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How Common Are Potential Habitable Worlds In Our Galaxy?

Centauri Dreams welcomes Ravi Kopparapu, a research associate in the Department of Geosciences at Pennsylvania State University. He obtained his Ph.D in Physics from Louisiana State University, working with the LIGO (Laser Interferometer Gravitational-wave Observatory) collaboration. After a brief stint as a LIGO postdoc at Penn State, Ravi switched to the exoplanet field and started working with Prof. James Kasting. His current research work includes estimating habitable zones around different kinds of stars, calculating the occurrence of exoplanets using the data from NASA’s Kepler space telescope, and understanding the bio-signatures that can potentially be detected by future space telescope missions. Dr. Kopparapu’s website is http://www3.geosc.psu.edu/~ruk15/index.shtml.

by Ravi Kopparapu


Imagine this scenario: You are planning to buy a new house in a nice neighborhood. The schools in the area are good, the neighborhood is very safe, but you want to know the ‘kid friendly’ area (so that your kids can have friends). You drive around, looking at the available houses, watching for any ‘kid signatures’. You notice that a good proportion of the homes in your neighborhood show some ‘potential’ to have kids. Based on your observation, you estimate the percentage of houses with kids.

A very similar process is currently being carried out in the field of ‘exoplanets’: planets orbiting other stars. The past two decades have seen a rapid increase in the discoveries of exoplanets (although, if you follow the International Astronomical Union’s definition of a planet, exoplanets are not technically ‘planets’. But that discussion is for another time). Just this year, the number of confirmed exoplanets has almost doubled in number. The discoveries in the first decade and a half were dominated by the radial or Doppler velocity technique, where an orbiting planet causes the star to wobble, and the light from the star, split into various colors, also wobbles, providing some clues about the mass and orbital period of the planet. But the floodgates to the exoplanet discoveries really opened up after the launch of NASA’s Kepler space telescope in March 2009.

Kepler finds planets using the ‘transit’ method: A planet crossing in front of a star blocks a portion of the star’s surface from the observer’s view, causing the light from the star to decrease proportional to the area of the planet’s disc. Just this year, the Kepler mission has more than doubled the number of exopanets discovered. Although it is easy for Kepler to detect large planets, like Jupiter or Saturn, closer to their stars (because they can block a larger portion of the star’s disc), there has been a huge increase in the Earth-size planet population from this recent discovery. And there has been a modest increase in the Jupiter or Saturn type planet population. That means Kepler probably has detected most of the giant planets that it can detect, and a large population of Earth-size planets are just being discovered.

Now, what is of interest (to me, and hopefully to many of you) is how many of these Earth-size planets are potentially habitable? We do not know if these planets indeed have habitable conditions, but we do know how far away they are from their host star. And if they are at the “right” distance from the star, the so-called ‘habitable zone’ (HZ) where liquid water can be sustained on the surface of a planet with appropriate atmospheric conditions, then they are good candidates for potential habitable worlds. But, how do we estimate the habitable zone (HZ) around a star? ‘Not too hot, not too cold region’ is a nice guess, but a more rigorous approach is needed because we can’t measure a distant exoplanet’s surface temperature. Furthermore, the location of the HZ varies depending upon the star. And we are looking for those kinds of planets, which have similar atmospheric composition as Earth (because we know whatever Earth has, works for life and we know what kind of life signatures to look for). This is where climate models come into the picture.

Recently, I along with my group at Pennsylvania State University and collaborators from NASA Astrobiology Institute’s Virtual Planetary Laboratory, used a climate model to estimate HZs around various kinds of stars. We assumed an Earth-mass planet, with Earth-like composition, and determined the boundaries of HZ for different stars [1]. The results are shown in Figure 1. On the horizontal axis is the amount of ‘starlight’ a planet receives: For example, a value of ‘1’ represents a planet receiving the same amount of light from its star, as Earth does from the Sun. A value of 1.25 implies the planet receives 25% more light than Earth, and a 0.75 indicates it receives 25% less light than the Earth. The vertical axis shows stars of different sizes (or temperatures). Hotter stars are at the top, cooler stars at the bottom. The yellow curve with the label ‘runaway greenhouse’ implies that the planet is so hot (because it is closer to the star), that at that location all the water from the surface of a planet is evaporated and resides in the atmosphere (kind of what happened to Venus billions of years ago). This is the ‘conservative inner edge of the HZ’. There is an `optimistic inner edge of the HZ’, which is shown as the red curve labeled `Recent Venus’. This limit is based on the observation that Venus seems to have lost its water by 1 billion years (Gyr) ago, when the Sun was 8% less bright than today. Basically, an Earth-mass planet in the blue shaded area has a good chance of having liquid water on its surface (if it has the right atmospheric conditions), and it may have some water if it lies in the red shaded region.

[1] We also changed the planet mass to see how the HZ changes. Assuming similar composition as Earth, larger planets have wider HZs than do smaller ones.


Figure 1: Habitable Zones around various stars. The horizontal axis indicates the amount of starlight a planet receives compared to the Earth: A value of 0.75 implies the planet receives 25% less light than Earth does from the Sun. Similarly a value of 1.25 indicates the planet receives 25% more light and so on. The vertical axis is the star’s temperature. The blue and red shaded regions are conservative and optimistic widths of habitable zones, respectively. Some of the known exoplanets, that are potential habitable worlds, are also shown. Image credit: Chester Harman.

You can see from Figure 1 that many of the Kepler-discovered (and confirmed) exoplanets that are of Earth or near Earth-size reside in the HZ. So we can count them and obtain an answer to our question: “How common are potential habitable worlds?” But that counting doesn’t tell us the whole answer. The Kepler telescope finds a planet when the planet crosses in front of its star, observing the dip in the starlight. Not every planet does this: some planets may be aligned in such a way that they may not be crossing in front of the star. So we may be missing some of them. And we have not confirmed that all the Kepler detected planets indeed are planets, or even Earth-size. To complicate matters, there are some imposters: two stars orbiting around each other can produce a signal as though a planet is orbiting around a star. So one needs to be careful in considering all these issues when calculating the commonness of Earth-size planets.

In the past year, there have been some estimates of the occurrence of Earth-size planets in the HZs. Prof. David Charbonneau and his graduate student Courtney Dressing at Harvard University used Kepler data and calculated that about 15% of low-mass stars, the so-called M-dwarfs that are cool and red, in our Galaxy have Earth-size planets in the HZ. This is a big number and great news! Dressing and Charbonneau have done a phenomenal job of calculating this number. That means nearly 1 out of 7 M-dwarfs in our Galaxy may have a potential habitable planet. But even this number seemed low to me. M-dwarfs are the most prevalent stars in our Galaxy. About 77% of stars in our Galaxy are M-dwarfs. Within 30 light-years of the Sun, there are nearly 250 M-dwarfs (whereas, there are only about 20 Sun type stars). So I recalculated Dressing & Charbonneau’s estimate of the prevalence of potential habitable planets, using my newly determined HZs (Figure 1). And the number I got is a BIG increase from 15%: a conservative estimate showed that about 48% of M-dwarfs should have Earth-size planets in the HZ. That means, nearly 1 out of 2 M-dwarfs (i.e, approximately 50% of M-dwarfs in our Galaxy) may have Earth-size planets in the HZ!

Did you ever get that euphoric feeling when you discover something that is really cool? Well, I was in that moment and nearly jumped out of my chair! 48%, and that is a conservative estimate! I had absolutely no prior expectations of what that number could be, except that it may be bigger than 15%. For the first time in the history of human civilization, we not only know there are Earth-size planets around stars, but also that there are a good number of them that could be habitable!

The next obvious question is: “How common are potential habitable worlds around Sun-like stars?” A recent study by Eric Petigura and Geoff Marcy of University of California at Berkeley (with Andrew Howard at University of Hawaii) estimated that about 22% of Sun-like stars have Earth-size planets in the HZ. That is 1 out 5 Sun-like stars in our Galaxy have Earth-size planets in the HZ. That is an amazing discovery!


Figure 2: Planet size versus the amount of starlight incident on a planet. The green box shows the assumed HZ width in a study by Petigura et al. (2013) to calculate how common are Earth-size planets around Sun-like stars. Image credit: Petigura et al. (2013), Proceedings of National Academy of Sciences, 110, 48.

Petigura and collaborators assumed that for Earth-size planets around Sun-like stars, the inner edge of the HZ is at 0.5 AU, or at a distance where a planet receives 4 times the starlight the Earth receives from the Sun (See Figure 2.). Some people, including me, think that a planet at 4 times the Earth flux will be too hot to have liquid water on its surface. For example, Venus, which is the hottest planet in our Solar system, receives only 2 times the Sunlight that Earth does. And it is not a habitable place. Furthermore, looking at Figure 1 which shows HZs for different kinds of stars, even the most optimistic HZ estimate from climate models (based on the physics of atmospheres) indicates the inner edge of the HZ cannot be much closer than about 1.75 times Earth flux. So certainly 4 times as inner edge is too close to the star!

When we use the correct HZ limits from Figure 1, the Petigura et al. (2013) estimate for potential habitable worlds around Sun-like stars actually drops to about 10% (i.e, 1 out of 10 stars)! That looks like a low number, but note that Petigura et al. do not consider planets smaller than 1 Earth radii (because their analysis method, quite appropriately, is not sensitive to those small planets). But Dressing & Charbonneau, and my analysis of M-dwarfs, does consider planets smaller than 1 Earth-radii. So, if one wants a consistent comparison between all these studies, a reanalysis of the calculations should be made.

If not 22% (or 10%) for potential habitable worlds around Sun-like stars, what number can we expect? Well, recently, along with my colleagues Stephen Kane and Shawn Domagal-Goldman, I published a paper looking at how common Venus-like planets are. And we found that this number is about 45% for Sun-like stars. Figure 3 shows the “Venus Zone”, with some candidate planets detected by the Kepler mission. Interestingly, Petigura et al. find that the planet distribution remains flat at longer periods. What this means is that the number of planets at longer orbital periods remains the same. So if Venus-like planets, which are just near the Earth-like range, are as prevalent as 45% around Sun-like stars, then does it mean Earth-like planets are also as prevalent as 45%? At this point this is speculation based on non-rigorous analysis.


Figure 3: Similar to Figure 1 that shows the Habitable zone, this figure shows the “Venus zone”, the area around a star in which a planet is likely to exhibit Venus like conditions. Some of the candidate planets discovered by the Kepler mission are shown as yellow circles. The size of the circle is compared to the size of Venus. Image credit: Chester Harman.

Returning back to where we started about the analogy of buying a house: Now let’s say you purchased a house. You moved in with your family. You want to introduce yourself to your neighbors. So, you knock on your neighbor’s door. Nobody answers. You knock on your other neighbor’s door. No response. You try every house on your street and in your neighborhood. Silence is all you hear. There is no response from anyone. There are just houses, no people that you can see or talk to. This is more or less the current situation humanity is pondering. We see lots of houses (planets), but haven’t seen life yet. Maybe we needed to look harder. Finding life on a distant planet has a profound importance to humanity. It can unite us to work towards a common goal, and focus more on our strengths than weaknesses. We have to commit ourselves, to invest in technologies and telescopes, which can find inhabited worlds. We know potential habitable planets exist. We know they are quite common. We even know (or will know soon) where they are in our Sun’s neighborhood. What are we waiting for?


Comments on this entry are closed.

  • William Blight October 3, 2014, 11:15

    Let’s call the first life bearing planet we find Eureka.

  • Erik Landahl October 3, 2014, 12:27

    First I want to say to Ravi Kopparapu, excellent article!

    Second, I answer Dr. Kopparapu’s question – “What are we waiting for?” – with pessimism. Caveat – I dearly hope my pessimism is quickly countered by evidence that justifies an optimistic outlook.

    1) The current generation at the controls of civilization’s power structure has been raised to believe that serious space exploration with an accompanying commitment of considerable resources and money is an impractical dream which is not affordable. The current generation in power believes that providing barely enough resources to continue dipping humanity’s toes into the shallow tidepools of space is sufficient. Creating tax breaks for corporations, producing and supplying war provisions, and increasing the amount of purchases by consumers are the actions to which this generation is primarily committed.

    2) The next generation – that which will take control of civilization’s power structure in 10 to 20 years – is deeply committed to virtual reality and the computer-generated wonders and abilities which it provides to society at will. Actual reality, including space exploration, pales by comparison and is accordingly much less important. Increasing the spread, influence and faculty of virtual reality in all spheres of society will be far more important to the power-brokers of the next generation than the expansion of humanity’s presence in space.

  • Andrew LePage October 3, 2014, 14:51

    First I would just like to say what a great article this was and how much I admire Dr. Kopparapu’s work expanding Dr. Kasting’s original work on defining the extent of the habitable zones around stars of different types for planets of various sizes. I have frequently cited this work in my own research over the years.

    I do want to point out, however, that one must be very careful about the interpretation of the work on the occurrence rate of “Earth analogs” by Petigura et al. as well as more recent work by Daniel Foreman-Mackey, David W. Hogg, Timothy D. Morton (see “Exoplanet population inference and the abundance of Earth analogs from noisy, incomplete catalogs”, arXiv:1406.3020, Submitted June 11, 2014). These teams define “Earth analog” as being a planet with a radius between 1 and 2 times that of the Earth. Recent analysis by multiple teams using Kepler data strongly suggests that the majority of planets with radii larger than about 1.5 to 1.6 times that of the Earth are NOT rocky, terrestrial planets. Instead they are most likely mini-Neptunes or gas dwarfs that are distinctly un-Earth-like (save for their size only) and unlikely abodes for life.

    A fuller discussion of this work (especially the latest analysis by Leslie Rogers) with references can be found in the following article:


  • Ole Burde October 3, 2014, 17:00

    What are ”we’ waiting for ?
    What we ?
    Humanity is in this area divided into two distinct subspecies : A big majority that doesn’t realy care , and therefore are not waiting for anything (exept perhabs a certain entertainment value) , and a smallish minority that does care , but is limited in its abilies by the majority . Perhabs more efforts hould be devoted to developing the internal cultural identity of the minority …

  • Adam October 3, 2014, 17:24

    Drew’s concern is certainly worth a mention. Another question is whether such planets have received sufficient water for life – it’s an open question whether Venus is radically different to Earth because it began with a quite different water inventory, as some models of oligarchic accretion imply. A bit more water and it might’ve developed a habitable climate thanks to high albedo permanent cloud masses forming in the sub-solar point. Venus might’ve formed dry.

  • Alex Tolley October 3, 2014, 19:44

    Just like the Drake equation, difference assumptions and approaches result in wildly different results. This article (hat tip to ljk a few post back) gave a figure of just 100 million habitable worlds with complex life. Perhaps that last filter is the major constraint, leaving just 1 in a 1000 stars with such multi-cellular inhabited worlds. If correct, this implies that almost all rocky worlds in the HZ are either sterile or have only microbial life. I’m not sure is that is good or bad, but it does suggest that we need to think about bio-signatures for prokaryotic worlds, possibly even before the evolution of photosynthesis and its giveaway O2 signature. What might that be – methane as a component?

  • Joëlle B. October 3, 2014, 20:09

    @Eric Landahl

    To your second point, I’d say that’s a highly optimistic view of sociocultural evolution. The types of people in control now are the same types of people who have controlled societies since… well, forever. If a commitment is made to virtual reality (based in computational/analytic systems), playing as big a role as you predict, then it would not at all be a bad thing for space exploration (on the contrary it’d have a beneficial effect)–technologies would crossover and supplement one another; virtual reality is a complimentary (and continually contributing) friend of interstellar travel, considering our puny lifespans amongst the behemoth, relatively near-immortal lifespan of stars and their distances from one another.

    The real problem of each generation is much deeper than the human race, ourselves descending from pre-human conditions, all influenced by those damned behemoths. The constructs we develop around our societies are based solely in instinct, to survive, in the most immediate and seemingly convenient ways (though sometimes irrational and not all the time effective).

    Generally, computer cultures exist as a very visible counter-culture to rule of law/alpha ape in the jungle mentality–look at Jaron Lanier (a pioneer of virtual reality): the guy appears completely in a different world than those in power from his generation. (http://en.wikipedia.org/wiki/Jaron_Lanier)

    Virtual reality is not our enemy–just reality itself. I think the more control people feel they have over their lives (which simulated existence could provide or give a taste of), the more divergence we’d see in the way people view their place in space, ultimately encouraging the bringing about of changes they wish to perceive as real. For you, Eric, this may be interstellar travel–for someone else, it could just be life in a palace with servants, concubines and loyal subjects spanning the globe. Why take away either of those desires?–especially if they’re able to be experienced in a simulated environment without affecting others who may not want to be a part of those particular realities themselves?

    There’ll even be people opposed to allowing that possibility, entirely [of simulated existence]. It would appear nothing will ever be as reliable as we want, if we have to rely on others… We just have to proselytize and hope people convert and remain faithful to our cause, if it fails it fails, if it works it works. The blame only has one source, and we’re all a part of it. Escapism is sometimes a forward moving arrow in evolution, sometimes a backwards one.

    Maybe we haven’t seen or heard from our neighbors because once they got fed up with the neighborhood, they figured out all the other properties they had to choose from are, realistically, just as much a pain in the back-end. They asserted a ticket to the easy-way-out.

  • Joëlle B. October 3, 2014, 20:25

    Ole Burde is dead on: the fastest track to getting to where we wanna go is to empower the space culture. Although, it is safe to say that the same problems the space community face now will be amplified internally, instead of externally, eventually. Humanity won’t so easily shed its behavior just because one group has finally achieved what it initially wanted. We do get bored. …Actually, I think space enthusiasts have the biggest problem with looking too far ahead into the future, instead of acting on impulse like other cultures. Physical sciences are deeply based in a notion of realism, but also futurism. Since not everyone can agree on what is real or what the future should look like, I guess the issues we face are natural… unfortunately.

  • Ian October 3, 2014, 22:43

    An interesting and valuable article but at the end of the day what is it saying? I have become more and more convinced that the answer to the Fermi paradox is that the Drake equation term that characterizes the likelihood of intelligent life arising is exceedingly small. There may well be earth analogs out there, and I hope there are, but the probability of there being intelligent occupants seems to me to be vanishingly small. So let’s swallow our political correctness and think about terra forming … there, that should stir up some debate.

  • Ravi Kopparapu October 4, 2014, 8:16

    Thank you for the comments. With regards to the terrestrial planet size limit, as Andrew mentioned, about 1.5 seems to be the upper limit. I did mention in the article that if once takes Petigura’s estimate and use a limit of 1.4, once gets a larger occurrence rate for potential habitable planets around Sun like stars. The Foreman-Mackey et al. Paper obtains a very low estimate from Petigura et al. Data. I still need to study this carefully.

    With regards to Venus forming dry, there was a recent Nature paper by Hamano et al.(2013) which argues that there was a persistent magma ocean for about 100 million years, suggesting that Venus may have formed dry.

  • spaceman October 4, 2014, 12:50

    Thank you for posting! Very interesting paper.

    I remember reading in Lovelock’s “Gaia: A New Look at Life on Earth” that without the moderating influence of life and its associated biogeochemical cycles, the Earth would very likely be a somewhat less intense version of Venus– that is, in the absence of life the atmospheric pressure and temperature would be much higher than they are on Earth today but still significantly lower than they are on Venus today. Here’s his prediction of some of Earth’s basic physical properties if there was no life to help moderate things:

    Surface Temperature: 292 +/- 50 degrees Celsius
    Total pressure: 60 bars
    Atmosphere: 98% CO2

    Here is the same data for Venus: 477 degrees Celsius, 90 bars, 98% CO2

    This is of interest to me because it seems likely that we may find many earth-sized planets in the habitable zone in the coming decades, but without life (as we know it), the planets may likely end up being sterile, less severe versions of Venus. This underscores the importance of trying to determine if life is a bizarre chemical fluke unlikely to be repeated elsewhere in the observable universe or a likely outcome on most watery terrestrial planets in the HZ .

    Many assume that finding extremophiles on Earth raises the likelihood of us finding at least microbial life just about anywhere we look in the Universe, but the fallacy here is that this line of thinking fails to address the likelihood of life arising in the first place. In other words, the extremophiles had to come from somewhere…how likely was the chemical event(s) that gave rise to even the simplest unicellular life forms– is it likely to be repeated on nearly every watery earth-sized planet in the habitable zone of its star, or 1 in 100 of this type of planet, or maybe only one in a trillion? To me, this is the most crucial variable for us to gain a better understanding of in the Drake equation. If life is indeed a chemical fluke, then in addition to mastering interstellar travel we will also have to master terraforming both inside and outside of our solar system in order to survive as a species!

  • Eniac October 4, 2014, 13:18

    Alex: I am curious about how strong a distinction you think there is between “complex” and “non-complex” (?) life, and what the decisive difference is?

    Is it single vs. multicellular? If so, I would like to roll out the old fact that this particular transition has happened independently 14 times that we know of, give or take one.

    Is it prokaryot vs. eukaryot? Besides the obvious fact that these terms are specific to life as we know it, there is no solid reason that “complex” life could not evolve from prokaryots, or alien whatever-karyots.

    Whatever else could it be?

    In my view, all life is complex. The transition from non-life to life is the only clearly demarcated hurdle between habitable planets and technological advances. But what a formidable hurdle it is! Almost certainly the most rational explanation for the Fermi observation.

  • Alex Tolley October 4, 2014, 14:56

    I tend to agree that a majority of the population couldn’t care less about looking for ET life, bacterial or intelligent. Even supposing we do find evidence of non-intelligent multi-cellular-life, after the first dozen or examples there is likely be a collective loss of further interest, especially as there is no easy way to reach it. Only 2-way communication with ETIs might maintain public interest. Recall that congressional hearing about life in space and how poorly briefed the members were when commenting or asking questions of the experts. IMO, this means that funding is always going to be somewhat peripheral, unless something is discovered that would have an impact on society. The discovery of alien artifacts might galvanize spending to assess the threat. But ecosystems, 10’s to 1000’s of light years away, not so much. Find something close to home, and that may be different. But consider, would finding fossils of multi0cellular life on Mars mean more interest than terrestrial paleontology? Would a living ecosystem in Europa draw more attention than life in our ocean depths? After the initial excitement, I doubt it. The US government will look at funding big-science experiments (like the next generation of telescopes) rather like the “super collider” and ask, what is the bang for buck if we fund it. That is what the astronomers need to answer in a convincing way to the funders.

  • Alex Tolley October 4, 2014, 14:59

    @spaceman – I thought Kasting showed that an abiotic Earth would not have a runaway greenhouse like Venus, at least until the sun grew much hotter. Granted that this is based on simple climate models, but still.

  • spaceman October 5, 2014, 11:29

    @Alex– I was not aware of Kasting’s work on what the abiotic earth would look like. Thank you for sharing! Did he write about this in “How to Find a Habitable Planet” or somewhere else?
    If not a runaway greenhouse like Venus, what would an abiotic earth look like prior to when the Sun grows much hotter, according to Kasting?

  • R Kelley October 5, 2014, 11:51

    With respect to the probability of habitable planets:
    1. I am very pessimistic about the possibility of habitable planets around M dwarfs because of:
    a. A recent article talking about the star DG Canum Venaticorum which was found to have a super-flare lasting 11 days with a magnitude over 10,000 times the strongest solar flares.
    b. The fact that planets close enough to the star to be habitable are almost certainly going to be tidally locked. Because of the tidal locking, they will also not have a magnetic field and will be even more vulnerable to super-flares.
    2. With respect to solar-type stars:
    a. The article “A Statistical Reconstruction of the Planet Population Around Solar-Type Stars” by Eric Gaidos is more optimistic than the Foreman-Mackey paper. However, what is found most interesting about the Gaidos paper is that it points out strongly that Kepler was simply not sensitive enough to detect planets of 1.4 Earth radii and smaller in the habitable zone. Thus, it seems most work on the issue in question simply involves very educated extrapolations.
    b. The article “Terrestrial Planet Formation in the Presence of Migrating Super Earths” by Sean Raymond discusses the most common type of solar system (which contains super-earths close in to the star but no gas giants). It appears this type of solar system would have material in the habitable zone which could condense into terrestrial planets if either (i) the close-in super-earths and other bodies are formed in-situ (which he considers unlikely) or (ii) the close-in super-earths migrated very quickly through the terrestrial planet zone in the original migration process. However, if the migration process was slow (1,000,000 or more years), the material which could condense into terrestrial planets in the habitable zone will probably be dragged along with the super-earths into a close orbit around the star.

    What we really need is the Terrestrial Planet Finder which may never get funded by Congress.

  • Ole Burde October 5, 2014, 16:18

    If we are planning to have sucses in freeing man from his birthplanet , it won’t do to build our plans on finding alien artifacts or even telescopic evidence of life chemistry . The probability of theese events are compeletly unknown , impossible to calculate , and so useless for any planning purpose .
    IF something of the kind should happen ,it would for our purposes be a case of good luck , and ofcourse present powerful new possibilities to achieve a temporary boost in funding , until the thing lost its entertaiment value .
    What CAN be counted on for planning purposes is the steady support from the minority who cares , and this minority might have a much bigger potential capability in itself than has been achieved sofar .
    No wellorganized lobbying has been achieved , no concerted effort to influence the media , no build up of independant financial structures .. and as many other minorities it is in constant danger of being split up in warring factions acording to cultural-political afiliations …

  • advanced deep space propeller October 6, 2014, 3:51

    @ Ravi; the arxiv link for the venus like planets goes somewhere but not to the paper ;)

  • Ole Burde October 6, 2014, 4:34

    What about the James Webb telescope ? I have heard conflicting opinions about its capablities , with or without a far-out starshade of afordable proportions , so who’s right ? is it astronomys biggest ever white elefant in an era where finding an earth-twin seems (to me!) seems to be the most important objective , or is it astronomys best investment in the future ?

  • kzb October 6, 2014, 7:53

    Another interesting fact I learned about Venus recently:
    its net solar energy gain per unit area is LESS than that of Earth. This is because of its much greater albedo.
    Probably old news to you folks but I thought it was interesting…

  • spaceman October 6, 2014, 8:00

    Although there are a number of unresolved issues regarding the viability of M-stars as habitable planet hosts, what about K-stars? K-stars are not as common as M-stars, but they certainly exist in larger numbers than G-stars even though I know that their HZs are smaller than G-type stars. These orange stars probably have fewer big flares than M-stars, from what I remember reading years ago. Are K-stars likely to be as good, better, or worse candidates for hosting habitable planets than G-stars?

  • RobFlores October 6, 2014, 13:10

    The likelihood of finding Microbial life is probably too optimistic.

    Even if advanced single celled organism arose from a new world.
    It does not necessarily follow that it will be long lived. One
    strike from 45KM asteroid will sterilize all life forms, even those
    dwelling deep in the crust.
    We have only one example of relatively stable biosphere, it does not
    follow that most planets biosphere stay that way.
    The Kepler program cannot definitively prove that there is an abundance of Near Earth Twins out there, due to is large gap in it’s capabilities.
    Speaking of which I don’t think from the Kepler data that a final determination of the average eccentricity of planets has been made.
    In our system Mars and Mercury come to mind. Just as these planets are off kilter, it’s probably just chance that the Earth has a very slight eccentricity

  • ljk October 6, 2014, 15:01

    A Centauri Dreams article on possible habitable worlds around K class stars:


    And a rather recent one on F class stars:


    And why not white dwarfs while we are at it:


  • ljk October 6, 2014, 15:22

    One thing to consider: While certain types of stars may not be very friendly to native life forms as we know it, there could be advanced ETI visiting said star systems to survey, mine, terraform for colonization, fuel up, etc. Certain possibilities might be transmitting to other systems for various reasons such as data return to the home world and such.

    I know with our currently undersupported SETI programs it is probably too much to survey basically just about every star system in the galaxy, but perhaps we could thin things out a bit by targeting systems with, say, lots of planetoids and comets such as Tau Ceti. Even their relative youth may not be a deterrence to interstellar beings who want to exploit their resources or hide out during an interstellar conflict.

    Let’s be creative, folks! After all, these are hypothetical aliens and our scientific conjectures are often just this side of being WAGs. Plus SETI is pretty cheap, even properly equipped amateurs can join in, and who knows what else we might discover astronomically in the process. There are many, many star systems which professional astronomers have barely examined, don’t be fooled by their press releases.

    Regarding my comments about amateurs being able to do SETI:





  • Alex Tolley October 6, 2014, 15:59

    @spaceman – yes this was in his book. One part is in the Venus chapter 6(?). Then there is another chart later that shows runaway greenhouse about 1.2 billion years in the future as the sun gets hotter.

  • Wojciech J October 6, 2014, 16:01

    Alex Tolley “But consider, would finding fossils of multi0cellular life on Mars mean more interest than terrestrial paleontology? Would a living ecosystem in Europa draw more attention than life in our ocean depths? After the initial excitement, I doubt it”
    Actually such discovery would be highly interesting to governments across the world as potential alien lifeforms could represent enormous treasure trove of biochemical secrets and processes we could adopt in industry, medicine or even engineering. Thus the discovery of multi-cellar life under Europa’s ice crust or(very optimistically and rather unlikely) under Martian surface(I am thinking about very primitive floral organisms) would definitely ignite a small space race to get samples.

    Ian ” So let’s swallow our political correctness and think about terra forming … there, that should stir up some debate.”
    Terraforming for all its romantic vision and grandeur isn’t profitable economically, its probably better to construct artificial settlements in space where conditions are perfectly manageable and which take less time to construct while still providing habitats for billions.

  • Erik Landahl October 9, 2014, 12:00

    @Joelle B. – Please excuse my delayed response. Thank you for your thoughtful and insightful response. You thought about the subject in ways I had not, and you gave me much to be optimistic about – which I am grateful for!

    Having young children, I am often fearful and pessimistic about the world they will have to exist in, a world for which my parents’ generation and my generation is largely responsible.

    Global extreme climate change particularly worries me. It is an extinction-level threat several orders of magnitude more dangerous than an asteroid strike. Yet R&D has spent so much more money on preventing an asteroid strike than on removing CO2 from the atmosphere and eliminate CO2 production as a byproduct of our future energy use. As my 14-year old son said, we won’t ever get to the Centauri system if climate change wipes out most of humanity by the end of this century.

    But you certainly made my perception of the future a bit brighter, Joelle.

  • Eniac October 11, 2014, 15:50

    Rob Flores:

    strike from 45KM asteroid will sterilize all life forms, even those
    dwelling deep in the crust.

    I don’t think that is true, at all. It would take a moon sized object to heat the entire planet beyond where microbes can survive.

  • Mark Zambelli October 13, 2014, 11:46

    Even with a devastating impact that liquifies the crust I doubt this’ll be enough to sterilise a planetary surface… for long. With the sheer number of bacteria involved I’m sure some of them will be lobbed into orbit to rain back down on the surface some while later after hitching a ride in/on some material. It wouldn’t be ideal but ignoring life’s tenacity would be an oversight so enough organisms might survive to carry on after a ‘reset’ like this. Kind of like a localised, not-so-pan spermia.

  • Ronald February 5, 2015, 6:42

    Very late comment (I was very busy then), but for the record:
    “Interestingly, Petigura et al. find that the planet distribution remains flat at longer periods. What this means is that the number of planets at longer orbital periods remains the same.”
    The validity of this extrapolation could be the most relevant statement in this post and indeed of the Petigura paper.
    Though the corrected estimate of rocky planets in the conservative HZ (as per Kopparapu et al.) of about 10%, and even if corrected for R of about 5%, still seemed very high to me, the recent paper by Bovaird and Lineweaver (2015; Using the Inclinations of Kepler Systems to Prioritize New Titius-Bode-Based Exoplanet Predictions) confirms this: they come to an estimate of 10 – 30% for rocky planets (<= 1.5 RE) in the conservative HZ.

  • Ronald February 5, 2015, 10:18

    I take the liberty to also comment here on another older post for which comments are closed, but which is related:
    Three Regimes of Planet Formation
    based on a paper by Buchhave et al. (2014), which in turn is based on an analysis of Kepler data.

    I was and am still quite puzzled by this paper. Not by the 3 groups of planets in size and composition (seems almost logical by now), but by the authors’ conclusion that whereas solar metallicities favor (smaller) terrestrial planets, *gas dwarfs (mini-Neptunes, super-earths) are favored by higher (than solar)metallicity*!

    I would certainly agree with this if it concerned the true gas giants, which indeed, according to several studies, are strongly correlated with high metallicity. And indeed the paper also mentions this:
    “Our data indicate a statistically significant increase in metallicity at a
    comparable planetary radius, RP = 3.9R. This observation is in agreement with the well-established correlation between a star’s metallicity and its likelihood to host hot Jupiters, confirming that the formation regime for larger planets (>3.9R) requires exceptionally high metallicity environments”.

    However, my trouble with their conclusion is based on the following:

    – Several recent studies indicate that terrestrial planet occurrence is rather independent from metallicity (contrary to indeed gas giant occurrence).

    – The compact systems of medium-sized planets (Neptunes, gas dwarfs) are very common, possibly thé most common system for solar type stars, and demonstrably occur commonly among lower/low-metallicity systems.

    – There is, in the authors’ line of reasoning, even a contradiction in the combination of small terrestrials (lower metallicity) and a gas giant (high metallicity) in our own solar system.

    In my humblest opinion and without wanting to be too opinionated, I think that the authors have jumped to conclusions here, by extrapolating from the terrestrials to the gas dwarfs and/or from the true gas giants to the gas dwarfs, without it being sufficiently justified by their own paper and research.

    Again, in my humble and very (VERY) premature opinion, I have been getting the tentative impression lately that there may be another mechanism at work here, producing gas dwarfs and ice giants/Neptunes (i.e. the medium-sized category, roughly from 2 – 4 Re), particularly in (inner) compact systems;
    At higher metallicities ( > about 0.8-0.9 * solar, or about beyond -0.05 to -0.1 dex) there is usually a (real) gas giant which sucks up most protoplanetary material in the system, this being enhanced even more by inward migration. In the most extreme case (very high metallicity) this may result in a hot Jupiter which has absorbed nearly all material (as far it has not slung it into the star).
    The resulting upside of this greedy gas giant (as long as it is not a hot Jupiter) is that it leaves just enough planetary material in the inner system for small terrestrials to form.

    In this view, the gas dwarfs and Neptunes can form in the inner system *in the absence* of a true gas giant, because in that case sufficiently material is left there for their formation. In other words, in case of *lower*, not higher, metallicity.
    And indeed, there are plenty of examples of gas dwarf/Neptune planets and (compact, inner) planetary systems around low-medium metallicity stars. A few nice nearby examples of low-medium metallicity stars with super-earths/gas dwarfs and Neptunes are: Tau Ceti, 82 Eridani, Nu2 Lupi and 61 Virginis.

    As others here mentioned, what may further complicate the metallicity issue is the enrichment of stellar atmospheres by falling/in (proto)planetary material (pushed by gas giants), leading to misleadingly high stellar measured metallicities, *not* being a positive sign of terrestrial planets (see the post A New Marker for Planet Formation, https://centauri-dreams.org/?p=30660). This may be the case for Delta Pavonis with its very high metallicity, for which no planets have been detected yet.

    What is very relevant in the mentioned paper is the suggestion that most gas dwarfs and Neptunes in inner systems got there by inward migration. This would not bode well for the presence of small terrestrial planets in the HZ of such stars.

    Time will tell.