# Range of Earth's year within habitable zone?

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1. Jan 29, 2016

### Lapsangtea

Hi all,

So the (hypothetical) question I have is about the range of the Earth's length of orbit (number of days) within the Sun's habitable zone which could sustain human life. I.e. what would the length of the shortest 'habitable' year be, and the longest (in days)? What would our living conditions be like at either extreme?

By the same process, what would the range (in days) approximate to (in support of human life) for a K-type star?

Looking forward to your responses! :)

2. Jan 29, 2016

### DuckAmuck

According to wikipedia, the biggest theorized range of the habitable zone is 0.5 AU to 3 AU. We are at 1 AU now.
So the shortest year is ~183 days.
The longest year is ~1095 days.

Not sure about the temperature and how livable the conditions are for humans at those extremes.

3. Jan 29, 2016

### Staff: Mentor

3 AU would guarantee a pretty heavy bombardment ... This won't be habitable for long.

4. Jan 29, 2016

### Staff: Mentor

You mean because of the asteroid belt. Consider that the question may apply generally to other Solar Systems. Also, single variable questions like this are hard to give good answers to. For example, there is a hypothesis that the presence of the moon has stabilized the axial tilt of the earth, another that planets close to the primary star becomes tidally locked with one side "cooked" the other side "frozen". The list goes on - the point is not that these ideas are correct/not correct but that there are many possible parameters that come into play. Being in the 'Goldilocks Zone' is just one.

Or maybe this discussion is somewhat pointless if you accept this idea:
http://www.anu.edu.au/news/all-news/the-aliens-are-silent-because-they-are-extinct

Also note that we are assuming a G star not a K as was asked in the question. @Chalnoth could probably give us a decent way to calculate this for star in the main sequence. Or a paper that already has done that.

Last edited: Jan 29, 2016
5. Jan 29, 2016

### Staff: Mentor

I agree. To give the question, which was about earth, a glimpse of a sense I imagined we could in some future time increase earth's orbit as soon as our sun starts to become a red giant. In addition part of the question has been under which conditions we would live under an altered orbit.

6. Jan 29, 2016

### Janus

Staff Emeritus
A lot depends on what range you use for the habitable zone ( there have been a lot of estimates over the years) For example, one of them puts the habitable zone for our solar system as being from 0.99 to 1.688 AU. Knowing that a K class star can run from 0.48-0.8 solar masses, we can apply the mass-luminosity law for main sequence stars and get a luminosity range of 0.2304-0.64 that of the Sun. From this and using the range above as an example we can work out that for a K0 star the period for the habitable range orbit is from 0.157 yrs to 0.35 years and For a K9 star would range from 0.564-1.255 yrs.

7. Jan 29, 2016

### |Glitch|

You are mistaken to believe a star's habitable zone has anything to do with sustaining human life. A star's habitable zone has absolutely nothing to do with whether it is habitable for life, human or otherwise. A star's habitable zone only pertains to whether liquid water could be present on the surface of a planet. Which also means that more than distance from the star must be taken into consideration. For example, a planet must have an atmosphere with a high enough atmospheric surface pressure. If the planet's atmospheric surface pressure is too low, then there cannot be liquid water.

8. Jan 29, 2016

Staff Emeritus
While I like Janus' answer, I doubt very much that we understand the limits to the habitable zone to 1%, which is what the precision implies. I also think .99 is too big. I understand the concern about a runaway greenhouse effect from oceanic evaporation, but if you had an atmosphere that was 5 bars of N2, it would greatly mitigate the effect. (I also suspect that the .99 number might be intended more to inform modern political discussion than scientific discussion)

9. Jan 29, 2016

### |Glitch|

Kopparapu et al. (2014) puts the optimistic habitable zone range at 0.75 AU and 1.765 AU, and a conservative habitable zone range at 0.95 AU and 1.676 AU. However, it also assumes an Albedo of 30% and a high atmospheric surface pressure.

10. Jan 30, 2016

### |Glitch|

Using Sol's conservative inner habitable zone from Kopparapu et al. (2014) of 0.95 AU, Earth would have:
• An orbit of 338 days;
• An orbital velocity of 30.6 ± 0.5 km/s; and
• A mean surface temperature of 33.4°C ± 0.5°C
Using Sol's conservative outer habitable zone of 1.676 AU, Earth would have:
• An orbit of 792 days;
• An orbital velocity of 23 ± 0.5 km/s; and
• A mean surface temperature of -97.2°C ± 0.5°C.
It should be noted that the mean surface temperature of Earth at the conservative inner habitable zone was calculated using an Albedo of 30%, while the mean surface temperature of Earth at the conservative outer habitable zone used an Albedo of 60%. As ice begins to cover Earth's surface at a distance of 1.676 AU the Albedo would begin to increase. Therefore, the Albedo of 60% reflects an Earth that has been completely covered by ice.

Kopparapu et al. (2014) also assumes a lower atmospheric carbon dioxide level at the inner habitable zone and a higher atmospheric carbon dioxide level at the outer habitable zone. The results above are based upon an unchanging 400 ppm (3,098 Gt CO2) atmospheric carbon dioxide level, with an atmospheric mass of 5.1E+18 kg, and an atmospheric surface pressure of 98.2 kPa.

In order to calculate the habitable zone of a spectral type K star, more information is required. The effective surface temperature and luminosity (in solar units) of the star would need to be known.

11. Feb 1, 2016

### Bernie G

12. Feb 1, 2016

### Staff: Mentor

@Bernie G - This is an explanation of possibly why that is. It involves a large number of parameters that all have exist to keep life going - or least not repeatedly blast life back into single celled organisms like bacteria. Mars had liquid water for a measurable fraction of it's existence. If life evolved there anything that is now extant likely has to live under or near the surface. There is a large variety of very simple organisms on Earth living way down under the surface in rock or deep in caves with nasty (to us) atmospheres.

So the Drake Equation has to account for repeated geologic/ecological/cosmic catastrophes in addition to all the parameters it originally had. If you care to use it.

13. Feb 1, 2016

### Bernie G

For a planet to allow advanced lifeforms is probably not all that rare. But for those lifeforms to develop technology (printing, chemistry, optics, electricity) is probably exceptionally rare. On Earth its only happened one time. What our species has is a rare treasure.

14. Feb 1, 2016

### Staff: Mentor

@Bernie G - and your citation is, please? That is the exact opposite of what the paper cited above claims: All the alien life forms are extinct and/or never got beyond the refrigerator parasite stage.

Last edited: Feb 1, 2016
15. Feb 1, 2016

### Bernie G

I think you are right. In the Milky Way there must be/have been 1000 - 1 million planets with intelligent life. Did any of that intelligent life live long enough to develop technology to prevent extinction? Probably not. Very sad. Our species has an obligation to make things better.