Finally, just noted your update (italics) refer with a typo to “HR 4709″ rather than HR 4796(A). No biggie – but you know how possessive we astronomers are about our “favorite” stars — and HR 4796A is indeed my personal favorite.

HR 4796A may not be the prototype circumstellar debris system (Beta Pic is), but may be an archytpe for many. It certainly continues to inform our still (in detail) uncertain understanding of the formation and evolution of exoplanetray systems.

That reminds me of an old Sid Harris cartoon: “I think I know why we’re finding it so difficult to attract funding” or something like that.

You seem to have a keen physics intuition! Indeed, we are taught in astrobiology courses on planetary formation, as I remember it, that the size from small grains up to ~ 1 km large bodies is the unknown and unsolved “desert” where models rather fragment bodies than aggregate them. Smaller and larger bodies works well with traditional models from surface and gravitational forces.

However, I seem to remember having read somewhere recently that indeed a collisions with enough collisional sticking as you describe in your last suggestion solves the problem. I don’t think the hypothesis was that it comes from bodies thermal viscodynamic properties but how these aggregates works viscodynamically under the collisional process.

So I take from your answer that we’re reasonably sure life took under 800 Myr to evolve on the Earth- 4.6 Gyr Earth age, followed by (perhaps) 3.8 Gyr old fossils

That is the official value yes, and that is reasonable. But really sure would be ~ 1 Ga (4.54 Ga bp Earth birth – 3.5 Ga bp relative consensus fossils).

And there are reasonable hopes that we can push that figure a lot more.

I guess the real questions I can see here are exactly when life formed on Earth,

I hope it can be constrained as I laid out in my #11 comment.

Gene clocks can be used to predict dates, but a) they have historically been spreading dates a lot (less so with modern techniques) b) they usually need an earlier dating and don’t do well on extrapolation. However, it seems a quick and dirty clock could be in the right neighborhood in this case. It hasn’t so long to go either, from 3.8 (or 3.5) Ga bp.

did it/could it survive the formation of the Moon?

No way! That impact removed the first crust, ocean and atmosphere, which had to be reformed by gravitational settling and cooling.

With the time frames coming out now, I wouldn’t be surprised if the Earth-Moon impactor sterilized a first biosphere. If so, the evidence of the Death From The Skies may still be found on the Moon…

Wow. Although I wonder how long vents on the early, more active Earth lasted…

Quite. The talk I cribbed from reformulated that as “or ~ 10^15 ms” IIRC, presumably illustrating the average reaction time for organic compounds under those conditions. And of course there were many vents, so many attempts, and some of the metabolic networks could spread in between.

But that is the order of it. Miller once calculated the abiogenesis times for his then soup theories before they too had to become proliferating cells, and that ended up ~ 12 Ma. This is, ironically, the estimated average time before a volume of sea water would circulate through a hydrothermal vent in the Archaean, breaking down any passive circulating “broth”.

At this rate, I expect to see an actual surface map of an exoplanet within 30 years.

Well now that you mention it – have you seen this one which the BA blogged about here :

http://blogs.discovermagazine.com/badastronomy/2007/05/09/astronomer-make-first-map-of-extrasolar-planet/

Back in 2007 May the 9th or this one via the 80 beats blog from the Discover stable :

http://blogs.discovermagazine.com/80beats/2009/01/29/first-ever-weather-report-from-an-exoplanet-highs-of-2240-degrees/

with maps of the Comet-orbit planet HD 80606b? See also the BA’s blog post ‘Weather sizzles on a planet that kisses its star’ from back in 2009 January 28th for more on that eccentric Hot Jove.

@ 14. Marina : January 2nd, 2012 at 11:11 pm

THEY’VE LOCATED SAURON’S EYE!

But, hang on, this isn’t Fomalhaut! See :

http://blogs.discovermagazine.com/intersection/2008/11/13/eye-of-sauron/

I think that one takes priority & is a better likeness. Sauron’s *other* eye maybe?

@ 10. Torbjörn Larsson, OM & 9. DigitalAxis : Thanks – good points there.

I’m right there with ya! This makes me wonder how it would look if we were able to detect a ringworld like this.

[quote]That is the sole domain for eukaryotes, which have ~ 10^5 larger energy density from mitochondria symbionts for protein turnover, making and utilizing larger genomes.

The reason why mitochondrial symbionts can make the necessary simplified energy plants for eukaryotes is an oxidized atmosphere. [/quote]

I was going to suggest perhaps the ~100x increased flux from the star would give life forms the required energy for interesting life forms… then I remembered that’d only apply to a planet at 1 AU, and such energy would vaporize the oceans… You would need interesting chemistries to get around THAT problem.

So I take from your answer that we’re reasonably sure life took under 800 Myr to evolve on the Earth- 4.6 Gyr Earth age, followed by (perhaps) 3.8 Gyr old fossils. I guess the real questions I can see here are exactly when life formed on Earth, and did it/could it survive the formation of the Moon? If life really did originate in ~50 Myr (or even 100 Myr) that makes A-type stars much more interesting planetary hosts.

[quote]For example, the hydrothermal vent abiogenesis theorists sets the average lifetime for modern vents as their upper limit for chemical evolution to free cells. That is some ~ 30 ka (thousand years).[/quote]

Wow. Although I wonder how long vents on the early, more active Earth lasted… But I’m no geologist; for all I know the Earth settled down “quickly” on astronomical scales.

If you account for the migration of the habitable zones as the star ages and brightens, it’s a lot less than 300 Myr. I can’t recall if scientists have found any clear evidence of life on the Earth at that age.

Not clear evidence, but the question is interesting. [Astrobiology student here.]

For example, the hydrothermal vent abiogenesis theorists sets the average lifetime for modern vents as their upper limit for chemical evolution to free cells. That is some ~ 30 ka (thousand years).

The stated consensus is life “sometime before 3.8 Ga bp” (before present) which at times you see mentioned as “at 3.8 Ga bp” (see below on the LHB) or more correctly “sometime before 3.5 Ga bp” which is the most recent accepted date. (The ~ 3.8 Ga bp isotope ratios were argued, the fall back was several ~ 3.2 Ga bp observations of cell fossils, and at least one of the ~ 3.5 Ga bp observations of cells/stromatolites seems to be generally accepted. But that is a 2011 result, so the fallout is unclear to me.)

However contrary to what was firmly believed before 2010ish, actual models of the Last Heavy Bombardment (LHB) @ 4.1 – 3.8 Ga puts it recurrently as easily survivable. Cells proliferate and migrate faster than any realistic sterilizing bombardment can keep up with. And today we know of Golidilock survival zones ~ 1 km down the crust, that putatively can withstand even crust busters vaporizing the oceans. Life is a plague on a planet.

Similarly the Iron Banded Formations (IBFs) arguably but again recurrently dated to ~ 4.28 Ga bp of the Nuvvuagituuq Greenstone Belt may or may not be a result of photosynthetic bacteria. So the upper boundary for life gets pushed down over time.

Similarly, the lower boundary tend to get pushed up over time. Redating of the Apollo Moon rocks may put the Earth-Moon impactor as late as ~ 4.36 Ga bp.

Finally there are now coming out several gene family models that self-consistently puts important events from the Archaean Expansion (the gene family explosion) over the Great Oxygen Event (the oxygenation of Earth) to expansion onto land correctly in time. When I used them for a toy model gene family clock extrapolation in the fashion of physicists, I get that the first genome may date from ~ 4.31 Ga bp.

Assuming that the crust had to reform over some ten or tens of million of years if we accept the late Earth-Moon system age, that is rather consistent with hydrothermal vent abiogenesis theories, external observation can perhaps constrain abiogenesis to at most 40 Ma. So I would say that 300 Ma is more than enough for having biological evolution from chemical evolution, aka life.

This means HR4796′s main- sequence lifetime will be “only” around 300 million years and so any earth-like planets around it won’t – we think – have time to develop any indigenous lifeforms upon them before their Vega-like (& thus possibly very fast rotating and distorted) sun evolves into a red giant and fries the system.

Remember that life for most of our planet’s (pre~!)history was limited to microscopic bacteria and even the dinosausr were relative latecomers in the geological timescale.

More specifically, while prokaryotes can make multicellulars, even true ones with adhering and differentiated cells (cyanobacteria sheathed cell strands with nitrification cells), they don’t make complex trait multicellulars (tissues & body plans). That is the sole domain for eukaryotes, which have ~ 10^5 larger energy density from mitochondria symbionts for protein turnover, making and utilizing larger genomes.

The reason why mitochondrial symbionts can make the necessary simplified energy plants for eukaryotes is an oxidized atmosphere. The reason for life is to increase the hydrogenation of carbon to methane by catalyzing enzymes. It does so by taking oxygen from carbon dioxide and puts it onto iron. Even if we can tentatively trace back these Iron Banded Formations to ~ 4.28 Ga bp (billion years before present) in the arguable dating of the Nuvvuagituuq Greenstone Belt it took a long while before a a minute percentage* of that redox potential was freed as molecular oxygen.

As a matter of fact it took a decently wet and warm planet of Earth size some ~ 2 Ga (billion years), from ~ 4.5 – 4.3 Ga bp to ~ 2.3 Ga bp. So 0.3 Ga won’t make it, it is an order of magnitude too little time.

—————-
* Of the redox potential ~ 1-2 % has gone to oxygen, ~ 75 % has oxidized iron, and the rest sulfur. The latter part is what makes up much of terrestrials, especially their core.

Life is all about making as much rust as possible. (O.o)

If you account for the migration of the habitable zones as the star ages and brightens, it’s a lot less than 300 Myr. I can’t recall if scientists have found any clear evidence of life on the Earth at that age.

@7. Dragonchild:

I doubt we have all that much in common. A-type stars like HR4796 have formed (very quickly- less time for planets to form!) from much more massive clumps of material; their disks should be more massive and farther out (ice boils at greater distances from A stars than G stars)… Then there’s the slight difference that A-type stars aren’t thought to have convective outer layers, although what this does to their coronae and winds I couldn’t say.

We really have no idea what’s orbiting Sol at 22 billion km, but Pluto is by no means the edge of the Hill sphere. I can’t wait until the full results of the WISE survery come out, but even that’s not going to find everything (tho it will find or rule out anything big).

http://visav.phys.uvic.ca/~babul/AstroCourses/P303/BB-slide.htm

for Carl Sagan’s great way of comparing the Cosmological year ie all time from the Big Bang up til now with our own standard calendar years.