Planets Being Formed Now

An intriguing observation, recorded in Nature Astronomy3, 749 (2019) is that two planets are being formed around the star PDS 70, a star about 370 light years from Earth in the constellation of Centaurus. The star is roughly 76% the mass of the sun. Its temperature is 3972 degrees K, so it is a bit cooler than our sun, and is about 1.26 time bigger. What that means is it has yet to collapse properly. Because at least some of the accretion disk is still there, gas is still falling towards the star, and towards any planets that are still forming. Because giants have to form quickly, the huge amount of gas falling into them gets very hot, the planets glow, and if they are far enough away from the star, we can see them through very large telescopes. The first such planet to be observed in this system (Labelled PDS 70 b) is about 7 times as big as Jupiter, and is about 23 AU away from the star, i.e. it is a little further away from its star as Uranus is from our star. (1 AU is the Earth-sun distance.)

We can detect the gas flowing into the planet by the spectral signal Hα at 656.28 nm, and by selecting such a signal much of the general light is filtered out and gas can be seen streaming into planetary objects. The planet PDS 70 b was confirmed in the cited reference, but there is an addition: PDS 70 c, which is about 38 AU from its star, also with about 4 AU uncertainty. This is a bit further away from its star as Neptune is from ours, which suggests an overall system that is a bit more expanded than ours.

These planets are interesting in terms of generating a theory on how planets form. The standard theory is that dust from the accretion disk somehow accretes to form bodies about the size of asteroids called planetesimals, and through gravity these collide to form larger bodies, which in turn through their stronger gravity collide to form what are called embryos or oligarchs, and these are about the size of Mars. These then collide to form Earth-sized planets, and in the outer regions collisions keep going until the cores get to about ten times the size of Earth, then these start accreting gas, until eventually they become giants. Getting rid of heat is a problem, and consequently newly formed giants are very hot and seem like mini-stars.

The problem with that theory is timing. The further away from the star, the dust density is much lower simply because there is more space for it and even if you can form planetesimals, and nobody has any idea how they formed, the space between them gets so big their weak gravity does not lead to useful collisions. There is a way out of this in what is called the Grand Tack model. What this postulates is that Uranus and Neptune both grew a little further out than Saturn, and as they grew to be giants their gravity attracted planetesimals from further out. However, now the model argues they did not accrete them, but instead effectively pulled them in and let them go further in towards the star. By giving them inwards momentum, they got “lift” and moved out. They kept going until Neptune ran out of planetesimals, which occurred at about 32 AU.

Now, momentum is mass times velocity, which means the bigger the planet that is moving, the more planetesimals it needs, although it gets a benefit of being able to throw the planetesimal faster through its stronger gravity. That effect is partly cancelled by the planetesimals being able to pass further away. Anyway, why does a star that is about ¾ the size of our star have more planetesimals that go out almost 20% further. In fairness, you might argue that is a rather weak conclusion because the distances are not that much different and you would not expect exact correspondence. Further, while the masses of the giants are so much bigger than ours, you might argue they travelled then accreted the gas.However, there is worse. Also very recently discovered by the same technique are two planets around the young star TYC 8998-760-1, which is about the same size as the sun. The inner planet has a mass of 14 times that of Jupiter and is 162 AU from the star, and the outer planet has a mass of 6 times that of Jupiter and is 320 AU from the star. It is difficult to believe that one star of the same size as another would inadvertently have a huge distribution of planetesimals scattered out over ten times further. Further, if it did, the star’s metallicity would have to be very much higher. Unfortunately, that has yet to be measured for this star. In my opinion, this strongly suggests that this Grand Tack model is wrong, but that leaves open the question, what is right? My usual answer would be what is outlined in my ebook “Planetary Formation and Biogenesis”, although the outer planet of TYC 8998-760-1 may be a problem. However, that explanation will have to wait for a further post but those interested can make up their minds from the ebook.

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