Habitable zones

Aug 22, 2011
An artist's conception of the habitable zone (green ring) around 55 Cancri, a star known to have a large planet orbiting in this temperature-suitable region where water could be liquid. Credit: NASA/JPL-Caltech

(PhysOrg.com) -- The "habitable zone" is the region around a star where a suitable planet could sustain the conditions necessary for life. Most astronomers take it to be the region where the balance between stellar radiation onto the planet and radiative cooling from the planet allows water on the surface to be a liquid; this definition also presumes the planet has an atmosphere and a solid surface. In our solar system, the Earth is cozily situated in the middle of the habitable zone which, depending on the model, extends roughly from Venus to Mars.

The satellite has recently announced the detection of 1235 planetary candidates around other stars. How many of these exoplanets lie in their habitable zones and might (at least to this extent) be suitable hosts for life? The original Kepler paper concluded that fifty-four were in their 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 . 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 concentration, and finally, some details of the planet's atmosphere.

With some reasonable assumptions the scientists find that, in the case of the solar system, the habitable zone extends from the orbit of Venus to well beyond the orbit of Mars (nearly to the inner edge of the ). When they apply their models to the 1235 candidates in the current Kepler catalog they find that the original estimate of fifty-four planets was far too high.

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.


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antialias_physorg
5 / 5 (9) Aug 22, 2011
Most astronomers take it to be the region where the balance between stellar radiation onto the planet and radiative cooling from the planet allows water on the surface to be a liquid;

With bacteria already having been found underground and feeding on radiation energy from nuclear decay those prerequisites are a bit on the strict side. (Also there is plenty of life in liquid oceans)

Life does not only exist on the surface nor does it strictly require an atmosphere.

Other considerations: A planet with high internal radioactivity (i.e. internal heat generating mechanism) could be very far away from the sun and still support liquid water.

Dense clouds (like on Venus) can also extend the range because temperatures under a cloud of greenhouse gases can be dispoportionately hotter than would be accounted for by the distance to the sun. Conversely thick clouds can also block radiation extending the habitable zone inwards.

Conclusion: Life could be anywhere.
gopher65
3.7 / 5 (3) Aug 22, 2011
It's quite possibly true that life could be everywhere, antialias. But how would we find it? Until we know what we're looking for, the best bet is to search for life that is very similar to us.
hard2grep
1.3 / 5 (3) Aug 22, 2011
I would imagine that time is an important factor here too. Does a galaxy have a snow line too? What is the best time for the universe to give birth to life? Is it right now, or does life's span reach beyond a few billion years when considering all the galaxies? I would think that we could create a map of candidate galaxies for intergalactic travel and so forth. My guess is that math dictates we will not find anything until we have been here long enough to travel and populate other systems. Of course, we will also need time to learn proper navigation. We are not in the snow line of intergalactic time... Beam me up, Scotty...( Our snow lines must overlap.)
antialias_physorg
5 / 5 (5) Aug 22, 2011
But how would we find it? Until we know what we're looking for, the best bet is to search for life that is very similar to us.

Well, that's the point: as long as we keep thinking in 'habitable zones' we'll only ever look for life there.

There are other radiation sources except stars. E.g. Jupiter has a prtty hefty radiation output which could feed life on a close moon. Such potentially life bearing planet-moon systems could be anywhere in a solar system - not just within a certain distance bracket from its sun.

Eccentric orbits are also not necessarily a show stopper. Many microbes (and even some higher animals) are perfectly capable of surviving a frozen state more or less indefinitely. Life could be an on again-off again occurence on such planets.

We really need to get a few probes out to Io, Titan and Europa and a _serious_ mission to Mars.
LKD
not rated yet Aug 22, 2011
Well, that's the point: as long as we keep thinking in 'habitable zones' we'll only ever look for life there.


I agree, but I think that there is more a priority in the back of people's mind to find life we can communicate with and places we can live than bacterial aliens.
antialias_physorg
5 / 5 (5) Aug 22, 2011
I agree, but I think that there is more a priority in the back of people's mind to find life we can communicate with and places we can live than bacterial aliens.

Don't you think that any kind of lifeform that is in the business of / interested in communicating with other lifeforms has already spread beyond its home ken?

All other planets should be 'toxic' for us (and them) in one way or another
- temperature
- gravity
- radiation
- pressure
- and most of all: atmospheric composition - which is virtually guaranteed to be incompatible for human/alien life anywhere in the universe but on their respective planets of origin.

Any life that speads will, by neccesity, spread to environments that they make habitable by either
a) terraforming
b) building enclosed habitats
or
c) adapting themselves to survive under different environmental conditions

None of which requires that these places be situated in the 'habitable zone' around stars.
El_Nose
4.5 / 5 (2) Aug 22, 2011
With bacteria already having been found underground and feeding on radiation energy from nuclear decay those prerequisites are a bit on the strict side. (Also there is plenty of life in liquid oceans)

Life does not only exist on the surface nor does it strictly require an atmosphere.
....
Conclusion: Life could be anywhere.


lets be honest this is not the search for life -- it never has been -- this is the search for intelligent life.

It always has been.
rjh405
5 / 5 (3) Aug 22, 2011
lets be honest this is not the search for life -- it never has been -- this is the search for intelligent life.

It always has been.


I'd have to disagree, El Nose. Just the detection of an independent form of life would give us a valuable second data point. That would tell us much more about how life forms and evolves, just as observation of geology and weather on other planets, or the properties and behavior of other stars, allows us to learn more about the history of the Earth.
antialias_physorg
5 / 5 (4) Aug 22, 2011
lets be honest this is not the search for life -- it never has been -- this is the search for intelligent life.

Intelligent life would be the icing on the cake. But life by itself would already be revolutionary. And since the latter is probably far more likely than the former we should start with that and work our way up.
Nik_2213
not rated yet Aug 22, 2011
D'uh, where's their article's reference for us to read their detailed calculations and findings ? The edu site has no link, either...
Nanobanano
not rated yet Aug 22, 2011
Honest, I doubt very much that there would be any life on any of Jupiter's moons.

All the water is frozen, in most cases, and the surface temps or so cold that rubber freezes and becomes brittle at those temps.

The air pressure for those that do have a tiny outgassing of atmosphere, this is so low that water boils and freezes at the same time. How would anything even remotely resembling carbon-based life exist like that?
Gawad
5 / 5 (2) Aug 22, 2011
All the water is frozen, in most cases, and the surface temps or so cold that rubber freezes and becomes brittle at those temps.
Europa and Ganymede are thought to have sub-surface oceans as a result of tidal heating. Callisto possibly as well, though this is less certain as, not being in orbital resonance with Jupiter and the other 3 Galilean moons, it experiences far less tidal heating.
yyz
5 / 5 (2) Aug 22, 2011
"D'uh, where's their article's reference for us to read their detailed calculations and findings ? The edu site has no link, either..."

Nik, a preprint of the paper is available here: http://arxiv.org/...0861.pdf
El_Nose
not rated yet Aug 23, 2011
Oh i agree that any new life would be awesome -- but as scientists this is the main and more realistic persuit ... but to keep funding this has always been about life / something we might communicate with.

ET had a bigger effect than Spilberg ever dreamed -- and probably got NASA more money/funding than want to admit
LKD
not rated yet Aug 23, 2011
Don't you think that any kind of lifeform that is in the business of / interested in communicating with other lifeforms has already spread beyond its home ken?


Ken? Explain that usage, please?

Considering how vast space is, and how comparably slow light speed is in regards to traversing the galaxy and universe... I don't see how you can't imagine that someone within 50 light years of here (let alone anywhere in the Milky Way) would have no way to realize we're here, or vice versa.

As for terraforming, we can't reliably see the comparably sized planets to Earth to gain a good idea how many planetary systems are like our solar system and how many are not. So assuming all planets need to be tweaked to accommodate our presence is not a guarantee.
yyz
5 / 5 (1) Aug 23, 2011
"Ken? Explain that usage, please?"

Ken-noun:

1)knowledge, understanding, or cognizance; mental perception: an idea "beyond one's ken".

2)range of sight or vision.

[from dictionary.com: http://dictionary...owse/Ken ]

Nik_2213
not rated yet Aug 23, 2011
Thanks, yyz !!
antialias_physorg
5 / 5 (2) Aug 24, 2011
Ken? Explain that usage, please?

ken (short for kennel) = original habitat

I don't see how you can't imagine that someone within 50 light years of here (let alone anywhere in the Milky Way) would have no way to realize we're here, or vice versa.

Electromgnetic waves follow an inverse square law (at twice the distance you have only one quarter of the signal strength)

Beyond 2 light years out anything that has ever been sent from earth (intentionally or unintentionally) is undetectable - even by theoretically optimal receivers.
Our nearest neighbor is 4.2 light years away. So no: They won't detect us unless they are making a fly-by to take a peek. We also won't detect them that way (unless they are using a beacon with the power output of a sun - which seems an awful lot even for a spacefaring species)
antialias_physorg
5 / 5 (1) Aug 24, 2011
As for terraforming, we can't reliably see the comparably sized planets to Earth to gain a good idea how many planetary systems are like our solar system and how many are not. So assuming all planets need to be tweaked to accommodate our presence is not a guarantee.

We do know that oxygen in the atmosphere (which we are very dependent on) is not the product of inanimate processes but is the 'waste' of billions of years of bacteria doing their thing.

We are also dependent on oxygen being available in a very specific concentration. Slightly too low and we're dead (just climb a tall mountain). Slightly too high and we get oxygen poisoning.

Add to that that there are a whole host of gasses which must NOT be present in our atmosphere because of toxicity issues.

The circumstances we have adapted to are just so very specific that it's utterly unlikely we'll ever find another rock where we can live without a suit (barring biomorphing and terraforming).
LKD
not rated yet Aug 24, 2011
Perhaps. Perhaps not. We don't have enough information for now to understand the probabilities except that there are far too many stars for there not to be another Earth out there. So we look and catalog and learn. :)