Habitability: The Case for F-Class Stars

by Paul Gilster on March 27, 2014

When it comes to habitable planets, we focus naturally enough on stars like our own. But increasing attention has been paid to stars smaller and cooler than the Sun. M-class dwarfs have small but interesting habitable zones of their own and certain advantages when it comes to detecting terrestrial planets. K-class stars are also interesting, with a prominent candidate, Alpha Centauri B, existing in our stellar back yard. What we haven’t examined with the same intensity, though, are stars a bit more massive and hotter than the Sun, and new work suggests that this is a mistake.

Manfred Cuntz (University of Texas at Arlington), working with grad student Satoko Sato, has been leading work on F-class stars of the kind normally thought problematic for life because of their high levels of ultraviolet radiation. Along with researchers from the University of Guanajuato (Mexico), Cuntz and Sato suggest that we take a closer look at F stars, particularly considering that they offer a wider habitable zone where life-sustaining planets might flourish.

Cuntz thinks the case is a strong one:

“F-type stars are not hopeless. There is a gap in attention from the scientific community when it comes to knowledge about F-type stars and that is what our research is working to fill. It appears they may indeed be a good place to look for habitable planets.”

CompLifeZone

Image: The habitable zone as visualized around different types of star. Credit: NASA.

The team’s paper in the International Journal of Astrobiology makes this argument based on its studies of the damage that ultraviolet radiation can cause to the carbon-based macro-molecules necessary for life. Its estimates of the damage that would accrue to DNA on planets in F-class star systems covered calculations for F-type stars at various points in their evolution. Planets in the outermost regions of the habitable zone experience much lower levels of radiation. This UT-Arlington news release quotes the paper:

“Our study is a further contribution toward the exploration of the exobiological suitability of stars hotter and, by implication, more massive than the Sun…at least in the outer portions of F-star habitable zones, UV radiation should not be viewed as an insurmountable hindrance to the existence and evolution of life.”

F-type stars represent 3 percent of the stars in the Milky Way, as compared with G-class at about 7 percent and K-class at approximately 12. And then there are M-dwarfs, which may account for over 75 percent of all main sequence stars. In any event, the more we widen the prospects for astrobiology beyond stars like the Sun, the more we address the possibility of a galaxy suffused with life, even if we still have no direct evidence. Just as intriguing: If it turns out life is abundant, is intelligence abundant as well?

The paper is Sato et al., “Habitability around F-type Stars,” International Journal of Astrobiology, published online 25 March 2014 (abstract).

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{ 41 comments… read them below or add one }

Scott March 27, 2014 at 13:26

I actually thought stellar lifespan was a bigger concern than the UV radiation. It doesn’t matter how big the habitable zone is if the star is likely to die before life really gets a foothold.

Alex Tolley March 27, 2014 at 13:27

There is an arvix full paper here (although this may be an early version): http://arxiv.org/pdf/1312.7431.pdf

Alex Tolley March 27, 2014 at 13:42

The authors assume (in the arxiv paper) that the only UV mitigating factor investigated is the atmosphere. This is strange, as water is a very good UV protector. Given that we think a number of worlds have oceans, even are deep oceans, it seems rather quaint to imply/assume that life must also be terrestrial rather than purely aquatic.

Even land dwellers can move on the surface at night, and even live underground, or utilize crevices and caves for protection during the day. I would speculate that on such a world, with a biology similar to Earth’s, primary production would be limited to the oceans and lakes, whilst the higher trophic levels would be mainly aquatic, but with some terrestrial forms that lived protected underground with some forms being nocturnal to avoid the direct sunlight.

Life can be very adaptive, exploiting unexpected physical niches. Earth biology has explored many of these niches and I would expect them to do so even on planets orbiting stars with “unfriendly” em spectra.

RobFlores March 27, 2014 at 15:39

Interesting, what if the closest Earth Twin orbits an F type star.

Does having only 2-3 billion years as a main sequence star make such a planet a non-starter as far as an object of colonization of.

Example Theta Ursae Majoris Distance 44Ly Primary F6
Gravity .98 G Amosphere 90% N2, 8% O2, 2% CO2 and water vapor.
Rotation 23 Hrs , 65% water 35 % land
Native Photosynthesis established

Example CCE 223 (to call it something) Distance 154 LY Primary F6
Gravity 1.2G Atmosphere 93 N2, 7% combo of CO2, cyanogens, water vapor.
Rotation 20 Hrs , 50% land 50 ocean.;
Introduction of Photosynthesis needed.

What do we do ?
Assume we can only achieve 25% C. max in next millennium

James D. Stilwell March 27, 2014 at 16:21

Life will always find a way…
From Jurassic Park…

RobFlores March 27, 2014 at 16:41

CORECTION the 2nd farther away planet is assumed to orbit a G5 star.

Rob Flores March 27, 2014 at 19:40

Correctio: I meant for the primary system at 144 Ly, to be to be G5

andy March 27, 2014 at 19:43

Incidentally the nearest F-type star, Procyon A is probably a bad place to look for habitable planets: the system is a binary and the primary’s HZ is at the edge of the stable zone. Furthermore the companion Procyon B is a white dwarf, which implies that the binary was initially even tighter than it is today (mass loss during the red giant stage would tend to increase the binary separation).

On the other hand, Procyon B is metal-polluted, which suggests the system did at least get as far as producing asteroids.

swage March 28, 2014 at 6:52

“What do we do ?
Assume we can only achieve 25% C. max in next millennium”

Well… my current favorite is Gliese 667 Cc. Aside from achiving 84% in the earth-similarity index (most likely an ocean world a bit warmer than Earth and bigger and a just 28 day year), there are two more planets in the habitable zone, so that would increase chances considerably. Besides: its distance is 23.6 Ly, so at 25% C thats equal to ~100 years (well it istn’t that simple because the star moves and you have to accelerate and decellerate), which seems manageable in a generation ship. 2-3 generations seem feasible.

Ronald March 28, 2014 at 8:30

I went through the article and the published paper (referred to by Alex). Interesting, but I wondered what was really new.

Quite relevant are Fig 7 and Table 4:

They show the 2 main and related problems with F stars (and in general with hotter, early spectral type stars):
Not only is their stable, main sequence life span relatively short, but, even more importantly, because of their rapid evolution, the HZ moves outward rapidly, hence severely limiting the *continuous* HZ, defined as the part of the HZ which remains as such during the entire main sequence lifespan of the star.

They define the conservative HZ (CHZ, with limits of 0.95 and 1.37 AU; confusing, this is often also used for continuous HZ) and the general HZ (GHZ, from 0.84 – 1.67 AU; also confusing since this is also used for Galactic HZ). As recent update work on the HZ by Kasting et al. shows, this CHZ may be a bit strict on the outside, but it is definitely realistic on the inside (defined by onset of continuous water loss by photodissociation).

Table 4 and Fig. 7 both show that the continuous CHZ declines rapidly with increasing stellar mass (and hence brightness, evolution rate, etc.). Particularly telling are the blue horizontal lines in Fig. 7: the width between them is the continuous CHZ, and we see that this width narrows drastically moving up the stellar mass range. At 1.5 Msol (about F2) the continuous CHZ (oC minus iC) is reduced to almost zero, but already at 1.2 Msol (about F5/6) and beyond it is only about 0.2 AU or less (!). And that is for a total lifespan of 2.5 to 4 gy.
This aspect suddenly renders those F stars much less attractive with regard to habitability: their continuous HZ isn’t any wider than that of an early orange (K2/K3) star and for a MUCH shorter time period.

It is possible that the much more generous GHZ will do for microbial life. However, it seems likely from this paper that any star hotter/brighter/bigger than about F8/9 offers both a too narrow and a too short window of opportunity for higher life to arise and develop.

Ronald March 28, 2014 at 9:10

swage: Gliese 667Cc is probably not terrestrial at at least 4 Me, rather a mini-Neptune. And it is really very much on the outskirts of the HZ.

There are some really promising Kepler candidates, terrestrial planets orbiting in the HZ of G and K stars, that I will mention soon.

Ronald March 28, 2014 at 11:06

Further to my previous comment and swage, an advance:
The 5 most promising Kepler candidates by far that I could fund in the complete Kepler database are:
Kepler ID Planet Type ESI
K05904.01 G-Warm Subterran 0,82
K05545.01 G-Warm Terran 0,72
K05123.01 G-Warm Terran 0,93
K05927.01 G-Warm Terran 0,91
K05819.01 G-Warm Superterran 0,83

The Planet type and ESI (Earth Similarity Index) are from the Planetary Habitability Lab (PHL).

Talking about favorites: ‘Nearby’ Tau Ceti has two planets, super-earths/mini-Neptunes, on the very (VERY) outskirts of its HZ, and plenty of space for another, *hopefully* terrestrial, planet in between, that would be smack in the middle of the HZ.

Michael March 28, 2014 at 14:33

@Ronald March 28, 2014 at 8:30

‘However, it seems likely from this paper that any star hotter/brighter/bigger than about F8/9 offers both a too narrow and a too short window of opportunity for higher life to arise and develop.’

Higher UV emitters may actually aid the development of complex life by increasing the amount of oxygen in the atmosphere (hydrogen escape/organic formation) earlier on by photo dissociation. That is provided they develop strategies to cope with the damaging effect of UV. After all life took ~2 billion years to oxygenate the earths atmosphere so any earlier oxygenation would be a significant head start for complex life.

Buckaroo Bobsington March 28, 2014 at 22:12

The main necessity for life to adapt to harsh conditions is that there are tiered environments where it can start in very protected enclaves and then gradually expand to harsher ones, all the while complexifying. So as long as it can start somewhere stable, wet, warm, with a lot of UV protection and then gradually expand and “learn” to adapt, there’s no reason it can’t fill an F-star’s terrestrial planet. It would just end up evolving a lot more of the kind of chemical protections Earth life has – dark pigmentation, shells, exoskeletons, perhaps a strong underground orientation.

andy March 29, 2014 at 2:55

@Ronald: for GJ 667Cc, using the Kopparapu et al. estimates of the HZ it basically ends up either inside or outside the inner HZ border depending on which set of stellar parameters you use. Then again, that HZ model also shows Earth to be right up against the HZ inner edge as well. Then again, I agree that it should be assumed mini-Neptune until proven terrestrial.

As regards Tau Ceti, the outer planets in that system are on fairly shaky ground as regards their existence. The HARPS dataset only provided evidence for three planets, the other two resulted from combining the HARPS data with a noisier dataset (AAPS+HIRES) which by itself had evidence for none of the planets, a situation which reminds me uncomfortably of the infamous case of GJ 581g.

Michel Van March 29, 2014 at 5:49

The authors neglect one import an factor: Time
F-stars are more massive as our sun, means they burn faster it’s Hydrogen.
even if there life on a F-star planet, it got not much time to evolve.
after 2-3 billions years a F-star turns into red giant, killing it’s planets.
even G0-star goes into red giant after 4 billion years.

Lionel March 29, 2014 at 6:25

Nothing to add to the article but wanted to say that I always learn a lot from the high quality comments on this site, not to mention Paul’s excellent writing, and this keeps Centauri Dreams as my favorite place for space news

Paul Gilster March 29, 2014 at 8:09

Very kind of you, Lionel! For my part, it’s a daily pleasure to work with this community. Thanks to all.

William R. Alschuler March 29, 2014 at 8:13

I think the big issue with stars hotter than the Sun is their shorter lifetimes for evolution to take place. Any planet with a lot of water orbiting even a quite hot star would be self shielding from UV for marine life. Given enough time, marine life near the surface might evolve UV resistance, and if there is land and enough time, might eventually emerge. The oldest life we know of here is shown by fossil bacteria in mats and stromatolites, as old as 3.5 billion years, and bacteria-deposited bands of iron called red-banded or banded-iron formations, perhaps a bit newer. That suggests a minimum age for life to evolve of 750 million to a billion years after the star turns on, and a
bit less from the star’s arrival on the Main Sequence. In Earth’s case, from single celled life to multicelled life took about another 3 billion years, after which it was 0.5 billion years to us. So from a lifetime point of view, following our example, planets around Main Sequence A0 stars (MSLT=1.0 billion years) might barely have enough time to evolve single celled life, around F0 stars (MSLT=3 By) are likely to have evolved single celled life, around F5 stars (MSLT=5By) are moderately likely to have evolved multi-celled life, and around G0 stars (MSLT=9By) quite likely to have evolved intelligent life. The Sun of course is a G2 with a MSLT of about 9.5By.

Could marine life evolve to intelligence? Yes- see dolphins and octopi! Could they evolve tools and find out about the Sun and stars? Not yet known!

Anyhow, the argument about MS lifetime as the limiting factor to the evolution of intelligent life is not at all a new one. It leaves us with concentrating efforts on stars later/cooler than about F5.

sedjak March 29, 2014 at 13:06

William brings up a good point in laying out the evolution of life on Earth. Going multicellular is hard–very hard as indicated by the 3BY to get there from solo cells. So, unless there is a panspermia highway network we can expect any life in the galaxy to be predominately simple-celled life. That, somehow, may be the Greatest Filter for ETIs, unless conditions on Earth were atypical where perhaps many other exoplanets might have a richer soup and earlier O2 profile.

Just saw “Noah.” Just wondering where all that water went… Couldn’t be in the atmosphere or else we would have choked or been crushed a la Venus. It possibly could be photo dissociation after a very long spell of high output UV from the sun, which would have toasted things, but at least made for an oxygen rich biome. However, recent research indicates that the Earth’s mantle is much more enriched with water than previously thought. It just drained away. That would be the safest bet. (TIC)

CharlesJQuarra March 29, 2014 at 14:19

Since such systems are so short-lived that an abiogenesis event might seem unlikely (which we don’t know for sure given our lack of statistical samples), finding life markers in such systems might be a strong support for either panspermia or ETI migration: Most likely inhabitants are tourists

NS March 29, 2014 at 16:31

There are arguments from biologists that since life on Earth started fairly quickly, the initial appearance of simple life may be highly deterministic, driven by the laws of physics and chemistry (of course this assumes abiogenesis). Subsequent life developments (multicellularity, intelligence) however are the result of the vagaries of evolution and thus may (re-) occur rarely if ever. This suggests a simple life common/everything else rare scenario.

Alex Tolley March 29, 2014 at 18:20

Just saw “Noah.” Just wondering where all that water went…

Or it really was just a local event (and recalled from a much earlier one) and the water just drained away, probably into the Mediterranean Sea.

Did you see any dinosaurs drowning and settling into the muddy depths? If not, Ken Ham will be complaining…. ;)

Eniac March 30, 2014 at 0:11

Higher UV emitters may actually aid the development of complex life by increasing the amount of oxygen in the atmosphere (hydrogen escape/organic formation) earlier on by photo dissociation.

Don’t forget, though, that from an early life point of view, oxygen is far more dangerous than UV, so may very well be prohibitive rather than helpful.

Eniac March 30, 2014 at 0:18

Subsequent life developments (multicellularity, intelligence) however are the result of the vagaries of evolution and thus may (re-) occur rarely if ever.

Multicellularity has evolved independently many times on Earth. It is most likely that it’s late rise is due to geological factors, such as the long delay until oxygenation. Need to look for the great filter elsewhere…

Alex Tolley March 30, 2014 at 12:02

Multicellularity has evolved independently many times on Earth.

Good point. This does suggest that multicellularity is so useful that different clades have reinvented it many times using different approaches. Even though not multi-cellular, bacteria will form biofilms, indicating that tight packing of cells has advantages in some environments. We also know that bacteria have inter-cell communication which guides their behavior.

Since true multicellularity evolved after the great oxygenation event, around 2.5 Gya, it suggests that the evolution of photosynthesis was the key event that allowed this to subsequently happen. We can certainly imagine that if this was hard, then many planets may have anerobic, single cell, life. These cells may form biofims and mats, but not be multicellular. Of course this is based on a single instance, and may not be universally valid.

Ronald March 30, 2014 at 18:09

andy: yes, the Kopparapu et al. update of Kasting’s estimate of the HZ (in fact, remarkably similar, after 20 years) puts us frighteningly close to the inner edge of our HZ: the old estimate put the inner edge at 0.95 AU, the new estimate is 0.97 – 0.99 AU. We may only have a few hundred million years left before it gets too hot here for higher life!

Michel Van, William R. Alschuler: yes, that was exactly what I was referring to in my longer above comment: F stars and earlier probably have too short lifespans for higher life to develop plus the fact that even during that lifespan their HZ moves outward rather quickly, so that the continuous HZ is actually disappointingly narrow.

With regard to time window for microbial and higher life, see this very interesting earlier post on this site:
G-Class Outliers: Musings on Intelligent Life
http://www.centauri-dreams.org/?p=25359&cpage=1#comments
particularly the very illustrative image of different stellar spectra and time time windows for microbial and complex life.

I tend to agree with Eniac and Alex Tolley that the rise of multicelled and complex life probably depended on oxygenation, and the saturation of the oxygen sinks of the earth.
Hence, anything below about 3 gy may be too short for this event on an earthlike planet. And therefore, I am still inclined to think that around F9/G0 is about the (high end) limit for a star to be suitable for an earthlike planet to develop complex life.

Eniac March 30, 2014 at 21:13

Since true multicellularity evolved after the great oxygenation event, around 2.5 Gya, it suggests that the evolution of photosynthesis was the key event that allowed this to subsequently happen.

Photosynthesis followed by a billion or two years of crust oxidization, I think.

A propos photosynthesis, there are at least two independently evolved mechanisms to utilize light energy: The well-known photosynthetic reaction center using chlorophyll found in blue-green algae and plants, and the more primitive bacteriorhodopsin, a light activated proton pump using retinal found in halobacteria (and our eyes).

So, that one can’t really be the great filter, either.

Eniac March 30, 2014 at 21:43

I am not fully convinced that multicellularity depends on oxygen, though. You would think that it has evolved because it is advantageous for high energy organisms (animals) to band together and do things such as see and swim. But then you remember that low energy photosynthetic organisms also have multicellular forms: plants. I cannot explain why those did not arise much earlier than animals. There may be completely different factors involved that we are not thinking of. Perhaps a breakthrough in genetics, such as sexual reproduction?

Rob Henry March 30, 2014 at 23:18

One must be careful with abiogenic calculations. IF the probability of abiogenesis is uniform with time, and relies on ABSOLUTELY NO early factors, such as geothermal activity, meteorite impacts,, or a high energy prebiotic soup, THEN life’s early appearance on Earth can be used as evidence for a high value to Drakes p(l). Notice that this is a rather poor assumption in term of science and the probable background of most abiogenesis scenarios, but without it we cant even make an educated guess.

Meanwhile, the record of our planet indicates that life repeatedly underwent major advances just after oxygen levels seems to reach levels that allowed them. Some say it shows that at least three billion years are needed despite the clear absence of anything that looks like linear progress, but I never could see the evidence for that. I would love to see what would happen had the same Earth and abiogenesis suffered and oxygen build up about ten times as rapid. Would a billion years prove way more than enough?

Doug M. March 31, 2014 at 7:41

Coming a bit late to this, but a couple of points:

– “Oxygenation had to wait for Earth’s oxygen sinks to saturate” is not wrong exactly, but current thinking is that it’s simplistic. Oxygen production may have varied over an order of magnitude or more at different points in the Proterozoic, and the oxygen sinks themselves were probably highly dynamic as well. It’s probably more helpful to think of it as a billion-year-long race between production of oxygen by bacteria and algae on one hand, and production of oxygen-absorbing minerals by crustal processes on the other. One reason for the long stable period of low O2 levels in the Mesoproterozoic — the so-called “boring billion”, when atmospheric oxygen was present but stayed steady in the low single digits — is probably that these two factors were in dynamic equilibrium.

– HZ models are evolving so rapidly that I would really hesitate to draw conclusions too strongly from them right now. Note that Earth’s surface temperatures seem to have stayed roughly stable over astronomical time: Archaean Earth was very roughly about as warm as Earth today, despite getting only about a third as much sunlight. To give just one example of the complexities, there’s a theory that a mass glaciation at the onset of the Proterozoic was caused when the Great Oxygenation Event destroyed a pre-existing layer of greenhouse atmospheric methane.

– The real explosion of life can arguably be traced to the rise of eukaryotes. Eukaryotes are monophyletic, and the group includes all multicellular lifeforms on Earth. Unfortunately, the origin of Eukaryota is still rather obscure; we’re not even sure of the exact date to within a billion years. An important question is whether it predated the Great Oxygenation Event or not. Current thinking is that oxygen came first, but we’re really not entirely sure — there are no unambiguous eukaryote fossils before 2 billion gya (really, there’s not much unambiguous anything before 2 billion gya) but there are tantalizing, confusing hints in the geochemistry of possible eukaryote biomarkers in Archaean rocks.

Move the rise of eukaryotes back in time a billion years, and would everything else happen faster? Is that even plausible? Right now we really can’t say.

Doug M.

Michael March 31, 2014 at 14:10

@Doug M. March 31, 2014 at 7:41

‘One reason for the long stable period of low O2 levels in the Mesoproterozoic — the so-called “boring billion”, when atmospheric oxygen was present but stayed steady in the low single digits — is probably that these two factors were in dynamic equilibrium.’

There appears to be two major events that kept the oxygen level low and constant. First was the oxidation of the oceans which consumed all oxygen production for 1-2 billion years, where most of the oxygen produces where concentrated. The second started after oxidation was more or less completed in the oceans and rain waters carried dissolved excess oxygen to be deposited on the land to oxidise the continental surfaces. Only after these processes where more or less completed did the atmosphere get enriched to the level today. The current level of free oxygen of today took only 200-300 million years after the oxidative sinks where used up.

My point is here is that changes such as the amount of oxidation needed (oceans/land) have a huge effect, if you only have half of the oxidative material to start with (solar nebulae oxidised already) that wipes out around a billions years or two before complex life could make an ‘appearance’. Also there is more higher end wavelengths with hotter stars which offer higher energy gains which could favour earlier oxygen enrichment, this combined with more photo-dissociation could reduce the time for complex life to gain a foot hold significantly.

Ronald March 31, 2014 at 16:17

Doug: thanks for your reflections.
In particular: “Note that Earth’s surface temperatures seem to have stayed roughly stable over astronomical time: Archaean Earth was very roughly about as warm as Earth today, despite getting only about a third as much sunlight.”
This would suggest that within the margins of the HZ, an earthlike planet manages to maintain a very stable temperature over very long periods, mainly dependent upon the characteristics of that planet itself. I would say it emphasizes the importance of the stellar HZ concept.

Also, with regard to evolving HZ models: I have noticed that while the outer boundary of the HZ has indeed evolved significantly in models over time (it has moved outward quite a bit) the inner edge has remained remarkably similar in models and remarkably sharply delimited, varying only from about 0.95 to 0.99 in our solar system (at least for the more realistic runaway greenhouse and water loss models).

Doug M. April 1, 2014 at 3:29

“There appears to be two major events that kept the oxygen level low and constant. ”

Yes and no. Current thinking is that these were not so much “events” as dynamic processes, with the level of atmospheric O2 at any time representing an equilibrium point. It’s not that “oxidizing the oceans took 1-2 billion years” — if it were that simple, then we wouldn’t have seen a billion years with ~2% atmospheric oxygen. On one hand, that’s a tenth the modern level, but on the other it’s more than enough to oxidize dissolved minerals in the ocean pretty quickly. If it was just a question of filling up a huge oxygen sink, then atmospheric oxygen would have stayed well under 1% until everything that could be oxidized, was — and then it would spike suddenly upwards.

But that’s not the pattern we see in the geologic record. Instead we see the first appearance of very low levels of oxygen around 3.2 gya; the Great Oxygenation Event around 2.1 gya; rough stability for a billion years or so; and then a slow rise to near modern levels in the Cryogenian and Ediacaran. But also, complicating that pattern, a bunch of transient pulses and recessions all over the late Archaean and Proterozoic. (Paleochemists can argue bitterly over the details of these, but at this point pretty much everyone agrees that there were at least several of them.) For that matter, the “modern” level is a snapshot rather than an end state; atmospheric O2 levels seem to have varied by a factor of three or so over the last 500 million years. There’s no reason to think that’s going to stop.

Note that when you get past 1 gya, the datasets get very skimpy, patchy, and open to conflicting interpretations. So whenever you hear someone speaking with an authoritative voice about the details of geochemical processes in the Archaean, your response should include a healthy dose of skepticism. To give a single example, we’re not even sure if the Archaean had plate tectonics. The Earth’s heat output is very slowly declining over astronomical time, right? Well, it seems that when you extrapolate it backwards past about 3 gya and raise heat output to a bit over double the modern level, things abruptly go nonlinear, and the crust starts to behave in some very new and strange ways.

Point being, this area is still very much in the “warring models” stage, and probably will be for at least another generation. This is why I lard my statements with lots of “current thinking is,” “it seems that,” and “probably”.

Doug M.

Eniac April 5, 2014 at 18:56

Doug M.

Note that Earth’s surface temperatures seem to have stayed roughly stable over astronomical time: Archaean Earth was very roughly about as warm as Earth today, despite getting only about a third as much sunlight.

I think this is a fascinating piece of data, and I feel that there is a distinct possibility that this stability is maintained by none other than the presence of life. Here’s the rough narrative:

Photosynthesis tends to remove greenhouse gases, and decreasing temperatures tend to decrease photosynthesis (due to glaciation, mostly, I think). Together, these effects form a negative feedback cycle to stabilize temperature. The sun gets hotter, the Earth warmer, algae/plants get more productive. That means less greenhouse gas, and the heating effect is reversed.

This was good for a few billion years, but at some point you get to where there is no more greenhouse gas to remove (CO2 is at trace levels today), but the sun is still getting hotter. Eventually, it will get too warm for glaciation to have much effect, more heat is going to reduce rather than increase photosynthesis, and the negative feedback turns into a positive one. Eventually Earth climate may resemble that of Venus. We may still have a few billion years left, or maybe not….

Does this make any sense at all?

Michael April 6, 2014 at 4:42

@Doug M.

‘Note that Earth’s surface temperatures seem to have stayed roughly stable over astronomical time: Archaean Earth was very roughly about as warm as Earth today, despite getting only about a third as much sunlight.’

Do you mean 30% less light, a third as much would mean 70% less light.

@Eniac April 5, 2014 at 18:56

This was good for a few billion years, but at some point you get to where there is no more greenhouse gas to remove (CO2 is at trace levels today), but the sun is still getting hotter.’

Something a little more worrying, there will be no greenhouse gases and we are moving towards the thermal tipping point. If we are hit by a carbon containing asteroid or it hits a carbon bearing deposit and then we could be tipped over into the run away greenhouse effect much sooner.

xcalibur April 6, 2014 at 18:45

If we’re thinking very long term, the focus should be on G, K, and M class stars. As others have pointed out, it’s also more likely for life to arise when given a larger window of geologic time. That said, we shouldn’t completely neglect other solar systems in our pursuit of knowledge.

I have a couple questions:

1. Is it certain that the earth will eventually become too hot for most life, due to the sun’s increasing luminosity? Is large-scale planetary engineering and/or space colonization the only way to avoid destruction?

2. Even if the sun’s increased output doesn’t turn earth into venus, it will become a red giant eventually. Is the verdict in on whether earths orbit would be altered enough to not get cooked or swallowed up?

Doug M. April 7, 2014 at 5:20

1) Yes, I meant about a third less light (~70% as much), not a third as much light.

2) @Eniac, yes, that’s the very broad outline sketch in a nutshell. The picture is complicated considerably by clouds, ice caps, and the fact that Earth has three significant greenhouse gases (CO2, water vapor, and methane), all of which have probably varied dramatically over astronomical time. (Note, BTW, that current levels of CO2 are a lot more than “trace”. Yes, it’s only 400 parts per million, but that’s actually a lot as these things go. Raising or lowering that by just 100 ppm would have dramatic and obvious effects on global climate.)

Current thinking is that Earth will eventually, inevitably hit runaway greenhouse and turn into something depressingly similar to Venus, and that this will long before the Sun leaves the main sequence ~6 billion years from now. However, there is dramatic disagreement between the models. The most pessimistic ones give us only a few hundred million years; others, as much as a couple of billion. But there’s general consensus that, yes, the Earth is eventually going to cook.

Doug M.

Rob Henry April 7, 2014 at 21:25

I agree with Eniac over CO2 being a trace. We have thirty times more argon than CO2! More importantly, water vapour absorbs about a order magnitude more infra-red than CO2, the point being that CO2 is getting close to that limit whereby reducing its level would have insufficient effect on negative feedback loops (such as rock-weathering) to stabilise the system.

If that went it would allow us to become a vapour bath, with Earth suddenly becoming much hotter. I am sceptical that we would end up like Venus though. A high cold trap (one above the O3 layer) might lose water faster from uv then hydrogen loss, but its hard to explain rapid oxygen loss at a sufficient rate to end up as dry as Venus in just a few hundred million years. Thus I suspect the poles might still be a reasonable place to live for the following billion years at least.

Ronald April 9, 2014 at 7:49

With regard to the habitable lifespan of our planet and an inevitable runaway greenhouse effect: most astronomers used to put tje inner edge of our HZ at about 0.95 AU, the update of Kasting 1993, by Kopparapu et al. (2012, I think) even puts it at 0.97 – 0.99 AU.
And since our sun gets about 10% brighter per gy (or roughly 1% per 0.1 gy), this would mean that out planet may get too hot for higher life within about 300 – 500 million years, less than the time since the Cambrian life diversity explosion.

I have been thinking along the same lines as Michael: what if a few bad incidents and natural processes coincide, mutually reinforcing and accelerating a post-threshold runaway greenhouse? Such as a methane release explosion (like at the end of the Permian), a solar super-peak, and a carbon rich asteroid and/or a super-volcanic eruption, all occurring within about the same time-period (note: the asteroid and super-volcano would cool down the climate for a limited number of years, ‘cosmic/volcanic winter’, but after that the released greenhouse gasses may kick in).

This is all rather unlikely, but over very long periods of time even unlikely combinations of events become plausible. Not only does s**t happen, but very occasionally rare combinations of s**ts happen together.

This is not the most of our concerns, first we’ll have another ice-age. And no doubt a few pandemics. And some global (cereal) crop diseases. And some new and original and destructive wars. Oh, and I mentioned the super-volcanoes already.

All these are reasons to become a multi-planet civilization.

Aleksandar Volta July 27, 2014 at 15:20

Meanwhile, as we argue stats in UV and lifespans of certain star classes, in a far corner of space, an alien species that recently evolved to sapience on a planet orbiting an 8 billion year old dwarf is pursuing a similar debate about G-type dwarfs:

“Too much UV, too little time. Those stars are lethal to life as we know it, it would never arise there, because it wont have any time, and if it somehow does, too much UV would severely hamper its development to complexity, and consequently, intelligence. Thus, we are best to use our resources in exploring infrared rich systems.”

@Michael Van – I’m afraid you’re mistaken. G0V stars have a pretty similar lifespan like our sun 9-10 billion. 2-3 billion year lifespan might apply for the hotter, brighter, F1-F3 dwarfs. F5-F9 should have about 4 to 8 billion years lifetime.

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