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.