Astrobiology: The Necessity of Asteroids

by Paul Gilster on November 2, 2012

Let’s talk this morning about the ‘snow line,’ the boundary in the Solar System beyond which volatiles like water ice remain cold enough to keep intact. Rebecca Martin (University of Colorado) and Mario Livio (Space Telescope Science Institute) have been running simulations using models of planet-forming disks around young stars. The idea: To calculate the location of the snow line in these disks as measured against the mass of the central star. Their hypothesis is that asteroid belts in other solar systems will be located at the snow line, with implications for life.

Here’s the thinking on this. We know that asteroids, in addition to creating impact threats that can trigger world-changing events, may also have had a crucial role delivering water and organic compounds to the early Earth. Occasional asteroid impacts, says the theory of punctuated equilibrium, may have accelerated biological evolution, forcing species to adapt to rapidly changing conditions. And there are still other possibilities, as the paper notes (I’ve removed the internal citations for brevity):

The formation of large moons may also require an asteroid collision…although here again, a different origin for the impactor has been considered…Our Moon, for instance, stabilises the rotation axis of the Earth and prevents weather extremes that would have resulted from chaotic motion. This process may not be universal, since it depends on initial conditions. According to some hypotheses, life itself may have been delivered to Earth by an asteroid… Heavy elements, including some that are essential for life, were also probably delivered to the Earth’s crust through collisions… During the early times of formation the Earth was molten and its gravity pulled heavy elements to its core leaving the crust depleted of elements such as iron, gold and platinum.

The authors point out that if even one of these ideas is true, then the formation and evolution of asteroid belts may play a major role in the development of complex life.

But gas giants are also a major part of this picture. In our system, the gravitational influence of Jupiter plays a role in the development of the asteroid belt. The planet’s presence just beyond the snow line meant that nearby material inside its orbit could not form into planets, but instead settled into the belt of fragmented rocks that we see today. Livio explains it like this:

“To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt. If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve.”

Image (click to enlarge): This illustration shows three possible scenarios for the evolution of asteroid belts. In the top panel, a Jupiter-size planet migrates through the asteroid belt, scattering material and inhibiting the formation of life on planets. The second scenario shows our solar-system model: a Jupiter-size planet that moves slightly inward but is just outside the asteroid belt. In the third illustration, a large planet does not migrate at all, creating a massive asteroid belt. Material from the hefty asteroid belt would bombard planets, possibly preventing life from evolving. Illustration Credit: NASA, ESA, and A. Feild (STScI). Science Credit: NASA, ESA, R. Martin and M. Livio (STScI).

Back to the simulations. Martin and Livio put their models to work to test their proposition that asteroid belts in other solar systems would be located in the vicinity of the snow line. They examined 90 stars known from observations with the Spitzer Space Telescope to have warm dust that could indicate the presence of an asteroid belt, finding that the temperature of the dust was consistent with the snow line around these stars. The researchers also studied 520 giant planets outside our Solar System, finding only 19 of them residing outside the snow line.

The suggestion is that most of the giant planets that originally formed outside the snow line have migrated too far inward to preserve the kind of asteroid belt that would enhance evolution on inner, terrestrial worlds. The ideal circumstance is a gas giant placed just outside the snow line, like Jupiter, where the planetesimals of an early solar system are too gravitationally disturbed to coalesce into planets. Only four percent of the observed gas giants are found in this position.

If Martin and Livio are right, then our best chance to find living worlds is to find systems where gas giant planets exist outside the snow line. But even here, the conditions are narrowly defined:

Inward migration disrupts the asteroid belt. Terrestrial planets may still form from scattered planetesimals in the habitable zone after the migration, but if asteroid belts are indeed necessary for complex life to evolve, then such evolution is unlikely in these systems. On the other hand, a small amount of giant planet migration may be necessary to remove a significant fraction of the initial belt mass because otherwise there would be too many devastating impacts on the planet
for life to evolve. Consequently, there appears to be a very narrow “window of opportunity” of time during which the giant planet should form, in order for the correct amount of migration to take place – potentially making our solar system even more special.

In other words, too populous an asteroid belt would lead to too many impacts on inner worlds, while the lack of an asteroid belt could mean complex life would be unlikely to develop on these worlds in the first place. The placement of the gas giant becomes a critical parameter.

The authors are quick to note that selection effects may be at play in their data. While our radial velocity techniques are improving all the time, it is still difficult to observe planets in long-period orbits, which makes it likely that, over time, we will find more gas giants outside the snow line than the four percent referred to above. Whatever the actual percentage of gas giants outside the snow line turns out to be, the authors argue that systems with a gas giant outside an asteroid belt will still be the best places to look for complex life.

The paper is Martin and Livio, “On the formation and evolution of asteroid belts and their potential significance for life,” accepted for publication in Monthly Notices of the Royal Astronomical Society (preprint). A Royal Astronomical Society news release is also available.

tzf_img_post

jkittle November 2, 2012 at 10:33

This is a well considered report but it does have an underlying bias. We know OUR solar system is special in that it contains life that quite (most) likely originated here. There for it is natural to look at what makes it special, and the exact arrangements of planets and the exact size and stability of the sun , and their exact compositon are all likely “special” HOWEVER that does not make them necessary. We need DATA to decide if planets in other systems need these special circumstances to have life, not to decide if they have life with or without these circumstance. The argument is backwards. Still in the absence of Data speculation is interesting and informs us as to where to place our bets and how to interpret the data we do have. Case in Point I have heard many news reports declaring Mars CANNOT have life for any number of reasons ( lack of water, too acidic soil etc…) only to have these blown away by later observations or by other interpretations. ) We explore to learn, not to reinforce our prejudices..

Ron S November 2, 2012 at 11:02

“The researchers also studied 520 giant planets outside our Solar System, finding only 19 of them residing outside the snow line…The authors are quick to note that selection effects may be at play in their data.”

There’s no ‘may’ about it: the selection effects are well understood.

Also, migrating Jovians and asteroid belts (or dust) are not independent quantities: Jovians migrate for a reason, the presence of objects among which transfer of angular momentum occurs.

I would have to look at the paper, but I am unconvinced that such a scenario is needed to deposit metals into the crust. If bombardment is the mechanism, there is going to be lots of that going on anyway.

andy November 2, 2012 at 14:19

The second scenario shows our solar-system model: a Jupiter-size planet that moves slightly inward but is just outside the asteroid belt.

On the other hand, to reproduce the small mass of Mars and the compositional gradient of the asteroid belt it seems you need to migrate Jupiter inwards to 1.5 AU and then out again (the “Grand Tack” scenario).

In other words, too populous an asteroid belt would lead to too many impacts on inner worlds, while the lack of an asteroid belt could mean complex life would be unlikely to develop on these worlds in the first place.

A massive asteroid belt may also indicate that very little material is being lost from the belt (e.g. into HZ-crossing orbits). The large population of potential impactors does not necessarily lead to a larger impact rate in the inner system, despite this often being asserted in such “rare Earth”-style papers.

Interstellar Bill November 2, 2012 at 14:25

Long before we can go to other star systems,
we will know how few of 10,000 nearest systems
there are with habitable planets,
(i.e., it’s probably less than 10^-6)
so that most stellar destinations will be rated
according to total metal content
and also to total content of elements heavier than iron.

Any luckless system stripped of its elements by, say, an AGN
will find it a meager place to start a civilization.
Would Sirius or Procyon turn out be such a case?

Alex tolley November 2, 2012 at 14:53

This has too many underlying assumptions to be likely to be predictive of life on exoplanets. The assumptions of asteroid impacts pro and con for life is completely unknown, plus it assumes an Earth II, rather than a host of different types of worlds that could be life supporting. At best this analysis might help explain the special case of Earth. Let’s get some data, rather than speculate.

FrankH November 2, 2012 at 15:13

That’s a great article in part because it confirms my bias (“confirmation bias”) against gas giants in a star’s HZ. They are always going to be bad news for Earth-like planets (and life). It’s as if the three bears came home and found not Goldilocks but a blue whale in bed. Goldilocks may still be there – under the whale and not very presentable.

I’m also glad that they stressed that the selection effect favors short period gas giants in the current exo-planet census.

Young planetary systems are certainly not a peaceful, static event but chaotic and violent, even after the planets have formed. It’s possible that our solar system ejected a Neptune sized planet as the surviving gas giants jostled into position. Gas giant migration is probably the rule, not the exception. From reading the article the impression I get from this paragraph:

“On the other hand, a small amount of giant planet migration may be necessary to remove a significant fraction of the initial belt mass because otherwise there would be too many devastating impacts on the planet
for life to evolve. Consequently, there appears to be a very narrow “window of opportunity” of time during which the giant planet should form, in order for the correct amount of migration to take place – potentially making our solar system even more special.”

Is that even these perturbations may be enough to thin out the asteroid belt, which goes against the implications of the “dense asteroid belt” in the figure above.

Wojciech November 2, 2012 at 17:51

Selection bias, they are eliminating systems that create the only known result-our Earth.
We simply don’t know if life wouldn’t evolve anyway on a exo-moon or in clouds of gas giant.
We have too little samples of life.

Rob Henry November 2, 2012 at 18:09

I agree with Ron S’ comment that there is no maybe about the selection factor for Jovians, but I was rather surprised that enough data had been collected for sufficient time to find even 19 of them beyond the snowline. Some researchers must be very persistent in their study of a single star, and surely even those that do must be tempted to update their equipment after half a decade or so, and thus have a tricky time eliminating the possibility of van de Kamp type errors before they can be certain of a signal of such a long period. 19! That quite a stack!

Also notice how environmental change triggered from medium sized meteorite impacts are said to accelerate evolution because this seems to be the pattern from our fossil record, but variation due to greater axial tilt changes than Earth encounters (of which we can have no data to base any speculation on) is claimed very bad. So… widely separated massive rapid changes that happen on a timescale too fast for natural selection are good, and changes that occur at a rate that could skew the existing gene pool to a new equilibrium or facilitate the selection of novel mutations are bad. Gosh its so lucky we live on a planet like Earth isn’t it.

Ronald November 2, 2012 at 18:56

andy is right: a study last year (Walsh, O’Brien, Mandell. A low mass for Mars from Jupiter’s early gas-driven migration. Nature, 2011) indicated that Jupiter started out at 3.5 AU, then migrated inward to 1.5 AU (Mars distance), and then migrated outward again to its present position at 5.2 AU.
The Grand Tack Scenario.

So, the idea of a giant planet staying just outside the asteroid belt is already flawed.

jkittle is also right: it is an old and deterministic fallacy to think that those large impacts would be necessary for evolution. Nonsensical, evolution takes place driven by any environmental pressure and large impacts are very rare events. In fact, they mainly function as great erasers, from which life slowly and gradually recovers. There is no indication that complex, or even multi-celled life arose as a result from impacts, rather the opposite, life became possible after the late heavy bombardment and complex life after atmospheric O2 increased to sufficiently high levels.

Enzo November 2, 2012 at 19:06

Regarding hot, warm and cold jupiters, we’ll know a lot more by 2017 when Gaia finishes its mission. It is supposed to detect all jupiters within 600 y, a very large sample.
So far cold jupiters look rare and it’s not just the RV bias.

GaryChurch November 2, 2012 at 20:29

As I posted in another thread, Ed Lu wrote about the effect of asteroid belts on civilizations. A little too much asteroid belt and no intelligent life can evolve, not enough and earth like planets will not form. And of the few planets that manage not to keep being reset to microbes by impacts, they might destroy themselves at our stage of technological development. It would explain the great silence. It may not have much data to support it but it is still another warning to humankind to have a defense against both external impacts and internal self-destructive events. Or we will just join that great silence eventually.

Rob Henry November 2, 2012 at 21:25

Ronald said “There is no indication that complex, or even multi-celled life arose as a result from impacts, rather the opposite, life became possible after the late heavy bombardment and complex life after atmospheric O2 increased to sufficiently high levels.”

That is a bit unsettling as it seems to equate LHB impacts that were large enough to raise the temperature over Earth’s entire crust to over 1000K, to much smaller ones such as that at the K-T boundary. Also, for those who still believe that an ocean fill of high energy prebiotic chemicals is the answer to abiogenesis, you are in need of a major and short lived source of energy to wind these chemicals up… such as a massive LHB impact.

Ronald November 3, 2012 at 6:36

Enzo: do you mean all Jupiters within 600 ly? And do you have some more specifics on this: up to what orbital distance (AU) or period, down to what mass?

Arecibo November 3, 2012 at 9:09

And surprisingly we back to rare earth after smashing it for years lol.

Well, if among 600 hundred exoplanets found only 1 have esi index above 0.80 yet still below 0.90, we should lowering our expectation of meeting alien life that can use technology.

Dave Moore November 3, 2012 at 19:56

There is some evidence that extinction events promote evolutionary diversity by opening up the ecological niche space, but the biggest extinction event in the last 500 million years, the Permian-Triassic event was caused by global warming leading to ocean current stagnation and oxygen depletion, which led to the return of sulphur-cycle organisms and a poisoning the biosphere. I’m of the opinion that planets can find there own ways of disrupting life without outside help (global ice-ages, massive volcanism), and that an asteroid belt is superfluous to the evolution of complex life.
The problem with the asteroid belt theories is that they are an untestable assumption until we find a lot of planets with complex life on them, and then the question is moot. So this whole line of investigation is irrelevant.

Talking about giant planets inside the snowline, I have just finished reading Raymond & Armitage’s paper: “Debris disk as a sign post to terrestrial planet formation”, where they model systems with 3 giant planets outside the snowline. In two thirds of the cases, there is planet-planet scattering an you finish up with a giant planet inside the snowline, which in most cases results in the destruction of any terrestrial planets (neat animation included).

What the paper did show is that giant planet scattering also disrupts the Kuiper belt, so if you have an older star system with a dense, undisrupted Kuiper belt, you will almost certainly have the undisrupted formation of terrestrial planets. 16% of older stars have dense Kuiper belts. However, a depleted Kuiper belt, as in the case with our solar system, does not necessarily point to the absence of terrestrial planets.

henk November 4, 2012 at 9:27

Arecibo

we should lowering our expectation of meeting alien life that can use technology.

i already lowered that to zero a long time ago. I can not lower it more than that. Intelligent aliens are something from star trek. In reality they are very rare. We need to accept that we never going to meet other Intelligent lifeforms.

Eniac November 4, 2012 at 12:51

Enzo:

So far cold jupiters look rare and it’s not just the RV bias.

How so? I though it was just the detection bias.

Enzo November 4, 2012 at 17:03

@Ronald
The source for this is this article :
http://www.thespacereview.com/article/2167/1

I previously hear of 150 ly, not sure what changed. Gaia is an astrometric mission so one should be able to detect the wobble regardless of the inclination.

@Eniac
My source for this is astronomer Greg Laughlin’s blog, Systemic and, in particular, this article :
http://oklo.org/2011/02/13/an-analogy/

The argument goes like this : since the first Marcy’s detection of a hot jupiter, some 17 years have passed. Plenty of teams have been making radial velocity measurements for a variety of stars. There’s been plenty of time to detect jupiter analogues and few detections.
I believe that RV measurements for jupiters is not too hard and people must have really good coverage if even unassuming stars like Iota Horologii have been observed and a warm jupiter found.
If cold jupiters are rare, so is our solar system that has two. Of course, the bigger the sample, the better and Gaia will do just that.

In the same post Greg makes another point about “compact” (i.e. Kepler 11 like) systems. Very interesting post.

GaryChurch November 4, 2012 at 18:08

” I’m of the opinion that planets can find there own ways of disrupting life without outside help (global ice-ages, massive volcanism), and that an asteroid belt is superfluous to the evolution of complex life. ”

I have to disagree with that. One big rock and we get reset to the microbe level. It is sometimes called a planet killer and while it is nice to ignore the possibility by citing strings of numbers- if we see one coming a couple months away we are out of luck. Any civilization monitoring the sudden end our civilization via our radio emissions would simply comment, “another one too stupid to survive.”
There is no excuse for not having a fleet of atomic spaceships to defend this planet against impacts and to establish off-world colonies as insurance against an engineered pathogen.
No excuse except stupidity.

jkittle November 4, 2012 at 18:33

If there is an ejected Neptune-size planet out beyond the Kuiper belt is it likely beyond 200Au ( or even 500AU) or we would have seen it. If it was shot out more that 10,000 AU it may have been further pulled away by passing star systems.

Finding an ejected neptune would REALLY help us understand solar system formation and provide a convenient distant target to aim probes at. This might help catalyze the development of more advanced propulsion systems.. Just hoping …
( Science is Not dispassionate, just passionately fact seeking)

Holger November 5, 2012 at 14:30

@Enzo:
Regarding cold Jupiter frequencies, a recent study came to an opposite result to Laughlin’s guesstimate:
In http://www.spacedaily.com/reports/Kepler_Statistical_Analysis_Suggests_Earthlike_Planets_Very_Rare_999.html ,
John Rehling states that the de-biased frequencies of Jupiter-sized planets in the Kepler sample goes UP with increasing distance from the Sun, at least until a ~500 days period – which is closer than Jupiter, but would be a cold Jupiter for many smaller stars.

(And of course finding a true “second Jupiter” with period >4 years would require a RV or transit study of ~10 years to get to 3 orbits, so a null result as of now says little about their real frequency.)

Dave Moore November 5, 2012 at 17:13

To Gary Church
My references to the asteroid belt were in respect to it as a determinant of planetary habitability. I suspect that a dynamically unstable large asteroid belt would preclude higher life as the impact rate on a planet would be too high, but I think higher life would develop just fine on a planet with no major bolide strikes.

Looking for systems with just the right amount of asteroids does not strike me as one of the more fruitful ways of looking for habitable planets.

As with respect to Earth’s current situation, that is a different story.

Dave Moore November 5, 2012 at 17:47

To Enzo,
Commenting on Greg Laughlin’s blog and compact planetary systems that have planets inside Mercury’s orbit. In some of the cases where their is a giant planet in a close orbit like 51 peg or planets in eccentric orbits , they got there most likely through dynamic instability, but in other cases such as 55 Cancri, which have regular systems in close orbit, I suspect they were formed in situ.

55 Cancri is a high carbon star. In stars, like our sun, with more oxygen than carbon, the carbon is stripped out of the hot inner nebula in the form of Carbon monoxide, which then condenses or reacts in the cooler outer nebula. If there is a surplus of Carbon, though, then it condenses as graphite particles. We know this from carbon stars, cool red gaints with a surplus of carbon. They have a graphite smog in there out layers.

Our solar nebula had an inner “snow line,” condensation line, at the 1500 deg. C, the distance at which the first silicates condensed out. A high-carbon nebula has an inner condensation line at about 3500 deg. C , which would mean you would get planetismals forming from just about the inner most stable orbit outwards, and hence a whole collection of planets with carbon cores inside the orbit of Mercury.

Enzo November 5, 2012 at 18:47

@Holger

I’m aware of the peak in the Kepler data for warm juptiers and that makes habitable exomoons a possibility. It shows in the RV data too. it keeps going up with a maximum around 1000 days orbital period but it then it drops off quickly.
You can see two distinct groups of jupiters in this in the diagram at about 6 min 38 sec in this Seti talk :
http://www.youtube.com/watch?v=JZFED5dvMVo&list=PL7B4FE6C62DCB34E1&index=3&feature=plcp

The talk is interesting in its own right.

@ David Moore
What you are saying about super earths, especially in compact systems being formed in situ is supported by the speaker of the same talk above.

Enzo November 5, 2012 at 19:01

Actually, rather than a peak at 1000 days is more like a blog between 500 and 1000 days.

GaryChurch November 5, 2012 at 22:13

“As with respect to Earth’s current situation, that is a different story.”

I was not attaching any stupidity to you- just to our species collectively.

Eniac November 6, 2012 at 1:19

The reason we do not know if our system is special is simple: None of the methods we have at this point is sensitive to detect any of our own planets even from neighboring stars. They are either too small, too far away, or have periods that are too long. Just a few posts ago we observed that none of our planets would currently be detectable around Alpha Centauri, for example. Jupiter would require 33 years of observation to get 3 periods, for another example. Please correct me if I am wrong.

No amount of extrapolation can change the fact that we just do not know, right now.

Ronald November 6, 2012 at 11:18

Enzo, fascinating presentation.

The essentials were:
- As you already mentioned the two distinct populations of giant planets: the ‘normal’ distance giant planets that formed in situ, and the hot giant planets that resulted from inward migration.
- The hot giant planets are actually rather rare, only about 0.5% occurrence among stars. But they were the easiest and first to be detected using RV, hence the initial observational bias.
- Close orbit super-earths (and subgiants/Neptunes) are very common, maybe even the norm, about 50% occurrence among stars.
- These super-earths and Neptunes have formed in situ, even the hot ones among them.
- Mass loss due to stellar irradiation is only a significant problem for the smallest (about moon-Mercury) and at the same time closest (within a few % of 1 AU) of planets, for any planet bigger and/or farther away mass loss is insignificant even over entire planetary lifetime, it could at most loose (part of) its atmosphere.

Ronald November 6, 2012 at 11:24

Interestingly, when asked, the speaker, Dr. Chiang, stated that he did not know the reason for this predominance of compact super-earth/Neptune systems. It probably has to do something with initial protoplanetary disc mass and density (and maybe composition).
Which, I say, may in turn be correlated with stellar metallicity and elemental abundances.

Ronald November 6, 2012 at 11:55

Dave Moore: what do you mean by 55 Cancri having a ´regular system´?

Although I immediately agree that the 51 Peg hot giant planet (hot Jupiter) is most probably the result of inward migration (Gliese 86 is another good example), it seems to me that at least some of the inner planets of 55 Cancri are also inward migrations: two of the four innermost panets are warm/hot giants (well within Mercury’s orbit), plus the fact that the whole region between 0.8 and 5.8 AU is empty, well, at least no giant planets there. But I admit this is a contestable case .

I would rather suggest that examples of in situ giant planets are the Mu Arae, 47 Ursae Majoris and Epsilon Eridani systems, with giant planets in more ‘normal’ orbits, without hot giants.
And good examples of (in situ formed) compact systems of super-earths/Neptunes are: 82 Eridani, 61 Virginis, Nu 2 Lupi and Gliese 302.

andy November 6, 2012 at 14:49

The argument goes like this : since the first Marcy’s detection of a hot jupiter, some 17 years have passed. Plenty of teams have been making radial velocity measurements for a variety of stars. There’s been plenty of time to detect jupiter analogues and few detections.
I believe that RV measurements for jupiters is not too hard and people must have really good coverage if even unassuming stars like Iota Horologii have been observed and a warm jupiter found.

Well actually the Jupiter-analogues are really only just starting to come out of the longer RV datasets. One of the interesting things so far is that very massive superjovians at such distances appear to be quite rare, as noted by Boisse et al. (2012).

There is also the issue that the period for a Jupiter analogue is similar to that expected for stellar activity cycles. According to Xuesong Wang et al. (2012) (who also report the detection of a Jupiter-analogue at HD 37605) there is already cause for concern about the claimed Jupiter analogue at HD 154345.

It’s also worth noting that the estimates of Jupiter analogue frequency from microlensing seem to be somewhat higher than those estimated from RV. So far microlensing has produced two detections of what appear to be scaled-down Jupiter-Saturn analogues, OGLE-2006-BLG-109Lbc, OGLE-2012-BLG-0026Lbc.

Enzo November 6, 2012 at 19:11

@Eniac

I don’t think you need 3 periods to announce a RV planet.
That would not explain the detection of 55 Cancri d and Mu Arae e, both announced in 2002.
If the signal is clear enough, then it’s enough to announce it. And, for large planets, it normally is.
I think the 3 periods rule was the older Kepler’s rule, now being revised to 4, 5 or more since the noise is higher than expected (and are not finding many earths where they’d like them to be :-)).

Dave Moore November 6, 2012 at 23:53

Ronald:
Looking at the inclinations of 55Cnc d and 55 Cnc e, I guest you can’t call the system too regular; however, I still think 55Cnc b, c & f could have formed there.
I watched Chiang’s presentation too, and if you go with what he proposed and assume the healthy formation of graphite planetismals within 0.3 au, then these would rapidly coalesce into larger planets (The shorter the orbital time, the faster the accretion process goes), which would accumulate large atmospheres quickly.
55Cnc e with its inclination at 81 deg. probably got where it was by been scattered.
55Cnc d has a listed angle of inclination of 53 deg. This could explain a lack of planets between 0.8 and 5.8 au. If a planet’s orbit was inclined at any angle relative to 55 Cnc d then it would be subject very strongly to the Kozai effect, so a lot of the mass from that region may have been thrown into the inner system, beefing up those planets already there. (Of course, there may also be undetected bodies in that region.)

Ronald November 7, 2012 at 7:56

The first article quoted by andy (Boisse et al, 2012) gives some very relevant results and conclusions:
- “a decrease in frequency of giant planets at larger distance (> 5-6 AU), which is a solid prediction of the core accretion theory”
- Giant gaseous planets (in wide orbits > 4 AU) appear to occur significantly around stars that are more metal-rich than average (minimum metallicity > -0.15). Well, we already knew that for gas giants in general.
- Giant gas planets seem much more common in close orbits than in wide orbits, even when corrected for observational bias, which is most likely a result of inward migration and again in line with core accretion theory.

The third article quoted by andy (Gould et al, 2012) also has a very relevant conclusion:
“a first estimate of 1/6 for the frequency of solar-like systems”.

Remarkably and maybe coincidentally that last one corresponds very well with research results by Melendez, Ramirez and Asplund (various recent publications), based on elemental abundance patterns, indicating that about 15% of solar twins and solar analogs have terrestrial planets in the inner system, comparable to our solar system.

All this, though still based on very preliminary results and small numbers statistics seems to indicate that our type of solar system, with small terrestrial planets in the inner system and large (gas) planets on the outside, is a real cosmic minority, though still significant.
The majority of planetary systems are either “compact super-earth/ice giant (Neptune)” systems, or “warm/cool giant gas planet” (i.e. not hot Jupiter, but well within 4 AU) systems, or a combination of these two.
What we do not and cannot know yet, is the abundance of systems with only small planets, both in the inner system and on the outside, possibly a result of low (but not too low) metallicity.

Ronald November 7, 2012 at 12:45

Dave Moore: thanks, very interesting what you mention about 55 Cancri and carbon-rich stars. Do you have a source for that? I would like to learn more about it. I cannot remember that Chiang mentioned this in his talk.

andy November 7, 2012 at 14:34

55Cnc e with its inclination at 81 deg. probably got where it was by been scattered.
55Cnc d has a listed angle of inclination of 53 deg.

Well yes the two inclination values are quite different, but you have to consider the source for the inclination of the 53 degree value. Turns out it is McArthur et al. (2004) – at this stage 55 Cancri d had not completed a full orbit at the time of the observations (and indeed they note that the astrometric measurements only covered a small arc of the orbit), and the best fit assigned it an eccentric orbit (e=0.327). That paper references as the source of the astrometric measurements McGrath et al. (2003) who note that “The phasing and time span of the observations preclude us from saying anything about the longer-period RV companion to ρ¹ Cnc that has been recently verified”.

McArthur et al. (2004) mentions a follow-up paper where the astrometry would be further discussed, this has to my knowledge never materialised (indeed the Winn et al. 2011 paper announcing the discovery of transits for planet e note that the final results of the astrometric analysis have not been announced). So at the current time it does not seem to me that the inclination of 55 Cancri d is at all well-determined.

Dave Moore November 7, 2012 at 22:16

Ronald:
I don’t know of any papers about graphite condensation in planetary nebulas. I came up with the idea while thinking about the concept of snow lines. It seemed to me to be a plausible idea given there is an observational history of Carbon monoxide chemistry and graphite condensation in Carbon Stars.

andy November 8, 2012 at 14:48

@Ronald: for simulations of terrestrial planet formation around stars of varying elemental abundances (including 55 Cancri), see Bond et al. (2010).

Eniac November 8, 2012 at 22:55

I would be interested in what percentage of stars that have been looked at have not been found to have planets. Those would all be candidates for being just like our own system. Unless this number is really small, I am impressed by the ability of some to extrapolate data into a realm of which it cannot contain any examples, and conclude such systems are uncommon.

Ronald November 9, 2012 at 5:27

andy, thanks for the Bond paper, interesting.
Dave, this paper also contains some interesting suggestions on C-rich stars and close-in planets.

ljk November 9, 2012 at 10:06

Comet collisions every six seconds explain 17-year-old stellar mystery

By Stuart Wolpert

November 08, 2012

Category: Research

Every six seconds, for millions of years, comets have been colliding with one another near a star in the constellation Cetus called 49 CETI, which is visible to the naked eye.

Over the past three decades, astronomers have discovered hundreds of dusty disks around stars, but only two — 49 CETI is one — have been found that also have large amounts of gas orbiting them.

Young stars, about a million years old, have a disk of both dust and gas orbiting them, but the gas tends to dissipate within a few million years and almost always within about 10 million years. Yet 49 CETI, which is thought to be considerably older, is still being orbited by a tremendous quantity of gas in the form of carbon monoxide molecules, long after that gas should have dissipated.

“We now believe that 49 CETI is 40 million years old, and the mystery is how in the world can there be this much gas around an otherwise ordinary star that is this old,” said Benjamin Zuckerman, a UCLA professor of physics and astronomy and co-author of the research, which was recently published in the Astrophysical Journal. “This is the oldest star we know of with so much gas.”

Zuckerman and his co-author Inseok Song, a University of Georgia assistant professor of physics and astronomy, propose that the mysterious gas comes from a very massive disk-shaped region around 49 CETI that is similar to the sun’s Kuiper Belt, which lies beyond the orbit of Neptune.

The total mass of the various objects that make up the Kuiper Belt, including the dwarf planet Pluto, is about one-tenth the mass of the Earth. But back when the Earth was forming, astronomers say, the Kuiper Belt likely had a mass that was approximately 40 times larger than the Earth’s; most of that initial mass has been lost in the last 4.5 billion years.

By contrast, the Kuiper Belt analogue that orbits around 49 CETI now has a mass of about 400 Earth masses — 4,000 times the current mass of the Kuiper Belt.

“Hundreds of trillions of comets orbit around 49 CETI and one other star whose age is about 30 million years. Imagine so many trillions of comets, each the size of the UCLA campus — approximately 1 mile in diameter — orbiting around 49 CETI and bashing into one another,” Zuckerman said. “These young comets likely contain more carbon monoxide than typical comets in our solar system. When they collide, the carbon monoxide escapes as a gas. The gas seen around these two stars is the result of the incredible number of collisions among these comets.

“We calculate that comets collide around these two stars about every six seconds,” he said. “I was absolutely amazed when we calculated this rapid rate. I would not have dreamt it in a million years. We think these collisions have been occurring for 10 million years or so.”

Using a radio telescope in the Sierra Nevada mountains of southern Spain in 1995, Zuckerman and two colleagues discovered the gas that orbits 49 CETI, but the origin of the gas had remained unexplained for 17 years, until now.

Full article here:

http://newsroom.ucla.edu/portal/ucla/comet-collisions-every-six-seconds-240565.aspx

Rob Henry November 9, 2012 at 18:44

Dave Moore, I your idea is interesting, but you have failed to address the following complication…

Even in high C/O cases most of that available proto-planetary carbon should be locked up as CO. The main difference is that virtually all the oxygen is to. This lack of water would make other high refectory chemicals more common than around our system, such as metallic iron, silicon carbide, nitrides, and even some pure silicon. I suspect that graphite will never dominate.

Ronald November 11, 2012 at 6:03

Further to Chiang’s presentation and the formation of compact systems of super-earths and Neptunes, by far the most common type of planetary system, Dr. Greg Laughlin has a very interesting post on this on his systemic website:
http://oklo.org/2012/11/10/the-mmen/
Also referring to a recent publication by Chiang and Laughlin:
The Minimum-Mass Extrasolar Nebula: In-Situ Formation of Close-In Super-Earths: http://arxiv.org/abs/1211.1673
The entire PDF is downloadable.

The essential point is that these compact systems of super-earths (and Neptunes) have most probably formed in situ, without any significant further migration. It confirms that our type of planetary system is indeed rather exceptional.
The systemic site shows two great diagrams of which the first one was already well-known, showing the known size/orbit populations of planets. It clearly shows 3 distinct and real populations of planets: the hot Jupiters, the ‘normal’ exo-Jupiters (giant planets) and the (cose-orbit) super-earths and Neptunes.
What still remains to be seen is to which extent these super-earths and Neptunes continue toward wider orbits, as well as the occurrence of small terrestrial planets on wider orbits. This is the right bottom part of the diagram which is still empty as an observational bias, because of present technical limitations.
What struck me most now is the a gap between the super-earths/Neptunes and the normal exo-giants. In other words, these seem to be two really distinct populations, with little continuation between the two.

Comments on this entry are closed.