How to make planets

On Jan. 1, 2019, the New Horizons space probe flew past a very important object in the outer solar system. 

It was a leftover lump of the original construction material from which all the bodies in the solar system were made. The images that came back underlined our need to update our ideas as to how planets form.

The sun and all the other bodies in the solar system formed from the collapse of a huge cloud of gas and dust that took place about 4.5 billion years ago. Not all the material was used up. Some of it is still orbiting the sun as a disc of dust that at certain times of the year we see glowing in the sunlight as the "zodiacal light." 

Some of it wound up as asteroids: would-be planets that had their formation interrupted by the gravitational attraction of the giant planet Jupiter. 

However, the bulk of it remains on the outskirts of the solar system as a large collection of lumps of various sizes, in a formation known today as the Kuiper Belt.

Objects moving around in space tend to travel at high speeds: many kilometres a second. It seems, therefore, logical to see the coming together of these small lumps to form planets as a progression of very high-energy collisions, generating enormous amounts of heat and throwing off debris in all directions. 

So It was a huge surprise when we managed to pass spacecraft close to comets and asteroids and saw images of what were obviously pairs of bodies gently joined together. Could something have happened to them since they formed? 

However, when the New Horizons spacecraft sent back that image of one of the bodies in the Kuiper Belt, we had to change our ideas. This object, now named Arrakoth, consists of two roughly spherical bodies gently joined together. The two have been named Ultima, with a diameter of 15.9 km, and Thule, 12.9 km. In this case, we could not argue unusual circumstances; this object marked a very early stage in the formation of planets. The bodies did not always smash together; they gently touched and then stuck. Like many things in science, the new picture seems logical, in hindsight of course.

The cloud from which everything formed had to have some sort of rotation due to particles orbiting around the cloud's centre of gravity, otherwise it would have collapsed. When that delicate balance of forces was disrupted, perhaps by a nearby supernova explosion, the rotation did not vanish. So the collapsing material tended to settle down into a rotating disc. Particles moving in other paths had to pass through that disc twice each orbit, where they were gradually filtered out by collisions with the disc particles. Similarly, anything in highly elliptical orbits in the disc would have been swept up by collisions and their orbits circularized.

The basic physics as set down by Newton state that all objects in circular orbits around a common body at the same distance are moving at the same speed. Collisions with neighbours would have been low-speed events, making it possible for the energy of impact to be absorbed by crumpling and crushing, allowing the objects to stick together rather than just bouncing apart. This would lead to growing planets sweeping out a path in the disc rather than sending fragments blasting everywhere, which is what we see in our images of planetary systems forming around distant stars, and also in the rings around the planet Saturn.

The cosmic clouds are loaded with organic chemicals that are widely believed to be the foundation for life on Earth, and almost certainly other places too. The question is how can they survive the planet formation process. However, the possibility our planet formed through mostly low-speed impacts would certainly make it easier.

  • Venus shines brightly in the southwest after sunset, looking beautiful against the blue. 
  • Mars, Jupiter and Saturn lie low in the southeast before dawn. The moon will be full on the 9th.  


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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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