‘Super-Earth’ in a Triple Star System

by Paul Gilster on February 2, 2012

GJ 667C is an M-class dwarf, part of a triple star system some 22 light years from Earth. Hearing rumors that a ‘super-Earth’ — and one in the habitable zone to boot — has been detected around a nearby triple star system might cause the pulse to quicken, but this is not Alpha Centauri, about which we continue to await news from the three teams studying the prospect of planets there. Nonetheless, GJ 667C is fascinating in its own right, the M-dwarf being accompanied by a pair of orange K-class stars much lower in metal content than the Sun. The super-Earth that orbits the M-dwarf raises questions about theories of planet formation.

Thus Steven Vogt (UC Santa Cruz), who puts the find into context, noting that heavy elements like iron, carbon and silicon are considered the building blocks of terrestrial planets:

“This was expected to be a rather unlikely star to host planets. Yet there they are, around a very nearby, metal-poor example of the most common type of star in our galaxy. The detection of this planet, this nearby and this soon, implies that our galaxy must be teeming with billions of potentially habitable rocky planets.”

Image: An artist’s impression of a super-Earth planet. Credit: ESO/M. Kornmesser.

Nearby is right, and assuming we do eventually develop the technologies to send probes to distances of tens of light years out, this super-Earth would surely gain a place high on the target list. The reference to the most common stars in the galaxy is drawn from estimates that 75 to 80 percent of all stars in the Milky Way may be M-dwarfs, although the relative percentages will surely be adjusted as we get a better handle on the distribution of brown dwarfs. Vogt refers to multiple planets because in addition to the super-Earth orbiting GJ 667C, there appears to be evidence for additional planets, perhaps as many as three, orbiting the same star.

Building on earlier work on a different super-Earth in the same system (GJ 667Cb), too close to the star to allow liquid water to exist on its surface, the research team found the signature of the new planet GJ 667Cc, with an orbital period of 28.15 days and a minimum mass of 4.5 times that of Earth. Guillem Anglada-Escudé (Carnegie Institution for Science) calls the latter world “…the new best candidate to support liquid water and, perhaps, life as we know it,” though we have to add the usual caveats about flare activity on M-dwarfs and their potential for disrupting life.

Those other worlds in the same system? A possible gas giant and yet another super-Earth are candidates, but neither has been confirmed. Meanwhile, GJ 667Cc is thought to absorb about the same amount of energy from its star that the Earth draws in from the Sun, leading to the possibility of Earth-like temperatures and, if conditions are right, the potential for liquid water. As with all such results, further information about the planet and its atmosphere are needed before we can run too wild with such speculation, but GJ 667Cc clearly deserves a much closer look.

The paper is Anglada-Escudé et al., “A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone,” accepted at the Astrophysical Journal Letters (preprint).

tzf_img_post

Eniac February 5, 2012 at 22:23

jkittle:

( the o2 in the earths atmosphere is mainly from H escape, not carbon sequestration by biological processes)

amphiox:

The O2 in earth’s atmosphere is from carbon sequestration from geological processes, not biologic ones, and H escape. You need both.

Perhaps either of you has a reference to share documenting these extraordinary claims?

Neither of them agree with conventional wisdom. Photosynthesis is the dominant source of oxygen in the oxygen cycle. See http://en.wikipedia.org/wiki/Oxygen_cycle and references therein.

Eniac February 5, 2012 at 23:34

…though we have to add the usual caveats about flare activity on M-dwarfs and their potential for disrupting life.

I am not sure I understand these “usual caveats”. The only effect of flares likely to penetrate an atmosphere is increased light and UV, perhaps. What kind of increase in light are we talking about? 20% or ten-fold? Anything that a cloud cover could not take care of?

Eniac February 5, 2012 at 23:34

…though we have to add the usual caveats about flare activity on M-dwarfs and their potential for disrupting life.

I am not sure I understand these “usual caveats”. The only effect of flares likely to penetrate an atmosphere is increased light and UV, perhaps. What kind of increase in light are we talking about? 20% or ten-fold? Anything that a cloud cover could not take care of?

Eniac February 6, 2012 at 1:34

@Scott G:

If we assume that our solar system is nothing special, then why can’t we apply this rate of occurrence elsewhere in the galaxy to give a prediction of the total number of occurrences of life in the galaxy?

Because of observational bias.

Your argument would be valid had you picked a random planet, looked for life on it, and found it. However, this is not the case. This being our own planet, the existence of life on it is a foregone conclusion, since we ARE life. It is much like taking a poll with the question “Are you dead or alive?” If you are not careful about observational bias, you might conclude from the results that there are no dead people.

We will only have real information once we can actually look for life on arbitrary planets other than Earth. So far, such investigation has been scarce, and entirely negative. Contrary to amphiox’s shouting, I do believe that tells us something, but it is not much, for now.

Eniac February 6, 2012 at 9:40

I found this on red dwarf flares:

During a deep survey of 215,000 red dwarf stars for planets called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) using NASA’s Hubble Space Telescope, astronomers found some 100 stellar flares over just a week of observations. Constituting the largest continuous monitoring of red dwarf stars ever undertaken, the astronomers announced on January 10, 2011 that their survey results suggest that even “fairly old stars” which are several billion years old can flare violently and become as much as 10 percent brighter in a short time, with an average flare duration of 15 minutes.

( http://www.solstation.com/stars/pc10rds.htm )
10 percent brighter for 15 minutes is not exactly going to singe the tops of lifeforms. In addition, it appears that UV is much less of a problem with red dwarfs compared to sun-like stars, flares or not. All other radiation is going to be blocked very effectively by the atmosphere. Therefore, hiding on the dark side or in valleys and crevices, albeit available, does not appear to be necessary. The only thing I see left is that frequent flares or a generally stronger stellar wind (is that even true?) may gradually erode the atmosphere away. Likely, this is easily counteracted by a denser atmosphere as would be found on a more massive planet, such as the one we are talking about.

Since, I believe, the “freezing out of the atmosphere” on the dark side of tide-locked planets has by now also been relegated to myth, the case for non-habitability around red dwarfs has become a very difficult one to make, in my opinion.

Scott G. February 6, 2012 at 11:05

@Nick

Yes, I’m aware of the “Lottery” argument. But doesn’t that argument itself also suggest multiple (many, in fact) occurrences of life? It takes at most a handful of attempts before someone wins the lottery. SOMEONE always eventually wins. They win when the right “conditions” (or numbers) occur. Why does someone always win? Because there’s only a FINITE set of possibilities that can occur.

The same thing happens in nature. There’s only a finite set of conditions (combinations of molecules, temperature, pressure, radiation environment, etc.) that can occur out there. We already know that complex organic molecules are found throughout space and we know that there’s likely in excess of 100 billion available environments out there as well. So, if you want to talk odds… then I think things look very favorable in terms of the same conditions that occurred on the early Earth occurring elsewhere. And if you have the exact same initial conditions, then the exact same response should occur, because everything in the universe is governed by the same set of physical laws. I think that life is undoubtedly a forgone conclusion and that it’s very hard to argue otherwise.

Scott G. February 6, 2012 at 11:32

One final point about the “Lottery Argument”. It seems to me, that this argument needs an update. Why is it presumed that we’re only running a single lottery in the universe (which carries the obvious implication that there can be only one winner, i.e. us)? Why not turn this reasoning on its head and run one lottery for every habitable patch of real estate in the universe? That seems like a better representation of what’s actually occurring in nature. Why not run one lottery for every square foot of habitable real estate in our galaxy? Well, people don’t do that because with potentially 1,000′s of trillions of square feet of habitable real estate, you still end up with millions or billions of winners.

kzb February 6, 2012 at 14:11

Eniac, the trouble is that a planet in the HZ of a red dwarf is almost inevitably tidally-locked, which means its rotation is slow and its magnetic field low. Earth’s atmosphere has been shielded from stripping by its magnetic field, but these planets don’t have this shielding, hence the fears about their atmospheres.

Personally though, I do wonder how Venus has such a thick atmosphere if this fear is true.

andy February 6, 2012 at 15:12

If you have an oxygen-rich atmosphere then the flares may well trigger ozone buildup anyway, e.g. this simulation of the effect of the large flare observed on AD Leonis in 1985 that suggests ultraviolet levels would exceed those on Earth for maybe only a couple of minutes or so. Of course this presumes that the atmosphere contains large enough amounts of free oxygen.

Then again young G-dwarfs also have high activity levels (there is some evidence that they can also undergo violent flare events), so I have to wonder about what was happening on early Earth before the oxygen levels rose to current levels.

andy February 6, 2012 at 16:58

Eniac, the trouble is that a planet in the HZ of a red dwarf is almost inevitably tidally-locked, which means its rotation is slow and its magnetic field low.

I have to wonder how much it is possible to justify this statement about the magnetic field, or whether it is one of those bits of “folk wisdom” that gets mentioned in this kind of discussion without taking a look at what is going on in our solar system (rather like “moons close-in to a gas giant are inevitably highly volcanic”).

The magnetic planets in our solar system (excluding the case of induced fields in oceanic moons) are the four gas giants, Mercury, Earth and Ganymede*. Once you get beyond “gas giants are magnetic”, you’re left with a fairly odd selection: why is Mercury magnetic but not Mars, why is Ganymede magnetic but not Io, why Earth but not Venus? And coming up with a theory that handles all of them is not an easy task.

* Ganymede has both an induced field and an intrinsic one, so it makes the list.

Nick February 6, 2012 at 17:17

Scott G., you should combine my comment about lotteries with Eniac’s about observational bias. The fact that we are here on this planet tells us nothing about what the odds of the lottery that got us here are, except that they are not exactly zero. It doesn’t tell us that the probability isn’t extremely close to zero.

Of course the lottery is “run” on every habitable patch. But 10^-22 probability per patch * 10^20 patches per galaxy still gives us independent life originating only once in every hundred galaxies. Two independent pieces of evidence, both based on large amounts of data, argue that the probability is indeed this low or lower: the lack of blatantly observable ETI (which the Malthusian imperative combined with technological ability predicts), and the vast genomic complexity of the simplest known closed ecosystem. Both indicate that the probability of abiogenesis per patch is indeed nearly as extremely low as the observable universe is vast in number of patches.

Eniac February 6, 2012 at 17:35

@Scott G: You must be kidding. What if the lottery has 100 numbers instead of 6 or 7? Will somebody always win, still? A “finite” number of conditions? How about 10^1000 conditions, is that finite enough for you? It does not take much paper to print the lottery ticket with 1000 numbers that would achieve such odds. Now imagine molecules instead of numbers, and you will see that “finite” could easily be a lot more than you bargain for.

Rob Henry February 6, 2012 at 17:37

Eniac, aren’t you missing amphiox’s and jkittle’s oxygen build-up perspective. They are only interested in long-term trends, not short term maintenance mechanisms, and these are much harder to calculate. However that is also why I join in you call in the need for references.

Actually, some aspects of applying Earth’s case to all others worry me. The cold trap is not sufficient to prevent water loss through hydrolysis on a planet our size, we also need nucleation of rain drops, and that seems very poorly understood. I am also suspicious on how Venus is meant to have lost so much water, and yet been provided with enough reduced material by geological processes to oxidise it all. Also how did far lower ozone levels through much of Earth’s existence effect this process, or do we assume that it didn’t?

Rob Henry February 6, 2012 at 17:44

Scott G, you ideas of what constitute an improbable event seem entirely drawn by events that occur in everyday life. Mathematics and science regularly produce examples that are in an altogether different league.

FrankH February 6, 2012 at 20:53

@kzb
I don’t know where this “magnetic field protects atmospheres” meme came from, but it’s mostly wrong. The temperature of a planet’s atmosphere and the planet’s escape velocity determine the density and composition of its atmosphere, over geologic time. Not having a magnetic filed just means that the solar (or stellar) wind will heat up the atmosphere directly; this may cause some (possibly significant, depending on the two items I mentioned above) gas loss, but it’s not the primary reason planets loose gases.
Venus has a thick atmosphere mainly because its massive enough to hold on to the components of its current atmosphere, despite the high temperatures and direct contact with the solar wind. Venus lost all of its water because the water vapor was broken down into oxygen (which reacted with other components of the atmosphere, although some was probably lost) and hydrogen (which escaped).

kzb February 7, 2012 at 8:37

FrankH and Andy: about the magnetic field and atmosphere stripping. I was just giving what I perceived to be the conventional wisdom. But I do have problems with it myself, hence the comment about Venus’s atmosphere.

Frank H, it’s the temperature of the very outer layer of the atmosphere that is important in this regard. In fact, individual atoms or molecules could be individually hit by solar wind protons and gain escape velocity ?

Eniac February 7, 2012 at 9:37

Eniac, aren’t you missing amphiox’s and jkittle’s oxygen build-up perspective. They are only interested in long-term trends, not short term maintenance mechanisms, and these are much harder to calculate. However that is also why I join in you call in the need for references.

No, I do not think I am missing anything. You take away the dominant source, the oxygen will disappear. The more short term the source, the more quickly it will disappear, but it will still remain gone forever. That is long term enough for me.

Oxygen is too reactive, it is not meant to be free. It will bind to hydrogen, silicon, carbon, and most any metal. You would have to have an excess over the combination (i.e. more oxygen than hydrogen, carbon, silicon, and all other metals combined) before you see inorganic free oxygen, long term. Not very likely.

Ronald February 7, 2012 at 10:00

@Eniac (February 5, 2012 at 22:23: ), in response to jkittle and amphiox:

“Perhaps either of you has a reference to share documenting these extraordinary claims?”

Unless backed up bu some extraordinary evidence these extraordinary claims are simply and plainly wrong: all evidence points to a biological origin of atmospheric O2.
Even more so, O2 is so reactive that our atmosphere is in a chemical disequilibrium: without replenishment most of the O2 would disappear rather quickly (I have read estimates between 100 and 200 thousand years).

Scott G. February 7, 2012 at 10:45

@ Nick, Rob, Eniac — So we’re all in agreement that [the likelihood of extraterrestrial life] all comes down to the probability of formation of the first self-replicating molecule (at its most fundamental level). People often site the infinitesimally small odds that such complex molecules could arise from random processes. But my contention is that the processes leading to the formation of the first “living” molecule are not entirely random. Julius Rebek has shown, in fact, that a primitive form of natural selection can occur on abiotic molecules. And since selection is a non-random process, this HUGELY increases the probability of formation of complex molecules; once we recognize that they are not entirely formed via random processes. See the “Self Assembly” section of the Wikipedia article on Rebek for details and links to references: http://en.wikipedia.org/wiki/Julius_Rebek#Self_assembly

Rob Henry February 7, 2012 at 16:27

Scott G, self-assembly process help a little if they are exactly the right sort, and hinder otherwise, but they can’t get us far along the apparent complexity route.

FrankH February 7, 2012 at 16:53

@kzb
“In fact, individual atoms or molecules could be individually hit by solar wind protons and gain escape velocity ?”

Sure, the solar wind can provide enough energy for some gases to escape. If a molecule is on the right side of the bell curve and it gets enough extra energy, it’ll go. But this still depends on the planet’s escape velocity. A planet like Mars (with a relatively tenuous grasp on some gases which have an average velocity that is close to Mars’ escape velocity) is going to loose more than Venus, which would require a LOT more atmospheric heating to loose the gases it now has.

Rob Henry February 7, 2012 at 17:44

Ronald, you’re right that O2 N2 and water are an unstable combination, and I am interested that you heard that its time scale was 100,000 years for the following reason.

On Titan life has recently been hinted at by evidence of a 0.4 mg/s/sqkm inflow of H2
For Earth, your figures would give a 600mg mg/s/sqkm inflow of O2.

So to an outside observer, Earth would be the better target on which an ETI may concentrate their search – but wait! An ETI does not know the details of our biosphere or geochemical cycles, just as we can only make a rank guess about those on Titan. The only source of O2 loss that I can see them calculating directly is due to nitrogen fixation by lightning, and that removes 0.4mg/s/sqkm of O2. So to a hydrogen breathing ETI from GJ 667Cc which is the better target for them to concentrate on?

Titan and GJ 667Cc are most probably lifeless but, unlike the Fermi paradox, there is no reason here to preempt further investigation.

Rob Henry February 7, 2012 at 17:45

Ronald, you’re right that O2 N2 and water are an unstable combination, and I am interested that you heard that its time scale was 100,000 years for the following reason.

On Titan life has recently been hinted at by evidence of a 0.4 mg/s/sqkm inflow of H2
For Earth, your figures would give a 600mg mg/s/sqkm inflow of O2.

So to an outside observer, Earth would be the better target on which an ETI may concentrate their search – but wait! An ETI does not know the details of our biosphere or geochemical cycles, just as we can only make a rank guess about those on Titan. The only source of O2 loss that I can see them calculating directly is due to nitrogen fixation by lightning, and that removes 0.4mg/s/sqkm of O2. So to a hydrogen breathing ETI from GJ 667Cc which is the better target for them to concentrate on?

Titan and GJ 667Cc are most probably lifeless but, unlike the Fermi paradox with radio SETI, there is no reason here to preempt further investigation.

Eniac February 8, 2012 at 0:09

@kzb

In fact, individual atoms or molecules could be individually hit by solar wind protons and gain escape velocity ?

Since the solar wind particles are themselves protons, it is not even clear to me whether their impact on the upper atmosphere would constitute a net gain or loss of hydrogen. It would have to be worked out. In any case, it is likely that either would be negligible. That leaves a rise in temperature due to solar wind, but it is again not obvious to me that such would not also be negligible.

The solar wind is awfully tenuous. Does anyone know of a serious treatment of these mechanisms, or are they off-the-cuff “memes” from some book I have not read, about the Rare Earth, perhaps?

Rob Henry February 8, 2012 at 17:08

Kzb and Eniac, I am also puzzled by current atmospheric models, and for me the problem is helium. At normal temperatures and pressures we should be able to retain it. Sure molecules hitting the upper atmosphere can easily take it past escape velocity, but one in ten of those impacting molecules should be helium, compared to 0.0005% of molecules in our atmosphere. Each molecule that hits must be thousands of times better at heating others to escape than other at cooling the impactor to retention velocity. Admittedly scale height factors should greatly enrich helium at the exobase, but most of those incoming molecules should plough much deeper than that before stoping. So it seems that we are all missing something, but what?

Eniac February 9, 2012 at 9:40

So it seems that we are all missing something, but what?

I agree with you on these inconsistencies. My instinct is that what we are missing is that solar wind effects are insignificant, magnetic field or not. That is the parsimonious explanation, and until I find a deeper treatment showing otherwise, I will go with that.

Nick February 10, 2012 at 15:30

“I am also puzzled by current atmospheric models, and for me the problem is helium. ”

Isn’t it much easier to lose helium than water? Most of the hydrogen is locked up in water which we keep, and we lose the dihydgrogen and helium.

Rob Henry February 10, 2012 at 19:16

Nick asks “Isn’t it much easier to lose helium than water?” and, if we restrict this question as only applying to modelling Earth’s atmosphere such that it matches the available data the answer is “no”.

The exobase temperature is already high enough so that escape is so rapid for H that diffusion from the lower atmosphere is the rate limiting step. This is not so for helium. For hydrogen we can adjust such parameters as ancient ozone levels, and greater difficulties with rain drop nucleation in the early Earth, and its effect on the cold trap. Helium is much harder to explain.

PS. here is something to make your hair stand on end. We have no idea what causes the current climate stability that constitutes the Holocene stability, but I’ve seen it speculated that if our exosphere heated by at least 500K in a way never seen before for at least 2% of the time (and it would need to be in a way that does not add too much helium at lower levels in our atmosphere), the problem might be rendered soluble. Now that’s disturbing!

andy February 11, 2012 at 10:31

@Rob Henry: regarding helium, as I understand it nonthermal processes are thought to be the main mechanism for removal of helium, with photoionisation followed by escape along open magnetic field lines being one of the primary suspects.

Rob Henry February 11, 2012 at 20:36

@Andy, I have also heard that suggestion, including as a speculative possibility in a peer reviewed paper. However I have never seen a reference given to any work suggesting that the level of this type of loss is (or even could be) sufficiently high on Earth to explain He-4 levels.

I have the strangest feeling that for this mechanism to solve the problem, it may also require the Sun or Earth to often be significantly different from observations that we have ever made of them.

ljk February 14, 2012 at 10:02

Scientists Find New Clues About the Interiors of ‘Super-Earth’ Exoplanets

by Paul Scott Anderson on February 13, 2012

As we learned in science class in school, the planet Earth has a molten interior (the outer core) deep beneath its mantle and crust. The temperatures and pressures are increasingly extreme, the farther down you go. The liquid magmas can “melt” into different types, a process referred to as pressure-induced liquid-liquid phase separation. Graphite can turn into diamond under similar extreme pressures.

Now, new research is showing that a similar process could take place inside “Super-Earth” exoplanets, rocky worlds larger than Earth, where a molten magnesium silicate interior would likely be transformed into a denser state as well.

Full article here:

http://www.universetoday.com/93536/scientists-find-new-clues-about-the-interiors-of-super-earth-exoplanets/#

Comments on this entry are closed.

{ 1 trackback }