A Planet Destroyer

Probably everyone now knows that there are planets around other stars, and planet formation may very well be normal around developing stars. This, at least, takes such alien planets out of science fiction and into reality. In the standard theory of planetary formation, the assumption is that dust from the accretion disk somehow turns into planetesimals, which are objects of about asteroid size and then mutual gravity brings these together to form planets. A small industry has sprung up in the scientific community to do computerised simulations of this sort of thing, with the output of a very large number of scientific papers, which results in a number of grants to keep the industry going, lots of conferences to attend, and a strong “academic reputation”. The mere fact that nobody knows how to get to their initial position appears to be irrelevant and this is one of the things I believe is wrong with modern science. Because those who award prizes, grants, promotions, etc have no idea whether the work is right or wrong, they look for productivity. Lots of garbage usually easily defeats something novel that the establishment does not easily understand, or is prepared to give the time to try.

Initially, these simulations predicted solar systems similar to ours in that there were planets in circular orbits around their stars, although most simulations actually showed a different number of planets, usually more in the rocky planet zone. The outer zone has been strangely ignored, in part because simulations indicate that because of the greater separation of planetesimals, everything is extremely slow. The Grand Tack simulations indicate that planets cannot form further than about 10 A.U. from the star. That is actually demonstrably wrong, because giants larger than Jupiter and very much further out are observed. What some simulations have argued for is that there is planetary formation activity limited to around the ice point, where the disk was cold enough for water to form ice, and this led to Jupiter and Saturn. The idea behind the NICE model, or Grand Tack model (which is very close to being the same thing) is that Uranus and Neptune formed in this zone and moved out by throwing planetesimals inwards through gravity. However, all the models ended up with planets being in near circular motion around the star because whatever happened was more or less happening equally at all angles to some fixed background. The gas was spiralling into the star so there were models where the planets moved slightly inwards, and sometimes outwards, but with one exception there was never a directional preference. That one exception was when a star came by too close – a rather uncommon occurrence. 

Then, we started to see exoplanets, and there were three immediate problems. The first was the presence of “star-burners”; planets incredibly close to their star; so close they could not have formed there. Further, many of them were giants, and bigger than Jupiter. Models soon came out to accommodate this through density waves in the gas. On a personal level, I always found these difficult to swallow because the very earliest such models calculated the effects as minor and there were two such waves that tended to cancel out each other’s effects. That calculation was made to show why Jupiter did not move, which, for me, raises the problem, if it did not, why did others?

The next major problem was that giants started to appear in the middle of where you might expect the rocky planets to be. The obvious answer to that was, they moved in and stopped, but that begs the question, why did they stop? If we go back to the Grand Tack model, Jupiter was argued to migrate in towards Mars, and while doing so, throw a whole lot of planetesimals out, then Saturn did much the same, then for some reason Saturn turned around and began throwing planetesimals inwards, which Jupiter continued the act and moved out. One answer to our question might be that Jupiter ran out of planetesimals to throw out and stopped, although it is hard to see why. The reason Saturn began throwing planetesimals in was that Uranus and Neptune started life just beyond Saturn and moved out to where they are now by throwing planetesimals in, which fed Saturn’s and Jupiter’s outwards movement. Note that this does depend on a particular starting point, and it is not clear to me  that since planetesimals are supposed to collide and form planets, if there was an equivalent to the masses of Jupiter and Saturn, why did they not form a planet?

The final major problem was that we discovered that the great bulk of exoplanets, apart from those very close to the star, had quite significant elliptical orbits. If you draw a line through the major axis, on one side of the star the planet moves faster and closer to it than the other side. There is a directional preference. How did that come about? The answer appears to be simple. The circular orbit arises from a large number of small interactions that have no particular directional preference. Thus the planet might form from collecting a huge number of planetesimals, or a large amount of gas, and these occur more or less continuously as the planet orbits the star. The elliptical orbit occurs if there is on very big impact or interaction. What is believed to happen is when planets grow, if they get big enough their gravity alters their orbits and if they come quite close to another planet, they exchange energy and one goes outwards, usually leaving the system altogether, and the other moves towards the star, or even into the star. If it comes close enough to the star, the star’s tidal forces circularise the orbit and the planet remains close to the star, and if it is moving prograde, like our moon the tidal forces will push the planet out. Equally, if the orbit is highly elliptical, the planet might “flip”, and become circularised with a retrograde orbit. If so, eventually it is doomed because the tidal forces cause it to fall into the star.

All of which may seem somewhat speculative, but the more interesting point is we have now found evidence this happens, namely evidence that the star M67 Y2235 has ingested a “superearth”. The technique goes by the name “differential stellar spectroscopy”, and what happens is that provided you can realistically estimate what the composition should be, which can be done with reasonable confidence if stars have been formed in a cluster and can reasonably be assigned as having started from the same gas. M67 is a cluster with over 1200 known members and it is close enough that reasonable details can be obtained. Further, the stars have a metallicity (the amount of heavy elements) similar to the sun. A careful study has shown that when the stars are separated into subgroups, they all behave according to expectations, except for Y2235, which has far too high a metallicity. This enhancement corresponds to an amount of rocky planet 5.2 times the mass of the earth in the outer convective envelope. If a star swallows a planet, the impact will usually be tangential because the ingestion is a consequence of an elliptical orbit decaying through tidal interactions with the star such that the planet grazes the external region of the star a few times before its orbital energy is reduced enough for ingestion. If so, the planet should dissolve in the stellar medium and increase the metallicity of the outer envelope of the star. So, to the extent that these observations are correctly interpreted, we have the evidence that stars do ingest planets, at least sometimes.

For those who wish to go deeper, being biased I recommend my ebook “Planetary Formation and Biogenesis.” Besides showing what I think happened, it analyses over 600 scientific papers, most of which are about different aspects.

Star and Planetary Formation: Where and When?

Two posts ago, as a result of questions, I promised to write a post outlining the concept of planetary accretion, based on the current evidence. Before starting that, I should explain something about the terms used. When I say something is observed, I do not mean necessarily with direct eyesight. The large telescopes record the light signals electronically, similarly to how a digital camera works. An observation in physics means that a signal is received that can be interpreted in one only certain way, assuming the laws of physics hold. Thus in the famous two-slit experiment, if you fire one electron through the slits, you get one point impact, which is of too low an energy for the human eye to see. Photomultipliers, however, can record this as a pixel. We have to assume that the “observer” uses laws of physics competently.

The accretion of a star almost certainly starts with the collapse of a cloud of gas. What starts that is unknown, but it is probably some sort of shock wave, such as a cloud of debris from a nearby supernova. Another cause appears to be the collision of galaxies, since we can see the remains of such collisions that are accompanied by a large number of new stars forming. The gas then collapses and forms an accretion disk, and these have been observed many times. The gas has a centre of mass, and this acts as the centre of a gravitational field, and as such, the gas tries to circulate at an orbital velocity, which is where the rate of falling into the star is countered by the material moving sideways, at a rate that takes it away from the star so that the distance from the centre remains the same. If they do this, angular momentum is also conserved, which is a fundamental requirement of physics. (Conservation of angular momentum is why the ice skater spins slowly with arms outstretched; when she tucks her arms in, she spins faster.

The closer to the centre, the strnger gravity requires faster orbital velocity. Thus a stream of gas is moving faster than the stream just further from the centre, and slower than the stream just closer. That leads to turbulence and friction. Friction slows the gas, meaning it starts to fall starwards, while the friction converts kinetic energy to heat. Thus gas drifts towards the centre, getting hotter and hotter, where it forms a star. This has been observed many times, and the rate of stellar accretion is such that a star takes about a million years to form. When it has finished growing, there remains a dust-filled gas cloud of much lower gas density around it that is circulating in roughly orbital velocities. Gas still falls into the star, but the rate of gas falling into the star is at least a thousand times less than during primary stellar accretion. This stage lasts between 1 to 30 million years, at which point the star sends out extreme solar winds, which blow the gas and dust away.

However, the new star cannot spin fast enough to conserve angular momentum. The usual explanation is that gas is thrown out from near the centre, and there is evidence in favour of this in that comets appear to have small grains of silicates that could only be formed in very hot regions. The stellar outburst noted above will also take away some of the star’s angular momentum. However, in our system, the bulk of the angular momentum actually resides in the planets, while the bulk of the mass is in the star. It would seem that somehow, some angular momentum must have been transferred from the gas to the planets.

Planets are usually considered to form by what is called oligarchic growth, which occurs after primary stellar accretion. This involves the dust aggregating into lumps that stick together by some undisclosed mechanism, to make first, stone-sized objects, then these collide to form larger masses, until eventually you get planetesimals (asteroid-sized objects) that are spread throughout the solar system. These then collide to form larger bodies, and so on, at each stage collisions getting bigger until eventually Mars-sized bodies collide to form planets. If the planet gets big enough, it then starts accreting gas from the disk, and provided the heat can be taken away, if left long enough you get a gas giant.

In my opinion, there are a number of things wrong with this. The first is, the angular momentum of the planets should correspond roughly to the angular momentum of the dust, which had velocity of the gas around it, but there is at least a hundred thousand times more gas than dust, so why did the planets end up with so much more angular momentum than the star? Then there is timing. Calculations indicate that to get the core of Jupiter, it would take something approaching 10 million years, and that assumes a fairly generous amount of solids, bearing in mind the solid to gas ratio. At that point, it probably accretes gas very quickly. Get twice as far away from the star, and collisions are much slower. Now obviously this depends on how many planetesimals there are, but on any reasonable estimate, something like Neptune should not have formed. Within current theory, this is answered by having Neptune and Uranus being formed somewhere near Saturn, and then moved out. To do that, while conserving angular momentum, they had to throw similar masses back towards the star. I suppose it is possible, but where are the signs of the residues? Further, if every planet is made from the same material, the same sort of planet should have the same composition, but they don’t. The Neptune is about the same size as Uranus, but it is about 70% denser. Of the rocky planets, Earth alone has massive granitic/feldsic continents.

Stronger evidence comes from the star called LkCa 15 that apparently has a gas giant forming that is already about five times bigger than Jupiter and about three times further away. The star is only 3 million years old. There is no time for that to have formed by this current theory, particularly since any solid body forming during the primary stellar accretion is supposed to be swept into the star very quickly.

Is there any way around this? In my opinion, yes. I shall put up my answer in a later post, although for those who cannot wait, it is there in my ebook, “Planetary Formation and Biogenesis”.