I Why do we need "planetary equilibrium temperature"?

[GW150914]

I mean, currently it seems that scientists are using equilibrium temperature of exoplanets (calculated assuming an Earth-like albedo) to determine whether a planet is habitable or not. But aren't there other more accurate ways to determine surface temperatures of exoplanets? I learned Wien's displacement law in basic physics, and I know we have the ability to isolate the planet's spectrum from the star's while transiting, then why can't we use it to get more accurate temperature data, or even components of atmosphere of those planets?

 

[PabloAMC]

I know that the the components of the atmosphere can be determined using spectroscopy, and temperature too. For example, take a look at these papers
http://iopscience.iop.org/article/10.1088/0004-637X/707/1/24/pdf
http://iopscience.iop.org/article/10.1086/527475/meta
http://www.aanda.org/articles/aa/abs/2006/10/aa3861-05/aa3861-05.html
However, the transit is not the only method for detection of detection, though it is the most successful one. The second most productive one is radial velocity which was until 2010 the most successful one, and this does not give any information about the spectra of the exoplanets.
I think that the biggest problem for temperature determination is the low temperature of the exoplanet, some absorption features on certain ranges that modify the blackbody spectra and that the biggest amount of exoplanets detected have been very lately, more or less since 2014
However, I guess that it is promising, but more difficult than simulations. By the way, you may found interesting
http://www.hzgallery.org/
They have a catalog of exoplanets, and some papers concerning temperature estimates

 

[Ken G]

I think the answer can be found in this snippet from the abstract of the third paper cited above:
"According to our calculations, the signatures of planetary atmospheres represent an absorption of a few parts-per-million (ppm) in the stellar flux. The atmospheres of a few Earth-like planets can be detected with a 30-40 m telescope."
So it certainly sounds like the technology is not readily available to determine the temperatures of the kinds of planets we are interested in being habitable. So perhaps the problem is a confusion between what we can do to detect temperatures of large planets with heavy atmospheres, which we do not regard as habitable in any event, with earthlike planets.

 

[|Glitch|]

I mean, currently it seems that scientists are using equilibrium temperature of exoplanets (calculated assuming an Earth-like albedo) to determine whether a planet is habitable or not. But aren't there other more accurate ways to determine surface temperatures of exoplanets? I learned Wien's displacement law in basic physics, and I know we have the ability to isolate the planet's spectrum from the star's while transiting, then why can't we use it to get more accurate temperature data, or even components of atmosphere of those planets?

They are using more than just "planetary equilibrium temperature" to determine whether a planet is within the habitable zone. Besides Albedo, which you mention, they are also making the assumption that an exoplanet has a suitable atmospheric pressure and the exoplanet is between 0.1 M_⊕ and 5 M_⊕. If the temperature and atmospheric pressure of an exoplanet is not within the triple point of water, then there can be no liquid water on the surface of the exoplanet, and anything larger than 5 M_⊕ would not be considered rocky. Other studies have suggested anything larger than 1.6 R_⊕ may not be considered rocky.

A modified version of the Stefan-Boltzmann Law is currently used to determine the habitable zone of a main sequence star, where the luminosity and effective surface temperature of the star are the primary considerations. Although I see no reason why Wien's Displacement Law could not be substituted and still achieve the same result.

Sources:
Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass - The Astrophysical Journal Letters, Volume 787, Number 2, May 15, 2014 (free issue)
Most 1.6 Earth-Radius Planets are not Rocky - The Astrophysical Journal, Volume 801, Number 1, March 2, 2015 (free issue)

 

[GW150914]

They are using more than just "planetary equilibrium temperature" to determine whether a planet is within the habitable zone. Besides Albedo, which you mention, they are also making the assumption that an exoplanet has a suitable atmospheric pressure and the exoplanet is between 0.1 M_⊕ and 5 M_⊕. If the temperature and atmospheric pressure of an exoplanet is not within the triple point of water, then there can be no liquid water on the surface of the exoplanet, and anything larger than 5 M_⊕ would not be considered rocky. Other studies have suggested anything larger than 1.6 R_⊕ may not be considered rocky.

A modified version of the Stefan-Boltzmann Law is currently used to determine the habitable zone of a main sequence star, where the luminosity and effective surface temperature of the star are the primary considerations. Although I see no reason why Wien's Displacement Law could not be substituted and still achieve the same result.

Sources:
Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass - The Astrophysical Journal Letters, Volume 787, Number 2, May 15, 2014 (free issue)
Most 1.6 Earth-Radius Planets are not Rocky - The Astrophysical Journal, Volume 801, Number 1, March 2, 2015 (free issue)

Thanks for your responding. My point is, since we have the ability to determine the surface temperature of some exoplanets (discovered through transit or direct imaging methods), I think it's not necessary to use planetary equilibrium temperature to test the habitability of exoplanets. Besides, though the sizes and atm. pressures of those planets are indeed important, if they don't have proper surface temperatures, they are still unhabitable.

 

[Ken G]

My point is, since we have the ability to determine the surface temperature of some exoplanets (discovered through transit or direct imaging methods), I think it's not necessary to use planetary equilibrium temperature to test the habitability of exoplanets.

I think you will find that we currently do not have the ability to directly determine the surface temperature of any planets we would regard as habitable.