Steinn Sigurðsson (Pennsylvania State) has been reporting from the Greek island of Santorini, where he is attending the Extreme Solar Systems conference. I want to send you at once to Steinn’s Dynamics of Cats weblog, where updates are being filed and will presumably continue through today, the conference’s last day. It sounds like a terrific gathering filled with the energizing news of discoveries, its theme being, in addition to finding Earth-like planets, the study of exoplanets in tricky places like dense star clusters, near giant stars and orbiting pulsars.
That last is fitting enough, given that the first extrasolar planets of Earth-like mass were discovered 15 years ago around the pulsar PSR 1257+12; in fact, the conference meets on the occasion of that anniversary and celebrates as well the sixtieth birthday of Alex Wolszczan, the discoverer of those worlds. All of that and beauteous Santorini too, though as Steinn reports, the heat has been the worst since 1916, as per locals who should know.
Steinn has to honor embargos and there are things he can’t speak about, but among the more interesting things to emerge is that the California-Carnegie team has found a true Jupiter analogue around a G8V main sequence dwarf, orbiting at about 4.4 AU. From Steinn’s notes:
This is a Jupiter – a cold gaseous giant planet in the right place, which does not look to have migrated or done anything messy. It is of course a fabulous target for low mass rocky planets interior to the current known giant, including in the habitable zone. It is also a very promising indicator that the large number of known “trending” systems being monitored will resolve out to be solar system analogs – maybe 20-30% of stars being monitored may be solar system like if this all pans out – but that is speculative at this stage.
Some other intriguing notes:
- Nine nearby white dwarfs are now known to have confirmed warm debris disks;
- Planets around giant stars seem to be proliferating. The California-Carnegie team has reported three confirmed Jovian planets and four other candidates; the East-Asian Planet Search Network (EAPSNET) likewise has a detection around a K giant in an open cluster and several candidates; and the Pennsylvania State-Poland search team also announces several detections with a whopping thirty candidates.
- A number of low-mass planet candidates are emerging around K and M stars, but no formal announcements as yet. Take note of Steinn’s provocative comment: “I don’t think the “Rare Earth” hypothesis is holding up well, the pieces of the argument are being dismantled wholesale as we find more systems and gain more understanding.”
And this should interest those interested in the nearest stars, quoting Steinn again:
There is now data on Barnard’s star and Proxima Cen with good velocity sensitivity (~ 3 m/sec). Barnard’s star (old nearby M dwarf) is active and velocity variability correlates with photometric variability – Proxima Cen is very variable, but the fluctuations in velocity do not correlate with the fluctuations in brightness. Maybe something there if the data can be dug into. Should have better than 1 m/sec observations with UVES spectrograph on the VLT telescope. This will get to sensitivity of one earth mass in the “habitable zone” (which isn’t really, the star is variable). Maybe there is a low mass planet in moderately short period orbit around Proxima Cen – be interesting if that turnsout to be the case.
I don’t want to go any further here (we’ll be discussing many of these findings here in days to come), but I do urge you to read through Steinn’s postings as Santorini winds down. There is much of interest — the Swiss group alone has twelve new planets — and the chance to follow a conference from afar at this level of detail is most welcome. A glance at photos of Santorini from the conference site as well as Steinn’s own snapshots has me thinking about how to get there myself one day soon.
Comments on this entry are closed.
Way, way premature to dis the Rare Earthers. RE has to do with Earthlike worlds, none of which have been found yet, nor should they have been due to the limited capabilities we have. It is interesting if we finally come up with solar system analogs with a Jupiter like planet several AU from the sun in a circular orbit. I maintain that only a miniscule % of systems so far observed are solar analogs. Anyone care to post a link contradicting that statement? Again, solar system analogs are at the very edge of our capability to detect. The long orbital periods and the tiny displacements because of the wide orbits make it difficult. Point being that the RE statement was not justified by facts.
Take note, though, that Sigurðsson says this in the context of a nearby statement that other Jupiter-analogs (in terms of distance and orbital eccentricity) are beginning to emerge. Announcements not ready to be made yet, but it looks as though we’re on the edge of detecting systems more analogous to our own. Whether we will or not is, as you say Phil, still speculative.
We are only finding the Hot Jupiters because they are the
easiest to detect with our current instruments.
A variation on the drunk looking under the street lamp
even though he lost his keys far away because the light
is better there.
I would not write off the “Rare Earth” hypothesis yet. Even if we find lots and lots of “earth-sized” planets in the proper goldilocks orbits, we still have no way of telling if these are actually habitable. I see the core argument of RE as that of plate tectonics. Plate tectonics makes a habital planet because of the water and CO2 recycling. Without plate tectonics, there is no CO2 recycling. Worse, I think plate tectonics allows for a stready controlled release of the internal energy through vulcanism and what not along the plate boundaries. Without tectonics, the energy builds up with out any release until a global resurfacing event occurs. These resurfacing events release all kinds of heat and CO2 (which creates more heat) to create a Venus-like environment.
A central tenet of RE is that it was the giant impact (admittedly a rare event) and the large resultant moon that created the plate tectonics of Earth. The pre-impact Earth was likely a “waterworld” or was on its way to becoming a venus as well. Hense, an “Earth-like” world without a large moon is likely to become a Venus. This is the core of the Rare Earth hypothesis.
It will be several decades before with have the telescopes and instruments to detect “Earth-like” worlds in near by star systems and to confirm that those “Earth-like” worlds are not “Venus-like” hellholes.
I accept the RE hypothesis (with the caveat that the Solar System offers much forensic evidence for not only the LHB impacts but an earler, even more massive set of impacts, so such ‘Big Whacks’ may actually be rather ordinary), but would fo further, and argue that the ‘Habitable Zone’ is not as normally described – as noted above, had the early Earth not lost a large part of it’s volatiles then we’d presently see a cool Venus (100 degrees, water, 5atm) where the Earth is today.
Earth is *outside* Sol’s Habitable Zone!
Hi Bob & Kurt9
We really don’t know what exactly happened to Venus – even the “cataclysmic resurfacing” idea has been challenged in recent discussions. But there’s certainly a lot to the idea that a habitable Earth has only existed for a small fraction of its lifetime. It was a different place in the Hadean and Archean, slightly improved in the Proterozoic, and really quite variable even through out the Phanerozoic – 50 C temperatures in the Caribbean have been recorded in the isotopic record even in the Cretaceous, while through much of “dinosaur time” oxygen was quite low. How Earth got from A to B is not obvious and might’ve required stochastic changes in geology & biology that aren’t certain.
Rare Earth makes sense insofar as for a rocky terrestrial you need some kind of mechanism like plate tectonics and volcanism to recycle carbon to the surface – otherwise it’s all likely to end up at the bottom of the ocean after half a million years or so. For us, a large moon plays a role in keeping that tectonic/carbon cycle going ( as well as taking the brunt of impacts that might otherwise have kept life on earth at the microbial level permanently) But if you had a rocky terrestrial in orbit around a gas giant, then tidal flexing might be the mechanism that keeps carbon and other essential elements constantly in cycle. There should still be enough possible mechanisms to make many other earths possible out there.
Simulations of environment on ocean-planets suggests that at Earth’s distance from the Sun, the ocean surface would be at boiling point, so if you define your habitable zone solely in terms of volatile-rich planets, then sure, the Earth is outside the habitable zone. (Though if you’re willing to accept a boiling water ocean as habitable, the habitable zone extends inwards to about 0.85 AU, closer than that and the ocean is supercritical).
However, it is by no means certain that planets such as Venus did NOT suffer similar giant impacts to the one that formed the Moon. Such impacts might be efficient at removing volatiles, but only a subset may be able to form moons. (Note the giant impact hypothesis for Mercury’s anomalously large core, but Mercury does not have a moon!) I’m not sure if a large moon is required to maintain plate tectonics (given the sample size of one of planets with plate tectonics, speculations are fairly dangerous), but it is not unreasonable that giant impacts sufficient to initiate the process occurred on all four terrestrial planets.
Furthermore, it is not clear that the Earth is especially volatile-poor: Earth has more than enough volatiles to enter a moist greenhouse phase, but mostly they are locked up in rocks. On Venus, temperatures are high enough that this could not happen, hence the runaway. Bear in mind also the faint young sun – easier to set up habitable conditions and various feedback cycles).
Got any references for the boiling ocean results? I’ve seen climate modeling attempts that indicate either an Icehouse or a super-critical fluid ocean, and nothing in between, for Earth. Clearly there’s a lot of heat-transport happening to stop the runaway conditions from stabilizing. GCMs are becoming more accessible as modeling tools and I would be interested in how putting realistic heat transport into a model changes the outcome.
As for Venus, while a wet early Venus has interesting astrobiological possibilities, modeling work indicates that Venus formed essentially dry and the current water & deuterium inventories are the steady-state end-point of comet supply. We’d know a hell of a lot more if we could sample the gas flow from a current Cytherean volcano.
One more point the impact of Theia was at an unusually low energy, whereas Mercury’s impactor much faster. I suspect that Theia formed at one of Earth’s L-4/5 points, and hit a mass at which is was destabilised, beginning a walking orbit that eventually crashed into Earth – at almost zero hyperbolic excess. Other objects in our Solar System might have formed similarly – Mercury’s impactor, Titan, Pluto/Charon – so it may not be that rare, but the specifics are by no means deterministic.
A few years ago Robin Canup did Monte Carlo simulations of the crash with Theia which indicated a Moon-making event roughly once in every 4 systems, but about 1/3 of those matched the Earth-Moon configuration – thus odds of about 0.08 to make an Earth-Moon pair. And of course it has to be right planet-moon pair in the habitable zone to be an Earth-analogue.
I am not so convinced that a moon is necesary for a habitable terrestrial planet, and in particular for plate tectonics. In my knowledge, but correct me if I am wrong, the earth’s plate tectonics are purely a result of the movements of the core and resulting mantle convection flows. The moon has relatively little (tidal) impact on that.
Isn’t it true that venus also has plate tectonics, or at least volcanism?
Small, cooled-down planets like Mars don’t have plate tectonics (anymore).
I believe the argument that a moon is required for plate tectonics is several parts. First, the impact itself resulted in a thinner crust, which makes plate tectonics easier. Secondly, the impact itself broke up the crust so that plate tectonics can occur. Once the Earth-moon system formed, the tidal effects of the moon are not that dramatic, but are believed to be just enough to keep tectonics in motion.
There is a recent paper indicating that the Earth’s crust does move westward 1-2 cm per year. This would be due to the moon.
Robin Canup’s simulations, if correct, are quite profound. If “moon creation” collisions are as common as he suggests, then there are likely to be far more habitable planets than there other wise would (like 5-6 magnitudes of order more). This, in turn, would render REH irrelevant.
The only way to know is to make the telescopes and instruments that can detect “Earth-like” planets out to, say, 200-300 light-years away and to characterize them (temp, atmospheric composition, etc.). I believe some of the instruments planned for deployment in the next 15 years or so will be able to do this.
Oh, then there’s the matter of making the “magic wand” so that we can get out there and check these planets out.
What I found troubling about the Rare Earth hypothesis is that it seems to assume that some parameters have to be just right for life to emerge. It may be possible to say that an Earth sized planet in a similar orbit but without a moon or plate tectonics would be uninhabitable. But what about a slightly larger planet?
How about moons to Neptune sized planets around M dwarfs etc.
The problem I see is there’s a large parameter space and you can only look at some trivial examples (no water=bad) and cases which are in most ways very similar to Earth or one of the other solar system planets.
So when we get to the point where we have a large sample of terrestrial planets and hard data on their atmospheres we can start to think about why some cases allow life while others don’t. It’s amazing to think that that might happen in 15 years.
Speaking of ‘bad climate history’ for the Earth, here’s an interesting chart that graphs the Earth’s temperature for a bit less than the last billion years.
What’s scary is how WARM this ecosystem is typically. We’re in one of the rare Icehouse Earth episodes and coming out FAST! Usually, this planet has not had polar ice caps since the PreCambrian ~550+ million years ago.
Even if there were lots of Earthlike analogies out there, there’s no guarantee that life itself isn’t the rarity. Maybe they’re all empty…
Robin Canup is a she, and an accomplished ballerina as well as an astrophysicist.
BTW Brian May, of “Queen” fame, is also an astrophysicist – he just recently completed his PhD (extrasolar Zodiacal dust, I believe) which he put on-hold back when “Queen” started hitting the big-time.
Talented lot these astrophysicists!
Reference for the boiling ocean (pdf file). Transition between hot and boiling ocean seems to be ~1.03 AU.
So you found the scotese website as well? You also noticed that most of its time, the Earth’s mean temperature is usually around 23degC. Today its 13degC. This means that the Earth has usually been tropical since the PreCambrian, with our time being one of the exceptions. Also, the sun tends to grow and get warmer, like all main sequence stars, as it burns it hydrogen fuel and starts to burn helium.
At a mean temp of 23degC, much of the Earth is tropical. In order to get a nice, cool Seattle-like climate, you have to go to the shores of the Arctic Ocean, and even this had a water temperature of 20degC. Everywhere was like, maybe, Taiwan or Thailand. Imagine that!
I’m always amazed at the ignorance of even scientists when blathering on about the Earth’s climate. To most of them the Pleistoscene ice age is as far back as they go in their thinking.
That being said, I say it’s wise not to mess around bigtime with mother nature since we remain essentially clueless about what can really trigger a phase change.
Ah, the Kuchner paper. Interesting that one. Notice he’s talking about a 30 bar water greenhouse causing a super-critical “ocean” of steam – actual oceans form further out than 1.03 AU. But there’s a few assumptions involved in his computations. Steam atmospheres would be liable to collapse if their surfaces drop below the critical point (647 K) and I suspect cloud formation would utterly destabilise such a scenario. I’ll have to read further.
Not sure if it would be relevant to the situation, but Venus has no problems maintaining temperatures above the critical point of water, despite global cloud cover. Then again, it isn’t the carbon dioxide forming the clouds.
Studying Venus was a gestault experience wherein we viscerially became aware of greenhouse effects and runaway. I think that the study of exoplanet atmospheres later in this 21st century will gain us increased insight into Earth’s mysterious climate. Who says there’s no practical applications of pure science astronomy?
Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations
Authors: Eric L. Nielsen (1), Laird M. Close (1), Beth A. Biller (1), Elena Masciadri (2) ((1) Steward Observatory, University of Arizona, (2) INAF-Osservatorio Astrofisico di Arcetri, Italy)
(Submitted on 28 Jun 2007 (v1), last revised 1 Jul 2007 (this version, v2))
Abstract: We examine the implications for the distribution of extrasolar planets based on the null results from two of the largest direct imaging surveys published to date. Combining the measured contrast curves from 23 of the stars observed with the VLT NACO adaptive optics system by Masciadri et al. (2005), and 47 of the stars observed with the VLT NACO SDI and MMT SDI devices by Biller et al. (2007) (for a total of 59 unique stars), we consider what distributions of planet masses and semi-major axes can be ruled out by these data, based on Monte Carlo simulations of planet populations. We can set this upper limit with 95% confidence: the fraction of stars with planets with semi-major axis from 20 to 100 AU, and mass greater than 4 M_Jup, is 20% or less. Also at the 95% confidence level, with a distribution of planet mass of dN/dM ~ M -1.16 between 0.5-13 M_Jup, we can rule out a power-law distribution for semi-major axis (dN/da ~ a alpha) with index 0 and upper cut-off of 17 AU, and index -0.5 with an upper cut-off of 46 AU. For the distribution suggested by Cumming et al. (2007), a power-law of index -0.61, we can place an upper limit of 73 AU on the semi-major axis distribution, again at the 95% confidence level.
In other words, given these assumptions for the semi-major axis distribution, and using the models of Burrows et al. (2003), giant planets are rare past 73 AU. With our current observations, we cannot reject the Ida and Lin (2004) models for the masses and semi-major axes of giant planets with better than 50% confidence.
In general, we find that even null results from direct imaging surveys are very powerful in constraining the distributions of giant planets at large separations, but more work needs to be done to close the gap between planets that can be detected by direct imaging, and those to which the radial velocity method is sensitive.
Comments: 44 pages, 17 figures, submitted to ApJ
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0706.4331v2 [astro-ph]
From: Eric Nielsen [view email]
[v1] Thu, 28 Jun 2007 22:08:04 GMT (489kb)
[v2] Sun, 1 Jul 2007 11:23:24 GMT (480kb)
I think the average distance between two habitable worlds is around 1580 light years away. We may not find anything unless we have some “new toys” in our hands. We also assume that ETs behave like human, what if they do not, like they never used radio or tv then we can’t detect any signal from them and our signals won’t come in another 1500 years. I wonder if we can detect any signature from fusion/antimatter propulsions from 1500 light years away.
Hiro, how did you come about with the figure of 1,580 light years?
hiro i find it interesting that you surmise that we might be able to detect advanced propulsion signatures from so far off.really,that is a good thought since it is not the most easy thing to detect intelligent alien life at a great distance as we all have come to know.but as to the 1,580 light years part i must ad my voice to ljk’s…where did you come up with such a figure?! thank you very much my friend,heck you could have an idea there we have not thought of yet and it might be a really good one! such things have been known to happen. well any how thanks, george
I can understand that the calculations used to postulate earth like planets are educated guesses at best at this point. One of the things that I think we are underestimating is the probability of moons of larger planets being viable as life bearing and habitable. The discussion of moons being a necessary ingredient in the “habitability soup” because of plate tectonics pretty much takes care of itself if the habitable “planet” is actually a moon. Just because there are no earth size moons in our system, by no means diminishes the likelihood of the widespread existence of water bearing satellites in a habitable zone that may easily include so – called super Jupiters. I postulate that the masses of the planets we are discovering, must by nature of the way we a finding them would include the mass of the satellites around them, which even in our Jupiter’s case would be including the mass of about three earths or more (I don’t have the figures on the masses of Jupiter’s satellites). I think we have little or no chance with current technology to locate satellites of these newly discovered planets. Perhaps someday.
I understand there seems to be a correllation between planetary mass and (total) satellite mass, so if that holds true, then some ‘super-Jupiters’ may be expected to have some Earth-like moons (‘moon of Endor’ idea). However, close to the giant planet radiation may be a problem as is the case with our Jupiter.
In fact there has been at least one serious publication on terrestrial moons of ginat planets, I saw it but forgot where (I think astrophysical abstracts).
And I wonder if it wouldn’t be very hard (practically impossible with present/foreseeable technical means) to distinguish the spectral signature of such a moon from its mother planet.
I posed this issue before under a coronograph post.
There have been papers on the detection of exo-moons, but the method generally relies on directly observing the planet (which is a difficult task in itself!) and then detecting eclipses/transits of moons.
As for whether habitable moons are possible, the scaling relation for our solar system suggests that super-Jupiters might be expected to host moons up to a few times the mass of Mars (which might be sufficient for habitability). Of course, many known exoplanets orbit stars with greater metallicity than our sun. What that would do for moon formation, I don’t know – there’d be more material available to form moons, however the primary limiting process on moon mass in models seems to be orbital migration causing them to spiral into their parent planet (which, judging by the occurrence of hot Jupiters is favoured in high metallicity systems).
Also, if jovian planets form beyond the ice line, their moons probably would too, which would give them high volatile contents. This might make large moons ocean worlds if they end up in the habitable zone.
Well, we all know that the distribution of planetary systems around 10,000 light years from the galactic center is way higher than regions 40,000 light years away from the center, so the average distance between two habitable planets might range from 500-4000 light years. I think the possibility of life exists in the region around 5000 light years from the super massive black hole is very low.