaller, not bigger. There are a modest number of such families of detritus asteroids.

The further out the lower the concentration of matter, simply because there is a lot more space. A Jupiter-sized body has to grow fast because it has to get big enough for its gravity to hold hydrogen, and then actually hold it, before the disk gases disappear. Even accreting gas is not as simple as it might sound, because as the gas falls down the planetary gravitational field, it gets hot, and that leads to some gas boiling off back to space. To get going quickly, it needs more material, and hence a Jupiter type body is argued (correctly, in my opinion) to form above the snow line of water ice. (For the purposes of discussion, I shall call material higher up the gravitational potential “above”, in which case “below” is closer to the star.) It is also held that the snow line is not particularly dependent on stellar mass, in which case various planetary systems should scale similarly. With less material around the red dwarf, and as much space to put it in, everything will go a lot slower and the gas will be eliminated before a planet is big enough to handle it. Accordingly, it seems that according to standard theory, this planet should not form, let alone be 0.036 A.U. from the star.

The distance from the star is simply explained in any theory: it started somewhere else and moved there. The temperature at that distance is about 520 degrees C, and with solar wind it would be impossible for a small core to accrete that much gas. (The planet has a density of less than 1, so like Saturn it would float if put in a big enough tub of water.) How would it move? The simplest way would be if we imagined a Jupiter and a Saturn formed close enough together, when they could play gravitational billiards, whereby one moves close to the star and the other is ejected from the system. There are other plausible ways.

That leaves the question of how the planet forms in the first place. To get so big, it has to form fast, and there is evidence to support such rapid growth. The planet LkCa 15b is around a star that is slightly smaller than the sun, it is three times further out than Jupiter, and it is five times bigger than Jupiter. I believe this makes our sun special – the accretion disk must have been ejected maybe as quickly as 1 My. Simulations indicate that oligarchic growth should not have led to any such oligarchic growth that far out. My explanation (given in my ebook “Planetary Formation and Biogenesis”) is that the growth was actually monarchic. This is a mechanism once postulated by Weidenschilling, in 2004 (Weidenschilling, S., 2004. Formation of the cores of the outer planets. Space Science Rev. 116: 53-56.) In this mechanism, provided other bodies do not grow at a sufficient rate to modify significantly the feed density, a single body will grow proportionately to its cross-sectional area by taking all dust that is in its feed zone, which is augmented by gravitation. The second key way to get a bigger planet is to have the planetary accretion disk last longer. The third is, in my theory, the initial accretion is chemical, and the Jupiter core forms like a snowball, by water ice compression fusing. Further, I argue it will start even while the star is accreting. That only occurs tolerably close to the melting point, so it is temperature dependent. The temperatures are reached very much closer to the star for a dwarf. Finally, the planet forming around a dwarf has one final growth advantage: because the star has a lower gravity, the gas will be drifting towards the star more slowly, so the growing planet, while having a less dense feed, also receives a higher fraction of the feed.

So, in my opinion, apart from the fact the planet is so lose to the star, so far there is nothing surprising about it at all, and the mechanisms for getting it close to the star are there, and there are plenty of other “star-burning” planets that have been found.

Why has the monarchic growth concept not taken hold? In my opinion, this is a question of fashion. The oligarchic growth mechanism has several advantages for the preparation of scientific papers. You can postulate all sorts of initial conditions and run computer simulations, then report those that make any sense as well as those that don’t (so others don’t waste time.) Monarchic growth leaves no real room for scientific papers.