I wasn’t too happy when the first observed estimates of disk lifetime came out back in the 90’s and they were running around 10^6 years. It now looks like times of a few million years are fairly common for the gas, and that solid debris can last a lot longer than that. These are quite consistent with models of both giant and terrestrial planet formation.

Well, the article the Phil linked to says the following:

“Kenyon added that while Earth took about 20 to 30 million years to reach its final mass, Jupiter was fully grown in only 2 to 3 million years.”

That sort of gives the impression that the dust debris disk is only around for maybe 30 million years or so (enough time for Earth and the other terrestrial planets to form) and then it’s pretty much gone. That’s why I gave the example of Vega which is half a billion years old and still has a sizeable debris disk with possible planet formation going on right now. So I am a bit skeptical of some of these low time ranges being handed out. Perhaps the computer models say it works that way but in reality we find much older debris disks.

First off, Tom’s comment about Beta Pic, Vega, etc. These are all debris disks, and as such they have very little gas. So you could keep making terrestrials and icy planets, but gas accretion is not likely in any of these older systems. That’s one of the characteristics of debris disks.

For the dust clearing, there are two competing approaches to removing the fine dust: photoevaporation, which does work from the inner disk outward, and efficient accretion which operates more evenly throughout the disk. Probably both were in operation, but at least one talk I saw today argued that the latter process dominated more than 50% of the time (at least in one star-forming region).

On a related note, the whole discussion of extrasolar planets has just exploded in the last several years. Once upon a time this was a couple of talks at the end of the planetary sessions (itself a small part of the AAS meeting). Now there are several sessions devoted just to planets around other stars in both the poster and oral talks. It almost makes me wish I’d stayed in research.

I would have thought that the distance from the star has something to do with the longevity of the gas/dust disk. We would expect the stellar wind to sweep the area within 1 AU clear of gas before it sweeps the entire area within 10 AU. It’s interesting that the article didn’t talk about distance as a factor and just focused on the number 5 million years.

Some other data points are these:

Beta Pictoris has an approximate age of 12 million years with a debris disk out to 1800 AU. Some interesting notes from Wikipedia:

“Studies made with the NASA Far Ultraviolet Spectroscopic Explorer have discovered that the disk around Beta Pictoris contains an extreme overabundance of carbon-rich gas. This helps stabilise the disk against radiation pressure which would otherwise blow the material away into interstellar space. Currently, there are two suggested explanations for the origin of the carbon overabundance. Beta Pictoris might be in the process of forming exotic carbon-rich planets, in contrast to the terrestrial planets in our solar system, which are rich in oxygen instead of carbon”

So apparently having carbon in the gas can stabilize it and make it last much longer than ordinary. I have no explanation for how that is supposed to work but it might explain why the disk hasn’t dissipated yet.

Vega has an approximate age of 500 million years and a debris disk extending out to 800 AU. A perturbation has been found in the debris disk at about a distance of 70 AU which may be a planet during formation.

So 5 million years is not a hard and fast rule for planetary formation. It depends on many different factors.

No, but this thread is.

I didn’t think this was (just) an astronomy blog. Did you?