Earth-sized exoplanet spotted in star’s habitable zone

[reenmachine]

http://www.nasa.gov/ames/kepler/nas...-zone-of-another-star/index.html#.U1BUR1csSuQ

Don't know if this is the right section to post this , but I'm a sucker for any news on exoplanets so here goes.

 

[mfb]

As the NASA website is very unreliable (at least for me)*:

The Kepler-186 system is home to Kepler-186f, the first validated Earth-size planet orbiting a distant star in the habitable zone—a range of distance from a star where liquid water might pool on the planet's surface. The discovery of Kepler-186f confirms that Earth-size planets exist in the habitable zones of other stars and signals a significant step toward finding a world similar to Earth.

The size of Kepler-186f is known to be less ten percent larger than Earth, but its mass and composition are not known. Kepler-186f orbits its star once every 130 days, receiving one-third the heat energy that Earth does from the sun. This places the planet near the outer edge of the habitable zone.

*edit, clarification: I cannot reach the website sometimes due to DNS server issues. This is a technical issue and has nothing to with the content.

 

[Drakkith]

Sweet!

 

[Greg Bernhardt]

How easily can the composition be known if at all? I suppose that is the next great signal.

 

[DHF]

They are saying even after the James Webb comes online, it might not be able to get a good read on this planet because the star could be too dim.

 

[mfb]

Then we can still hope for the E-ELT.
Even if not, it is just a matter of time until we can do this, PLATO will launch in ~10 years, it should find a lot of those planets at stars closer to our own.

 

[DHF]

very exciting times.

 

[|Glitch|]

Kepler-186f does not reside in the habitable zone of its star.

The habitable zone can be calculated for any star. If you know the luminosity of the star then it is merely a simple calculation:

r_i = √ (Star's Luminosity / 1.1)
r_o = √ (Star's Luminosity / 0.53)​

According to the paper where the discovery was announced the luminosity of the Kepler-186 M1V star is 0.0412. Which puts the habitable zone of the star at between 0.19353 AU and 0.27881 AU. Yet the paper places the semi-major axis of the planet at 0.3926 AU. Well outside the habitable zone of the star.

Sources:
Formation, Tidal Evolution & Habitability of the Kepler-186 System - arVix 1404.4368v1 [PDF]
Calculating the Habitable Zone

 

[DHF]

Does it have an elliptical orbit that puts it within the habitable zone at any point?

 

[mfb]

The habitable zone can be calculated for any star. If you know the luminosity of the star then it is merely a simple calculation:

r_i = √ (Star's Luminosity / 1.1)
r_o = √ (Star's Luminosity / 0.53)​

The units do not fit (they do fit in the source, however), and the numerical constant of 0.53 would indicate a precision the current models do not have.

Does it have an elliptical orbit that puts it within the habitable zone at any point?

I guess that would make things even worse due to extreme seasonal variations (together with issues like the long-term stability of the orbit) - and the average would still be similar.

 

[|Glitch|]

Does it have an elliptical orbit that puts it within the habitable zone at any point?

Possibly. All they can say is that Kelper-186f has an eccentricity of less than 0.34. So it might fall within the habitable zone for a time, or it may never fall in the habitable zone.

 

[|Glitch|]

The units do not fit (they do fit in the source, however), and the numerical constant of 0.53 would indicate a precision the current models do not have.

I guess that would make things even worse due to extreme seasonal variations (together with issues like the long-term stability of the orbit) - and the average would still be similar.

What do you mean "the units do not fit?" I am using their data from their paper, and the planet does not fall within the habitable zone of the star. Not by a long shot. Despite their claims to the contrary. Apparently they are more interested in publicity than actual science.

 

[mfb]

What do you mean "the units do not fit?"

r is a length and needs a unit of length (like meters or AU), unless you specify something like "in meters" or (here) "in AU". The same applies to the luminosity.

I am using their data from their paper, and the planet does not fall within the habitable zone of the star. Not by a long shot. Despite their claims to the contrary. Apparently they are more interested in publicity than actual science.

The claim comes from the Kepler collaboration and they are certainly interested in actual science. That leaves multiple possible options:
- error from Kepler
- error from Tom E. Morris (or the sources he bases his formulas on)
- error from you combining both
- error in our interpretation, as an example the definition of "habitable zone" varies a bit between different scientists
- something else (there is always something else)

 

[|Glitch|]

r is a length and needs a unit of length (like meters or AU), unless you specify something like "in meters" or (here) "in AU". The same applies to the luminosity.

As I previously posted, r_i and r_o is in AU.

The claim comes from the Kepler collaboration and they are certainly interested in actual science. That leaves multiple possible options:
- error from Kepler
- error from Tom E. Morris (or the sources he bases his formulas on)
- error from you combining both
- error in our interpretation, as an example the definition of "habitable zone" varies a bit between different scientists
- something else (there is always something else)

I am using only the data the discoverer's provided in their paper. I am not taking any other source. There are actually several different sources for Kelper-186 and Kepler-186f, and they do not all have the same data in common. Which is why I specifically chose to use their source paper and their data.

If they want to redefine the size of a habitable zone around a star, fine. But at the very least they could have explained why in this particular case they are expanding it to include this exoplanet. It should not be expanded merely because they want to be the first to find an Earth-like exoplanet. They call that sensationalism, not science.

When you find an error in my math, then you can rightly blame me. However, if my math is correct and I provided the sources for both the calculations and the data, then I am hardly to blame. Your hostility towards me is misplaced.

 

[DHF]

The distance from the star alone may not exclude it from habitability. depending on the composition of the atmosphere. if it has a very thick CO2 atmosphere it could still maintain suitable temperatures for plant life. Being further away from its star might also be a benefit as it might not be tially locked.

 

[mfb]

I don't get the original publication to load here, but I assume the wikipedia editors managed to copy the numbers correctly:

The habitable zone for this system is estimated conservatively to extend over distances receiving from 88% to 25% of Earth's illumination (from 0.22 to 0.40 AU). Kepler-186f receives 32%, placing it within the conservative zone but near the outer edge

=> different definition of "habitable zone".

 

[Bandersnatch]

Kepler-186f does not reside in the habitable zone of its star.

The habitable zone can be calculated for any star. If you know the luminosity of the star then it is merely a simple calculation:

r_i = √ (Star's Luminosity / 1.1)
r_o = √ (Star's Luminosity / 0.53)​

According to the paper where the discovery was announced the luminosity of the Kepler-186 M1V star is 0.0412. Which puts the habitable zone of the star at between 0.19353 AU and 0.27881 AU. Yet the paper places the semi-major axis of the planet at 0.3926 AU. Well outside the habitable zone of the star.

Sources:
Formation, Tidal Evolution & Habitability of the Kepler-186 System - arVix 1404.4368v1 [PDF]
Calculating the Habitable Zone

Using this calculator:
http://depts.washington.edu/naivpl/sites/default/files/HZ_Calc.html
and accompanying paper(Kopparapu et al.): http://xxx.lanl.gov/abs/1301.6674
For a 3500K, 0.0412 solar luminosity star you get the conservative habitable zone between 0.22 and 0.41 AU.

The above calculation method is mentioned and linked to in the second link you provided(near the bottom) as an "alternative approach". Judging from the numbers mfb dug up, it is clear that it was the method used.

 

[Mordred]

This paper may also be of interest

Habitable Zones Around Main-Sequence Stars: New Estimates

http://arxiv.org/abs/1301.6674

 

[DHF]

good article, thanks.

 

[|Glitch|]

Using this calculator:
http://depts.washington.edu/naivpl/sites/default/files/HZ_Calc.html
and accompanying paper(Kopparapu et al.): http://xxx.lanl.gov/abs/1301.6674
For a 3500K, 0.0412 solar luminosity star you get the conservative habitable zone between 0.22 and 0.41 AU.

The above calculation method is mentioned and linked to in the second link you provided(near the bottom) as an "alternative approach". Judging from the numbers mfb dug up, it is clear that it was the method used.

Unfortunately I could not get their calculator to work properly. Every time I entered the values for the star's surface temperature and the star's luminosity, it kept getting reset to Sol's values. However, I am in the process of reading the paper "Habitable Zones Around Main-Sequence Stars: New Estimates." Thanks.

 

[Bandersnatch]

Unfortunately I could not get their calculator to work properly. Every time I entered the values for the star's surface temperature and the star's luminosity, it kept getting reset to Sol's values.

Yeah, it does that for some reason. Try entering a value and then clicking on the same box(you'll see the calculated values change) before proceeding to the next one.

 

[Mordred]

I particularly like the atmospheric conditions correlations in that article.

 

[|Glitch|]

I particularly like the atmospheric conditions correlations in that article.

Apparently H₂O and CO₂ clouds are the primary reason for the change in calculating the habitable zone of stars. The Kasting (1993) model that I used does not take clouds into consideration at all. Clouds would certainly change the habitability of the planet. An exoplanet with clouds could be further away from its star and still have the same surface temperature than an exoplanet without clouds.

Frankly, I think it is just as wrong to assume an exoplanet has no clouds as it is to assume an exoplanet has clouds. We simply do not know one way or the other ... yet. If we ever do find an exoplanet with clouds, that would certainly be a very good argument to extend the habitable zone of the star. If an exoplanet has an atmosphere, then it seems likely that the exoplanet will also have clouds of some sort. Even Mars has CO₂ clouds, albeit rather wispy. If the exoplanet does not have an atmosphere, then it really does not matter where it lies in its solar system, there will not be any liquid water on its surface.

 

[mfb]

If we ever do find an exoplanet with clouds, that would certainly be a very good argument to extend the habitable zone of the star.

Done
And another one

(What's wrong with Earth and venus as examples of planets with clouds?)

 

[|Glitch|]

Done
And another one

(What's wrong with Earth and venus as examples of planets with clouds?)

Very cool links, thanks, but not exactly to what I was referring. A planet with clouds would extend the habitable zone for that planet, when compared to a planet without clouds.

Hence, the Kastings (1996) model, which does not take clouds into consideration at all, puts the furthest habitable zone radius for Kepler-186 at 0.2788 AU. Conversely, the updated habitable zone estimates (2013), which assumes H₂O or CO₂ clouds, places the furthest habitable zone radius for Kepler-186 at 0.4026 AU.

While the Kastings (1996) model relies solely upon the star's luminosity, the 2013 updated habitable zone model depends upon the cloud cover of the specific planet.

The reality is that we do not know anything about the exoplanet, other than a good idea about its diameter and orbit. Which means, for that particular exoplanet, it could be just barely inside the habitable zone, or well outside the habitable zone. We do not have enough information yet to nail it down one way or the other.

 

[Bandersnatch]

Hence, the Kastings (1996) model, which does not take clouds into consideration at all, puts the furthest habitable zone radius for Kepler-186 at 0.2788 AU. Conversely, the updated habitable zone estimates (2013), which assumes H₂O or CO₂ clouds, places the furthest habitable zone radius for Kepler-186 at 0.4026 AU.

While the Kastings (1996) model relies solely upon the star's luminosity, the 2013 updated habitable zone model depends upon the cloud cover of the specific planet.

The paper cited does no such thing. Have you actually read it? It says twice in the abstract alone that it's a cloud-free model.

 

[mfb]

The habitable zone is the distance where a planet with liquid water could exist, it does not depend on the planet itself. A planet can have clouds, so we have to consider clouds for the habitable zone. If we discover that the planet has no clouds (or even no water at all), this would mean the planet is not habitable (in the usual definition -> liquid water), but still in the habitable zone.

 

[|Glitch|]

The habitable zone is the distance where a planet with liquid water could exist, it does not depend on the planet itself.

That was Kastings argument, which is why his model is based solely upon the luminosity of main sequence stars.

A planet can have clouds, so we have to consider clouds for the habitable zone. If we discover that the planet has no clouds (or even no water at all), this would mean the planet is not habitable (in the usual definition -> liquid water), but still in the habitable zone.

So very true. Conversely, if we do find an exoplanet with H₂O or CO₂ clouds just outside the habitable zone as determined by Kastings (1996), then according to the 2013 new estimates it could very well still be habitable (meaning the surface temperature of the exoplanet is still in the range to support liquid water, 1°C to 99°C). But in order to make that determination we need to know at a minimum whether or not the exoplanet has clouds of some sort.

If we just used the luminosity of the star as the sole determining factor, then Kepler-186f would not fall in the habitable zone. Not even close.

 

[Bandersnatch]

Glitch, go and read Kopparapu et al.! It discusses how it differs from Kasting's model. It is most empathically not about the inclusion of cloud coverage(there is none!). Both models are qualitativelly similar, the new one simply uses updated coefficients for absorption, Rayleigh scattering and heat capacity, as well as extending the range of calculations to include colder stars.
As discussed in the article, cloud coverage is hypothesised to extend the HZ beyond what the updated model already shows.

 

[|Glitch|]

The paper cited does no such thing. Have you actually read it? It says twice in the abstract alone that it's a cloud-free model.

I read the paper, and while the abstract may describe their model as cloud-free, their paper does not:

"H2O and CO2 clouds are neglected in the model, but the effect of the former is accounted for by increasing the surface albedo, as done in previous climate simulations by the Kasting research group (Kasting 1991; Haqq-Misra et al. 2008)."​

They make the claim that their model is cloud-free, yet in the same sentence state that clouds are factored into their model by increasing the albedo of the planet.

"...we assumed an Earth-mass planet with an H₂O (IHZ) or CO₂ (OHZ) dominated atmosphere for our base model."​

Everything about their model is dependent upon the climate of the planet in question.

"The difference is caused by increased atmospheric absorption of incoming solar radiation by H₂O in the new model."​

Furthermore, the paper goes on to reestablish the habitable zone for Sol, putting the inner radius at 0.9928 AU. Which means that the Earth would only be in the habitable zone for half the year. As the Earth makes its closest approach to Sol, it would no longer be in the habitable zone. Earth, at its perihelion, is 0.9833 AU from Sol.

 

[Chronos]

I'm stuck on the oxygen thing. Any exoplanet with detectable amounts of ozone in the atmosphere would be very curious. Ozone is not particularly stable. That signals oxygen is continuously replenished by a process that looks biogenic.

 

[|Glitch|]

I'm stuck on the oxygen thing. Any exoplanet with detectable amounts of ozone in the atmosphere would be very curious. Ozone is not particularly stable. That signals oxygen is continuously replenished by a process that looks biogenic.

I agree. Any traces of methane in the atmosphere might also indicate some form of life, since that is the waste product of living organisms on Earth. Ozone, however, is a dead give-away for molecular oxygen.

There is one consideration however, under an M type star most of its electromagnetic radiation will be in the IR part of the spectrum, while it requires the electromagnetic radiation to be in the UV part of the spectrum to create/destroy ozone. So I would not expect M type stars to have planets with a significant amount of ozone, assuming those planets had molecular oxygen in their atmosphere to begin with, certainly less than our ozone layer.

Since Kepler-186f transits its star every ~130 days, it should not be long before we are able to get a spectrum of its atmosphere.

 

[DHF]

I agree. Any traces of methane in the atmosphere might also indicate some form of life, since that is the waste product of living organisms on Earth. .

How can we differentiate between Methane produced as a waste product and Methane occurring naturally? For Example Titan contains between 1-2% Methane in its atmosphere.

 

[mfb]

Methane is an indication only together with specific other components, especially with oxygen. Those two don't live long together...

 

[|Glitch|]

How can we differentiate between Methane produced as a waste product and Methane occurring naturally? For Example Titan contains between 1-2% Methane in its atmosphere.

That is a very good question, unfortunately I do not have an equally good answer. I would think that we want to look for the most volatile elements in the atmosphere because they have to be continually produced to stay in the atmosphere. Indicating on-going activity of some sort. Possibly volcanic, or possibly organic.

 

[enorbet]

Aren't we "jumping the gun" by trying to extrapolate too much too soon? It seems to me this discovery is a milestone in that a rocky planet has been discovered and catalogued that has neither an orbit around it's star measured in mere days, or in numerous years. Previously only near-Jupiter sized planets were detectable and afaik, none even remotely close to any "Goldilocks Zone". It's just a nice step in the right direction but I suppose in these days of Infotainment, that's not sexy enough. It is fun to speculate, but I think it is important to remember that we are speculating until we have more data.. Soon come.

 

[|Glitch|]

Aren't we "jumping the gun" by trying to extrapolate too much too soon? It seems to me this discovery is a milestone in that a rocky planet has been discovered and catalogued that has neither an orbit around it's star measured in mere days, or in numerous years. Previously only near-Jupiter sized planets were detectable and afaik, none even remotely close to any "Goldilocks Zone". It's just a nice step in the right direction but I suppose in these days of Infotainment, that's not sexy enough. It is fun to speculate, but I think it is important to remember that we are speculating until we have more data.. Soon come.

It is "jumping the gun" to claim that Kepler-186f is within the habitable zone of its star. That too is based entirely upon speculation. Kepler-186f could only be in the habitable zone if it had a heavy cloud layer composed of CO₂. Since we know nothing about the atmospheric content of the exoplanet, one has to speculate that those CO₂ clouds exist in order for the exoplanet to be within the habitable zone.

 

[Bandersnatch]

@Glitch:

Whether the planet is habitable is indeed unknown, but it does lie within the HZ as defined by both Kasting 1993 and Kopparapu 2013.
Both define HZ as the region where liquid water can exist on a terrestial planet with N2-H2O-CO2 dominated atmosphere.

For a planet to be habitable while lying in the outer HZ region, it does indeed need to have maximum greenhouse effect provided by CO2. But it doesn't mean that you need a special planet with just the right amount of CO2 present to be habitable. There can be various feedback mechanisms that change the composition of the atmosphere as the planet changes its place in the HZ(due to stellar evolution increasing output of the central star and orbital migration). Kastings describes one of such feedback mechanisms - the Carbonate-Silicate cycle.



I've been reading Kasting's paper that you said you used to calculate the low HZ limits in post #23, and I can't find any reference to the calculation method(with numerical constants) presented on the Planetary Biology page that you linked to earlier in post #8(even though they cite the paper as the source). In fact, Kasting calculated HZ for an M0 star in that paper, and it's pretty close to what you get from Kopparapu's(outer is about 0.47 AU for a 3700K star).
If you know where to find it, give us a shout.

Kasting's paper is here:
http://adsabs.harvard.edu/abs/1993Icar..101..108K
It's behind the paywall, but googling the title and authors quickly nets a few places from which a free copy can be obtained.

All the complaints about the Kopparapu paper that you've voiced so far apply to this paper as well. I don't understand why you said in post #23 that it is somehow different. Both papers parametrise H2O cloud effects on the inner HZ without actually modelling cloud cover(which doesn't affect outer HZ, by the way, and that's where Kepler-186f lies). Both use the same definiton of HZ, both calculate the effects of atmospheric gasses on the surface temperature.
In fact, Kopparapu's is merely an update on Kasting's methods with more refined data.

I couldn't access the other source Planetary Biology used(Whitmire et al.) as it's from a textbook, I believe. Maybe that would shed some light on the reason for the calculations they used, but it seems odd that their other source doesn't appear to support them.

 

[|Glitch|]

@Glitch:

Whether the planet is habitable is indeed unknown, but it does lie within the HZ as defined by both Kasting 1993 and Kopparapu 2013.
Both define HZ as the region where liquid water can exist on a terrestial planet with N2-H2O-CO2 dominated atmosphere.

For a planet to be habitable while lying in the outer HZ region, it does indeed need to have maximum greenhouse effect provided by CO2. But it doesn't mean that you need a special planet with just the right amount of CO2 present to be habitable. There can be various feedback mechanisms that change the composition of the atmosphere as the planet changes its place in the HZ(due to stellar evolution increasing output of the central star and orbital migration). Kastings describes one of such feedback mechanisms - the Carbonate-Silicate cycle.



I've been reading Kasting's paper that you said you used to calculate the low HZ limits in post #23, and I can't find any reference to the calculation method(with numerical constants) presented on the Planetary Biology page that you linked to earlier in post #8(even though they cite the paper as the source). In fact, Kasting calculated HZ for an M0 star in that paper, and it's pretty close to what you get from Kopparapu's(outer is about 0.47 AU for a 3700K star).
If you know where to find it, give us a shout.

Kasting's paper is here:
http://adsabs.harvard.edu/abs/1993Icar..101..108K
It's behind the paywall, but googling the title and authors quickly nets a few places from which a free copy can be obtained.

All the complaints about the Kopparapu paper that you've voiced so far apply to this paper as well. I don't understand why you said in post #23 that it is somehow different. Both papers parametrise H2O cloud effects on the inner HZ without actually modelling cloud cover(which doesn't affect outer HZ, by the way, and that's where Kepler-186f lies). Both use the same definiton of HZ, both calculate the effects of atmospheric gasses on the surface temperature.
In fact, Kopparapu's is merely an update on Kasting's methods with more refined data.

I couldn't access the other source Planetary Biology used(Whitmire et al.) as it's from a textbook, I believe. Maybe that would shed some light on the reason for the calculations they used, but it seems odd that their other source doesn't appear to support them.

Thanks for the link, and I was able to find a free copy. I apologize for taking so long to respond to your post. I too was searching for the source of those two constants (1.1 and 0.53) on the Planetary Biology web page. This was the basis for my original calculations, and why I determined that Kepler-186f was not in the habitable zone. I was not able to find any references to such constants in Kasting (1993 or 1996) or anywhere else. Since I do not know how those constants were derived, I cannot use those equations on the Planetary Biology web page.

As far as the "habitable zone" is concerned, that is only the area where water can be in a liquid state on the surface of a planet. Which means that anywhere that the surface temperature is between 1°C (274°K) and 99°C (372°K) is considered to be in the "habitable zone."

As mfb mentioned in post #27, "The habitable zone is the distance where a planet with liquid water could exist, it does not depend on the planet itself." I happen to agree, it should not depend on the planet at all. Everyone, including the Kasting and Kopparapu papers are fixated on liquid water actually being present instead of just the temperature range at which liquid water could be present.

For example, using a black-body object (absorbs 100% of the energy it receives) with no atmosphere, will still have a surface temperature in the range between 1°C (274°K) and 99°C (372°K) at some point for any given star. Using Sol as an example, and using the Stefan–Boltzmann law, the inner "habitable zone" radius would be 0.5548 AU and the outer "habitable zone" radius would be 1.0337 AU. That assumes a black-body planet with no atmosphere.

As we both know, no planet is a perfect black-body. Even Mercury has an albedo of 0.06. However, now we are getting into the characteristics of the specific planet, which should not be a determining factor.

The habitable zone for any given star should be based solely on the surface temperature of the star and the star's radius, as follows:

T_E = T_S x √ R_S / 2a_o​

Where:

T_E = Black-Body Surface Temperature (Kelvin)
T_S = Star Surface Temperature (Kelvin)
R_S = Star Radius (Meters)
a_o = Distance from Star (Meters)​

Using Sol as an example:

5,778 x √ ( 695,500,000 / ( 2 x 83,000,180,000 )) = 372°K
5,778 x √ ( 695,500,000 / ( 2 x 154,639,700,000 )) = 274°K​

In the case of the star Kepler-186, no radius is given. However, the Stefan–Boltzmann law can be used to approximate the radius, as follows:

R / R_☉ ≈ ( T_☉ / T )² x √ ( L / L_☉ )​

( 5,778 / 3,788 )² x √ ( 0.0412 / 1 ) ≈ 0.4723 x 695,500,000 ≈ 328,458,781 meters​

This would put the inner habitable zone radius for Kepler-186 at ~0.1138 AU, and the outer habitable zone radius at ~0.2098 AU. Again, that assumes a perfect black-body planet with no atmosphere.

We should be determining the habitable zone for a given star, not for a given planet. A planet with a high albedo would have to be much closer to its star. A planet with a dense CO₂ atmosphere could be much further away and still be warm enough to support liquid water on its surface. However, until we know more about the planet in question, it is entirely conjecture.

 

[Bandersnatch]

Oh, I fully agree that the surface temperature calculations would be different for a bare rock, and it's a good first appoximation. But that's like calculating whether liquid water can exist on a planet without water. It's just a bit silly, don't you think?

 

[mfb]

A perfect grey body has the same equilibrium temperature as a perfect black body. To get deviations, you need different absorption coefficients in the visible and the infrared range. And this model is not realistic anyway.

 

[|Glitch|]

Oh, I fully agree that the surface temperature calculations would be different for a bare rock, and it's a good first appoximation. But that's like calculating whether liquid water can exist on a planet without water. It's just a bit silly, don't you think?

We should not be trying to calculate whether liquid water can exist on a planet. We should be calculating where in the solar system water can be in a liquid state.

I think it is even more silly to manufacture ideal planetary conditions to support liquid water when we have absolutely no information about any of the planets in a given solar system. Such as in the papers by Kasting and Kopparapu, which creates an oxygen rich atmosphere for their inner habitable zone radius calculations, and a carbon dioxide rich atmosphere for their outer habitable zone radius calculations. Particularly since we have never discovered a planet that matches either of those descriptions in their models.

If all the information we have is about a given main sequence star, then that is what the habitable zone should be initially based upon. As you say, as a "first approximation." As we get more information on the planets in that solar system, the habitable zone can be adjusted to accommodate the specific planets in question. Rather than making up data for some imaginary planet that does not exist.

Kasting and Kopparapu are creating a fictitious planet where liquid water could exist, and then assuming all planets within a given solar system fits that established model. Kepler-186f, for example, could only have liquid water on its surface if it has a low albedo, high atmospheric pressure, high levels of CO₂ in its atmosphere, and strong radiative forcing. That is a lot of blind assumptions. It borders more on science fiction than actual science.

 

[Damo ET]

Glitch, I may be completely missing the point you are trying to make, but finding 'Earth' sized planets around any star is juuuussstttt becoming a reality now. You seem to be criticizing approximations and best guesses made 20 years ago about the places to logically look for habitable 'Earth like' planets which fall in the zone to maintain oceans of liquid water. No one is saying liquid water doesn't exist on other planets or moons for that matter, but we don't even know for certain that liquid water is exclusive to the Earth in our own solar system, let alone wanting to redefine the searching parameters for all stars and bodies we can't even begin to see. Until sensitive enough equipment is made which can effectively check atmospheres for particular content around these other rocky orbiting planets (that we are yet to consistently find), redefining the search is pointless.

The HZ of a star is just that, the zone around the star where liquid water is possible, forget cloud cover and atmosphere.


Damo

 

[|Glitch|]

Glitch, I may be completely missing the point you are trying to make, but finding 'Earth' sized planets around any star is juuuussstttt becoming a reality now. You seem to be criticizing approximations and best guesses made 20 years ago about the places to logically look for habitable 'Earth like' planets which fall in the zone to maintain oceans of liquid water. No one is saying liquid water doesn't exist on other planets or moons for that matter, but we don't even know for certain that liquid water is exclusive to the Earth in our own solar system, let alone wanting to redefine the searching parameters for all stars and bodies we can't even begin to see. Until sensitive enough equipment is made which can effectively check atmospheres for particular content around these other rocky orbiting planets (that we are yet to consistently find), redefining the search is pointless.

The HZ of a star is just that, the zone around the star where liquid water is possible, forget cloud cover and atmosphere.Damo

It is precisely because we do not have enough information concerning exoplanets that I am being critical. In order for Kepler-186f to fall within the habitable zone of its star it must meet certain criteria. If, for any reason, Kepler-186f does not fit that criteria then the planet cannot be in the habitable zone. For example, if Kepler-186f does not have a dense CO₂ atmosphere, then it cannot be in the habitable zone.

My chief criticism is that they are basing the habitable zone upon the planet, not the star alone, with little to no information. I consider that to be reckless sensationalism, not science. Science is suppose to be based upon actual observations, not wishful thinking.

If they had reported that Kepler-186f may be in the habitable zone if they eventually find out that it meets certain conditions, that would have been far more accurate than coming flat out and stating that the planet was in the habitable zone, when in reality it may not be.

For the record, I am not the one that is factoring clouds, albedo, CO₂ or H₂O atmospheres, and radiative forcing into the habitable zone equations. That would be Kasting and Kopparapu. I am arguing that the habitable zone should be determined solely by the temperature and radius of the star, and not be concerned with factoring in planetary conditions (at least until we have some information about the planet). Nor is the information 20 years old. These equations have been revised several times over the years. The most recent revision was Kopparapu (2013).

 

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In order for Kepler-186f to fall within the habitable zone of its star it must meet certain criteria.

Only for a definition of "habitable zone" no one apart from you uses. Everyone else uses it in a different way.

No one is claiming "every star in a habitable zone has to have the right temperature". This is just not the point of the habitable zone. The habitable zone is the distance range around a star where liquid water is possible, given the right conditions on the planet. We know that, according to our models, liquid water is possible on Kepler-186f, therefore it is in the habitable zone.