Kepler's 'Habitables'

[CraigS]

.. A bit of an update on our previous threads about Kepler's discovery of 1235 planetary candidates, of which they estimated 54 were in the 'Habitable Zone'...

Habitable zones

CfA astronomers Lisa Kaltenegger (now at the Max Planck Institute for Astronomy) and Dimitar Sasselov have explored in more detail the conditions necessary for a planet to lie in its habitable zone. They take into account more carefully five factors: the incident stellar flux and its spectral character, the planet's eccentricity (how its distance from the star differs during its orbit), the planet's reflectivity including the effects of partial cloud cover, the greenhouse gas concentration, and finally, some details of the planet's atmosphere.
...
A more accurate estimate finds that only six of the Kepler exoplanetary candidates could be in a habitable zone, assuming that they have atmospheres. The results are another important step in refining the search for Earth-like planets (not just Earth-sized planets) around other stars.

More stringently define this thing called 'Habitable Zone', and you get a smaller answer than the original announcements … curious !

Anyway, I think of the 1235 candidates, about 17 have now been 'confirmed' with a total of 573 extrasolar planets now having been 'identified'.

Cheers

[renormalised (Carl)]

More stringently define this thing called 'Habitable Zone', and you get a smaller answer than the original announcements … curious !

Not really...it would be normal for the number of candidates for inclusion to fall once you start putting more restrictions on the conditions. The more stringent they become, the less candidates will fall within the accepted limits. But, then you have to ask yourselves, "how stringent do we really have to be". If you want to be arbitrary about it, you could make the conditions for life so stringent that you'd only be left with the Earth as the only habitable planet. It's the same old problem...we're basing all our predictions on the one example we have and a few not so informed "guesstimates" based on broad, educated deliberations.

The only way they're going to resolve this is through more observation and empirical study. Find and confirm the planets, study their atmospheres and look for the signatures of life. Once they're found, then they can move forward with somewhat more confidence. Other than that, they're taking stabs in the dark.

Even on Earth, the more restrictions we put on how and where life thrives, the more often that life totally surprises us and makes us look like fools. The same will probably be evident with other planets.

[renormalised (Carl)]

What a pity other solar systems don't have giant green rings denoting their habitable zones, like you see in artist's impressions. It would make it so much easier to find these other Earth like planets!!!!

Just point your scope and count the green rings!!!

[CraigS]

I think the same author has recently produced this paper, (17 Aug 2011), and they come to the conclusion that the exoplanet HD 85512b (3.6 xMearth) could be potentially habitable if it has 50% cloud cover, (the surface might be cool enough for liquid water .. if water is present).

The super-earth Gliese 581d (20 lyrs) is the other most likely candidate for habitability.

This appears to be the paper applicable to the OP article:
Exploring the Habitable Zone for Kepler planetary candidates. (4th May 2011).

Cheers

[CraigS]

I think the idea here, is to provide guidance for further study by Kepler, narrowing the search space.

I mean, the 'Habitable Zone' for moons orbiting a hot Jupiter, would be completely different from the 'Habitable Zone' for a planet orbiting a star, which would be different from the 'Habitable Zone' for a planet (somehow) orbiting the centre of mass of a binary system, also (??).

Just because Kepler can't detect exo-moons, doesn't imply that there's no habitable zones at smaller scales .. and the same goes all the way down the scale ladder (as Carl points out in the case of right here, on Earth).

At the end of the day, I think they're only trying to provide theoretical guidance to the exo-planet hunters, within their detection capabilities.

Aside from water, oxygen in the atmosphere would also seem to be important, although I read the other day about some bacteria which metabolises sulphur .. which could be present where there is no water, or oxygen … the diversity is almost limitless .. and this latter point seems to be consistently reinforced with more solar system and exo-solar system planetary/moon research

Cheers

[renormalised (Carl)]

There are also bacteria which metabolise hydrogen and ammonia and others which live in the cooling pools and pressure vessels of nuclear reactors. I also know of bacteria which metabolise heavy metals...they're quite common in active garbage tips, old reclaimed dumps and mines. Sulphur metabolising bacteria are very common around underwater volcanic vents and in boiling mud pools, hot springs and the like. In actual fact, they help stabilise and form various types of ore deposits, especially the volcanogenic, sediment hosted deposits.

Life is a very tenacious thing and is far more widespread than many even realise.

[SkyViking (Rolf)]

Originally Posted by CraigS

Aside from water, oxygen in the atmosphere would also seem to be important, although I read the other day about some bacteria which metabolises sulphur .. which could be present where there is no water, or oxygen … the diversity is almost limitless .. and this latter point seems to be consistently reinforced with more solar system and exo-solar system planetary/moon research

Free oxygen in an exoplanet's atmosphere would be an indication of possible oxygen breathing life. It should not be a criteria though, since the Earth's free oxygen only appeared 1.7Gy ago while life itself has been present here for at least 3.5Gy. The first life forms on Earth didn't breathe oxygen at all.
But it really depends on what question we want to answer? To search for 'intelligent', and supposedly large, life forms it seems likely that oxygen breathing would be a criteria. But if we want to know if some form of life arises simply when the conditions are favourable then we don't need to limit the search to oxygen-rich planets.

[renormalised (Carl)]

But the presence of free oxygen is a sure indicator that there is life on that planet. Finding the unambiguous byproducts of anaerobic bacterial metabolism is much harder. So, it should most certainly be an indicator.

[CraigS]

I think the complexity of natural systems is still grossly underestimated, even by experienced scientists.

Even the effects of one cell on another, seems to be a daunting task in medical research, let alone the wider environmental effects on the development of life !

The sheer diversity of environmental permutations, and of the permutations life 'bits', is the one constant coming out of all research thesedays. Its staring us right in the face, but with reductionist analysis, followed by categorisation, I really do think the main focal points seem to get lost.

In which field is the focus on diversity of permutations, maintained against the enormous size of the universe ? To me, this seems to be the main message for us to be paying attention to, when we compare life to the universe !

Cheers

[CraigS]

Originally Posted by renormalised

But the presence of free oxygen is a sure indicator that there is life on that planet. Finding the unambiguous byproducts of anaerobic bacterial metabolism is much harder. So, it should most certainly be an indicator.

Oxygen can be liberated by photolysis of water, but this is a slow process, (compared with the absorption of oxygen by a rocky crust). I also think Europa has an oxygen atmosphere for this reason.

Cheers

[renormalised (Carl)]

And that's our big problem....we have this obsession about being reductionist in our approach and pigeon holing everything we find. It helps a lot in quite a few respects but it's also rather limiting in others. We tend to lose sight of the bigger picture quite a lot and miss things, which usually end up being found out some time down the track, usually years later.

[renormalised (Carl)]

Originally Posted by CraigS

Oxygen can be liberated by photolysis of water, but this is a slow process, (compared with the absorption of oxygen by a rocky crust). I also think Europa has an oxygen atmosphere for this reason.

Cheers

True, but the main problem is the reactivity of Oxygen. In conditions like on Europa, the fact that the Oxygen is there is due to the low temps and the relatively dim light. The rates of reaction are very slow. On Earth like planets, whilst photolysis rates are much higher than they would be for your average outer solar system conditions, the fact that Oxygen is so highly reactive and the conditions on these planets drive the reactions rates to substantially higher levels, would mean that photolysis is close to negligible so far as Oxygen production is concerned. To sustain a substantial amount of Oxygen in an atmosphere of an Earth like planet, you would need vast amounts to be continuously supplied to its atmosphere. There's only one way of doing that....life.

[CraigS]

Originally Posted by renormalised

There's only one way of doing that....life.

.. or … 'there's only one way we presently know of doing that .. life.'

The complexity of atmospheric physical processes in exo-planets is something we have zero knowledge of, as yet.

The complexity of atmospheric physical processes in solar-system planets, is something we have a few observations of.

The complexity of atmospheric physical processes on Earth, is debated.

The diversity of different atmospheric physical processes within our Solar system is enormous.

The diversity of different atmospheric physical processes on Earth, is enormous.

The constants common to all of the above, is the complexity and diversity of possible interactions and, the known chemistry.

Cheers

[renormalised (Carl)]

Oxygen is a highly reactive element, regardless of any of the points you've mentioned. Unless it's being replenished, it cannot remain free as a gas in any atmosphere. As a matter of fact, all of those processes would be conducive to oxygen being taken up just as rapidly as it would be on Earth, without the presence of life.

Most abiotic processes which liberate Oxygen from minerals are highly endothermic and require quite a bit of energy input to begin with. Any planet which exhibited such characteristics would most certainly not be habitable unless it was by silicon based organisms. To release enough Oxygen into an atmosphere via these processes (to sustain it at high levels) would melt the surface of the planet. Actually, it would most likely melt the planet right through. The normal range of processes are inadequate when it comes to producing and sustaining Oxygen in substantial quantities in planetary atmospheres.