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 galactic center. The difference is large: Estimates using the metallicity of the galactic disk over time place the Sun’s formation at an average of 5 kiloparsecs from the center, migrating to its current position 8.2 kpc out (a kiloparsec is 3261.56 light years).
Features like the ‘galactic bar,’ an elongated formation of stars and star-forming material, as well as the spiral arms so evident in photographs of spiral galaxies, have much to say about the dynamics of the galaxy at large. The bar is thought to have been present when the Sun formed, and earlier papers have considered that the Sun likely originated in a place where the effects of the galactic bar would have been pronounced. Our star evidently migrated outward despite its location within the galactic bar. Modeling the energies at work within this co-rotating frame allows the authors to investigate the question of habitable orbits being modulated by this dynamic system.
Image: This image from the NASA/ESA Hubble Space Telescope shows the broad and sweeping spiral galaxy NGC 4731. It lies in the constellation Virgo and is located 43 million light-years from Earth. The image uses data collected from six different filters. The abundance of color illustrates the galaxy’s billowing clouds of gas, dark dust bands, bright pink star-forming regions and, most obviously, the long, glowing bar with trailing arms. Barred spiral galaxies outnumber both regular spirals and elliptical galaxies put together, numbering around 60% of all galaxies. The visible bar structure is a result of orbits of stars and gas in the galaxy lining up, forming a dense region that individual stars move in and out of over time. Credit: ESA/Hubble & NASA, D. Thilker.
These stellar movements are important because they would have led to changes in the surrounding environments of the Solar System and thus affect planetary habitability.
One way is through radiation hazards, which change over time. From the paper:
We examine how the solar system’s migration through the Milky Way has altered radiation hazards, focusing specifically on the star formation rate (SFR) density and GRB event rates, both of which significantly influence planetary habitability. High SFRs are associated with frequent supernovae, as massive stars rapidly reach the end of their lifetimes. These supernovae can substantially impact their surrounding environments, especially through lethal GRBs. GRBs are divided into two types: short-duration GRBs (SGRBs), originating from compact object mergers (E. Berger 2014) and common in older stellar populations, and long-duration GRBs (LGRBs), resulting from massive star collapses (S. E. Woosley & J. S. Bloom 2006) in star-forming regions. Both types of GRBs pose significant risks to life by exposing planets to intense high-energy radiation.
Although the authors don’t probe deeply in the direction of giant molecular clouds, they do note that the work of other astronomers shows that stars moving through the galactic plane closer to galactic center encounter more of these, meaning that they are exposed to supernovae on a more frequent basis. Another factor I find intriguing is the number of comets entering the planetary region of the Solar System, which clearly would affect the supply of life-building materials. Tidal forces from the galaxy itself and encounters with other stars would disrupt the orbits of long-period comets in the Oort Cloud. Rich in prebiotic molecules and organic materials, these clearly affect the conditions for life to develop on a planetary surface.
The modeling in this paper shows that stars born in the same region can experience “vastly different environments for the habitability and evolution of planetary systems” as they follow different orbital migration paths. Scientists have previously considered a galactic habitable zone in terms of distance from the center, but to my knowledge this is the first attempt to model and quantify the effects of the migration of entire stellar systems. In other words, we need to abandon the idea of fixed ‘zones’ of habitability and think in terms of stellar movement rather than regions.
What emerges here is the new term I referenced above: galactic habitable orbits. These are:
…pathways through the Milky Way offering varying conditions for life’s development based on evolving galactic dynamics. By considering the dynamical effects of the Galactic bar and spiral arms, we can better understand habitability in the Galactic context. Examining the differences in radiation environments and the supply of life-building materials encountered along different migration pathways provides a more nuanced understanding of how the dynamic nature of the Milky Way impacts planetary habitability.
Obviously habitability discussions begin first with criteria based on life’s development on Earth, which only makes sense given that we have only this example to work with. Whether similar mechanisms are at play in other stellar systems is something we’re only beginning to learn as we investigate exoplanet atmospheres in search of biosignatures. So it’s clear that this early discussion of galactic habitability will be enriched with time as we learn if there are other pathways to supporting life. But the overall contribution is clear. Think in terms of dynamic orbits rather than static zones for life to develop in a galaxy that is in incessant motion and possible astrobiological evolution.
The paper is Baba et al., “Solar System Migration Points to a Renewed Concept: Galactic Habitable Orbits,” The Astrophysical Journal Letters Vol. 976, No. 2 (26 November 2024), L 29 (full text). You might also find Galactic Habitability and Sgr A* interesting. It’s an article I wrote in 2018 covering Balbi and Tombesi, “The habitability of the Milky Way during the active phase of its central supermassive black hole,” Scientific Reports 7, article #: 16626 (2017). Full text. And I have to add Charles Lineweaver’s seminal discussion of galactic habitability in “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” Science Vol. 303, No. 5654 (2 January 2004), pp. 59-62, with abstract here.
It seems to me that the Baba paper shows that the dynamic GHZ is wider than the earlier GHZ. The migration of stars can be both outward like our sun, or inward.
If so, then how does this help us narrow down the search? Stars with high metallicity could end up around 3 kpc in a high radiation environment inimicable to “life as we know it”.
Therefore, our search time should be allocated to suitable, relatively nearby stars, where we can detect planets with atmospheres and look for possible biosignatures. A dynamic GHZ doesn’t seem to narrow the wider search space but rather increase it. That may be informative in the distant future if/when we map out the galaxy-wide systems with high probability biosignatures.
The immediate issue is whether life of any sort exists beyond our solar system. Once that question is resolved, we can then widen the initial search to place a value of f_sub_l in the Drake equation, and from there map out the galaxy (assuming that is possible) for the density distribution of systems supporting life, possibly even the type of life.
If none of the stars in our galactic neighborhood indicate that they have a biosphere, then would any further effort be made for a wider search?
Conversely, if a number of stars relatively nearby have high probability biosignatures, would the best strategy be to focus observations and new technology on those planets? I can imagine that building high-resolution telescopes to image planets with suspected biospheres would be the desirable direction to take. We would certainly want to know whether they have complex life, and even life capable of using technoligies that may be visible remotely, such as agriculture, cities, and other larger scale features that could be visible (c.f. Sagan and Shklovskii’s “Intelligent Life in the Universe”).
We are at the beginning of a very interesting period in our search for ET life. Is it rare (or even unique to Sol) or ubiquitous? Is life mostly unicellular or is complex life common?
Yes, I was thinking along similar lines. Use ‘oddball’ metallicity as a search parameter to find other stars which appear to have had a similar ‘habitability journey’ to our’s and concentrate on searching those for planets. And preferentially listen to them in SETI.
This brings up a good question: What SETI programs are currently operational? What frequencies are they tuned into? How often are they scanning the skies – and which parts, for that matter?
Why do I even need to be asking this question?
I did conduct my own search for these searches, and this is what I found…
https://science.psu.edu/event/current-seti-programs-allen-telescope-array
https://www.inverse.com/science/search-for-extraterrestrial-intelligence-faq
https://www.space.com/searth-extraterrestrial-life-major-funding-boost-seti
https://en.wikipedia.org/wiki/Search_for_extraterrestrial_intelligence
Not quite what I was looking for. I know The Planetary Society let their SETI balls drop a good while ago and attempted to quietly make sure no one would notice.
Their dedicated SETI page isn’t exactly up to date…
https://www.planetary.org/sci-tech/seti
There is NO excuse today, especially with modern technology, why we should not have dedicated telescopes, both radio and optical, conducting SETI 24/7. Otherwise, we are still just doing the token and sporadic efforts just like they did in the 1960s.
It may be the other way around…
It’s important to remember a few key points when considering our understanding of planet Earth. Only 3.75 percent of the Earth’s surface is populated by humans, while approximately 75 percent is covered by oceans and large bodies of water. The remaining areas consist of deserts, mountains, jungles, arctic regions, and other regions that are less frequently observed. Although there are ocean shipping routes and airline paths that traverse some of these areas, their coverage is limited and varies by time. Additionally, cloud cover can obscure vast regions for days, and military observations are typically focused only on select targets. Most individuals rarely look up at the sky, likely doing so less than 1 percent of the time, and telescopes only provide views of a very small portion of the universe. Consequently, much of the Earth remains available for observation and exploration by any extraterrestrial civilization, often without our knowledge.
That figure is very susceptible to how it is measured, whether structures or land that is altered by humans. As regards observing the Earth by aliens, it really depends on how much land surface is unseen by humans over time. A small craft could easily make a night landing and stay hidden even from casual passersby. OTOH, a large craft descending from orbit could be detected almost wherever it attempts to land within many km of a human observer. Landing at the poles or over the ocean is the best bet to remain unobserved, although it would need stealth technology to evade electronic detection.
BTW, did you read about the kerfuffle over “drones” flying over military airfields in Britain and over military bases in the East USA? I find the various reactions, conspiracy theories, and general public “concern” of politicians fascinating. Russians, Chinese, aliens, danger to US security, etc, etc.
Electronic detection refers to raw radar systems, not the identification friend or foe (IFF) technology often depicted in movies: https://en.wikipedia.org/wiki/Identification_friend_or_foe.
These systems have limited use and do not provide global coverage. I worked at NORAD, and during that time, the entirety of the US, Canada, and Alaska was monitored by air surveillance radars due to the threat posed by large Russian nuclear bombers in the 1950s. This network of multiple air surveillance radars was developed by the Massachusetts Institute of Technology (MIT) Radiation Laboratory, which created microwave radar technology during World War II. It was first implemented in the controlled airspace over the Manhattan Project area to safeguard against aerial intrusion by foreign powers during the development of the atomic bomb. I also contributed to the development of stealth fighters and cruise missiles at the Tonopah radar test ranges.
The air surveillance radars extended only 300 miles off the US coast.
Regarding the so-called “dog and pony show,” I believe it serves primarily as a promotional effort to justify military expenditures concerning a nonexistent threat from miniature nuclear bombers, reminiscent of the significant threat posed by Russian nuclear bombers in the 1950s.
The 3.75 percent figure comes from various articles stating that 15 percent of the land area is occupied by the human population in the form of cities, suburbs, and infrastructure, excluding agricultural areas.
Most people work in enclosed environments, such as factories and office buildings. When they are outside, they often look at their cell phones or drive through narrow corridors of tall office buildings. Additionally, light pollution makes it difficult to see anything near cities at night, except for Musk’s army of 7,000 Starlink satellites.
The cities may have polluted skies, but most are not like Manhattan, NY, but are relatively open to the sky. The population density almost assures that someone will be looking up or noticing something. For the sparse population in the agricultural areas and countryside, pollution is far lower, and sightlines are unimpeded for those checking the sky for the local weather.
Apart from military aircraft misidentified over cities, e.g. Denver decades ago, most reports (hoaxes, misidentification?) are from the countryside, the very places that are supposedly easy for alien craft to land.
Bottom line, I think the idea that the US is mostly unpopulated and easy to approach without detection is unwarranted. In reality, a large object trying to make a landing without some sort of effective visual stealth mode is unlikely to be sure that it is unobserved as it approaches. Landing well offshore is a different matter. But like supernatural “sightings”, the state of the observer and their interpretation of what they experience, affects the reporting of an observation. As the majority of the US population believes in angels, I don’t have much faith in the accuracy of people’s interpretation of observations. Maybe that helps any alien craft landings avoid being correctly interpreted.
If the sun and solar system spent lots of time in such dense, dynamic interaction rich galactic environments, it could be interesting from a panspermia perspective too.
More comet bombardment could have elevated the exchange of rocks between different worlds (and planetary systems?).
If stars were packed closer together, travel times for such exchanges could be reduced.
This article about life survived continually in an ancient igneous rock makes this feel more plausible.
https://doi.org/10.1007/s00248-024-02434-8
Sorry, but I’m not talking about the center of the universe “NYC”. I live in the other 96.25 percent of the world for most of my life. That world does not even know were to report strange objects in the sky or even had a cell phone 15 years ago.
Well, I lived in London for half my life. No skyscrapers, no cellphones, and there were various authorities and non-authorities if you wanted to report strange objects in the sky. I’ve lived in and visited other cities, mainly in Europe, and again, until the 1990s, also no cellphones or dense skyscrapers occluding the sky. Back then, few places in the world had any skyscrapers outside of the USA, and none were close-packed like in NYC. NYC might not be the center of the world, but for all intents and purposes, it was Rome to the modern world until relatively recently.
But amazingly, pocket cameras became available, and we had *gasp* landline telephones to communicate with organizations. Worst case, use Shank’s pony to get to a place to report a strange object in the sky or on the ground. (When Wells wrote his fiction “War of the Worlds” they didn’t even have telephones, yet still managed to find the spacecraft in a S E England Common.) It was amazing what you could still achieve with mid-to-late-20th century communication technology and sound and image recording devices. We even had computers in the last quarter of that century. Of course, the authorities would not take you seriously reporting a spacecraft, but you could get in touch with a UFO society if you really wanted to. We Brits are very eccentric and cater to all sorts of ideas. ;-)
Oh I known since childhood how and to where report things, I’ve told about the satellite in Molniya orbit in the past. An event that happened during the 1960’s. Which I later reported to the UFO people after it had been solved what it was so they would not mistake it for something else. They were remarkably uninterested in learning the fact.
Several years later in the early 1970’s I did see one oddity which moved at first with just a faint light but then got very bright and moved like it had been chased by the trolls.
This time I contacted the military, since it had been lower in the atmosphere and a got in touch with a man who were extremely unhelpful – which put me on the track. A friend of mine had a father who worked as one air traffic controller so I got in touch, and even though he did not work that sector he later brought me a photocopy of flight in that area.
It turned out to be a military aircraft, which presumably had turned on the afterburner. And not some kind of rocket test which I first had assumed.
But after these two events in my youth, and having spent both springs, summers and autumns doing fieldwork in very remote location for 40 years. I have to report I have not seen a single thing worth reporting to anyone.
But there’s been two very good fireballs, one which only skipped the atmosphere and then the grand price comet Hyakutake which at closest span a large portion of the sky – yes that seen under a very dark sky and with averted vision. (The closest passage to Earth happened to the Eastern hemisphere so readers from the USA missed this marvelous sight.)
And: Happy X-mas!
Last month, I was reading through the updates from the Interstellar Research Group when I came across an article titled “A Link Between Rocky Exoplanet Composition and Stellar Age.” On the same date, another article, “Solar System Migration Points to a Renewed Concept: Galactic Habitable Orbits,” was also featured. The first article discussed how denser rocky planets tend to be found around younger stars.
There are two perspectives on this finding. One interpretation is that as stars evolve, they produce heavier elements, leading to the formation of denser planets. The alternative interpretation relates to the migration of our solar system through regions that may have caused significant bombardments, similar to the Late Heavy Bombardment (LHB) that occurred between 4.1 and 3.8 billion years ago.
AI notes that the LHB was a cataclysmic event during which shifts in the orbits of outer planets resulted in icy space rocks colliding with inner planets, including Earth. However, it is also possible that as our solar system passed through a dense galactic bar, it encountered a multitude of objects, gas, and a rain of material from that region.
This raises the question: could such large impacts and the influx of significant amounts of water and organic material have lowered Earth’s density by plunging these substances deep beneath its surface?
If life can exist on Venus, it can exist almost anywhere…
https://astrobiology.com/2024/12/a-new-strategy-for-the-exploration-of-venus.html
A New Strategy for the Exploration of Venus
By Keith Cowing
Status Report
The VEXAG Exploration Strategy Study Analysis Workgroup
December 11, 2024
A New Strategy for the Exploration of Venus – The VEXAG Exploration Strategy Study Analysis Workgroup
The 2023-2032 Planetary Science and Astrobiology Decadal Survey Origins, Worlds, and Life recommended that “NASA develop scientific exploration strategies, as it has for Mars, in areas of broad scientific importance, e.g., Venus… that have an increasing number of U.S. missions and international collaboration opportunities” (OWL, p.22-10).
In NASA’s initial responses to that Decadal Survey, the agency asserted that “…specific scientific exploration strategies should be community generated by bodies such as the Analysis Groups,” thus placing the onus on the planetary community to generate and support these exploration strategies.
In late 2022, the Venus Exploration Analysis Group began a project to develop a new exploration strategy for Venus, reflecting the 2021 selections of the VERITAS, DAVINCI, and EnVision missions and the sweeping comparative planetology recommendations relevant to Venus in Origins, Worlds, and Life.
This is that strategy.
Taking a broad look at the scientific, technological, and programmatic advances required to address the key outstanding questions that Venus poses, and predicated on VERITAS, DAVINCI, and EnVision flying as planned in the early 2030s, this report outlines a set of actions available to NASA, VEXAG, and the planetary science community at large to establish a sustained program of Venus exploration in the years and decades ahead.
Key to this approach is recognizing Venus as a unique setting where multiple, cross-disciplinary, Decadal-level planetary, Earth, heliophysics, and exoplanet science questions can be addressed, as well as being a worthy target of exploration in its own right.
This report offers Assessments of the current state of Venus exploration, and Actions for the U.S. and international Venus community, as well as NASA, to consider. This strategy is a living document and should be updated as warranted.
The VEXAG Exploration Strategy Study Analysis Workgroup
Comments: A New Strategy for the Exploration of Venus
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2412.06830 [astro-ph.IM] (or arXiv:2412.06830v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2412.06830
Focus to learn more
Submission history
From: Paul Byrne
[v1] Fri, 6 Dec 2024 22:58:32 UTC (9,425 KB)
https://arxiv.org/abs/2412.06830
Astrobiology
So what difference does it make?
Even if we can confidently establish that some parts of the galaxy are more or less hospitable to life, stars do move around. It probably takes life billions of years to establish itself anywhere (particularly intelligent life) but a typical orbital revolution around the galactic center is about an order of magnitude less. Our own solar example is instructive. Life may arise in life-hostile places, or vice-versa, and the stellar system can then move to some other part of the galaxy to some more or less hospitable place. The same can be said for the presence, or lack of, high metallicity. Some regions my be generally metal free, but rapid stellar evolution in those sterile zones can quickly seed star births in subsequent generations to enjoy local access to plenty of metals for rocky planet formation. For example, if the solar system first appeared in a nebula where little was available in the way of metals, it is certainly possible that a nearby massive star quickly evolved and provided a highly metallic environment (through supernova or planetary nebula formation) and might provide plenty of metals for the new solar system. After all, we’ve found evidence if planets in globular clusters, haven’t we?
I suppose if there are regions of the Milky Way particularly suitable (or not) for the forming of life-bearing planets (even if only for relatively short periods) then it may be more or less likely for biosignatures to be found there, but they can’t be guaranteed or ruled out in either case.
If life, intelligent or otherwise, is common, then it will be found everywhere. We are better to concentrate our searches based on the distances and times at which we can easily observe its expected biosignatures rather than on some conditions that may have existed only temporarily very far away or very long ago.
Don’t worry about it.
Some minor corrections:
Our star evidently migrated outward despite its location within the galactic bar.
No, the Sun undoubtedly formed (and remained) outside the bar. (The argument is that the Sun could have formed closer to the bar than it is now [the radius of the Sun’s orbit is about 8 kiloparsecs, while the bar radius is roughly half that], so that its orbit would have been more strongly affected by the bar than is possible now.)
The visible bar structure is a result of orbits of stars and gas in the galaxy lining up, forming a dense region that individual stars move in and out of over time.
This is partly incorrect. Unlike the case with spiral arms, most of the stars in a bar remain in the bar; they do not “move in and out of [it] over time”. They follow high elongated oval orbits within the bar, so sometimes they are closer to the center of the galaxy and sometimes they are further out — but they stay part of the bar.
Good stuff, Dr. Green. Thanks for the corrections.
Unless there’s a natural process to systematically move stars outward in the galaxy, this interesting article reminds us there’s probably randomness to a star’s trajectory depending upon how it happens to pass massive objects.
This, and the assumption life is significantly more likely to happen in some regions than others suggests that historically-life-developing regions have diverged and are mixing with more-barren star systems.
Perhaps someone has modeled how fast clusters of stars diverge over galactic orbits and perhaps the results might influence search strategies but it does seem worthwhile to try to identify members of our sibling planetary system that may still be nearby and spend spend extra resources studying them.