# Will a planet facing its star always support life ?

1. May 23, 2014

### vrmuth

I think this is the right forum to start this thread, if not i am sorry please somebody change it .
suppose a planet is in the right distance from its star and it is rocky , has water and atmosphere but the only thing is it's not rotating itself , i mean it's always facing it's star , then will it support life to evolve ?

2. May 23, 2014

### Staff: Mentor

This is definitely not social sciences, I am moving the thread.

I doubt we know enough about abiogenesis to be able to answer this kind of question.

I recall reading theories postulating tides are an important factor (that is, presence of a shore that is washed twice a day). I don't think you can have serious tides on a tidally locked planet.

3. May 23, 2014

What kind of atmosphere?
There have to be the right amount of gases to support life.

4. May 23, 2014

### Staff: Mentor

5. May 23, 2014

### jim mcnamara

Planets like that (Example: mercury) have very different environmental properties on the sun-facing side versus the out-facing side, specifically solar energy input.

Nobody knows definitely, but the possibilities for abiogenesis would be very different than what were on planet Earth 4+ billion years ago. Atmospheric differences would be large from side to side. Water and some atmospheric gases would be liquified or frozen on the dark side.

Here is a paper modeling atmospheres on earthlike planets with synchronous orbits:
http://crack.seismo.unr.edu/ftp/pub/gillett/joshi.pdf

They claim a 'habitable' atmosphere can exist on part of the surface, as I read it.

6. May 23, 2014

### Bandersnatch

That paper is almost 20 years old. Check out this one from 2013:
Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets
Their model shows that tide-locked planets may be even advantageous from the habitability range standpoint.

Last edited: May 23, 2014
7. May 23, 2014

### bobze

Since we have no real examples we cannot say for sure. I've thought about this before. I don't see why it would be impossible given the correct conditions. I think it would depend on lots of factors, such as atmosphere composition, distance to star, etc.

Maybe the twilight zones on either pole would be good places for life to exist on such a planet of extremes.

8. May 26, 2014

### vrmuth

yes .
I mean it has all the things that are essential for life on earth , the only thing is that the planet is not rotating itself .

9. May 26, 2014

So what? Do you mean that life can start even when the atmosphere is say,100% $SO_2$?

10. May 26, 2014

### Staff: Mentor

I don't know, do you?

Besides, it is not what the question is about, which you apparently missed. See the latest post by OP.

11. Jul 20, 2014

### Torbjorn_L

Likely, yes.

What on them would prohibit life? The presence of tidal lock changes the HZ boundaries a bit as well as it changes the ideal size and starting atmospheres for life emergence. E.g. it induces a runaway greenhouse easier so the HZ edges moves, it makes the atmosphere more convective so you may want to go for more massive and more dense atmospheres to efficiently move heat without super-convection. (E.g. on the nearly tidal locked Venus the surface winds are still lenient enough that you may stand up.)

If such effects change the number of potentially habitable planets much is unknown. In a linear estimate tidal lock wouldn't mean much for habitability.

Not serious tides, but the tidal bulge would move around a bit, compare with Moon as seen by LRO.

Tides may or may not have advanced land life, they do allow for more nutrients but also a harsher strand zone. The time to invade land from the tidal zone was short, so the effect is likely minute either way.

There are a few chemical cycles that are proposed to augment emergence of life, e.g. abiotic polypeptide linking, that are based on tides. But today they seem unnecessary since we have plenty of chemical pathways. So their importance are open.

The question was about habitable planets in general though.

12. Jul 29, 2014

### Chronos

I think you could make a case that life on a tidally locked planet would be possible. It would, however, face different challenges. Some might confer advantages, others, not so much.

13. Jul 30, 2014

### Torbjorn_L

I think those challenges would make good research at the moment.

- Higher likelihood of haze perhaps, lowering the photosynthesis yield and prohibiting anything else than radar astronomy among intelligent species.

- Higher surface pressure of 10-100 atmospheres to spread heat would perhaps not constrain habitable ocean depth much (at thousands of atmospheres), but with more volatiles the oceans may be deeper and the habitability biosphere would bottom out in a way not seen here.

- Higher surface winds (and superrotating upper atmospheres on high atmosphere density planets Venus, Titan, ... up to neptunes) would mean squatting organisms, perhaps awesome fliers and problems to discover nuclear rocketry to explore the system. (Since I doubt chemicals would be enough, especially on superEarths.)

And so on.

14. Jul 30, 2014

### Czcibor

vrmuth:

When talking about planets, at this moment it is to early to say whether there is any life, so that part of discussion is guessing.

However, the scientific question is whether such planets are "habitable", what means "whether they have liquid water on surface, which is a requirement for life as we know it". This one can be answered - nowadays the dominating answer is: "Unless the atmosphere and hydrophere is really thin, then heat transfer is possible to the dark side. If that is possible, then the whole atmosphere would not freeze on the dark side, so there would be no clear obstacle for existence of life".

15. Jul 31, 2014

### Torbjorn_L

I agree with Czcibor on the constraints at hand (habitability instead of inhabitation et cetera), but I have a problem with the "guessing" part.

We do have one example of life (duh), so we know it isn't pure guessing.

Moreover, unless the process of emergence of life was _extremely_ finetuned (unnatural), it will have ended in more than one instance. Even inflation, what resulted in our local universe, wasn't finetuned enough to likely result in only one universe (of which our observable universe is a small part of). I don't know of any such finetuned process, so I would say that it is extremely unlikely.

Reversely, we know from observation that life emerged rapidly. E.g. there was a habitable Cool Early Earth @ 4.4 Ga bp (billion years before present), and with a similar impact mass flux as the late bombardment life could arise at least @ 4.35 Ga bp. Otherwise unconstrained phylogenies place the first dateable clade split at average 4+ Ga bp. (Check Timetree for archaea vs bacteria).

That means the process was frequent and/or easy. In either case, that is the benchmark to use whenever an environment like Hadean Earth was present. That seems to be the case in terrestrials in the radiative habitable zone on 0.5 - 1.4 Earth radius bodies. (Though I would think most < 1 Earth radius bodies would dry out too much for a surface biosphere (Mars, Venus).)

If it is not guessing, is it quantitative? Well, not in ordinary statistics. But stochastic processes has funny properties of observability and being controllable as all other processes. And for control under loose constraints of a modeled process (say, Poisson) just 1 sample is enough. So I would claim, arguably, that the one observation we have is enough constraint to be quantitative.

16. Jul 31, 2014

### Czcibor

I'd not say pure guessing. I would not say: "we're alone". However in practice, whole Drake equation still contains mostly pure guesses. Here simply the knoledge is lacking and proper disclaimer is necessary. You may even add info about extermofiles and that there are hipothesis concerning life that is not based on carbon chains in water solvents.

17. Jul 31, 2014

### Torbjorn_L

Maybe we have a language issue. By pure guessing I would say non-informative, unconstrained claims. Then there is a discussion to be had what to do with bayesian betting odds and whether or not qualitative claims can really be tested.

Personally I would say that the Drake equation is poorly constrained. We can put numbers on all factors but they will lead to orders of magnitude spread. A shortened version which doesn't include civilizations is useful since it doesn't suffer from false negatives, and since the evolution of language capability is arguable. (I.e. non-communicative civilizations.)

Since it was some time I did it, and I have a better idea of the figures, let me try without error bars:

N = R*fp*ne*fl*fi*fc*L numbers of civilizations

R ~ 1 star/year; fp ~ 1 planet system/star; ne ~ 1 habitable planet/planet system; fl ~ 1 life at some point as per rapid emergence process; fi ~ 10^-5 evolve language capability, a rare trait like the elephant trunk and ~ 10^5 mammal species at any given time*; fc ~ 1 civilization that gives trace; L ~ 10^6 years for average species lifetime.**

* This can be done by integrating over all mammal species through a history of some 100 million years and ~ 1 million year lifetimes, so happens with 10^-13 frequency over species. Integrating over time gives 10^-5 as rate/planet with at least 1 mammal equivalent lineage.

** Assumes genomes and biospheres like ours as benchmark.

-> N ~ 1*1*1*10^-5*1*10^6 = 10 contemporary civilizations at a minimum in today's relatively mature galaxy (i.e. without bothering with the galactic habitable zone).