Stagnant interiors suppress chances of super-Earths supporting life

Sep 25, 2012
Artist’s impression of a trio of super-Earths discovered by an European team using the HARPS spectrograph on ESO’s 3.6-m telescope at La Silla, Chile. The three planets, having 4.2, 6.7, and 9.4 times the mass of the Earth, orbit the star HD 40307 with periods of 4.3, 9.6, and 20.4 days, respectively. Credit: ESO

(Phys.org)—Exoplanet hunters estimate that there could be billions of super-Earths—planets with a mass of up to ten times that of Earth—orbiting stars in the Milky Way alone. But do super-Earths really deserve their name and would they be capable of hosting life? A study of the thermal evolution of rocky super-Earths suggests that they may bear very little resemblance to our home planet. Dr. Vlada Stamenkovic will present the results at the European Planetary Science Congress on Wednesday 26th September.

"We are discovering planets orbiting distant stars that are similar to Earth in composition but more massive than Earth. The major question is: are they just scaled-up versions of Earth, or are they fundamentally different? We especially want to know if rocky super-Earths have thick atmospheres, volcanic activity, magnetic fields or ," said Stamenkovic, a researcher at the Massachusetts Institute of Technology. "Some of these features are crucial for determining if a planet might be capable of supporting surface life."

On Earth, plate tectonics and volcanic activity help to regulate the climate and to release and recycle nutrients for life. Our planet's magnetic field, driven by a liquid , possibly protects the atmosphere from being stripped away by solar and .

Stamenkovic and his colleagues have found that the viscosity and the of are strongly affected by pressure. In massive super-Earths internal pressures are tens of times greater than those in the terrestrial interior and can lead to large viscosities and melting temperatures—which furthermore can negatively impact the of a planet.

Surprisingly, the team's calculations suggest that rocky super-Earths may even be undifferentiated—not separated into a metallic core and like Earth.

"Current understanding is that the in our solar system formed rapidly—in about the first 50 million years. The time scale of core formation depends strongly on viscosity. The high melting temperatures and the large viscosities that we've calculated for super-Earths suggest either a slow core formation or no core formation at all. This raises doubts about whether super-Earths could generate magnetic fields," said Stamenkovic.

Even if those super-Earths are differentiated, the research indicates that convection would be sluggish or that stagnant layers could form deep in the mantle, putting an effective lid on heat flow from the core. Strongly depending on the initial conditions, conduction may become the dominant form of heat transport. This would reduce the cooling rate of the core, again potentially quashing dynamo action.

The team has found the propensity of plate tectonics to rather decline with planetary mass. But they also find that water in the lithosphere can easily buffer these effects. Hence plate tectonics on super-Earths is not inevitable, but rather depends on a set of unknown planetary characteristics, which can't be observed on exoplanets in the near future.

The atmosphere of the early Earth is thought to partially result from the differentiation of the planet and subsequent release of gases through volcanic eruptions. The team found that the duration of volcanic outgassing and the production of molten rock generally decrease with increasing planetary mass, with a sharper decline for larger viscosities. This could limit the timescales for ongoing on super-Earths. This on the other hand could have severe impacts on climate regulation during global ice ages.

"Our work highlights the importance of understanding the thermal evolution of planets—moreover, it shows that super-Earths are more diverse than expected. We will only be able to fully answer questions by gathering more data from high-pressure experiments and from spectroscopic observations of super-Earth atmospheres orbiting close-by bright stars. Theory shows the possibilities, which are far larger than previously thought, but remains full of uncertainties," said Stamenkovic.

Explore further: Astronomers find 'cousin' planets around twin stars

More information:

'The influence of pressure-dependent viscosity on the thermal evolution of super-Earths', V. Stamenkovic, L. Noak, D. Breuer & T. Spohn, Pub: The Astrophysical Journal, 748:41, 20 March 2012.

'Thermal and transport properties of mantle rock at high pressure: Applications to super-Earths', V. Stamenkovic, D. Breuer, T. Spohn. Pub: Icarus 216 (2011) 572-596.

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Lurker2358
1.7 / 5 (12) Sep 25, 2012
If the planet is greater than or equal to Earth mass then there ought to be zones inside which meet each of the basic pressure thresholds of our own core, even if they are not in the exact same configuration.

Consider and 8 Earth mass planet with the exact same composition has twice the surface gravity, twice the radius, and 32 times the gravitational binding energy.

"Somewhere" above the first Earth radius and below the surface, there ought to be a molten mantle which should be differentiated eventually, even if the inner core somehow manages to not be differentiated.

Just find spherical shells with pressures similar to each Earth benchmark:
Inner core/outer core boundary
Outer core/mantle boundary
mantel/crust boundary

The curve for where these benchmarks are met(intersect) should be solvable with a TI graphing calculator. Unfortunately, mine is broken.

Rocky undifferentiad planet-sized core with a metallic outer core and then a mantle and crust.

cont...
Lurker2358
1.7 / 5 (12) Sep 25, 2012
Because of the extra gravity, pressure increases very fast as you go down, so the Mantle and outer core may be very close to the surface.

The inner core would be ultra-dense rock and stuff(if it's undifferentiated) which is just smashed into a dense crystalline or meta-metal form. This zone will perhaps be as large as the Earth itself, possibly even 50% larger in radius.

Outside this is the metallic outer core, because the mantle will differentiate anyway, it will just be shallower relative to scale, but it will have 4 times the surface area, give or take, on an 8m planet. The crust would sit on top of this as always on the Earth.

Think about this carefully, each pressure value inside the Earth also occurs on a large super earth, it just occurs with a much different gradient.

Next, atmosphere and oceans. Even given the non-differentiation of the core, there should be far more atmosphere and ocean on a super-earth than on a regular earth AND see below...
Lurker2358
1.4 / 5 (11) Sep 25, 2012
Because the surface gravity will be 2 times higher on an 8 Earth mass super-earth, the atmosphere will weigh 2 times more per unit mass, but it is also likely to be 8 times as massive as a whole, but distributed over 4 times the area.

All of that is is to say the atmosphere above any one square meter of surface will be about twice as massive, and twice as heavy per unit mass, therefore 4 times the surface pressure, and this ignores any re-melting or re-heating effects from the whole heat/pressure relationship.

finally, about the same relationship could be expected from water.

however, if as they claim the core isn't differentiated, than the fraction of non differentiated water and "earth atmosphere components" which are trapped inside the non-differentiated inner core would not count towards the oceans or atmosphere.

Hope that makes sense.
rkolter
3 / 5 (2) Sep 25, 2012
I don't know why the deep interior of a super-earth would be undifferentiated, if the planet starts off like planets like Earth did - growing from smaller objects - at least until it grows significantly past the mass of the Earth, shouldn't a super-earth be differentiated? Or is the model showing that when two mega-earths collide and merge, they are too massive to effectively differentiate?
Lurker2358
1.7 / 5 (11) Sep 25, 2012
Study it from the outside in, rather than inside out, and you'll see why I'm right.

Each Earth internal pressure benchmark MUST be met at some depth on the super-earth. Of course it will also be greatly exceeded a few kilometers lower, etc.

You should have more differentiated material on an 8 Earth-mass Super-Earth than on Earth, even if 80% of the planet's mass is non-differntiated in a core (somehow).

Differentiation in the outer shells would be unavoidable, assuming an accretion model formation, internal pressures, and certainly internal radiation from nuclear decay.
Lurker2358
1 / 5 (8) Sep 25, 2012
I don't know why the deep interior of a super-earth would be undifferentiated, if the planet starts off like planets like Earth did - growing from smaller objects - at least until it grows significantly past the mass of the Earth, shouldn't a super-earth be differentiated? Or is the model showing that when two mega-earths collide and merge, they are too massive to effectively differentiate?


I thought about the same things, and honestly I think the guy is over-thinking it.

But as I've shown, he's wrong even if he's right.

The planet is definitely differentiated (the surface of a sphere ensures it as does common sense from a top-down approach for internal pressure,) therefore it probably has anywhere from 1 to 4 Earth masses of differentiated material, even if his theory is TRUE.

If his theory is false, then it will be even more differentiated than the Earth...
marble89
3 / 5 (2) Sep 25, 2012
Until we can successfully model our neighboring worlds like Venus how can we put much faith in models of planets which we can never hope to observe in a detail ?
Lurker2358
1.5 / 5 (10) Sep 25, 2012
Until we can successfully model our neighboring worlds like Venus how can we put much faith in models of planets which we can never hope to observe in a detail ?


That's why I used a "top down" approach to thinking about differentiation, because you CAN know the internal pressure, approximately, based on the gravity formula and the average expected density of the material (which should be about the same as the Earth if we are talking about a true Super Earth).

Because each Earth pressure gradient WILL be found in the top-down approach, you can be sure that earth-like internal conditions exist at the corresponding depth.

Also, the only main ting that makes Venus hard to model is we really don't have much data on it, and it's so hot it destroys our probes. If it was in the so-called "goldilocks" zone it would be easier to study, but then again we wouldn't exist.
marble89
1 / 5 (1) Sep 25, 2012
These superearths could have massive global oceans we will never detect knowing only the bulk density. Given the purported importance of H2O in the lithosphere it seems a folly to use these models to infer much of anything
Lurker2358
1.2 / 5 (10) Sep 25, 2012
These superearths could have massive global oceans we will never detect knowing only the bulk density. Given the purported importance of H2O in the lithosphere it seems a folly to use these models to infer much of anything


Of course. This is already known.

If they are differentiated, and assuming an initial composition similar to Earth, most Super-Earths should be covered to several miles depth by their oceans. This was already discussed and modeled several years ago, and even appeared on various learning networks.

Very deep liquid ocean with an ice-7 bottom at the barrier between the water and the sediment, as ice-7 is formed at depth under extreme pressure, unlike ordinary ice.

Less than perfect differentiation may actually serve to make the planet more habitable by preventing too much oceans and atmosphere from forming.

All of this is assuming we're talking about a "goldilocks zone" super-earth, as opposed to a "hot super-earth" or a "cold super-earth".
Lurker2358
1.4 / 5 (11) Sep 25, 2012
Hey.

Whoever is the negative giver.

Educate yourself:

55 Cancri E:

http://www.youtub...YHgRmxUw

almost exactly 8 earth masses an similar density.

But this is a "hot" super-earth, so lots of tings bake out of it that would not in a "goldilocks" super-earth.

Nevertheless, 1/5th of the planet's mass is ocean and atmosphere.

I know what I'm talking about.

I didn't make up anything on this thread.
Lurker2358
1.4 / 5 (11) Sep 25, 2012
Piss off.

Do some research, idiot, and you'll see I know what I'm talking about.

You mad now?

You mad because I could prove I was right?

You mad because I could instantly prove the professional astrophysicists was at least partially wrong?

You mad?

Maybe you're mad that I knew as much or more than the professional, so it makes you jealous, so you negative me for no reason.
marble89
2.3 / 5 (3) Sep 25, 2012
"Dont't feed the trolls" - I was warned
NOM
2.3 / 5 (6) Sep 25, 2012
Because the surface gravity will be 2 times higher on an 8 Earth mass super-earth, the atmosphere will weigh 2 times more per unit mass, but it is also likely to be 8 times as massive as a whole, but distributed over 4 times the area.

Not sure about the atmosphere. But assuming the ratio of radioactive isotopes is similar, there will be 8 times the heat given off but only twice the surface area to dissipate.
Lack of differentiation would probably make loss of this heat slower (no mantle convection).
So a super-Earth would probably be much hotter inside.

@Lurk
Not sure why Lite doesn't like you, as he has never posted. You may have pissed off one of his sock-puppets in the past.
LED Guy
3.5 / 5 (2) Sep 26, 2012
@Lurker
You should reconsider your estimates for atmospheric mass, density and pressure. Gaseous atmospheres are very compressible. The surface pressure will not scale as you indicated even if the composition is the same.

Also consider that the mass and density of the atmosphere on Venus is much higher than Earth even though surface gravity is almost identical.

Otherwise interesting ideas . . .
barakn
3.3 / 5 (7) Sep 26, 2012
Just find spherical shells with pressures similar to each Earth benchmark: Inner core/outer core boundary Outer core/mantle boundary mantel/crust boundary -Lurker2538

Complete bullshit. Every material has its own equation of state which is a variable of temperature, density, and pressure. You have made the giantly stupid assumption that within the Earthlike pressure regime A) the same materials are present B) the temperature profile is the same C) the density profile is the same. Wow. Count them. 1, 2, 3. Three variables that you willfully ignored. Luckily the actual scientists doing these studies aren't stupid enough to make the same mistake. You have a lot of reading to do to catch up. Here's the search term to start with: "equation of state"
Lurker2358
1.3 / 5 (7) Sep 26, 2012
Barakin:

On a planetary scale when you're dealing with several Earth masses of "terrestrial material" it is a certainty that there will be plenty of melt material.

It's the assumption of a SUPER-EARTH.

By it's fucking definition, a SUPER EARTH is made of the same basic materials and "ballpark" ratios as Earth, though not necessarily the same distribution. That's the whole point of the classification; a significantly greater than Earth-mass terrestrial planet which is not a gas giant, and not a "hybrid" planet, and not made of some exotic material.

Even as differentiated as the Earth is, our own mantle and crust still has every imaginable material in it in molten or "plastic" form, and since almost everything on the crust seems to have been molten at least one time in it's history, it obviously was not hard for melting to occur.

In this case NOM also has a very, very good point about the radioactive isotopes, due to the mass vs surface area ratio.
Czcibor
2 / 5 (7) Sep 26, 2012
Lurker2358:
It seems for me that you must be disliked by someone with multiple accounts.

Except NOM objection (which I agree with) concerning simply multiplying the amount of volatiles I see no weak points in the reasoning that you presented. (of course when treated as approximation)

If I had to point factors limiting habitability of superearths I'd think about something different - as the only surface feature there should be usually one huge ocean with isolating ice at the bottom, right? Would there be enough nutrients diluted in water at the top for plankton-like life forms?
Torbjorn_Larsson_OM
4.3 / 5 (3) Sep 26, 2012
Without any serious constraints these results do not "suppress chances". For example, the larger gravity well and likely thicker atmosphere of a superEarth, even considering a shorter timescale of volcanism, would efficiently retract from the need of a magnetic field to protect the atmosphere over geological time.

This class of planets will still have life. How much and what kind is what we want to find out.

@ Lurker2358:

That the larger mass makes a difference in convection vs conduction is expected, the analogous happens for thin vs thick ice layers of ice moons. You could use the same idea, "there ought to be zones inside which meet each of the basic pressure thresholds" for the thin ice conduction case. Yet the thick ice convect.

This is why physicists distinguish between linear and nonlinear effects. Physics 101.
Torbjorn_Larsson_OM
5 / 5 (2) Sep 26, 2012
@ marble89:

We are observing differentiated bodies in detail. Vesta was the latest such object.

As for water, it is readily observed in planetary spectra of atmosphere many light years out(say, obtained by differential spectra of transiting planets behind respectively in front of the star). Enough for future statistics.
GSwift7
2.6 / 5 (7) Sep 26, 2012
to Lurker:

You can kinda figure out who gave you those negative ratings if you click on your own name here in the comments, then go to the activity tab there. It shows the complete list of people who rated your comment, though it doesn't show how each of them rated you. For example, the following people all rated you 1/5 on one of the comments in this thread: barakn | Mayor__Dooley | SCVGoodToGo | atomsk | lite | trapezoid |

I don't really pay attention to the ratings, and I don't ever rate anyone any more. You should just ignore them; they are a joke. Whoever gave you those 1's is probably getting a big woody right now because you actually got upset about it. Don't give him the pleasure. The internet isn't serious business.
GSwift7
3.9 / 5 (7) Sep 26, 2012
Back on topic:

Everything I have read makes me think that there's probably a cut-off point for planetary mass that will be truely earthlike. I don't see how anything much beyond 2 or 3 earth masses would be a fun place for humans to live. Mass seems to be connected to atmosphere thickness and therefore surface air pressure. That changes the chemistry in the atmosphere. At higher pressures you change the solubilities and boiling points, for example.

That being said, I doubt wether the interior composition matters that much. There seem to be multiple showstopping characteristics when planetary mass is too big or too small.

However, I also think we will find plenty of earth-sized planets when our detection gets better. What we have observed so far seems to hint that our solar system isn't too far from average, though I bet our moon is highly extraordinary.
barakn
3.7 / 5 (6) Sep 26, 2012
Quantum Conundrum was banned for trolling and now GSwift7 is recommending to QC's sockpuppet that he keep trolling. That's just great. So, G, why don't you go over to the thread where Lurker is passionately defending the notion that Noah's flood consisted of ice and give him a bunch of 5s. That ought to show those woody-sporting 1-givers the whatfor.
Osiris1
1 / 5 (4) Sep 27, 2012
How do we know about planets that we cannot see? And often can only infer. We thought we knew a lot about planets in THIS system many years ago. I remember tables of temperatures of planets...all were dead wrong, and about Venus especially. From what we know about planets here, the larger the rocky part, the more the core activity and faster the rotation of all parts...Look at Jupiter. Five will get you ten it started with a large rock maybe several times size of the earth..large enuf to gather gasses...and then ballooned. Same for other gas giants. Steven Hawking said as much...look for rules elsewhere..start from what you know, right at home....common sense- maybe something new and different..like the sound of it as an engineer...common sense. Just like I here every yahoo with a diploma greater than drivers-ed parroting that 'c' is the universal speed limit..but know one really can say ...why. And just playing simon says...Albert says, is not enuf. We need thinkers not stinkers!
GSwift7
3.8 / 5 (4) Sep 27, 2012
How do we know about planets that we cannot see? And often can only infer. We thought we knew a lot about planets in THIS system many years ago


Nobody claims that we "KNOW" these things. When you read things like the story here, you must understand the uncertanty. That means you need to understand quite a bit about the methods they are using to make these estimates. Unfortunately, that requires quite a bit of reading.

Some of it is based on comparisons to things we know about our own solar system (comparative planetology). Some is based on math and chemistry as we know it. Some is a process of elimination (narrow it down to as few possibilities as you can, then look for ways to eliminate any of them).

There's LOTS of room for errors here, and the people in the field know this. Think of it more like reasoning out what is most likely to be true in a situation where you only know some of the facts.
Widdekind
1 / 5 (1) Oct 04, 2012
The Raleigh number for convection:

Ra = p g L^3 / (D u) ~ g (p R^3) / (D u) ~ g M / (D u) ~ GPE / Du

Inexpertly, the convection index of planets scales with their GPE ~ GM^2/R. Super-earths would have super-convection, unless the Diffusivity & Viscosity somehow increased more.

In addition, would not super-earths probably retain their giant gassy atmospheres? To have an actual super-earth, would require that its host star stripped its atmosphere away (despite its strong gravity, and magnetic fields). (?)
marble89
1 / 5 (1) Oct 04, 2012
earth's atmosphere is extremely thin for a planet of it's mass. Potentially, a planet with our mass could keep a firm grip on an atmosphere thousands of times more massive. If you think of our oceans as just a condensed component of earth's atmosphere we begin to approach this number.