Re: Our rocky solar system may be rare

Robert J. Bradbury (
Sat, 11 Sep 1999 15:17:00 -0700 (PDT)

On Sat, 11 Sep 1999, Brian Manning Delaney wrote:

I should remind myself *never* to start a discussion with someone who 'enjoys' discussing philosophy.

[lots of snips]

> Two explanations come to mind:
> 1) They don't exist.
> 2) They exist but we aren't aware of them (for obvious reasons).

I think I was trying to make the point that "abscence of evidence" is not "evidence of abscence".

> That is a fun question indeed, but I was getting at a different one.
> IF #1 WERE true, what would be the explanation of this?
> Two explanations come to mind:
> A) Advanced civilizations (generally) destroy themselves.
> B) Advanced civilizations never (or once, or more rarely than
> we'd thought) get going in the first place.
> If solar systems that can support the development of life are
> far more unusual than we'd realized (that is, if [a] the finding
> from the New Scientist is correct, and [b] rocky solar systems
> are necessary or highly advantageous to the development of life,
> or technological life), we then can assign #B a greater
> probability than #A.

I'll take apart the New Scientist claim, which is basically that a GRB within ~300 ly, around 4.5 billion years ago "aggregated" the objects in our solar system, initiating planet formation. Since we don't have the actual journal article yet, this is difficult.

Standard stellar models say that only massive stars are going to be able to turn themselves into the massive objects (neutron stars or black holes) that can produce GRB. Where do you get massive stars? In massive dust clouds. Now, the models for the creation of stars says that in massive dust clouds where you get massive stars you will also get many smaller stars. Models for the evolution of galaxies say that massive dust clouds, massive stars and GRB should have been more prevalent earlier in the history of the galaxy. So there are at least 2 holes in the N.S. theory -- (a) the likelyhood of many Sun-like stars in the vicinity of the GRB that made our solar system; (b) the fact that there could have been a time in galactic history when lots of GRB were occuring and creating lots of solar systems similar to ours. What we really need to discuss this further is the frequency at which GRB occur.

And that says nothing about how systems without GRB may condense into planets or other objects that concentrate matter such that other forms of life might evolve.

>From the article:
> For over a century, astronomers have tried to understand what made
> clumps of dust circling the young Sun melt into chondrules--rocky beads
> rich in iron and silicon minerals that make up the bulk of stony meteorites.

> they also think that the dense chondrules settle quickly into the plane
> of a protoplanetary disc and speed the formation of planets

They are solving the problem of "why do asteroids" (meteors) have this funny structure. They are not saying that solar systems without GRB *cannot* form planets, just that GRB may speed up planet formation. For stars with masses slightly smaller than the sun, planet formation could take an extra billion years and it wouldn't make much difference because the stars live so much longer.