A Bit More Detail

Proxima Centauri is the star other than the Sun nearest to us, a mere 4.2 light years away. The star is also commonly known as Alpha Centauri C, for the majority of recent studies suggest that Proxima is bound in a distant orbit a half-million years long around the A-B pair. A and B are Sun-like stars; Proxima, in contrast, is a dim red dwarf, with a total luminosity fractions of a percentage of Sol’s even when it flares.

Centauri Dreams has made a couple of posts about Proxima. In the post yesterday, Paul Gilster discusses the nature of Proxima’s orbit around A-B, and the apparently emerging consensus that Proxima does orbit the A-B pair in a distant orbit. If Proxima is gravitationally bound, this has implications for potential life on planets orbiting either star.

Given our age estimates of these stars, that would mean Proxima has orbited Centauri A and B roughly 6500 times. Its presence, note Laughlin and Wertheimer, introduces a mechanism for dislodging comets from outer orbits and pushing them into the inner system(s), allowing for the water they might otherwise lack. Even in terms of astrobiology, then, Proxima Centauri may play a role in making planets around Centauri A and B interesting, not to mention what it offers up in its own right.

What of planets orbiting Proxima Centauri itself? No planets have been definitively sighted in orbit of Proxima, but the state of the astronomical art does set upper limits on the worlds that could orbit it.

p[A] planet larger than Neptune could conceivably still be there around Proxima Centauri, but the odds do not favor it. We also learn from Endl and Kürster that no super-Earths have been detected larger than about 8.5 Earth masses in orbits with a period of less than 100 days. As for the habitable zone of this star — thought to be 0.022 to 0.054 AU, which corresponds to an orbital period ranging from 3.6 to 13.8 days — we can rule out super-Earths of 2-3 Earth masses in circular orbits. Here we pause again: The authors stress that their mass limits apply only to planets in circular orbits. Planets above these mass thresholds could still exist on eccentric orbits around this star.

So no planets yet around Proxima Centauri, and we’re beginning to rule out entire categories of planet here. We also have the possibility of smaller worlds in interesting orbits. The encouraging thing is that the radial velocity work on Proxima is getting better and better, and the authors see us closing in on planets of Earth size[.]

Might an Earth-mass planet orbit Proxima in the habitable zone? Sure, but it would be exposed to flares, great upsurges of radiation that would complicate life.

More, doubtless, to come.

Centauri Dreams’ Paul Gilster reports on the search for Earth-size–and potentially Earth-like–planets orbiting one of the stars of Alpha Centauri, nearest star system to our own. Alpha Centauri A and B are both broadly Sun-like stars, A brighter than B, are roughly as old as our sun, and computer models suggest that rocky planets could form around the two stars and enjoy stable orbits. According to Gilster, the discovery is just a matter of time and money.

A warm and cozy planet around the K-class Centauri B would be just the ticket, and the planet hunt continues. One thing we’ve learned in the past decade is that neither Centauri A or B is orbited by a gas giant — planets of this size should have shown up in the data by now. We’ve also learned that stable orbits reach out maybe 2 AU from either star. Remember that while Centauri A and B are separated by almost 40 AU at their widest point, they close to within 11 AU, thus disrupting outer orbits, as demonstrated by computer simulations. We should expect planets, if they exist, to be no further out than the main asteroid belt in our own system.

Debra Fischer (Yale University) has been working on the Alpha Centauri problem at Cerro Tololo (Chile) in addition to her efforts at improving instrument sensitivity for planet hunting at the Keck and Lick observatories. The goal is to reach the precision needed to turn up planets the size of the Earth with radial velocity methods. If we’re going to get a Centauri detection, odds are it favors Centauri B because A does not seem to be as stable as B, and the latter is more likely to be the first to yield what Fischer calls the ‘tiny whisper’ that would flag an Earth-like world. Usefully, the 79 degree orbital plane of these stars means that planets in this system, assuming they share this tilt, should be generating a reflex velocity close to the line of sight from the Earth.

Radial velocity methods, in other words, should work here if we can attain sufficient sensitivity. The detection effort calls for telescope time at the Cerro Tololo Inter-American Observatory this spring and summer, and The Planetary Society is campaigning to raise money to support the effort. What Fischer needs is 20 nights of observing time, but the team’s NASA and NSF grants cannot be used to pay for telescope time, which at Cerro Tololo runs to $1650 per night. A total of $33,000 will do it, then, money the community should be able to raise. Have a look at the Planetary Society’s donation page and let’s see if we can’t make this happen.

Anyone involved with The Planetary Society is probably already aware of Fischer’s work with astronomer and Tau Zero practitioner Geoff Marcy (UC-Berkeley) on FINDS Exo-Earths (Fiber-optic Improved Next generation Doppler Search for Exo-Earths). The collaboration has resulted in a high-end optical system installed on the 3-meter Lick Observatory telescope and is now feeding the FINDS 2 effort to provide advanced optics for the Keck Observatory in Hawaii. Marcy and Fischer are working with a fiber optics array that adjusts light entering the telescope’s spectrometer and an adaptive optics system that offers the best signal to noise ratio.

FINDS worked out well at the Lick Observatory, improving the ability to detect Doppler velocities from the pre-existing 5 meters per second down to the 1 meter per second range, allowing us to detect smaller planets. Fischer and Marcy are hopeful of attaining precisions down to 0.5 meters per second with their work at Keck, which should get us into the range of Earth-sized planets. FINDS 2 will then be used with Keck to provide follow-up data about planets found by the Kepler mission, ruling out false positives in the ongoing hunt for planets like our own. The work on FINDS has led directly into the commissioning of a new spectrometer at Cerro-Tololo.

• A BCer in Toronto’s Jeff Jedras argues that the Liberal Party should try to become the party of federalists, inside Québec particularly.
• Centauri Dreams links to astudy suggesting that elliptical galaxies, older galaxies with less dust than our Milky Way, could still support planets and potential life.
• Geocurrents reports on the various problems–economic, environmental, political–facing the timber industtry in the Russian Far East.
• Marginal Revolution’s Tyler Cowen questions Ross Douthat’s arguments about the decline of religious practice and its imports in the United States by wondering how, given the social and economic changes of the post-war period, this could have been prevented.
• Naked Anthropologist Laura Agustín takes issue with a recent New York Times article on the sex trade in Spain. Unquestioned narratives are not good analysis.
• At Personal Reflections, Paul Belshaw considers definitions of the Enlightenment and civilization as seen from different places–West versus non-West, England versus Scotland–with links.
• Registan’s Nathan Hamm comments on the unseemly ties between Susan G. Komen Uzbekistan Race for the Cure, a breast cancer charity that recently featured in the American culture war, and various charities run by Gulnora Karimova, daughter of Uzbekistan’s dictator.
• Torontoist’s Jamie Woo makes the point that Rob Ford’s disinterest in doing anything with Pride doesn’t speak to his being very up-to-date.
• Kenneth Anderson at the Volokh Conspiracy notes that the background of the emergent war between the Sudans over oil pipelines proves that clear property rights can diminish conflict.

The latest news item announcing that the experiments used by the Vikings Mars landers of the 1970s to determine if there might be Mars actually did detect life, just life that we did know how to identify back in the 1970s before we learned of the extremophiles of Earth, something that Livejournaler absinthe-dot-ca linked to, just as james-nicoll did. The former linked to the paper, “Complexity Analysis of the Viking Labeled Release Experiments”.

The only extraterrestrial life detection experiments ever conducted were the three which were components of the 1976 Viking Mission to Mars. Of these, only the Labeled Release experiment obtained a clearly positive response. In this experiment 14C radiolabeled nutrient was added to the Mars soil samples. Active soils exhibited rapid, substantial gas release. The gas was probably CO2 and, possibly, other radiocarbon-containing gases. We have applied complexity analysis to the Viking LR data. Measures of mathematical complexity permit deep analysis of data structure along continua including signal vs. noise, entropy vs.negentropy, periodicity vs. aperiodicity, order vs. disorder etc. We have employed seven complexity variables, all derived from LR data, to show that Viking LR active responses can be distinguished from controls via cluster analysis and other multivariate techniques. Furthermore, Martian LR active response data cluster with known biological time series while the control data cluster with purely physical measures. We conclude that the complexity pattern seen in active experiments strongly suggests biology while the different pattern in the control responses is more likely to be non-biological. Control responses that exhibit relatively low initial order rapidly devolve into near-random noise, while the active experiments exhibit higher initial order which decays only slowly. This suggests a robust biological response. These analyses support the interpretation that the Viking LR experiment did detect extant microbial life on Mars.

At best, this is provocative stuff, and makes the case for a followup mission to Mars. Comments in Jason Major’s Universe Today item make the point that retroactive analyses of the data are great at picking up patterns, just patterns that are imposed by the researchers combing through the data again as much as patterns that actually exist. One of the authors of the paper, Gilbert Levin, designed some of the Viking probes’ life-detection experimental kit and has since argued at length that NASA scientists almost went of their way to interpret the evidence as proof of an absence of life.

I think that io9 may have overreached in titling a post. The post “Why we won’t find Earth-2 around a red dwarf star” links to a very interesting paper regarding unconsidered problems facing potentially Earth-like planets around red dwarf stars, “Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating” by Barnes, Mullins, et al., but the paper consider a specific known exoplanet orbiting a red dwarf–Gliese 667C c, covered by me back in February here–and concludes that it could be habitable after all.

What’s going on? It all has to do with the habitable zones around stars, the set of orbits in which a planet could plausibly support an Earth-like climate friendly to liquid water. Traditional calculations of a habitable zone have considered the radiant energy produced by a star. For red dwarfs–dim, low mass stars–a planet in the habitable zone would be closely bound by gravitation to its star, quite possibly with one side forever facing its sun in much the same way that one side of the Moon forever faces the Earth and the other forever faces away. This degree of tidal locking wouldn’t prevent such a planet from being habitable, as atmospheric models suggest that an atmosphere only slightly denser than Mars would be capable of transporting enough heat to prevent the planet’s atmosphere from freezing on the dark side. Other constraints, however, might exist. The authors identify the heat produced by the gravitational tides exerted by a star on such a close planet as a major source of heat.

As a planet moves from periastron, its closest approach to the star, to apoastron, the furthest point, and back again, the gravitational force changes, being inversely proportional to distance squared. This difference creates an oscillating strain on the planet that causes it to undergo periodic deformation. The rigidity of the planet resists the deformation, and friction generates heat. This energy production is called tidal heating.

Tidal heating is responsible for the volcanism on Io (Strom et al. 1979; Laver et al. 2007), which was predicted, using tidal theory, by Peale et al. (1979). Io is a small body orbiting Jupiter with an eccentricity of 0.0041, which is maintained by the gravitational perturbations of its fellow Galilean moons, that shows global volcanism which resurfaces the planet on a timescale of 100 – 105 years (Johnson et al. 4 1979; Blaney et al. 1995; McEwen et al. 2004). The masses of Jupiter and Io are orders of magnitude smaller than a star and terrestrial exoplanet, and thus the latter have a much larger reservoir of orbital and rotational energy available for tidal heating. Moreover, some exoplanets have been found with orbital eccentricities larger than 0.9 (Naef et al. 2001; Jones et al. 2006; Tamuz et al. 2008). Thus, the tidal heating of terrestrial exoplanets may be much more effective than on Io (Jackson et al. 2008c,a; Barnes et al. 2009a, 2010; Heller et al. 2011). This expectation led to the proposition that terrestrial exoplanets with surface heat fluxes as large or larger than Io’s should be classified as “Super-Ios”, rather than “Super-Earths” (Barnes et al. 2009b).

The authors go on to calculate that it’s quite possible for some planets closely orbiting red dwarf stars, especially worlds orbiting low-mass red dwarf stars (less than 20% the mass of our sun, perhaps) and worlds with very eccentric orbits, to be located within the “classical” habitable zone of their star but nonetheless be so heated by the tidal forces exerted by their star as to become “Tidal Venuses”, becoming superheated worlds which lose their water to evaporation in space in just hundreds of millions of years. The aforementioned Gliese 667C c is not likely to be such a planet, according to the team’s calculations, as its orbit is too distant. Other worlds, as yet undiscovered, may not be so lucky.

HIP 11952 has made the news recently for its planets. This star 375 light-years away is noteworthy since, although it’s broadly similar to the Sol mass-wise–it’s only one-sixth less massive than our Sol, perhaps comparing to nearby famous Tau Ceti–it is a remnant of the very early universe. Our sun is 4.57 billion years old, our universe 13.75 billion (plus or minus 110 million years), but HIP 11952 formed in the youth of the universe, and is 12.8 billion years old. Because of its extreme age, when HIP 11952 was formed in the infant Milky Way it did so in an environment where there just hadn’t been enough nuclear fusion in stars to make the elements heavier and helium that dominate our terrestrial worlds; spectrograms indicate that this Population II star has 1% of the “metals” of our own sun. There was a consensus in astronomy that stars with so few metals shouldn’t have been able to support the formation of planets.

But HIP 11952 has two Jovian-mass planets regardless.

It is widely accepted that planets are formed in disks of gas and dust that swirl around young stars. But look into the details, and many open questions remain – including the question of what it actually takes to make a planet. With a sample of, by now, more than 750 confirmed planets orbiting stars other than the Sun, astronomers have some idea of the diversity among planetary systems. But also, certain trends have emerged: Statistically, a star that contains more “metals” – in astronomical parlance, the term includes all chemical elements other than hydrogen and helium – is more likely to have planets.

This suggests a key question: Originally, the universe contained almost no chemical elements other than hydrogen and helium. Almost all heavier elements have been produced, over time inside stars, and then flung into space as massive stars end their lives in giant explosions (supernovae). So what about planet formation under conditions like those of the very early universe, say: 13 billion years ago? If metal-rich stars are more likely to form planets, are there, conversely, stars with a metal content so low that they cannot form planets at all? And if the answer is yes, then when, throughout cosmic history, should we expect the very first planets to form?

Now a group of astronomers, including researchers from the Max-Planck-Institute for Astronomy in Heidelberg, Germany, has discovered a planetary system that could help provide answers to those questions. As part of a survey targeting especially metal-poor stars, they identified two giant planets around a star known by its catalogue number as HIP 11952, a star in the constellation Cetus (“the whale” or “the sea monster”) at a distance of about 375 light-years from Earth. By themselves, these planets, HIP 11952b and HIP 11952c, are not unusual. What is unusual is the fact that they orbit such an extremely metal-poor and, in particular, such a very old star!

For classical models of planet formation, which favor metal-rich stars when it comes to forming planets, planets around such a star should be extremely rare. Veronica Roccatagliata (University Observatory Munich), the principal investigator of the planet survey around metal-poor stars that led to the discovery, explains: “In 2010 we found the first example of such a metal-poor system, HIP 13044. Back then, we thought it might be a unique case; now, it seems as if there might be more planets around metal-poor stars than expected.”

What does this mean for planetary formation? What are the minimum requirements for planetary formation? When did the first planets in our galaxy form?

Centauri Dreams’ Paul Gilster notes that this has implications from the perspectives of galactic history and the possibilities of extraterrestrial life. If planets formed so early in the universe’s history, well, where is everybody?

The idea of ‘deep time’ exerts an abiding fascination. H.G. Wells took us forward to a remote futurity when his time traveler looked out on a beach dominated by a red and swollen Sun. But of course deep time goes in the other direction as well. I can remember wanting to become a paleontologist when I discovered books about the world of the dinosaurs, my mind reeling from the idea that the world these creatures lived in was as remote as any distant star. Paleontology was a grade-school ambition I never followed up on, but the Triassic and Jurassic eras still have a hold on my imagination.

In a SETI context, deep time presents challenges galore. Charles Lineweaver’s work offers up the prospect that the average Earth-like planet in our galactic neighborhood may well be far older than our own — Lineweaver calculates something like an average of 1.8 billion years older. Would a civilization around such a star, if one could survive without destroying itself for so long, have anything it wanted to say to us? Would it have evolved to a level where it had merged so completely with its environment that we might not be able to recognize its artifacts even if we saw them?

[. . .]

Are there, then, more planets around metal-poor stars than we have previously thought? We need to find more planetary systems in this age bracket to learn more, but Anna Pasquali (Heidelberg University), a co-author of the paper on this work, says “The discovery of the planets of HIP 11952 shows that planets have been forming throughout the life of our Universe,” a thought that reminds us to be careful about drawing hasty conclusions about planet formation. Don’t be surprised, either, if the idea doesn’t once again bring Dr. Fermi knocking at the door.

A commenter at Centauri Dreams suggests that gas giants, which are made substantially of the non-metals that were in exclusive abundance early in the universe’s history, would likely form more easily than rocky planets like the Earth. Against this, a commenter at the first news link wonders–jokingly–if HIP 11952 is the home system of the Vorlons.

Universe Today’s Nancy Atkinson shares the awesome news.

Could there be ‘tens of billions’ of habitable worlds in our own galaxy? That’s the results from a new study that searched for rocky planets in the habitable zones around red dwarf stars. An international team of astronomers using ESO’s HARPS spectrograph now estimates that there are tens of billions of such planets in the Milky Way galaxy, with probably about one hundred in the Sun’s immediate neighborhood, less than 30 light years away.

“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet,” said Xavier Bonfils, from IPAG, Observatoire des Sciences de l’Univers de Grenoble, France, and the leader of the team. “Because red dwarfs are so common — there are about 160 billion of them in the Milky Way — this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone.”

This is the first direct estimate of the number of smaller, rocky planets around red dwarf stars. Add this to another recent finding which suggested that every star in our night sky has at least one planet circling it — which didn’t include red dwarf stars – and our galaxy could be teeming with worlds.

This team used the HARPS spectrograph on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile to search for exoplanets orbiting the most common kind of star in the Milky Way — red dwarf stars (also known as M dwarfs). These stars are faint and cool compared to the Sun, but very common and long-lived, and therefore account for 80% of all the stars in the Milky Way.

The HARPS team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths (planets with masses between one and ten times that of Earth) were found, including two inside the habitable zones of Gliese 581 and Gliese 667 C respectively.

By combining all the data, including observations of stars that did not have planets, and looking at the fraction of existing planets that could be discovered, the team has been able to work out how common different sorts of planets are around red dwarfs. They find that the frequency of occurrence of super-Earths in the habitable zone is 41% with a range from 28% to 95%.

Bonfils and his team also found that rocky planets were far more common than massive gas giants like Jupiter and Saturn. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1000 times that of the Earth).

However, the rocky worlds orbiting red dwarfs wouldn’t necessarily be a good place to spend your first exo-vacation – or for harboring life.

“The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the star than the Earth is to the Sun,” said Stéphane Udry from the Geneva Observatory and member of the team. “But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely.”

• 80 Beats reports that NASA’s Mercury probe Messenger has determined that the innermost planet in our solar system is almost entirely solid iron, with a much thinner mantle and crust than had been believed before.
• Centauri Dreams describes how self-replicating probes might set up–might already have set up?–an interstellar communications network, slowly spreading out from a point.
• Daniel Drezner makes the point that books claiming to trace the origins of economic prosperity in certain policies can’t be overly reductive–how did North Korea keep up economically with South Korea until the mid-1970s, for instance?
• Extraordinary Observation’s Rob Pitingolo is unimpressed by playwright/performer Mike Daisey’s claims that, notwithstanding actual errors of facts and near-certain lies on his part in his piece on workers issues at an Apple manufacturer’s plant in China, he speaks to a deeper truth.
• Geocurrents reports on conflicted responses to immigrant childrearing practices in Norway and Argentina’s Tierra del Fuego electronics manufacturing industry.
• Language Hat reports that defenders of Chomsky’s theory of language are responding to anthropologist Dan Everett’s apparent disproof of Chomsky’s thesis with the language of the Piraha by getting him banned and calling him a racist. Not cool.
• Steve Munro links to and summarizes a recent city report making the case for light rail in Scarborough, as opposed to subway extension.
• Torontoist points out that Rob Ford’s call for a referendum on subway construction was legally ill-founded and near-pointless.
• Towleroad links to a neat video on life on the isolated South Atlantic island of Tristan da Cunha.

“Clarke’s fourth law” is the name of Gerry Canavan’s post linking to the blog Next Nature’s post “Any Sufficiently Advanced Civilization is Indistinguishable from Nature”. In the titles of their posts, the two blogs are referring to Clarke’s three laws.

1.When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.

2. The only way of discovering the limits of the possible is to venture a little way past them into the impossible.

3. Any sufficiently advanced technology is indistinguishable from magic.

Next Nature suggests–briefly–that as human technology advances it will stop being a force distinguishable from and opposed to nature, but will instead make use of nature’s techniques and environments in more harmonious ways.

Western cultures, nature is a cosmological, primal ordering force and a terrestrial condition that exists in the absence of human beings. Both meanings are freely implied in everyday conversation. We distinguish ourselves from the natural world by manipulating our environment through technology. In What Technology Wants, Kevin Kelly proposes that technology behaves as a form of meta-nature, which has greater potential for cultural change than the evolutionary powers of the organic world alone.

With the advent of ‘living technologies’, which possess some of the properties of living systems but are not ‘truly’ alive, a new understanding of our relationship to the natural and designed world is imminent. This change in perspective is encapsulated in Koert Van Mensvoort’s term ‘next nature’, which implies thinking ‘ecologically’, rather than ‘mechanically’. The implications of next nature are profound, and will shape our appreciation of humanity and influence the world around us.

The Universe of Things, by the British science fiction writer Gwyneth Jones (2010) takes the idea of an ecological existence to its logical extreme. She examines an alien civilization whose technology is intrinsically alive. Tools are extrusions of the alien’s own biology and extend into their surroundings through a wet, chemical network.

The idea of existing in a vibrant, organic habitat is an increasingly realistic prospect as living technologies are now being designed to counter the ravages of global industrialization. These can even be implemented at a citywide scale. For example, Arup’s Songdo International Business District, in South Korea, is being built on 1,500 acres of land reclaimed from the Yellow Sea. Incorporating rainwater irrigation and a seawater canal, this design suggests that the building industry is aspiring to use living technologies to revitalize urban environments via geoengineering. The Korean artist Do Ho Suh had proposed to build a bridge that connects his homes in Seoul and New York by harnessing natural forces and using synthetic biologies to literally ‘grow’ a trans-Pacific bridge.

The apparent science fictional nature of ecological-scale projects has prompted science fiction author Karl Schroeder to observe that the large-scale harnessing of ecologies might explain our current lack of success in encountering advanced alien civilizations. Schroeder explains the Fermi Paradox – the apparent contradiction between the likelihood that extraterrestrial civilizations exist and the lack of evidence for them – by speculating that we have not yet encountered our cosmic neighbors because they are indistinguishable from their native ecology.

This imagining of a technology that surpasses our crude mechanisms to make use of the dynamics of life itself is common, for which see the organic technologytof Babylon 5‘s Vorlons and Shadows, or of 2300AD‘s Pentapods.

My quibbling with the paradigm of superior organic technology is, firstly, that ecologies are dynamic systems which can be shaped profoundly by the actions of component species and the nature of their changing environments, and secondly, that the organic/technological distinction is increasingly arbitrary. Do living cells already make use of nanotechnology, for instance, with their chemical solvents and autonomous mechanisms? Isn’t nanotechnology being shaped by models from the living world? Also, are we at all justified in making any claims about the nature of galactic ecologies, inasmuch as we’re only beginning to detect planets and their environments and developing informed speculations about non-Earth environments and ecologies? Why wouldn’t disequilibria of some scale be present in any ecology, inasmuch as even virgin ecosystems see shifting imbalances of predator and prey? Et cetera.

What say you?

Space Daily’s John Rehling shares news of a recent analysis of data from the Kepler space telescope, charting extrasolar planets by the thousands, which suggests that Earth-like worlds are–contrary to earlier analyses–relatively rare. The Kepler program, which basically detected planets via their eclipses of their stars, was biased towards the detections of close-orbiting worlds and of large worlds (larger than the Earth). It turns out that doesn’t mean that Earth-like worlds, of the right size and in the right orbit, are common.

This release shows two favorable trends. As a statement from the Kepler team puts it, “With each new catalog release a clear progression toward smaller planets at longer orbital periods is emerging. This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant.”

However, the fine details of this latest release favor more pessmistic projections. Although the release shows, in comparison to the previous release, one favorable trend regarding planet size and another favorable trend regarding planet distance from the star, these trends are, unfortunately, unfolding in an either-or respect: We see more Earth sized planets which are very close to their stars, and therefore likely very hot; and, separately, we see more giant planets which are located farther out from their stars.

This table shows the estimated frequency of planets per star in each bin [category]. The planet sizes are presented in terms of Earth radius, Re, while the orbital periods are in days (d). The bins are populated with frequency estimates based on the de-bias factor, times the observed candidate count, where that count was 4 or greater. In addition, more tentative numbers appear in parentheses in certain bins, based on a smaller n or the n that would have resulted from a single discovery. In each row, the maximum value appears in bold.

This shows that for each terrestrial planet size catgeory, we have observed the frequency max out at a very short period between 4 and 16 days, then exhibit decreasing frequency for longer periods.

[. . .]

For the bin corresponding precisely to the Earth, the projected frequency is 0.7%, a far cry from Traub’s projection of 34%, owing in part to differences in the size of the bin: If we consider the 3×3 collection of bins that surround Earth’s bin, the current projection rises to 7.8%. By including also the smaller Mars-sized planets, to 9.0%.

This does disprove some earlier optimistic estimates, although these estimates were made without any data at all.

In a memorable scene of Carl Sagan’s Cosmos, Sagan considers the Drake Equation and estimates that perhaps 1/4 of all stars have planets and that each such system might have two planets suitable for the development of life. That estimate was recorded before any planet outside our solar system had been discovered.

With the current Kepler release, we find that the number of earthlike planets per star is likely to be considerably lower than the 0.5 implied by Sagan’s estimate. Of course, the picture is considerably complicated: We may find habitable worlds, whether earth-like or Europa-like, orbiting giant planets. We may find earth-like worlds harbored as Trojans of giant planets in the habitable zone.

The frequency as a function of orbital period may have a second peak or flatten and fail to drop with still longer periods. Comfortable temperatures may also be found at planets close in to dim red dwarf stars, although that may likely result in the world being tidally locked, and therefore subject to one daylight hemisphere and one in eternal night. And if nothing else, the pessmistic characteristic of these results suggest that to find earth-like worlds elsewhere, we should prepare to look hard – and quite possibily very hard for decades if not centuries.