Defining the Habitable Zone

(Some posts have been split from the MEarth-1b topic)

There's an interesting blog entry at oklo.org describing the implications of the first MEarth results for the presence of low-mass planets around M dwarfs, and perhaps by extension, sun-like stars.

http://oklo.org/2009/12/20/m-for-all-and-all-for-m/

Greg's post is a little bit misleading.

I thought that there is still no consensus onthe habitability zone around red dwarfs. Some groups have reported the width as being vanishingly small.

Two majors issues are tidal locking and frequent stellar flares.

Admittedly a thick atmospheric envelope may help moderate these extreme situations but still it is an extreme environment.

Also the same atmosphere would prevent light from reaching the surface, other than a dull Infrared glow.

Also no spin likely = no magnetic field, with its own implications for radiation protection.

All in all while it might be useful to look here because it is easier, there does not seem to be much of a future in these planets.

I believe Greg was referring to the traditional habitable zone, where the temperature of the surface of a planet (considering only stellar irradiation) is sufficient to allow water to be a liquid.

Of course, as you point out, the true concept of a "habitable zone" is much more complicated.

However putting this star in Celestia with coordinates available, it turns out a dim star much close to several Solar-kins: 18 Scorpii, Lambda Serpentis, Gamma Serpentis (a bit less Sun-like but...)

Well of course, Gliese 1214 is a red dwarf.

It is the statement that we are 4 times more likely to catch a transit of a potentially habitable planet around a Red Dwarf that I take issue with.

Very misleading.

The key word TheoA is missing or choosing to ignore is "potentially".

At present the issue of red dwarf habitability is not settled... some studies claim flares will pose an insurmountable problem, others claim they could provide the necessary high-energy photons to drive the chemistry necessary for life. Then again, the situation around solar-type stars is by no means settled either: there exist predictions that the habitable zone is extremely narrow as well. Plus there are issues of whether large moons are needed, the role of outer Jupiter-class planets...

I don't think there can be any equivalence between a terrestrial type planet around a Sun like star with one around a Red Dwarf.

This is comparing apples to oranges.

The only living system we know of is around a Sun like star. There are no living systems known around a Red Dwarf.

How can both be classified as potentially habitable. This what caused all that consternation over Gl581 c/d amongst the lay public.

The classification system needs to be clearer than this.

We just eliminated an entire planet in the name of clarity in exo-planet classification.

We can't go around calling anything that moves and looks warm potentially habitable. Greg should know better.

TheoA wrote:

I don't think there can be any equivalence between a terrestrial type planet around a Sun like star with one around a Red Dwarf.

While it may be harder for a planet in the traditional habitable zone of a red dwarf to be Earth-like, I don't see why it would be impossible. Some of the early M dwarfs are warm enough to push the traditional habitable zone further away than like those found at Gliese 581 et al.

TheoA wrote:

The only living system we know of is around a Sun like star. There are no living systems known around a Red Dwarf.

Well of course, but with this same reasoning before the detection of exoplanets, one could argue that only sun-like stars host planets. Of course we know of a life-bearing planet orbiting a sun-like star, but we don't have the ability to detect such a thing orbiting other sun-like stars.

When reasoning about extrasolar planetary systems as a whole, it may be better to just ignore our solar system. It's such a outlying data point in so many ways, perhaps due to our technological capabilities. Ignoring our solar system so as to scrutinize the planets with the same blindfold of technological progress here.

Recall that until recently, no super-Earths were known to orbit sun-like stars (Taking type G as sun-like here). With your reasoning here, I might as well rule out life-bearing planets around G-type stars and say that since M and K dwarf planets host super-Earths, they are the only hopes for life.

TheoA wrote:

How can both be classified as potentially habitable. This what caused all that consternation over Gl581 c/d amongst the lay public.

I certainly agree that the issue was over-hyped. But what if there's a 1 Earth-mass planet between Gliese 581 c and d? It'll be right in the habitable zone. It's got a better chance of having not too thick an atmosphere, and Gliese 581 is a quiet, old star.

There are many variables that determine if a planet is "habitable", the stellar spectral type is just one of them.

TheoA wrote:

The classification system needs to be clearer than this.

I'd love to have a nice long discussion about that, but it's outside the scope of this thread (Perhaps, since this thread is getting off-topic, the off-topic posts can contribute the start of such a thread).

TheoA wrote:

We just eliminated an entire planet in the name of clarity in exo-planet classification.

My only problem with the IAU decision is that it actually isn't applicable to the extrasolar planets (PSR B1257+12 D excepted) and likely won't for a very long time. The debate regarding the whole issue seemed motivated by the discovery of large bodies in the Kuiper Belt.

TheoA wrote:

We can't go around calling anything that moves and looks warm potentially habitable. Greg should know better.

Often, scientists use lingo that isn't 100% true, simply because it's the lingo. Many call Gliese 581 b a Neptune, when in fact, Neptune is 20 light years away from it. We call Gliese 876 d a super-Earth when in fact, it probably has nothing in common with Earth. We call most of those 400 planets "planets" when in fact they're just candidates for planets. "Habitable" is another one of those terms. It's easier to say "habitable" than it is to say "in the traditional habitable zone." As such, often you'll hear "habitable (super-)Earths" being thrown around more so than you would expect.

Just like we don't habitually call the exoplanets "planet candidates", we don't call terrestrial planets in the traditional habitable zone "habitable planet candidates". Laziness? Perhaps. Inaccurate? Probably.

If the audience is one that isn't aware of that (over)simplification, then I agree, the term "habitable" should not be used. Greg's blog always struck me as being aimed at an audience that was more informed about extrasolar planet science and likely to be aware of the terms that float around the field.

Such terms like "habitable", "super-Earth", "Neptune" and such are generally used with the understanding that they are not 100% what their names would imply.

TheoA wrote:

I don't think there can be any equivalence between a terrestrial type planet around a Sun like star with one around a Red Dwarf.

This is comparing apples to oranges.

The only living system we know of is around a Sun like star. There are no living systems known around a Red Dwarf.

How can both be classified as potentially habitable. This what caused all that consternation over Gl581 c/d amongst the lay public.

Actually the Gliese 581c issue was caused by misusing the concept of effective temperature to determine habitability. You are conflating the issues.

What is being meant by potential habitability means that certain conditions we can determine suggest that such planets could support a climate which enables the presence of liquid water. There are other variables which may affect this, but at present their effects are not yet precisely determined enough to make a call one way or the other. Hence the term "potential" habitability - given what we know, these worlds could be habitable, but there are other factors involved.

Then again, I am sure you know this already. My guess is that you want another opportunity to accuse scientists of deliberately trying to mislead the public or being involved in some kind of conspiracy. It would fit with your reaction to the Kepler amplifier issue. If you feel so strongly that it is misleading why not post a comment to his blog directly, instead of skulking around here to make your criticisms? I note you didn't bother to contact the Kepler team when you were accusing them of all kinds of fraud...

Sirius_Alpha wrote:

While it may be harder for a planet in the traditional habitable zone of a red dwarf to be Earth-like, I don't see why it would be impossible. Some of the early M dwarfs are warm enough to push the traditional habitable zone further away than like those found at Gliese 581 et al.

Potential warm zone, perhaps, Habitable not so much. These are not habitable in anything close to the way we perceive it here on Earth.

By this definition certain layers of Jupiters atmosphere are warm enough to be habitable, as are regions of Venus's atmosphere. Something Greg himself has posted about before.

Sirius_Alpha wrote:

When reasoning about extrasolar planetary systems as a whole, it may be better to just ignore our solar system. It's such a outlying data point in so many ways, perhaps due to our technological capabilities. Ignoring our solar system so as to scrutinize the planets with the same blindfold of technological progress here.

That is quite a bold statement. Ignoring the single data point we do have. Yes intelligent life is exceptional here but life itself has been on Earth through all kinds of environmental changes and catastrophes. What if it is not not such an outlier? The environment here has been relatively stable for 4 Billion years, or a third of the entire universes existence. This is likely to be important for life as well.

Sirius_Alpha wrote:

Recall that until recently, no super-Earths were known to orbit sun-like stars (Taking type G as sun-like here). With your reasoning here, I might as well rule out life-bearing planets around G-type stars and say that since M and K dwarf planets host super-Earths, they are the only hopes for life.

Calling it potentially habitable is such a loaded term. Call it a warm wet planet, or even a Liquid water zone planet which is after all where all this speculation is coming from. Why jump to habitability so prematurely when we know so little.

Sirius_Alpha wrote:

But what if there's a 1 Earth-mass planet between Gliese 581 c and d? It'll be right in the habitable zone.

Again, liquid water zone. There appears to be liquid water on Europa, we don't call that a potentially habitable moon. A warm zone around a Red Dwarf should not be called Habitable anything.

Sirius_Alpha wrote:

I'd love to have a nice long discussion about that, but it's outside the scope of this thread (Perhaps, since this thread is getting off-topic, the off-topic posts can contribute the start of such a thread.

Agreed. I like the title of this thread with a question mark. It could have very easily been called out as "Planet near potentially Habitable Zone". No need for loaded words.

Sirius_Alpha wrote:

My only problem with the IAU decision is that it actually isn't applicable to the extrasolar planets (PSR B1257+12 D excepted) and likely won't for a very long time. The debate regarding the whole issue seemed motivated by the discovery of large bodies in the Kuiper Belt.

They did set the Deuterium burn limit as well which was previously not so firmly set, they also categorized the Planemos. But true enough a final definition is awaited, hopefully soon.

Does this mean that these objects should not be called planets.

Sirius_Alpha wrote:

As such, often you'll hear "habitable (super-)Earths" being thrown around more so than you would expect.

Amongst the rating driven media types absolutely. Disappointed to see it amongst more professional groups.

Sirius_Alpha wrote:

Just like we don't habitually call the exoplanets "planet candidates", we don't call terrestrial planets in the traditional habitable zone "habitable planet candidates". Laziness? Perhaps. Inaccurate? Probably.

There has to be a slight ulterior motive. Scientists are human and they know anything linked to life sells.

Sirius_Alpha wrote:

Such terms like "habitable", "super-Earth", "Neptune" and such are generally used with the understanding that they are not 100% what their names would imply.

The Earth and Habitable terms are so much more loaded though. No one got excited with a 'Hot Jupiter' or 'Neptune like' other than the first one or two.

What annoys me is that there is push to take people eyes of the goal. Greg is trying to push Red Dwarfs as 4 time more likely to have a transiting potentially habitable planet. So he wants us to give 4 times the focus on these planets. Or is he saying for every one Earth like transiter we will have 4 time as many around Red Dwarfs.

TheoA wrote:

Greg is trying to push Red Dwarfs as 4 time more likely to have a transiting potentially habitable planet. So he wants us to give 4 times the focus on these planets.

ZING!!! Here's the conspiracy accusation! TheoA does not disappoint after his performance at the Kepler thread!!!

Lazarus wrote:

..certain conditions we can determine suggest that such planets could support a climate which enables the presence of liquid water.

Precisely.

So why not just say that.

TheoA wrote:

Potential warm zone, perhaps, Habitable not so much. These are not habitable in anything close to the way we perceive it here on Earth.

Unless you have a serious chunk of the M-dwarf planetary systems in the galaxy spectroscopically examined, it would be hard to make this statement. Until then, it is speculation. And because it is speculation, it leaves room for alternatives. As such, a Earth-mass planet (or moon of a gas giant) orbiting an M dwarf at a nice distance is potentially habitable. Potentially, because we haven't ruled it out yet.

TheoA wrote:

Ignoring the single data point we do have.

What's more bold? To say that all life-bearing worlds have a similar set-up to our solar system? Or to consider that ours may just be one of a wide range of possibilites, and to not make a decision as to what is, or what is not habitable based on it?

TheoA wrote:

The environment here has been relatively stable for 4 Billion years, or a third of the entire universes existence. This is likely to be important for life as well.

You're aware that M dwarfs live for trillions of years, right? As opposed to sun-like stars, that live billions of years? Take a nice quiet M dwarf, give it a trillion years, maybe some sort of life will develop on some of its planets or their moons.

TheoA wrote:

Calling it potentially habitable is such a loaded term. Call it a warm wet planet, or even a Liquid water zone planet which is after all where all this speculation is coming from. Why jump to habitability so prematurely when we know so little.

See, that's the thing. No one would (should) jump to the assumption that it is "habitable" just because it is potentially habitable. Again, as Lazarus reiterated, "potentially" is the key. It might be habitable, if nothing else because it hasn't been ruled out yet. I'm sure no one here would jump to such a conclusion, and I am sure Greg doesn't either.

TheoA wrote:

Again, liquid water zone. There appears to be liquid water on Europa, we don't call that a potentially habitable moon. A warm zone around a Red Dwarf should not be called Habitable anything.

I took a bit of effort to be clear and concise in my previous post, but it seems that I didn't do a good enough job at it. The traditional habitable zone is the region around a star where the irradiation from the star would be enough to allow water at the surface of the planet and thus whose temperature is determined by the energy received from the star, under the assumption that the surface atmospheric pressure is similar or equal to that of Earth (and thus a planet could potentially be Earth-like) in this zone. It is within these areas that we expect to find "Earth-like" planets.

TheoA wrote:

They did set the Deuterium burn limit as well which was previously not so firmly set, they also categorized the Planemos.

Here is the official IAU declaration of the definition of a planet.
http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf

TheoA wrote:

There has to be a slight ulterior motive. Scientists are human and they know anything linked to life sells.

Well, I'm human, and I have no problem using the term "habitable" outside the strictest of definitions, and I have nothing to gain from it. Why must there be an ulterior motive? I simply don't see why you would see it like this.

TheoA wrote:

What annoys me is that there is push to take people eyes of the goal. Greg is trying to push Red Dwarfs as 4 time more likely to have a transiting potentially habitable planet.

Not necessarily taking the eyes off the goal, but rather, working our way toward that goal. We don't have the technology to find Earth-like planets around sun-like stars. We're about to the point where we can begin to do that for red dwarfs though. So why not take a look? I'll say that it's not as likely for an M dwarf to have an Earth-like planet as a G dwarf would, but somebody's got to look. Besides, I personally am quite curious about the abundance of terrestrial planets around M dwarfs as well as G dwarfs.

TheoA wrote:

Greg is trying to push Red Dwarfs as 4 time more likely to have a transiting potentially habitable planet.

Yes. With of course, potentially being the key word.

TheoA wrote:

So he wants us to give 4 times the focus on these planets. Or is he saying for every one Earth like transiter we will have 4 time as many around Red Dwarfs.

I'm not sure what he's trying to imply about the focus of exoplanet surveys, but since potentially habitable worlds around M dwarfs are the only potentially habitable worlds we can detect, M dwarfs should definitely get a good amount of attention. I'm sure that Greg knows that M dwarfs will host less than 4 times as many Earth-like planets as sun-like stars, as it does seem harder (but perhaps not prohibitively so) for M dwarfs to host such planets. But rather, M dwarfs would have four times as many potentially habitbale transiting planets. This is four times as many candidates for Earth-like planets. No doubt, many if not most of them will turn out not to be Earth-like.

TheoA wrote:

Lazarus wrote:

..certain conditions we can determine suggest that such planets could support a climate which enables the presence of liquid water.

So why not just say that.

Because it takes up a whole line of text, and isn't needed as the meaning of "potentially habitable" is thought to be understood.

Lastly, let's all take a deep breath and count to three.

What I've seen about habitable zones is that people classify it as if Earth was around any star. But to me, habitable zone depend on each planet, not just on Earth.

To me, habitable life include planets that have life but are also different from Earth, including life as we don't know it, different temperature and air pressure.

In our solar system, if Earth had higher air pressure, the excessive accumulation of heat would larger the habitable zone, but if Earth had less air pressure, it would keep less heat, decreasing the size of its orbit. If we were adapted to cold climate and used substances to keep water liquid at temperature below zero, a colder Earth would be no problem and it orbit could be larger.

Another example is Europa. If it really has life under the ice, such world could orbit very far from the sun without affecting its life in underground oceans. But it would always have to orbit a massive planet in order to tidal effect heat the core and the water of such world.

If most lifeforms prefer cold climate, large orbit. If most lifeforms prefer hot climate, close orbit. Simple!

So, the habitable zone will depend on things such as the kind of life on that world, temperature and air pressure. Maybe even by the albedo. If a planet is mostly covered by land and have high albedo, it will have to orbit closer to the star. But if it has lower albedo, it can orbit farther the star, because it keeps more heat. So it's a much more complex thing than just putting Earth around another star.

Well , habitable is in fact a relative term. Consider Earth.

Vostok Lake , Antarctica : Life going along just fine at -20ºC
Deep Ocean : Hypertermophyles , thriving on high pressure water at over 100ºC
(this one would live happily on GJ 1214 b )
Candidatus Desulforudis audaxviator , a bacteria that lives by itself on the deep shaft gold mines of south africa , making a living off radioactive decay of uranium , 3 Km under the surface of the Earth

Rio Tinto , Huelva (Spain) , a river with a ph of 2.2 , where microorganism live happily oxidizing pyrite, not very far of what one would spect to find on Mars. Over 1.300 species are found so far .

Typically 'Habitable' in science and literature means liquid water on the surface and Earth like life on the surface.

The key is Earth like life on the surface. Meaning in the majority cases, Carbon based, sun tracking, evolving, life. Which could in certain situations lead to intelligence.

There are areas all over the solar system that are likely as benign as ciceron identifies. There typically not characterized as Habitable.

http://www.sciencedaily.com/releases/2009/12/091217183444.htm


Apparently the Red Dwarf planets are likely to be a bust. The moons on the other hand....

Quote :

An exomoon in the habitable zone wouldn't face this dilemma. The
moon would be tidally locked to its planet, not to the star, and
therefore would have regular day-night cycles just like Earth. Its
atmosphere would moderate temperatures, and plant life would have a
source of energy moon-wide.
"Alien moons orbiting gas giant planets may be more likely to be
habitable than tidally locked Earth-sized planets or super-Earths,"
said Kaltenegger. "We should certainly keep them in mind as we work
toward the ultimate goal of finding alien life."

I like the idea. One would need to consider that since the habitable zone of red dwarfs is further in, any planets in them will have smaller gravitational hill spheres, translating to not much room for planets. If indeed gas giant planets are the only planets to have such moons, then a moon would need to be far enough away from the planet to avoid the radiation environment, unless the moon has a way of dealing with that (magnetosphere or the like).

As suck, if earth-like moons exist around habitable zone gas giants, I would expect them to be more common around sun-like stars than red dwarfs as well.

A great plus for red dwarfs is their sheer ubiquity though.

Interesting that despite work done which shows that even quite modest atmospheres would be able to efficiently transport heat around the planet, the writers of these news articles still regard tidal locking as totally ruling out habitability.

Indeed. And even if the planet did not have an atmosphere with an effective heat transport mechanism, there should still be a strip of surface on the planet at the terminator where it could remain some definition of "habitable." The conditions at the equator would be the same at the polar areas assuming a true tidal lock (e=0).

Without a well conducting atmosphere, the temperature of a tidally locked world would vary wildly from the sub-stellar point to the anti-stellar point, with a temperature gradient at the terminator. Since the anti-stellar hemisphere of the planet would be quite cold regardless of the temperature of the sub-stellar hemisphere. So the temperature gradient would still encompass "habitable zone" temperatures for terrestrial planets in tight orbits around M dwarfs.

So perhaps planets like Gliese 581 e, Gliese 433 b (assuming e=0), and others like them may have habitable longitudes on their surface.

Maybe tidally locked worlds stand a better chance at hosting some sort of life around M dwarfs than non-tidally locked worlds.

An Earth-like planet orbiting a M star with higher air pressure and at safe distance will have temperature better spread (the poles will have temperature similar to the equator and terminator), and night side won't be too cold.

So in a planet like that there will be life not only in the terminator, but more next to the central zone. This region, being hotter, will have a sky always full of clouds, that will avoid the excess of heat on the surface, serving as a negative feedback. And behind that cloud layer temperatures may be still supportable.

Let's say a planet with air pressure of 7atm and average temperature of 30°C. The central zone will have temperature around 35°C and the center of night side will have temperature of 25°C. Ah, and the planet has it's own moon that due to cold climate is partially covered by ice, having albedo of 0.5, reflecting half the star light to the night side.

In this planet, one side is eternal day and the other side have "day" and night periods. The moon was formed by ancient impact, similar to that happened on Earth and have mass a bit higher than Mars. 2 more smaller moons formed at the same time, but having much more inferior mass.

The planet diameter is 30000km. It's not only a dwarf gas because it's relatively poor in volatiles and have density of 3.5. Due to the ancient impact, it lost most of it's atmosphere to space and later replenished a new one, this time more suitable for life and stabilized with that air pressure. The mysterious planet had mass superior to Earth, but the major planet was still hot and forming, so it could not be destroyed.

It only has low density because the core is relatively small to the huge thick mantle. But it does not mean that the surface is metal poor. It's metal rich due to intense bombardment of ancient asteroids that enriched the soil with heavy metals. Due to the low concentration of volatiles for a planet with such mass, about half of the surface is water and half is land, having giant oceans and lands.

One last detail... this planet is not in our generation of stars, but many billion of years in the future, when future planetary systems will be richer in heavier elements. So that new red dwarf systems will have more planets. And the chance of life in the universe will be even higher. In the future, red dwarf stars will be the rule for life.

The question is if Habitable Zone 'Temperatures' alone are sufficient.

The conditions of the planet count too, no.

For instance more water actually narrows the Habitable Zone, as run away effects on both inner and outer edge are easier to carry out.

TheoA wrote:

For instance more water actually narrows the Habitable Zone, as run
away effects on both inner and outer edge are easier to carry out.

On the other hand, more land surface means more chemical weathering and draw-down of CO₂... geodynamic habitability models tend to predict slightly wider and also longer-lasting habitability for oceanic worlds as opposed to land worlds, e.g. this paper about Gliese 581

I wonder if they might redo this paper now that water appears to play a major part in many super terrestrial type worlds.

Are they saying that the water contributes to a thicker atmosphere? Ergo more water means longer lasting thicker atmosphere?

All they say is more volatiles (could mean anything) means more atmosphere, preventing freezing.

On Earth CO2 & Water Vapor are such vanishingly small constituents. Here with CO2 of 5-10 Bar ~ 5-10 Sea Level Atmospheric pressure, these atmosphere would be dramatically different from Earth in composition.

They do mention that complex life of any kind is unlikely.