Saturday, 6 November 2021

Planetesimals or Pebbles

Planetesimals or Pebbles 

How do planets form? For decades, when asked, people like me would mutter something about planetesimals and quickly move on. THIS ARTICLE suggests that the word I should mutter is pebbles. And tell people to read the article!

Apparently there was some difficulty about planetesimals - it would take too long for them to form. A planet like Jupiter would take tens of millions of years to form a core, by which time the protoplanetary disc from which they would need to accumulate gas would no longer exist. Instead of planetesimals, measured in 100's of kilometres, pebbles, measured in millimetres and centimetres, were the answer. And todays telescopes could see them.

ALMA observations of protoplanetary disk around HL Tauri in 2014 revealed hidden structures, including the presence of pebbles in the disk.


The pebble theory rapidly gained acceptance for the giant planets but it has difficulties for smaller planets like the Earth. The article discusses this and comes to no definite conclusion. I suspect that debate will continue for years and, eventually a consensus will be reached. 

In the meantime the word is pebbles - probably!

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