Widening the Habitable Zone

by Paul Gilster on December 12, 2012

Finding a way to extend the classical habitable zone, where liquid water can exist on the surface of a planet, is a project of obvious astrobiological significance. Now a team of astronomers and geologists from Ohio State University is making the case that their sample of eight stars shows evidence for just such an extension. The stars in question, drawn from a dataset created by the High Accuracy Radial Velocity Planet Searcher spectrometer at the European Southern Observatory in Chile, were selected because they match up well with the Sun in terms of size, age and composition. Seven of the eight, however, show signs of much more thorium than found in our star.

It’s an interesting result, as seen in this Ohio State news release. The slow radioactive decay of elements like thorium, potassium and uranium, all found in the Earth’s mantle, helps to heat the planet. These are elements present at planetary formation and, according to Ohio State’s Wendy Panero, they are involved in producing enough heat to drive plate tectonics, which some believe to be a factor in maintaining our planet’s oceans. All this is in addition to the sources of heat convection found in the Earth’s core, which play a comparable role in crustal movement.

The work was presented last week at the American Geophysical Union meeting in San Francisco by Cayman Unterborn, a graduate student at Ohio State who is working under Panero. Unterborn’s thesis: More thorium in a stellar interior indicates that the interior of planets found around the star would be warmer than the Earth. He notes that one of the stars in the HARPS survey contains 2.5 times more thorium than the Sun. Unterborn’s calculations show that any terrestrial planets forming around that star would generate as much as 25 percent more heat than the Earth, providing a long-lasting driver for plate tectonics. The warmer planet would have a habitable zone farther from its star as well, supported by the additional internal heat.

Image: An artist’s impression of the ‘super-Earth’ HD 85512 b. Is it possible that a planet like this is warmer internally than the Earth, allowing life to form over a wider habitable zone? Credit: ESO/M. Kornmesser.

The cautious Unterborn is aware that he’s looking at a small sample:

“If it turns out that these planets are warmer than we previously thought, then we can effectively increase the size of the habitable zone around these stars by pushing the habitable zone farther from the host star, and consider more of those planets hospitable to microbial life. At this point, all we can say for sure is that there is some natural variation in the amount of radioactive elements inside stars like ours. With only nine samples including the sun, we can’t say much about the full extent of that variation throughout the galaxy. But from what we know about planet formation, we do know that the planets around those stars probably exhibit the same variation, which has implications for the possibility of life.”

The Ohio State researcher notes that thorium is more energetic and has a longer half-life than uranium. Planets rich in the element would tend to remain hot longer than the Earth, which gets most of the heat of its radioactive decay from uranium. As to why we wind up on the short end of the stick when it comes to thorium, Unterborn believes it harks back to the exploding stars that seeded local space with heavy elements long before our planet formed:

“It all starts with supernovae. The elements created in a supernova determine the materials that are available for new stars and planets to form. The solar twins we studied are scattered around the galaxy, so they all formed from different supernovae. It just so happens that they had more thorium available when they formed than we did.”

These results are clearly preliminary, and Jennifer Johnson (Ohio State), a co-author of the study, points to the need to expand its scope. One way to do that is to analyze noise in the HARPS data, something that is now on Unterborn’s to-do list, after which the team plans to expand the search to additional Sun-like stars. If there are indeed large numbers of stars whose basic composition creates planets that generate more internal heat than the Earth, then we need to consider the implications for how we analyze habitable zones and their longevity.

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jkittle December 12, 2012 at 10:11

Interesting to see how the discussion on habitable planets is begining to evolve as more data comes in from Harps and Kepler. this is because the data informs us, true… but also because there is now a career path for astronomers interested in the subject. I believe advances in this area, particularly starting to sort out the planetary size and composition factors, will be more rapid than most expect. The ever more expensive JWST should play a big role with its dep infrared capabilities.
my latest idea- a habitable moon of a super gas giant (5 to 10 10) Jupiter that is around a sun 2 to 5 times more massive than the sun ( higher temperature spectrum than our yellow sun) . The gas giant orbit would be way out there. maybe 10 to 30 AU . The moon would then have: light from the star; heat from the gas giant,; lots of atmosphere for shielding. -all equals a great vacation home for humans. ( spectacular views). A real fixer – upper!

Gregory Benford December 12, 2012 at 13:08

One sidelight of super Earths is that beyond 1.5 times our radius, it’s hard to get into orbit with chemical rockets. Using the best chem rocket payload percentages we’ve managed, it’s impossible to get out of such a deep grav well. With nuclear thermal, there’s little trouble, as they have Isp ~ 4 times our best H2-O2 rockets.
So smart life on such big worlds will have to get to nuclear to get to space. Not a big limitation–after all, we did both at about the same time.

Mike Lockmoore December 12, 2012 at 13:11

I have significant hope that thorium will become a practical, clean, and relatively safe nuclear fuel here on Earth (see energyfromthorium.com for boosterism). It is intriguing to think about planets that have much higher abundances of this useful element than we do, and speculate on the development of energy technology by any civilizations that arise there.

Thomas Mazanec December 12, 2012 at 14:02

One “Rare Earth” argument I once saw was that the sun formed just when the abundance of Uranium in the glaxy was supposedly at a maximum.

Ron S December 12, 2012 at 17:00

“The slow radioactive decay of elements like thorium, potassium and uranium, all found in the Earth’s mantle, helps to heat the planet.”

Yes, but not so much anything above the surface. Solar insolation, with Sun at the zenith is ~1 kW/m^2. Radiogenic heating is <0.1 W/m^2, although it keeps going all day and night at all latitudes.

So it drives tectonics but does not impact the HZ calculations. I suppose greater radiogenic heating could make mining more uncomfortable, and permafrost would be shallower in the polar regions.

andy December 12, 2012 at 17:42

So it drives tectonics but does not impact the HZ calculations.

Only if your HZ calculations ignore the effects of geodynamics (continental spreading, recycling of carbon dioxide back into the atmosphere, etc. should all have an impact). Attempts to take such effects into account has already been done for some models of planetary habitability (e.g. this paper for an example of an attempt to do geodynamic modelling for Gliese 581 g).

Astronist December 12, 2012 at 18:54

“Finding a way to extend the classical habitable zone, where liquid water can exist on the surface of a planet, is a project of obvious astrobiological significance.” — why should this be of particular astrobiological significance, when the sort of internal heat generation that you are talking about means that liquid water can exist subsurface on a planet or moon located essentially anywhere at all in interplanetary, interstellar or even intergalactic space?

Stephen

Ronald December 12, 2012 at 19:12

I agree with Ron S, that two different arguments should not be confused here: the one that due to the higher thorium content, plate tectonics will continue longer and hence the planet will remain a ‘living’ one longer, I consider valid. However, the other argument, that the planet will actually be significantly warmer at the surface, pushing the HZ outward, I have doubts about.
Besides, as the solar twin itself becomes brighter in the course of its MS evolution, this will probably be a more limiting factor for the habitable lifespan of the planet than the geophysics of the planet itself.

Question: does anybody know which 8 solar twins were studied here? I could not find that in the news article.

Gunnar Larsson December 12, 2012 at 19:18

@Ron S:
Could there be an indirect link through outgassing? If more carbon dioxide is outgassed the amount in the atmosphere is likely to be higher, which would create a greater green house effect and therefore a larger outer radius for the HZ ?

Does anyone now if, with regards to the number of available planets, it is likely to be a good or bad thing that the zone is pushed outwards?

coolstar December 12, 2012 at 19:34

Ron S makes a very good point above. It’s very hard to see how a relatively small increase in radiogenic heating can actually increase the size of the habitable zone by any significant margin. What it MIGHT do is to keep plate tectonics going on Mars sized planets going longer, and that might, indirectly, increase the radius of a habitable zone, for a while.

Abelard Lindsey December 12, 2012 at 21:29

All of the postulated elements of “Rare Earth” have been discredited EXCEPT for plate tectonics. The need for plate tectonics for a sustainable biosphere has been reinforced by research findings since the publication of “Rare Earth”. The open question is if the moon forming impact the Earth had is necessary for the initiation of plate tectonics or not.

Rob Henry December 13, 2012 at 0:34

Yes Ron S, that thorium has almost no direct effect on surface temperature. Apparently they were only speculating on how the internal structure of planets facilitated the biogeochemical cycles, and how, with more thorium, these planets could retain a vigorous bacterial community, with plate tectonics lasting until their sun heated up sufficiently to allow something more interesting. Thus their rather misleading claim that this widened the HZ.

I would look at this a completely different way, and say that if life on Earth could be sustained for a shorter complex life phase than was typical for a life suitable planet, then factors that attempt to emphasis how long life takes to develop, and how rare major transitions are, should be rethought.

Eniac December 13, 2012 at 0:47

Ron:

Yes, but not so much anything above the surface. Solar insolation, with Sun at the zenith is ~1 kW/m^2. Radiogenic heating is <0.1 W/m^2, although it keeps going all day and night at all latitudes.

This may be true for the surface, but you don’t have to dig all that deep to get to a place where the internal heat is dominant and insolation does no longer affect temperature at all. Only a few meters down, maybe a few tens of meters? Not sure, but it is not many kilometers. So, in principle, if you include the subsurface (water or rock, both) as potential habitat for life, the habitable zone would extend all the way into interstellar space and include rogue planets.

amphiox December 13, 2012 at 1:50

Hmm. If the planet is warmed by an internal process, that would move the outer edge of the habitable zone outwards.

But wouldn’t that also move the inward edge outwards as well?

So how does that count as “expanding” the habitable zone?

pinlighter December 13, 2012 at 2:21

More plate tectonics -> more vulcanism -> thicker atmosphere -> enhanced greenhouse effect -> higher mean temperature

kzb December 13, 2012 at 8:45

To add to what Ron S says, the solar energy intercepted by Earth is 176,000 Terawatt. Of that 30% is reflected back into space, but 70%, or 123,000TW, is absorbed.

Geothermal energy flux through the crust is 44TW. (Not all of this is supported by current radioactive decay, some of it is stored heat and some of it is tidal, but I’ll leave that because it does not affect the arguement)

So less than 0.04% of the energy at Earth’s surface is coming “from below”. I think you can see that if you increased this by 25% it is not going to affect surface temperatures directly.

What it might do is have the reverse effect on surface temperatures, because of increased vulcanism.

Wojciech J December 13, 2012 at 17:44

Gregory Benford December 12, 2012 at 13:08
“One sidelight of super Earths is that beyond 1.5 times our radius, it’s hard to get into orbit with chemical rockets”
If we imagine Super-Earths around Jovian planets several times size of our Jupiter it gets even more interesting. If a civilization living on such world manages to break free from its home world, it gains instant access to numerous worlds(moons) with diverse range of resources, allowing it relatively easy to transform into space based civilization. So while the initial cost might be very high, the rewards would be far more immediate and advantageous than in our case

Ron S December 14, 2012 at 11:10

To follow up, a couple of last questions to those who commented on the matter of tectonic activity and vulcanism:

- Radiogenic heat is only one source of interior heat, with the other, generally greater one, being the heat of gravitational contraction. This is still dominant within the Earth. The contribution of greater radiogenic heat to tectonics should therefore be less than the increase in that one component. By how much?

- Greater vulcanism does add to greenhouse gasses. However there is only so much carbon in the planet’s budget. Does an increase in tectonic activity over billions of years keep a greater proportion of the planet’s carbon in the atmosphere or does it merely reach equilibrium (or at least some long-term average) sooner?

Eniac December 14, 2012 at 13:19

All of the postulated elements of “Rare Earth” have been discredited EXCEPT for plate tectonics.

Then maybe it is time for this one, too. What is plate tectonics doing that cannot be either done in other ways or done without?

More plate tectonics -> more vulcanism -> thicker atmosphere -> enhanced greenhouse effect -> higher mean temperature

Let’s say each of these speculative conclusions has a chance of 80% of being correct and mandatory, than the entire argument will hold ~40% of the time. That leaves plenty of room for alternatives, I would say. Note also, as a counter example, that Venus has a thick atmosphere without plate tectonics.

Rob Henry December 14, 2012 at 16:47

Ron S says “- Radiogenic heat is only one source of interior heat, with the other, generally greater one, being the heat of gravitational contraction. This is still dominant within the Earth. The contribution of greater radiogenic heat to tectonics should therefore be less than the increase in that one component. By how much?”

And firstly we should straighten out a few terms. We usually express the release of gravitational potential energy in terrestrial planets as core or internal differentiation, since it typically involves much of this and little actual contraction. However for brown dwarfs and giant planets Ron S’ simplification would be appropriate.

Secondly, the statement that it is still the dominant release of heat for Earth is problematic in several respects.
- From what I have read the magnitude of terrestrial heat flow to space has never been measured definitively enough to have any sort of confidence that <50% of it is due to radiogenic processes.
- Most of that shortfall is said to be due to stored primordial heat from the core, than that could have its origin from the impact of Theia, or such an early formation that Al-26 heated it, etc rather than this being definitively due to “contraction”.
- If the dominant modern form of gravitational potential released from our system was from rearranging Earth’s internal structure, then the Earth’s spin rate should be increasing. Considering that there is fossil evidence that the year has fewer days than a few hundred million years ago, suggesting that tidal energy is now the more important contributor.

Thirdly, they were suggesting that even 250% more thorium would only increase the internal heat modestly, with the main effect being that cooling was slowed sufficiently for plate tectonics to “freeze out” much later

Eniac December 14, 2012 at 22:54

Ron:

Radiogenic heat is only one source of interior heat, with the other, generally greater one, being the heat of gravitational contraction. This is still dominant within the Earth.

Present, yes. Dominant, no. At least according to Wikipedia and its references:

The Earth’s internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[2]

http://en.wikipedia.org/wiki/Geothermal_gradient

Barney December 14, 2012 at 23:40

While I have little that is useful to contribute to this discussion, I’d like to say that I really like reading a well reasoned, polite debate in a comments section, without it turning into spiteful mud slinging. You should all be proud of yourselves. I dunno, maybe I’ve just been reading Youtube comments for too long.

But Amphiox, I’m inclined to agree, except in the instance of planets with low vulcanism in a closer orbit.

jkittle December 15, 2012 at 13:40

Benford-
We may not be a Rare Earth when it comes to habitability but we are likely Rare when it comes to the ability to explore space. The Relative ease of getting into orbit, the presence of fissionables in the crust, access to a major nearby moon with volatiles, a second planet on the edge of the habitable zone, and a relativity benign space environment ( few nearby asteroids, a low radiation star, etc>) and a favorable spot in the galaxy, are all factors that will eventually lead to success in exploring our solar system and perhaps beyond. Our distant offspring may look back at Earth and say they were evolved in rare cradle, indeed.

Ron S December 15, 2012 at 17:18

Eniac,

That’s what I get for relying on my memory rather than checking first. This would also apply to Rob’s latest comment.

Rob Henry December 15, 2012 at 21:10

Jkittle, our moon’s inventory of volatiles hardly seems a positive to me. Here you are arguing for lunar resources to massively boost the early phase of our space exploration. Extracting bound water and hydroxide from rock is only practical once a massive moon based economy is already in place, so is of no use here. This leaves the mining of ice of which the moon has only 600 million tons.

This is indeed a vast resource if it could be reserved for rocket fuel, but look at it this way… If I was the administrator of a New Zealand base there, I would want to transform it to a subterranean biosphere 2 facsimile, one day big enough to support our portion of a an almost self contained lunar economy. We would need about a thousand inhabitance, supported by a couple of hundred hectares, and containing half a million tons of water. That’s 0.1% of it gone just for our base!

Already I feel myself casting an envious eye to Mercury, which has 20 billion to a trillion tons of water – and an ice cap dense enough to bounce a radar image off.

Eniac December 16, 2012 at 1:06

jkittle:

The Relative ease of getting into orbit, the presence of fissionables in the crust, access to a major nearby moon with volatiles, a second planet on the edge of the habitable zone, and a relativity benign space environment ( few nearby asteroids, a low radiation star, etc>) and a favorable spot in the galaxy, are all factors that will eventually lead to success in exploring our solar system and perhaps beyond. Our distant offspring may look back at Earth and say they were evolved in rare cradle, indeed.

I find most of these doubtful. They are either not known to be rare or are not terribly necessary or even helpful for space travel. I do accept the one about escape velocity, though. Earth is in a narrow window between having no atmosphere at all and having no chance to escape it with chemical means. Larger planets could evolve life just as easily, but would be very hard to escape from.

Eniac December 16, 2012 at 1:13

I would also like to clarify that I think by “residual heat from planetary accretion” what is meant is stored heat, rather than heat still produced by ongoing gravitational processes such as contraction or differentiation. It is my impression that neither of these exist anymore, just some heat left over from them that hasn’t made it all the way out, yet.

This is another solution tot he “faint young sun” paradox. Maybe the sun grew hotter just in time to make up for the fading internal heat….

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