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?