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.

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From Whence Star-burning Planets?

This series started out with the objective of showing how life could have started, and some may be wondering why I have spent so much time talking about the cold giant planets. The answer is simple. To find the answer to a scientific problem we seldom go directly to it. The reason is that when you go directly to what you are trying to explain you will get an explanation, however for any given observation there will be many possible explanations. The real explanation will also explain every connected phenomenon, whereas the false explanations will only explain some. The ones that are seemingly not directed at the specific question you are trying to answer will nevertheless put constraints on what the eventual answer must include. I am trying to make things easier in the understanding department by considering a number of associated things. So, one more post before getting on to rocky planets.

In the previous two posts, I have outlined how I believe planets form, and why the outer parts of our solar system look like they do. An immediate objection might be, most other systems do not look like ours. Why not? One reason is I have outlined so far how the giants form, but these giants are a considerable distance from the star. We actually have rather little information about planets in other systems at these distances. However, some systems have giants very close to the star, with orbits (years) that take days and we do not. How can that be?

It becomes immediately obvious that planets cannot accrete from solids colliding that close to the star because the accretion disk get to over 10,000 degrees C that close, and there are no solids at those temperatures. The possibilities are that either there is some mechanism that so far has not been considered, which raises the question, why did it not operate here, or that the giants started somewhere else and moved there. Neither are very attractive, but the fact these star-burning giants only occur near a few stars suggests that there is no special mechanism. Physical laws are supposedly general, and it is hard to see why these rare exceptions occur. Further, we can see how they might move.

There is one immediate observation that suggests our solar system is expected to be different from many others and that is, if we look again at LkCa 15b, that planet is three times further from the star than Jupiter is from our star, which means the gas and dust there would have more than three times less concentrated, and collisions between dust over nine times rarer, yet it is five times bigger. That star is only 2 – 3 My old, and is about the same size as our star. So the question is, why did Jupiter stop growing so much earlier when it is in a more favourable spot through having denser gas? The obvious answer is Jupiter ran out of gas to accrete much sooner, and it would do that through the loss of the accretion disk. Stars blow away their accretion disks some time between 1 and 30 My after the star essentially finishes accreting. The inevitable conclusion is that our star blew out its disk of gases in the earliest part of the range, hence all the planets in our system will be, on average, somewhat smaller than their counterparts around most other stars of comparable size. Planets around small stars may also be small simply because the system ran out of material.

Given that giants keep growing as long as gas keeps being supplied, we might expect many bigger planets throughout the Universe. There is one system, around the star HR 8799 which has four giants arrayed in a similar pattern to ours, albeit the distances are proportionately scaled up and the four planets are between five and nine times bigger than Jupiter. The main reason we know about them is because they are further from the star and so much larger, hence we an see them. It is also because we do not observe then from reflected light. They are very young planets, and are yellow-white hot from gravitational accretion energy. Thus we can see how planets can get very big: they just have to keep growing, and there are planets that are up to 18 times bigger than Jupiter. If they were bigger, we would probably call them brown dwarfs, i.e. failed stars.

There are some planets that have highly elliptical orbits, so how did that situation arise? As planets grow, they get gravitationally stronger, and if they keep growing, eventually they start tugging on other planets. If they can keep this up, the orbits get more and more elliptical until eventually they start orbiting very close to each other. They do not need to collide, but if they are big enough and come close enough they exchange energy, in which case one gets thrown outwards, possibly completely out of its solar system, and one gets thrown inwards, usually with a highly elliptical orbit. There are a number of systems where planets have elliptical orbits, and it may be that most do, and if they do, they will exchange energy gravitationally with anything else they come close to. This may lead to a sort of gravitational billiards, where the system gets progressively smaller, and of course rocky planets, being smaller are more likely to get thrown out of the system, or to the outer regions, or into the star.

Planets being thrown into the star may seem excessive, nevertheless in the last week it was announced that a relatively new star, RW Aur A, over the preceding year had a 30 fold increase in the amount of iron in its spectrum. The spectrum of a star comes from whatever is on its surface, so the assumption is that something containing a lot of iron, which would be something the size of a reasonably sized asteroid at least, fell into the star. That means something else knocked it out of its orbit, and usually that means the something else was big.

If the orbit is sufficiently elliptical to bring it very close to the star one of two things happen. The first is it has its orbit circularized close to the star by tidal interactions, and you get one of the so-called star-burners, where they can orbit their star in days, and their temperatures are hideously hot. Since their orbit is prograde, they continue to orbit, and now tidal interactions with the star will actually slowly push the planet further from the star, in the same way our moon is getting further from us. The alternative is that the orbit can flip, and become retrograde. The same thing happens as with the prograde planets, except that now tidal interactions lead to the planet slowly falling into the star.

The relevance of all this is to the question, how common is life in the Universe? If we want a rocky planet in a circular orbit in the habitable zone, then we can eliminate all systems with giants on highly elliptical orbits, or in systems with star burners. However, there is a further possibility that is not advantageous to life. Suppose there are rocky planets formed but the star has yet to elimiinate its accretion disk. The rocky planet will also keep growing and in principle could also become a giant. This could be the reason why some systems have Neptune-sized planets or “superEarths” in the habitable zone. They probably do not have life, so now we have to limit the number of possible star systems to those that eliminate their accretion disk very early. That probably elimimates about 90% of them. Life on a planet like ours might be rarer than some like to think.