Earth-sized exoplanet spotted in star’s habitable zone

[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).

 

[mfb]

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.