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Omega Centauri: Improbable Venue for Life?

Although it appears in the same constellation as seen from Earth (Centaurus), Omega Centauri has nothing to do with the Alpha Centauri stars that so interest interstellar flight theorists. The brightest globular cluster visible in our skies, Omega Centauri is anything but close (16,000 light years out) and, containing several million stars, is the largest globular cluster in our galaxy. We may in fact be looking at the core of a dwarf galaxy once absorbed by the Milky Way.

But although it’s quite distant, Omega Centauri may be the source of the relatively nearby Kapteyn’s Star, just 13 light years from the Sun. Where this gets intriguing is that Kapteyn’s Star, (named after Dutch astronomer Jacobus Kapteyn) is known to have at least two planets, one of them considered the oldest known potentially habitable planet — let’s call it a ‘temperate Super-Earth’, as Guillem Anglada-Escudé and team have done — at 11 billion years old.

Steven Kane (UC-Riverside), working with graduate student Sarah Deveny (San Francisco State) has now produced a paper taking a deep dive into Omega Centauri and its own prospects for habitability. Beginning with 470,000 stars in the cluster’s core, the duo focused on a subset of 350,000 of these, using color as a gauge of temperature and age in making their choice. The idea was to calculate habitable zones around each star, in which a rocky world might have liquid water on the surface.

Image: There are colorful stars galore, but likely no habitable planets, inside the globular star cluster Omega Centauri. Credit: NASA, ESA, AND THE HUBBLE SM4 ERO TEAM.

The paper Kane and Deveny produced makes the point that most exoplanet searches have occurred around field stars, but globular clusters are attractive hunting grounds given the age of their stars and the possibility they offer to study planet formation and evolution as affected by the cluster. But so far planet hunting in such clusters has not proven profitable. A 2000 study using the Hubble instrument to study 34,000 stars in the core of the cluster 47 Tucanae revealed not a single transit, nor did follow-ups in the less crowded outer regions of the cluster.

Nor have transiting planets been found in the cluster NGC 6397. Undaunted, Kane and Deveny here analyze Hubble data on the core of Omega Centauri and calculate habitable zones for the observed stars. The idea is to determine how conditions in the Omega Centauri core affect the potential habitability of planets around main sequence stars, the latter being determined by a color-magnitude diagram with data drawn from filters described in the paper. Luminosity and effective temperatures were calculated using Hubble instrument filters from the Dartmouth Stellar Evolution Database, assuming 11.5 billion years as the age in the applied model.

Kane and Deveny use habitable zone relationships and methodologies found in the literature, going on to calculate the conservative habitable zone (CHZ) and optimistic habitable zone (OHZ), boundaries that Kane previously used in creating a catalog of Kepler habitable zone planets. Given the star properties determined by the above analysis, which showed that the sample under consideration was dominated by low-mass stars, the researchers found that about 50 percent of the sample had an outermost habitable zone boundary within 0.5 AU of the star.

No surprise there, as most of the stars in the Omega Centauri core are red dwarfs, implying habitable zones much closer than those around G- and K-class stars. Compact planetary systems of the kind we see, for example, around small M-dwarfs like TRAPPIST-1 would seem to stand a better chance of survival from disruption by nearby stars, but conditions here are truly tight. Our Sun has a separation of over 4 light years from its nearest stellar neighbors, but within Omega Centauri’s core, the average distance is a scant 0.16 light years. That would set up frequent encounters between stars, on the order of one every 1 million years.

The result does not bode well for these planets, as the paper notes:

…the compact nature of the HZ regions is more than offset by the potential disruption of planetary systems, where close encounters of only 0.5 AU are expected to occur on average every 1.65 × 106 years. Though the large resulting population of free-floating terrestrial planets are intrinsically interesting from formation and dynamical points of view, the potential for habitability in the ω Cen core environment is significantly reduced by such scattering events. The primary lesson that can be extracted from this analysis is the underlining of the importance of quantifying the long-term dynamical stability of orbits inside HZ regions taking into account both internal (planetary) dynamics and external (stellar) interactions.

Image: The globular cluster Omega Centauri — with as many as ten million stars — is seen in all its splendor in this image captured with the WFI camera from ESO’s La Silla Observatory. The image shows only the central part of the cluster — about the size of the full moon on the sky (half a degree). North is up, East is to the left. This colour image is a composite of B, V and I filtered images. Note that because WFI is equipped with a mosaic detector, there are two small gaps in the image which were filled with lower quality data from the Digitized Sky Survey. Credit: ESO.

Because we have no firm knowledge of how common planets are in this environment, we’re left to speculate, but the paper’s calculations show that numerous compact planetary systems could exist in Omega Centauri. But even assuming a large population of rocky worlds, cluster dynamics would keep encounters between these planetary systems happening on a regular basis. The scenario of planets stripped from their host stars seems inimical to life.

But a possibility remains. Terrestrial-class worlds disrupted from their home systems could constitute a large population of free-floating worlds that might be analyzed through gravitational microlensing. There are some studies in the literature arguing that free-floating worlds with a thick hydrogen atmosphere could retain habitable conditions at the surface, so as the authors note, habitable planets cannot be entirely ruled out despite the problem of stability.

And, of course, encounter rates between stars will vary depending on the size and density of the cluster:

“The rate at which stars gravitationally interact with each other would be too high to harbor stable habitable planets,” Deveny said. “Looking at clusters with similar or higher encounter rates to Omega Centauri’s could lead to the same conclusion. So, studying globular clusters with lower encounter rates might lead to a higher probability of finding stable habitable planets.”

Such studies will doubtless be pursued. After all, the close separation among ancient stars at a globular cluster’s core would seem to give any civilizations that formed near them the opportunity to explore nearby systems and eventually the entire cluster. We’ve talked before about the work of Rosanne Di Stefano (Harvard-Smithsonian Center for Astrophysics), who makes this case (see Globular Clusters: Home to Intelligent Life?). Says Di Stefano:

“Interstellar travel would take less time… The Voyager probes are 100 billion miles from Earth, or one-tenth as far as it would take to reach the closest star if we lived in a globular cluster. That means sending an interstellar probe is something a civilization at our technological level could do in a globular cluster.”

In fact, Di Stefano argues that portions of many globular clusters other than the core can be considered ‘sweet spots’ where habitable orbits are stable for long periods. So the question of habitability and planet formation itself inside globular clusters is one of continuing interest.

One thing is clear. When we talk about habitable zones, we have to take into account the dynamical stability of the orbits involved, examining not just the planets themselves but the possibility of encounters with nearby stars. In dense environments, such interactions and the scattering they cause would be game-changers. Cluster outskirts may offer better opportunities.

The paper is Kane and Deveny, “Habitability in the Omega Centauri Cluster,” accepted at The Astrophysical Journal (preprint). The Di Stefano paper mentioned above is “Globular Clusters as Cradles of Life and Advanced Civilizations,” Astrophysical Journal Vol. 827, No. 1 (5 August 2016). Abstract.

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{ 24 comments… add one }
  • andy August 15, 2018, 13:38

    The interesting question then becomes how many stars are likely to have avoided passing through the core over the lifetime of the cluster?

    To my knowledge there has only been one detection of a planet in a globular cluster, around a millisecond pulsar/white dwarf binary in Messier 4. On the other hand, it looks like Kapteyn’s star and other members of the Kapteyn moving group did not originate in Omega Centauri based on their different abundance patterns.

  • ljk August 15, 2018, 13:42

    Globular star clusters would also be very attractive places to advanced ETI with an interstellar culture who originated from other parts of the galaxy, thus avoiding the issue of needing to be natives of the GSC and therefore subject to its presumed natural cosmic perils.

    The late Robert Bradbury had a paper on this very subject. Sadly, many of his papers that were online do not seem to be available.

  • ljk August 15, 2018, 13:56

    Keep in mind this possibility for life existing in Omega Centauri: The cluster may actually be the remnant of an ancient galaxy that collided with the Milky Way galaxy long ago and “lost”. OC is much more voluminous than all other such clusters in our galaxy.

    https://www.centauri-dreams.org/2008/05/20/omega-centauri-when-galaxies-collide/

  • John walker August 15, 2018, 14:54

    With all due respect to Dr Stefano, her paper is exemplary for many others with astrobiological asperations in its narrow focus. It is still early days in hard exoplanetary research and thus an array of speculative conclusions is not surprising. What I do not find acceptable however, is the degree to which many are willing to draw broad catagorical conclusions based on a review of often only one or two variables. As today’s paper clearly illustrates, even the one chosen variable, here the high density stellar population, can prove to be a double edged sword.
    As much as many of us wish for a vibrant living galaxy, populated with or habitable for our very personal preferences of life and perhaps civilizations, we would be well advised to keep this sort of emotional dreaming separate from our analysis of the physical laws governing the bodies in our universe.
    Today’s paper is an knock for those dreamers. Good. But at the same time the sobriety of the authors conclusions is not entirely merited in my opinion. True, the simulations are sound and strip the postulated cluster core stellar population many times over of all of their satellites. However, what the paper does not discuss is the long term stellar migration within the whole cluster. The population of the central cubic parsec is not static. There are long term migrations within globular clusters, the details of which I will not describe here. I am uncertain whether this will increase or decrease the chances of planets, habitable or not, within them. However, a simulation of the dynamics of a cubic parsec with 3000 stars in isolation is not representative of cluster dynamics as a whole.

    • ljk August 16, 2018, 8:54

      I can remember when astronomers were certain that binary and even more multiple star systems could not possibly sustain stable planetary systems.

      Before that, most of them assumed planetary systems would resemble ours, with little rocky worlds in front and big gassy ones in the back.

      And so on back to when Earth was the place that everything else circled around.

      And while it is nice to see professional astronomers dipping their toes into the SETI/Astrobiology waters if for no other reason than to seem hip and relevant, they tend to draw quick, broad conclusions based on really distant observing and a few data points. Their actions are like someone standing on a beach and staring at the ocean for a few minutes, then concluding there are no fish in those waters because they didn’t see any.

      So yes, I am sure globular star clusters have their particular drawbacks when it comes to evolving and supporting life. However, as I said above, we won’t know until we get there (or build much better telescopes) and GSCs may be very useful to advanced ETI because they contain so many star systems in such a relatively compact area.

      And the mainstream media’s tendency towards lousy headlines and parroting other news services while not actually understanding the subject matter does not help, either.

      • ljk August 16, 2018, 9:10

        One more example I must add: Astronomers were once certain that Venus and Mars had some form of life while the thought that places such as the moons of the Jovian planets couldn’t possibly sustain organisms (they were much too cold).

        Now we think that several outer world satellites are better places for alien life than either Venus or Mars. All because we went there directly and checked them out.

        Here is a brand new article on this very subject regarding Venus from The Planetary Society:

        http://www.planetary.org/blogs/guest-blogs/the-venus-controversy.html

        • ljk August 16, 2018, 9:18

          Quoting from the above TPS article:

          “Similarly, we have far better topographic data for the Moon and Mars than we do for Venus. Only the Earth’s ocean floor is as poorly imaged as Venus, with less than 10 percent of it mapped in detail. Venus, shrouded in clouds, and the ocean floor, insulated by water, remain the few places in the Solar System that are currently out of our technological reach.”

          Oceanographers thought as late as the mid-1970s that most of the ocean floors were near deserts when it came to harboring life. Then the submersible Alvin found those hydrothermal vents in 1977 where oversized clams and giant red worms thrived in boiling waters under enormous pressures.

          http://www.whoi.edu/feature/history-hydrothermal-vents/discovery/1977.html

      • andy August 16, 2018, 13:05

        The fact that people have gotten things wrong before and may still get things wrong in the future does not allow you to just dismiss results you don’t like. Science doesn’t work like that.

        • Ron S. August 16, 2018, 17:46

          Instead of calling them “wrong” I would say that they made reasonable guesses based on the available data and theory. That’s nothing to be ashamed of.

          • ljk August 17, 2018, 8:56

            I did not say they should be ashamed and I am certainly aware that they were using the limited data they had of their time. What I am opposing is when science sticks to its conservative guns a bit too long when they either know there are alternative ideas that can be tested or they simply do not want to row against their professional peer culture tide.

            Objective science is a very good thing, but its practitioners are human and therefore as flawed as anyone else. This needs to be taken into account, especially when it comes to the subject of extraterrestrial life.

        • ljk August 17, 2018, 8:52

          Actually science works like that all the time, they just pretend it doesn’t because there is this cultural image of the scientist as the robotic by-the-book nerd.

          And I don’t like what they are saying not because, say, the scientists here wore an ugly sweater or have a deeply opposing political views from me, but because their thinking is flawed and limited. Now that I have every right not to agree with.

    • andy August 16, 2018, 13:02

      You appear to have confused the papers. The one about the Omega Centauri core was by Stephen Kane and Sarah Deveny. The main problem I see with it is the title: if it were titled “Habitability in the Core of the Omega Centauri Cluster” rather than just “Habitability in the Omega Centauri Cluster” that would more accurately reflect the focus of the paper. A discussion of the habitability of the cluster as a whole would have to take into account more of the large-scale motion of the stars through the cluster, how many stars are expected to have avoided passing through the cluster core, etc.

      • John walker August 16, 2018, 19:53

        Hi Andy. No. I did not “confuse the papers”. But I can understand you thinking that. My first 2 paragraphs addressed Di Stefano’s paper from 2014. Paragraph 3 was dedicated to “today’s paper” ie Kane Deveny. I should have been more explicit.

  • Dmitry Novoseltsev August 15, 2018, 23:09

    think pessimistic assessments are premature.
    Under certain conditions, in a cluster at such small distances between stars, there may be a continuous, single “habitable zone” for the entire cluster, in which exoplanets drift like free electrons in metals.
    And the life formed in the conditions of the considerable climatic fluctuations caused by drift of exoplanets, and also at the considerable and uneven radiation background caused by strong irregular flashes on red dwarfs if it reaches rather high level of the organization, can be as a result more “tempered” and steady against adverse factors of space, than life known to us on the Earth.
    This could make it easier for her, even for the simplest forms (through mechanisms of panspermia), colonization all cluster and promotion of beyond.

  • ljk August 16, 2018, 14:53

    Could Binary Mergers Help Us Find Intelligent Life?

    By Susanna Kohler on 13 August 2018

    How do we find extraterrestrial intelligence (ETI) — not just life beyond Earth, but advanced extraterrestrial civilizations? One approach is to seek signals from ETI that may be attempting to communicate with us — but the problem of where, when, and how to look for such communications is a complex one.

    A new study explores one way we could optimize this hunt: by searching for communication signals that are synchronized with the merger of two neutron stars.

    https://aasnova.org/2018/08/13/could-binary-mergers-help-us-find-intelligent-life/

    This idea has also been applied to supernovae.

  • Geoffrey Hillend August 16, 2018, 17:32

    Most stars in globular clusters are in population II and old or off main sequence burning, so If any life evolved there it has long since left the globular cluster. I agree with the conservative view that the odds are against life evolving in a globular cluster.

    It might be an interesting place for life to start if it could. In 1979, I bought a book as a teenager filled with an imaginative view from the surface of the planets in our solar system which were paintings. The red sky of Titan and Saturn were painted which was before we knew that you couldn’t see Saturn from Titan’s surface due the thick atmosphere and before Voyager spacecraft images in 1980. The last page had a painting of what a view of the sky from the surface of a planet around a globular cluster might look like at night. The entire sky was bright even at night and filled with the orange glow of stars. Although I always liked space science fiction, astronomy, and planetary science from an early age, the book also inspired me to get more seriously into astronomy with the help of the whole space science fiction and exploration zeitgeist of the late 1970’s, with Wars and the TV series Battlestar Galactica, etc. I think the book was by Patrick Moore, but I am not sure.

    • ljk August 17, 2018, 8:59

      Let me repeat what I said above: We need to consider the possibility that there may be life in GSCs that are not natives but advanced interstellar-capable visitors who are attracted to the idea of countless star systems in close proximity to each other. Add the possibility that Omega Centauri was once a galaxy in its own right and things become even more interesting.

    • Alex Tolley August 17, 2018, 11:03

      The last page had a painting of what a view of the sky from the surface of a planet around a globular cluster might look like at night. The entire sky was bright even at night and filled with the orange glow of stars. Although I always liked space science fiction, astronomy, and planetary science from an early age, the book also inspired me to get more seriously into astronomy with the help of the whole space science fiction and exploration zeitgeist of the late 1970’s, with Wars and the TV series Battlestar Galactica, etc. I think the book was by Patrick Moore, but I am not sure.

      this one?

      Caption:

      Alien Life 72
      This was the final illustration in ‘Challenge of the Stars’ with Patrick Moore in 1972. David postulated a planet in a globular cluster of tightly-packed, old, reddish stars; it has a carbon dioxide atmosphere, and the alien sacs fill with oxygen, then rise and finally fly free. An earlier, earthlike version formed a cover for Hawkwind – see next two images.

  • ljk August 17, 2018, 9:51

    A recent paper on exoplanets in open star clusters, for comparison:

    http://sites.psu.edu/astrowright/2018/03/20/how-old-is-that-planet/

  • Geoffrey Hillend August 17, 2018, 17:42

    Yes. ” Challenge of the Stars” is the book by Patrick Moore. I had revised version with the “Death Star” spherical spacecraft colliding with a triangular ship on the cover. I saw the older version with the spherical asteroid base on the cover of it at the library. The painting is probably more imaginative and fictional than factual, but definitely inspirational for a young person as I was sixteen at that time. The rest of the paintings are all bright colors and inspirational and good space art.
    Thankyou Alex Tolley for posting the name the book. I have not seen it in over thirty years, and I forgot the name. Some of the art of that book can be found on Google. The book is out of print.

  • WDK August 29, 2018, 13:49

    In the discussion thus far, I notice that Omega Centauri is described as a home of a red dwarf stellar majority. Well, I suspect that it was not always the case, but an observational artifact of stellar evolution in an ancient globular cluster sending the main sequence O, B, G and K into retirement billions of years back. What that did to the cluster medium would be another consideration. During that earlier period local elemental abundances would be low, but they would accumulate in planets such as those around Kapteyn’s star, assuming the evidence for them holds up over time. White dwarfs that the Fs, Gs etc. evolved into would be hotter, but their radii would be much lower than M dwarfs, hence being out of the picture. But they would enrich the interstellar medium somehow.

    Also, one would have to wonder how dense the cluster was in its early history. With regard to our own world’s mortality we sometimes speculate on how humanity or life might escape the sun and Earth’s fate by setting off to an environment such as Trappist 1 (numerous planets in a habitable zone at a red dwarf). Possibly the Omega Centauri environment offers a lab for such a scenario to have unfolded numerous times. With close passages of stars, there is even the possibility of micro-biotic contaminations of the sort we contemplate with meteorites in the solar system If so, then there is even a chance that there is widespread life originating from several worlds in different star systems. And we are still just talking about one globular cluster.

    “Just in passing”: noticed that Kapteyn’s star was supposedly 7 light years away about 12000 years ago, but appears to be 11 giga-years old.
    Wonder if its distance ( or that of other such stars) from Earth oscillates?

  • ljk September 1, 2018, 14:41

    Dating the Evaporation of Globular Clusters

    By Kerry Hensley on 31 August 2018

    The most ancient stellar populations in our galaxy are being ripped apart. Globular clusters — massive gravitationally bound collections of hundreds of thousands of stars — have occupied the Milky Way halo for billions of years. Studying globular clusters can help us understand not only how our galaxy formed, but also how it has evolved over the history of the universe. As the Milky Way has evolved, its gravitational potential has changed as well — and the changes in our galaxy’s gravitational pull are recorded in the behavior of globular clusters.

    As the stars in globular clusters interact gravitationally, some gain enough kinetic energy to be ejected from the cluster entirely. The shrinking of globular clusters through this process is called evaporation. When the ejected stars escape the gravitational confines of the cluster, the gravitational pull of the Milky Way starts to take over. As the cluster orbits the galactic center, it experiences tidal forces. Much like an unwitting spacefarer approaching a black hole, globular clusters get stretched out by these tidal forces, stringing those escaped stars into a tidal tail or stream (see Figure 1).

    We see these tidal tails in many globular clusters, but the question remains: How can we figure out when the tidal disruption began?

    https://aasnova.org/2018/08/31/dating-the-evaporation-of-globular-clusters/

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