Redefining the Galactic Habitable Zone

To understand the Solar System’s past and to tighten our parameters for SETI searches, we need to consider habitability not only as a planetary and stellar phenomenon but a galactic one as well. The Milky Way is a highly differentiated place, its core jammed with older stars and Sagittarius A*, which is almost certainly a supermassive black hole. The gorgeous spiral arms spawn new stars while the globular clusters in the halo house ancient clusters. Where in all this is life most likely to form? And perhaps more to the point, in what ways do stars and their associated planets migrate in the galactic disk? Our Sun raises the issue by virtue of the fact that its metallicity, as measured by the ratio of iron to hydrogen (Fe/H), is higher than nearby stars that are of a similar age. In a new paper from Junichi Baba (Kagoshima University) and colleagues at the National Observatory of Japan and Kobe University, the authors offer this as evidence that the Solar System formed closer to the...

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Can Life Emerge around a White Dwarf?

My curiosity about white dwarfs continues to be piqued by the occasional journal article, like a recent study from Caldon Whyte and colleagues reviewing the possibilities for living worlds around such stars. Aimed at Astrophysical Journal Letters, the paper takes note of the expansion of the search space from stars like the Sun (i.e., G-class) in early thinking about astrobiology to red dwarfs and even the smaller and cooler brown dwarf categories. Taking us into white dwarf territory is exciting indeed. How lucky to live in a time when our technologies are evolving fast enough to start producing answers. I sometimes imagine what it would have been like to have been around in the great age of ocean discovery, when a European port might witness the arrival of a crew with wondrous tales of places that as yet were on no maps. Today, with Earth-based instruments like the Extremely Large Telescope, the Giant Magellan Telescope and the Vera Rubin Observatory, we can expect data from...

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Autumn Among the Galaxy Clusters

The idea of moving stars as a way of concentrating mass for use by an advanced civilization – the topic of recent posts here – forces the question of whether such an effort wouldn’t be observable even by our far less advanced astronomy. In his paper on life’s response to dark energy and the need to offset the accelerating expansion of the cosmos, Dan Hooper analyzed the possibilities, pointing out that cultures billions of years older than our own may already be engaged in such activities. Can we see them? I like Centauri Dreams reader Andrew Palfreyman’s comment that what astronomers know as the ‘Great Attractor’ is conceivably a technosignature, “albeit on a scale somewhat more grand than that cited.” An interesting thought! And sure, as some have pointed out, nudging these concepts around on a mental chess board is wildly speculative, but in the spirit of good science fiction, I say why not? We have a universe far older than our own planet with possibilities we might as well...

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A Look at Dark Energy & Long-Term Survival

If life can organize into sentient beings around stars other than our own, there are few assumptions we can make about the civilizations that would emerge. We’ve long ago given up on the idea that such creatures would look like us, just as we abandoned the concept of life on every conceivable astronomical object. William Herschel, among others, thought life might exist on the Sun, a notion that in different form may be coming back around, as witness the growing interest in panpsychism and stellar consciousness. But let’s talk about physical life forms rather than energy fields. Since we have to assume something somewhere, let’s posit that any civilization would select as its top priority its own survival. That seems obvious enough. Survival demands energy, and that demand increases as the civilization grows through the Kardashev scale, gradually using more and more of the energy of its star and ultimately going beyond that to look for energy sources elsewhere. Dyson sphere thinking...

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Close-up of an Extragalactic Star

While working on a piece about interstellar migration as a response to the accelerating expansion of the universe for next week, I want to pause a moment on a just announced observation. I’ve always had a fascination with the Magellanics, those satellite galaxies that are so useful to astronomers because their gravitational interactions with the Milky Way render both of them irregular in shape. That triggers waves of star formation and tells us something about how galaxies assemble themselves. The Small Magellanic Cloud (SMC), about 200,000 light years away, shows little structure, while the Large Magellanic (LMC) is a bit more organized but lacks symmetry – no smooth disk apparent there. Image: This beautiful image taken at ESO's Paranal Observatory shows the four Auxiliary Telescopes of the Very Large Telescope (VLT) Array, set against an incredibly starry backdrop on Cerro Paranal in Chile. The Auxiliary Telescopes are each 1.8 metres in diameter and work with the four 8.2-metre...

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Star Harvest: Civilizations in Search of Energy

Living a long time forces decisions that could otherwise be ignored. This is true of individuals as well as societies, but let’s think in terms of the individual human being. Getting older creates survival scenarios as simple as ensuring safety and nutrition for the elderly. But let’s extend lifetimes to centuries and beyond. In this thought experiment, we create a society of people so long-lived that their personal planning takes in events like a possible asteroid strike in 200 years. A person who could live for a billion years has to think in terms of surviving a dying Sun engulfing his or her planet. If we assume a kind of immortality, the individual and the society merge in terms of their key concerns. It’s hard to imagine biological beings living for lifetimes like these, but as we’ve often considered in these pages, non-biological machine intelligence, constantly upgrading and improving itself, should be able to pull it off. Because we know of no extraterrestrial civilizations,...

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Clearing the Air

I've played around a bit with Bluesky in the past, but have now decided it's time to make a move. Those of you who have been following the site on X will want to know about the change, as posted in my first new Bluesky 'skeet' in some time. The change in user experience is striking, a bit like walking in an alpine meadow after spending years in a subway car. Not that I have anything against subways... Have decided to move to BlueSky for good, having left X for obvious reasons. Future posts from my Centauri Dreams site will be linked here. Current post considers Clément Vidal's striking concept for using millisecond pulsars to move entire stellar systems. www.centauri-dreams.org/2024/11/14/a...[image or embed]— Paul Gilster (@gilster.bsky.social) November 16, 2024 at 8:30 AM Addendum: For those who have asked, this affects only those of you who are connected to me on X, which I no longer use. If you don't use X, nothing has changed in how you see Centauri Dreams.

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A Millisecond Pulsar Engine for Interstellar Travel

Suppose you want to migrate to another star, taking your entire civilization with you. Not an easy task given our technology today, but let’s remember that in the 13 billion year-plus history of the Milky Way, countless stars and their planets have emerged that are far older than our 4.6-billion year old Sun. If we imagine an intelligence that survives for a billion years or more, we can hardly put constraints on what it might accomplish. The idea of moving a star with its planetary system intact is out there on the edge of what science fiction can accomplish, if not yet science. There have even been SETI searches for such projects, though as with SETI at large, no hits. Why would you want to move a star? Consider that if you are a long-lived species with a simple interest in exploring the universe, setting up a journey in which you can take your culture with you – all of it – could have serious appeal. For one thing, you are also taking your primary energy source with you, and can...

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Habitability and a Variable Young Sun

Given our intense scrutiny of planets around other stars, I find it interesting how little we know even now about the history of our own Sun, and its varying effects on habitability. A chapter in an upcoming (wildly overpriced) Elsevier title called The Archean Earth is informative on the matter, especially insofar as it illuminates which issues most affect habitability and how the values for these vary over time. It’s also a fascinating look at changing conditions on Venus, Earth and Mars. We know a great deal about the three worlds from our local and planetary explorations, but all too little when it comes to explaining the evolution of their atmospheres and interior structures. But it’s important to dig into all this because as Stephen Kane, director of The Planetary Research Laboratory at UC-Riverside and colleagues point out, we seem to be looking at the end state of habitability on both Mars and Venus, meaning that our explorations of these worlds should yield insights into...

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Vega’s Puzzling Disk

Over the weekend I learned about Joseph Haydn’s Symphony No. 47, unusual in that it offers up some of its treasures in perfect symmetry. Dubbed ‘The Palindrome,’ the symphony’s third movement, Minuetto e Trio, is crafted to play identically whether attacked normally – moving forward through the score – or backwards. You can check this out for yourself in this YouTube video, or on this non-auditory reference. The pleasure of unexpected symmetry is profound, and when seen through the eyes of our spacecraft, can be startling. Consider the storied star Vega. We see this system from our perspective at a very low inclination angle relative to its rotational axis, as if we were looking down from above the star’s pole. This face-on perspective is profoundly interesting when examined through our space-borne astronomical assets. In the image below, we get two views of Vega’s disk, from Hubble and then JWST. Image: The disk around Vega as seen by Hubble (left) and Webb (right). Hubble detects...

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Monkeying Around with Shakespeare

Bear with me today while I explore the pleasures of the Infinite Monkey Theorem. We’re all familiar with it: Set a monkey typing for an infinite amount of time and eventually the works of Shakespeare emerge. It’s a pleasing thought experiment because it’s so visual and involves animals that are like us in many ways. Now we learn from a new paper that the amount of time involved to reproduce the Bard is actually longer than the age of the universe. About which more in a moment, but indulge me again as I explore infinite monkeys as they appear in fictional form in the mid-20th Century. In "Inflexible Logic," which ran in The New Yorker's February 3, 1940 issue, Russell Maloney tells the tale of a man named Bainbridge, a bachelor, dilettante and wealthy New Yorker who lived in luxury in a remote part of Connecticut, "in a large old house with a carriage drive, a conservatory, a tennis court, and a well-selected library." He has about him the air of an English country gentlemen of the...

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Deep Space Implications for CubeSats

The Hera mission has been dwarfed in press coverage by the recent SpaceX Starship booster retrieval and the launch of Europa Clipper, both successful and significant. But let’s not ignore Hera. Its game plan is to check on the asteroid Dimorphos, which became the first body in the Solar System to have its orbit altered by human technologies when the DART spacecraft impacted it in 2022. Hera is all about assessing this double asteroid system to see first-hand the consequences of the impact, which shortened the smaller object’s orbit around asteroid Didymos by some 32 minutes. That’s a pretty good result, some 25 times what NASA had defined as the minimum successful orbital period change, and we’re learning more about the ejecta, which involve tons of asteroidal rock. The collision occurred at 6.1 kilometers per second, to be more fully assessed by Hera’s twin CubeSat craft, which will make precise measurements of Dimorphos’ mass to analyze the efficiency of the impact. All this...

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Do You Really Want to Live Forever?

Supposing you wanted to live forever and found yourself in 2024, would you sign up for something like Alcor, a company that offers a cryogenic way to preserve your body until whatever ails it can be fixed, presumably in the far future? Something over 200 people have made this choice with Alcor, and another 200 at the Cryonics Institute, whose website says “life extension within reach.” A body frozen at −196 °C using ‘cryoprotectants’ can, so the thinking goes, survive lengthy periods without undergoing destructive ice damage, with life restored when science masters the revival process. It’s not a choice I would make, although the idea of waking up refreshed and once again healthy in a few thousand years is a great plot device for science fiction. It has led to one farcical public event, in the form of Nederland, Colorado’s annual Frozen Dead Guys Days festival. The town found itself with a resident frozen man named Bredo Morstøl, brought there by his grandson Trygve Bauge in 1993 and...

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Does Artificial Intelligence Explain the Fermi Question?

Science fiction has been exploring advanced machine intelligence and its consequences for a long time now, and it’s now being bruited about in service of the Fermi paradox, which asks why we see no intelligent civilizations given the abundant opportunity seemingly offered by the cosmos. A new paper from Michael Garrett (Jodrell Bank Centre for Astrophysics/University of Manchester) explores the matter in terms of how advanced AI might provide the kind of ‘great filter’ (the term is Robin Hanson’s) that would limit the lifetime of any technological civilization. The AI question is huge given its implications in all spheres of life, and its application to the Fermi question is inevitable. We can plug in any number of scenarios that limit a technological society’s ability to become communicative or spacefaring, and indeed there are dozens of potential answers to Fermi’s “Where are they?” But let’s explore this paper because its discussion of the nature of AI and where it leads is timely...

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Streams of Stars and What They Tell Us

A quick follow-up to yesterday’s post. The idea of a stream of debris or even large objects like comets or asteroids from another star continues to resonate with me. The odds on identifying such a stream in terms of origin seem stupendous, but the benefits of doing so would be obvious. I notice that another kind of stellar stream is in the news, one involving not debris but entire stars. The Icarus stream is a grouping of stars that seem to have been tidally disrupted by the Milky Way, probably from an earlier encounter between the parent galaxy and a dwarf galaxy. Digging a bit, I learned that we can carry the idea of stellar streams back to the work of Donald Lynden-Bell, who in 1995 proposed the stream concept to explain the long structure or filament of stars evidently tidally stripped from the Sagittarius Dwarf Spheroidal Galaxy, the latter being a satellite galaxy of the Milky Way. The Sgr dSph, as it is known, actually contains four globular clusters within it. It travels a...

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An Incoming ‘Stream’ from Alpha Centauri?

Here’s an interesting thought. We know that at least two objects from outside our Solar System have appeared in our skies, the comet 2I/Borisov and the still enigmatic object called ‘Oumuamua. Most attention on these visitors has focused on their composition and the prospects of one day visiting such an interloper, for it is assumed that with new technologies like the Vera Rubin Observatory and its Legacy Survey of Space and Time (LSST), we will be picking up more of the same. But consider origins. Extrapolating backward to figure out where either object came from quickly exhausts the most patient researcher, for it only takes the slightest changes in trajectory to widen the search field so broadly as to be useless. That’s especially true since we don’t know the ages of the objects, which may span hundreds of millions of years. Enter Cole Gregg (University of Western Ontario), who has embarked on a project to study the question from a different perspective. Gregg asks how likely it...

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SETI: Learning from TRAPPIST-1

Given our decades-long lack of success in finding hard evidence for an extraterrestrial civilization, it hardly comes as a surprise that a recent campaign studying the seven-planet TRAPPIST-1 system came up without a detection. 28 hours of scanning with the Allen Telescope Array by scientists at the SETI Institute and Penn State University produced about 11,000 candidate signals for further analysis, subsequently narrowed down to 2,264 of higher interest. None proved to be evidence for non-human intelligence, but the campaign is interesting in its own right. Let’s dig into it. The unique configuration of the TRAPPIST-1 planets allowed the scientists involved to use planet-planet occultations (PPOs). A cool M-dwarf star, TRAPPIST-1 brings with it the features that make such stars optimal for detecting exoplanets. The relative mass and size of the planets and star mean that if we’re looking for rocky terrestrial-class worlds, we’re more likely to find and characterize them than around...

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A Gravitational Wave Surprise

I think gravitational wave astronomy is one of the most exciting breakthroughs we’re tracking on Centauri Dreams. The detection of black hole and neutron star mergers has been a reminder of the tough elasticity of spacetime itself, its interplay with massive objects that are accelerating. Ripples in the fabric of spacetime move outward from events of stupendous energy, which could include neutron star mergers with black holes or other neutron stars. Earth-based observing projects like LIGO (Laser Interferometer Gravitational-Wave Observatory), the European Virgo and KAGRA (Kamioka Gravitational Wave Detector) in Japan continue to track such mergers. But there is another aspect of gravitational wave work that I’m only now becoming familiar with. It’s background noise. Just as ham radio operators deal with QRN, which is the natural hum and crackle of thunderstorms and solar events, so the gravitational wave astronomer has to filter out what is being called the astrophysical...

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Catches, Comets and Europa

If the public seems more interested in spaceflight as a vehicle for streaming TV dramas, the reality of both the Europa Clipper liftoff and the astounding ‘catch’ of SpaceX’s Starship booster may kindle a bit more interest in exploring nearby space. When I say ‘nearby,’ bear in mind that on this site the term refers to the entire Solar System, as we routinely discuss technologies that may one day make travel to far more distant targets possible. But to get there, we need public engagement, and who could fail to be thrilled by a returning space booster landing as if in a 1950’s SF movie? Europa may itself offer another boost if Europa Clipper’s science return is anything like what it promises to be. Closing to 15 kilometers from the surface and making 49 passes over the icy ocean world, the spacecraft may give us further evidence that outer system moons can be venues for life. We also have the European Space Agency’s Jupiter Icy Moons Explorer (JUICE), which will study Europa,...

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Go Clipper

Is this not a beautiful sight? Europa Clipper sits atop a Falcon Heavy awaiting liftoff at launch complex 39A at Kennedy Space Center. Launch is set for 1206 EDT (1606 UTC) October 14. Clipper is the largest spacecraft NASA has ever built for a planetary mission, 30.5 meters tip to tip when its solar arrays are extended. Orbital operations at Jupiter are to begin in April of 2030, with the first of 49 Europa flybys occurring the following year. The closest flyby will take the spacecraft to within 25 kilometers of the surface. Go Europa Clipper! Photo Credit: NASA. In less than 24 hours, NASA's @EuropaClipper spacecraft is slated to launch from @NASAKennedy in Florida aboard a @SpaceX Falcon Heavy rocket.Tune in at 2pm PT / 5pm ET as experts discuss the prelaunch status of the mission. https://t.co/Nq36BeKieX— NASA JPL (@NASAJPL) October 13, 2024

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Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).

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If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to others, I'm probably not going to run it.

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