Life Throughout the Galaxy?

by Paul Gilster on December 22, 2006

Is there a galactic habitable zone, a region within the Milky Way where conditions for life are optimum? If so, we want to know its parameters, as they would help us define the search area for living worlds. The concept has kicked around for a while, and now surfaces again in an interesting paper by Nikos Prantzos (Institut d’Astrophysique de Paris). Prantzos ponders the main variables and, while concluding that the galactic habitable zone is far from well understood, believes it conceivable that the entire galactic disk may, at this stage of its evolution, be suitable for life.

That conclusion goes further than Charles Lineweaver and team’s work in 2004, the latter having found that the zone for complex life exists in a ring a few kiloparsecs wide surrounding the galactic center and gradually spreading outward as the Galaxy evolves (our earlier story on that work and habitable zones in general is here). Like Lineweaver, Prantzos looks at planet formation in terms of stellar metallicity (the amount of elements heavier than hydrogen and helium in the body of the star), using later data based on simulations of planet formation.

Artist's conception of the Milky Way

Image: An artist’s conception of the Milky Way. Can the entire galactic disk now be considered a viable habitable zone? Credit: NASA, ESA and The Hubble Heritage Team (AURA/STScI).

So what do we really know about metallicity? Stars hosting planets show high metallicity compared to stars that have no planets, a trend that points to metallicity as a major factor in gas giant formation. But note: the effect of metallicity on Earth-like worlds is unknown, and some theories even suggest that low metallicity may stimulate terrestrial planet formation. If that work (still unpublished) bears out, then Earth-like planets may be common in low metallicity environments like the outer edges of the galaxy.

Another key factor is the risk of life being exterminated by nearby supernovae events. A hostile cosmic environment, in other words, could sharply limit the number of life-bearing worlds. But Prantzos argues that the supernova threat is hard to characterize. If all land animals on a planet die because of nearby supernova radiation, marine life will most likely survive. Here the author comments on the survivability question:

In the case of the Earth, it took just a few hundred million years for marine life to spread on the land and evolve to dinosaurs and ultimately, to humans; this is less than 4% of the lifetime of a G-type star. Even if land life on a planet is destroyed from a nearby SN explosion, it may well reappear again after a few 108 years or so. Life displays unexpected robustness and a cosmic catastrophe might even accelerate evolution towards life forms that are presently unknown. Only an extremely high frequency of such catastrophic events (say, more than one every few 107 yr) could, perhaps, ensure permanent disappearance of complex life from the surface of a planet.

Then factor in galactic evolution, which suggests that the rate of supernovae, although always higher in the inner disk, declines over time, and survivability in the inner disk regions goes up as the galaxy ages. The ‘ring’ of survivability is narrow early on but progressively migrates outwards, extending perhaps to the outer rim of the galaxy today. There follows this interesting consequence:

Thus, despite the high risk from SN early on in the inner disk, that place becomes later relatively ‘hospitable.’ Because of the large density of stars in the inner disk, it is more interesting to seek complex life there than in the outer disk; the solar neighborhood (at 8 kpc from the center) is not privileged in that respect.

Prantzos is no dogmatist. A clear-eyed theorist, he takes pains to note that these findings depend heavily on our assumptions about metallicity and planet formation (and the question of migrating ‘hot Jupiters’ weighs in significantly as well). But if most of the galaxy really is suitable for life, then the concept of a galactic habitable zone loses punch. We may find that our search for extraterrestrial life can roam broadly through the cosmos.

The paper is Prantzos, Nikos, “On the ‘Galactic Habitable Zone,’” slated for publication in Space Science Reviews and available as a preprint online. The Lineweaver paper is “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.


andy December 22, 2006 at 10:12

That just deals with the galactic disc. What about the galactic bulge, which can evidently form low-mass planets (e.g. OGLE-2005-BLG-390Lb)

Administrator December 22, 2006 at 10:49

Andy, Prantzos’s work implies that there is no privileged area within the galaxy in terms of life formation. Not at the current state of the galaxy’s evolution.

andy December 22, 2006 at 12:10

I see that, however they state: “Note that the evolution of the Galactic bulge (innermost ~2 kpc) is not studied here, since it is not well constrained”, though this does not matter for the properties of the disc.

Administrator December 22, 2006 at 13:18

Whoops! Sorry, Andy, I didn’t see that. In that case, I don’t have any good information re habitability in that region. Maybe someone else here can weigh in if there are some papers that study the issue.

ljk December 22, 2006 at 13:19

Published in Science, Jan 2, 2004

The Galactic Habitable Zone and the Age Distributionof Complex Life in the Milky WayCharles H. Lineweaver,1,2* Yeshe Fenner,3* Brad K. Gibson3*

1Department of Astrophysics, University of New South Wales, Sydney, NSW 2052, Australia.2Australian Centre for Astrobiology, Macquarie University, NSW 2109, Australia.3Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn, VIC 3122, Australia.

We modeled the evolution of the Milky Way to trace the distribution in space andtime of four prerequisites for complex life: the presence of a host star, enoughheavy elements to form terrestrial planets, sufficient time for biological evolutionand an environment free of life-extinguishing supernovae.

We identified the Galactic Habitable Zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago. This GHZ yields an age distribution for the complex life that may inhabit our Galaxy.

We found that 75% of the stars in the GHZ are older than the Sun.

Full article here:

Kurt9 December 22, 2006 at 13:29

I think the paper correct about the effects of catastrophies (supernovas and the like) on the development of life. This suggests that the inner part of the galaxy ought to be OK for life. I’m not sure about metallicity, however. There is a clear co-relation between metallicity and the existance of gas giant planets. However, gas giants are mostly light elements. Terrestrial planets, on the other hand, are comprised of heavier elements, which probably need high metallicity in the parent star in order to exist. Thus, I would think that a solar system that is of low metallicity will likely consist of KBOs and smaller planets with the elemental make up of gas giants.

Hense, the outer galaxy will still have poor prospects for Earth-like planets. In any case, there are a fair number of G and K stars with varying matallicity within 100 light years of us that can be imaged to detect terrestial-sized planets in the next few decades.

For the purposes of intersteller travel (both sub-light and FTL, if possible), the steller neighborhood is most relevant, and that is definitely within the “galactic habitable zone”.

ljk December 22, 2006 at 14:00

Astrophysics, abstract

From: Nikos Prantzos [view email]

Date: Thu, 21 Dec 2006 12:32:20 GMT (388kb)

On the early chemical evolution of the Milky Way

Authors: Nikos Prantzos (Institut d’Astrophysique de Paris)

Comments: 11 pages, 7 figures, Invited talk, “Nuclei in the Cosmos IX” (CERN Geneva, July 2006), to appear in Proceedings of Science (Eds. A. Mengoni et al.)

A few topics concerning the early chemical evolution of the Milky Way are critically discussed. In particular, it is argued that: 1) Observed abundance patterns of extremely metal poor stars (of Pop. II) do not constrain the mass range of the first generation (Pop. III) stars; the latter may well be normal massive stars (10-50 Msun) or very massive ones (140-1000 Msun) or a combination of the two classes. 2) The discrepancy between primordial Li abundance (after WMAP) and the observed “Spite plateau” cannot be due to astration by a generation of massive Pop. III stars, as recently suggested, unless if such stars eject negligible amounts of metals. 3) The observed halo metallicity disribution may well be understood in the framework of hierarchical galaxy formation, as shown here with a simple semi-analytical model. 4) Formation of the Milky Way’s halo from a myriad of smaller sub-haloes may have important implications for our understanding of the abundance patterns of r-elements, the origin of which remains still unclear.

Administrator December 22, 2006 at 14:14

Prantzos argues that there is an emerging case for the planet/metallicity connection to break down when it comes to terrestrial-sized worlds. Gas giants, yes, but there is no firm correlation yet in terms of rocky planets, and the unpublished work discussed in this paper points to an opposite conclusion — that terrestrial worlds may actually form more readily around stars of lower metallicity. Clearly, this question is in need of a lot more research!

Kurt9 December 22, 2006 at 15:09

Yes, I agree that the link between terrestrial planet formation and metallicity needs to be investigated, given that no such link currently exists. The next generation of instruments, either in orbit or on the ground, may be able to detect terrestrial-sized planets in the next decade. However, I think Parntzos is wrong about the possibility that an opposite corelation might exist.

Gas giant planets certainly have much more mase than terrestrial planets. However, it you strip off the “gas giant” part, which is mostly light elements, you end up with a solid core not much larger than the Earth. This core is comprised of the heavier elements (silicon, metals, you name it) that, if current theories of star formation are correct, require large amounts of steller “metals” in order to come into existance. There being a relatively fixed ratio of light elements to heavy elements. So, if there is not enough of the light elements to make gas giants, it seems to me that there certainly would not be enough of the heavy elements to make terrestrial-sized planets.

In any case, more research is needed.

ljk December 22, 2006 at 19:41

Regarding the Lineweaver et al paper posted above that states
perhaps 75% of the stars on the GHZ are older than Sol: Perhaps
instead of being among the first intelligent species in the galaxy to
evolve, we are actually just the latest.

If ETI are even just a few million years ahead of us, this might
also explain why we haven’t found any yet.

Christopher L. Bennett December 23, 2006 at 14:41

This new paper is good to see. I’ve always been skeptical of the “galactic habitable zone” idea. Even if the points it made were valid, they were still statistical points, stating that habitable-planet formation would be less probable in certain areas. With billions of stars to choose from, “less probable” certainly does not mean “impossible,” yet the GHZ hypothesis pretty much assumed that it did, talking about it as though there would be no intelligences or civilizations at all in regions outside the GHZ, rather than just a lower abundance of them. So I always thought “habitable zone” was a misnomer for it; it would make more sense to call it a galactic temperate zone, say.

Also I’m glad to see astronomers catching on to the idea that environmental stressors such as supernovae can actually promote evolution rather than preventing it. That was another of my main problems with the GHZ idea. The thing about life is that it adapts to its environment. If frequent radiation bombardments are part of a planet’s environment, doesn’t it stand to reason that the life there could evolve radiation-resistance mechanisms of sorts that Earthly life has simply never needed to evolve?

Administrator December 23, 2006 at 15:26

“Also I’m glad to see astronomers catching on to the idea that environmental stressors such as supernovae can actually promote evolution rather than preventing it.”

Well said! Life’s adaptability is extraordinary, and obviously a major factor in how we develop our habitable zone ideas.

ljk December 23, 2006 at 22:55

And please note that life on Earth began almost as soon as
the planet cooled at least 3.8 billion years ago. And even with
all the major extinction events, including the Permian-Triassic
which wiped out over 90 percent of all life on Earth everywhere,
life recovered and flourished repeatedly.

Add 400 billion star systems to the mix with who knows how
many planets (and don’t forget the 100 billion galaxies in the
known Universe).

pinlighter December 24, 2006 at 9:31

Bacterial life under a planet’s crust or in the core of a comet or planetismal would obviously not be affected by supernova.

If such life can spread between solar systems the chances are that it predates the formation of the Earth and was already present in the planet-forming disk round the sun.

There would be rich but transient zones in the disk where bacterial life could thrive, not in vacuum but at depths inside forming planetismals where the temperature rose above freezing point.

This could produce trillions of tons of bacterial biomass distributed throughout the disk, and persisting over a period of millions of years.

This era of rich early life would end when the planets formed and most of the bacteria were boiled or crushed to death, but residual bacteria would remain to seed the planets with life and escape on hyperbolic comets to fertilise other disks.

All this is obvious enough and not new, but if we consider that the majority of the galaxy’s raw biomass could reside in such planet-forming disks it changes the criteria for “galactic habital zone” quite a bit.

What criteria would make a planet-forming disk hospitable to bacterial life? I would have said high metallicity combined with a long duration of disk formation, but supernovae are almost irrelevant.

Matt January 15, 2007 at 16:29

Unless the life-supporting planet is orbiting the star that goes supernova, I don’t think that too much else could completely eliminate life on a planet. Even if the planet was destroyed somehow some form of life might be able to survive. I seem to remember that a Moon mission left bacteria up there and when it was retrieved it was still alive, only dormant in spores.

ljk January 15, 2007 at 17:41

That was Apollo 12, which landed on the Moon in late 1969
and returned pieces of the robot lander Surveyor 3, which
had landed in the same spot over two years earlier.

Supposedly there was bacteria found inside the lander’s
TV camera upon examination on Earth, but it was later
indicated that the Surveyor pieces were not always in sterile
conditions, so it is possible that the bacteria only appeared
on the lander part after their return to Earth, not before.

Could microbes survive inside an unheated spacecraft
for several years on the Moon? It is possible, considering
how hardy some microbes are, but the ones from Surveyor 3
are not conclusive at this time.

Scroll down this Web page on Apollo12 to find references on
this topic:

ljk October 31, 2007 at 16:18

How Rare Is Complex Life in the Milky Way?

“An integrated Earth system model was applied to calculate the number of
habitable Earth-analog planets that are likely to have developed primitive
(unicellular) and complex (multicellular) life in extrasolar planetary systems.
The model is based on the global carbon cycle mediated by life and driven
by increasing stellar luminosity and plate tectonics.”

ljk December 20, 2007 at 11:18

Astrobiological Effects of F, G, K and M Main-Sequence Stars

Authors: M. Cuntz, L. Gurdemir, E. F. Guinan, R. L. Kurucz

(Submitted on 19 Dec 2007)

Abstract: We focus on the astrobiological effects of photospheric radiation produced by main-sequence stars of spectral types F, G, K, and M. The photospheric radiation is represented by using realistic spectra, taking into account millions or hundred of millions of lines for atoms and molecules. DNA is taken as a proxy for carbon-based macromolecules, assumed to be the chemical centerpiece of extraterrestrial life forms. Emphasis is placed on the investigation of the radiative environment in conservative as well as generalized habitable zones.

Comments: 3 pages, 3 figures; submitted to: Exoplanets: Detection, Formation and Dynamics, IAU Symposium 249, eds. Y.S. Sun and S. Ferraz-Mello (San Francisco: Astr. Soc. Pac.)

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0712.3257v1 [astro-ph]

Submission history

From: Manfred Cuntz [view email]

[v1] Wed, 19 Dec 2007 19:54:21 GMT (87kb)

Astrobiology in the Environments of Main-Sequence Stars: Effects of Photospheric Radiation

Authors: M. Cuntz, L. Gurdemir, E.F. Guinan, R. L. Kurucz

(Submitted on 19 Dec 2007)

Abstract: We explore if carbon-based macromolecules (such as DNA) in the environments of stars other than the Sun are able to survive the effects of photospheric stellar radiation, such as UV-C. Therefore, we focus on main-sequence stars of spectral types F, G, K, and M. Emphasis is placed on investigating the radiative environment in the stellar habitable zones. Stellar habitable zones are relevant to astrobiology because they constitute circumstellar regions in which a planet of suitable size can maintain surface temperatures for water to exist in fluid form, thus increasing the likelihood of Earth-type life.

Comments: 4 pages, 4 figures; submitted to: Bioastronomy 2007: Molecules, Microbes and Extraterrestrial Life, eds. K. Meech, M. Mumma, J. Siefert and D. Werthimer, A.S.P. Conf. Ser

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0712.3260v1 [astro-ph]

Submission history

From: Manfred Cuntz [view email]

[v1] Wed, 19 Dec 2007 19:25:24 GMT (130kb)

ljk July 27, 2009 at 22:06



Planetary Science Institute, Research School of Astronomy and
Astrophysics, Research School of Earth Science,
Australian National University, Canberra, ACT 0200 Australia

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