Radio image of the galactic plane. Image credit: Axel Mellinger, Natasha Hurley-Walker, GLEAM
Most studies of the Galactic Habitable Zone (GHZ) focus on metallicity and catastrophic events such as supernovae to define where life might emerge in the Milky Way. However, far less attention has been given to the chemical precursors that actually enable prebiotic chemistry. In particular, the distribution of formaldehyde (H2CO) - a key building block for sugars and amino acids - had not been systematically studied in the inner Galaxy, a region long considered too hostile for fragile molecules to survive. H2CO was previously detection in the far outer galaxy, which extends the GHZ to such extreme distances (Blair et al., 2008).
By surveying over 200 molecular clouds using radio observations, H2CO was detected in about 40% of the sources, including many previously unreported detections (Baharin et al., 2025). Strikingly, these signals appeared at distances as close as 0.2 kiloparsecs from the Galactic Center, deep inside regions where the GHZ is usually thought to be truncated by intense radiation, stellar collisions, and frequent supernovae. This shows that dense molecular clouds can shield and preserve prebiotic molecules even in extreme environments.
Face-on view of the Milky Way showing locations of H2CO detections in BGPS molecular clouds, with the green annulus marking the traditional GHZ definition
These findings challenge the conventional view of the GHZ as a narrow ring between 7–10 kiloparsecs. Instead, they suggest that chemical markers like H2CO, alongside other molecules such as ammonia and hydrogen cyanide, should be integrated into how we define galactic habitability. By reframing the GHZ to include chemistry as well as astrophysical hazards, the results highlight that the ingredients for life may be more widely distributed across the Galaxy than previously believed.
References
Formaldehyde in the Far Outer Galaxy: Constraining the Outer Boundary of the Galactic Habitable Zone
Samantha K. Blair, Loris Magnani, Jan Brand, and Jan G.A. Wouterloot
We present a comprehensive study of formaldehyde (H2CO)
absorption and radio recombination line (H110α) emission in
215 molecular clouds from the Bolocam Galactic Plane Survey, observed
using the Nanshan 25Â m radio telescope. H2CO was detected in
88 sources (40.93%) with 59 being new detections, while H110α
emission was found in only 11 sources (5.12%), all coincident with
H2CO absorption. There exists a correlation of
H2CO fluxes with millimeter fluxes below a \(3~\mathrm{Jy}\) threshold and an increased
dispersion above it, suggesting the sub-cosmic microwave background
cooling of H2CO. Cross-matching with kinematic distance
catalogs revealed H2CO spanning galactocentric distances from
\(0.216\) to \(10.769~\mathrm{kpc}\), with column
densities ranging from \(7.82 \times
10^{11}\) to \(6.69 \times
10^{14}~\mathrm{cm}^{-2}\). A significant inverse correlation was
observed between H2CO detection fraction and galactocentric
distance, suggesting enhanced star-forming activity closer to the
Galactic Center. These findings challenge traditional Galactic Habitable
Zone (GHZ) models by demonstrating the presence of biogenic precursors
in the inner Galaxy, shielded within dense molecular clouds. Our results
underscore the importance of incorporating chemical tracers such as
H2CO, alongside physical constraints to refine the boundaries
of the GHZ and advance the research of prebiotic chemistry in the Milky
Way.
@article{Baharin2025,title={Rethinking Habitability Using Biogenic Precursors: Formaldehyde in Millimeter Molecular Clouds of the Inner Galaxy},volume={988},issn={1538-4357},url={http://dx.doi.org/10.3847/1538-4357/add691},doi={10.3847/1538-4357/add691},number={1},journal={The Astrophysical Journal},publisher={American Astronomical Society},author={Baharin, Nursyazela Badrina and Nazri, Affan Adly and Rosli, Zulfazli and Abidin, Zamri Zainal and Tajuddin, Hairul Anuar and Esimbek, Jarken and Li, Da Lei and Tang, Xiaoke},year={2025},month=jul,pages={36},}
