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Tracing Our Interstellar Relatives

The idea that life on Earth might have originated elsewhere, on Mars, for example, has gained currency in recent times as we’ve learned more about the transfer of materials between planets. Mars cooled before the Earth and may well have become habitable at a time when our planet was not. There seems nothing particularly outrageous in the idea that dormant bacteria inside chunks of the Martian surface, blasted into space by comet or asteroid impacts, might have crossed the interplanetary gulf and given rise to life here.

But what of an interstellar origin for life on Earth? The odds on meteoroids from a system around the average galactic field star not only striking the early Earth but delivering viable microbes are long indeed. But if we consider the Sun’s probable origin in a cluster of young stars, all emerging from the same collapsing cloud, the picture changes significantly.

Now we’re dealing with much smaller distances between stars and slow relative motion as well, conditions that could make such a transfer possible. A new paper makes the case that this process might well have been two-way:

…there is a definite possibility that bacteria carrying meteoroids of extrasolar origin have landed on the Earth. In reverse, it is possible that at least one other planetary system in our birth star cluster received a life-carrying asteroid from the Earth; and it is not excluded that the whole birth star cluster was ‘fertilized’ in this way by live bacteria from the Earth.

So write Mauri Valtonen (Turku University, Finland) and team in an upcoming paper. The authors believe bacterial exchanges between planets of different solar systems could only have occurred during the birth cluster stage of these systems, but given this constraint, it is possible that the Earth received large numbers of life-bearing bodies early on. The broad process of panspermia in which life spreads through an entire galaxy from a single source seems unlikely, but “…life-carrying bodies originating from our solar system may have found their way to our original neighbours, and …all conditions being optimal, life seeded by our system could have spread to many other solar systems.”

And now it gets interesting for future space missions. Because we may be getting into position to find our long lost relatives, the stars from the original cluster that gave birth to our Sun. They’ve long since moved away from us, but missions like Gaia may be able to track them down. Gaia will study a billion stars in the Milky Way, monitoring each some seventy times over the course of a five year period. The mission is expected to discover extrasolar planets, brown dwarfs and numerous other interesting objects, and will help us extend our picture of galactic structure three-dimensionally, perhaps pin-pointing our siblings.

Gaia is expected to be launched in 2011, a much improved version of the Hipparcos mission that did so much to catalog the more than 100,000 stars it looked at. It’s a significant upgrade, and other missions to be launched within the next two decades could go on to provide us with a look at planetary systems around the stars Gaia identifies as members of our original stellar family. If we ever confirm the existence of life on these planets, we may be looking at worlds that either gave birth to life here or received life’s impetus from Earth.

The paper is Valtonen et al., “Natural Transfer of Viable Microbes in Space from Planets in the Extra-Solar Systems to a Planet in our Solar System and Vice-Versa,” accepted by the Astrophysical Journal and available online.

Comments on this entry are closed.

  • david lewis September 12, 2008, 17:15

    Are the odds of panspermia really that low?

    If a life bearing world got smashed several billion years ago, maybe via repeated comet and meteor impacts, a lot of material could have been tossed into space. Maybe as much as trillions of cubic meters of material or more depending on the planet. In space, with much of the material at very low temperatures, some of the simple life would have been in suspended animation and might have survived for millions, if not hundreds of millions, of years. Protected from radiation by a few meters of rock/ice and at such low temperatures what would have destroyed it?

    Stars can move fast – tens of kilometers per second. Assuming a velocity of 10 km/s a star over 100 million years could travel over 3,000 light years. In that time it might have had multiple close encounters with other stars that could have yanked some of that material away from the original star – capturing some of it for itself and sending some of the rest randomly into space. If the original star was in an area where stars are close together, light months instead of years, it could have had a lot of such close encounters – it might have been such an encounter that destroyed the planet in the first place.

    It’s not as though such a projectiles would travel in a straight line and need to be aimed exactly at another planet to hit it. The gravity of the star would potentially grab it and trap it into the system where the odds of it hitting a planet would be greatly increased. Still low, but if trillions of cubic meters of material, forming hundreds of billions of projectiles, were originally cast from the original planet maybe the odds aren’t so low that a number of them might safely deliver the seeds of life to other worlds?

    If on that original world life was just getting started that life might have been well suited for conditions on primitive planets – planets in systems where things have not yet settled down. If so then a number of those worlds themselves might suffer massive impacts that would further distribute the original seeds of life through out space. If the material from the original world managed to seed x worlds and a number of those suffered massive impacts then the process could become exponential and quickly (a billion years or two) spread life through the entire galaxy.

    Such life probably wouldn’t do too well on a world with an existing thriving ecosystem.

  • Eric September 12, 2008, 18:50

    It still doesn’t explain where the life itself came from.

    Great, we’re from the star A1978! So, how did life get there in the first place? Where is the actual origin, and how was it created?

  • Adam September 13, 2008, 4:53

    Hi guys

    David, when interstellar meteoroids are passed between stars the speed has to be low enough so the meteoroids are captured and so they don’t smash into atmospheres too fast for microbes to survive the heat. Hard to do at high speed.

    Eric, panspermia doesn’t answer the origin question, but these days it isn’t meant to. The question is more about the potential size of the biosphere – if bacteria have “infected” the Sun’s birth nebula then the biosphere is HUGE.

  • Ronald September 13, 2008, 6:27

    @david lewis: fascinating idea! If this version of panspermia is indeed the case then it will show in the biochemistry, particularly the similarity of DNA as a genetic coding system.

    In the (distant) future we may be able to analyze and detect these common origins on various planets. However, in order to be able to do so, we would have to go there (or at least send very advanced probes capable of automated detailed analysis), because such biochemical detail can not be detected by means of telescopes analyzing spectral biosignature.

  • Michael Spencer September 13, 2008, 7:03

    I wonder just how close stars were at the point in time we are discussing, and what the conditions were like on the respective plant, too.

    Thanks, David, for putting some numbers together.

    Small point, Paul: in the third graph from the end, ‘because’ usually introduces a subordinate clause, so that sentence is a bit of a clunker, sticking out of your normal prose.

  • Administrator September 13, 2008, 9:07

    Michael writes:

    Small point, Paul: in the third graph from the end, ‘because’ usually introduces a subordinate clause, so that sentence is a bit of a clunker, sticking out of your normal prose.

    True enough, but I like the sound of the sentence. Just an idiosyncrasy :-)

  • andy September 13, 2008, 19:03

    It’d be very interesting to study planetary systems in an old open cluster like M67, which has similar age and metallicity to our Sun. Such clusters might be very interesting SETI targets.

    Another good laboratory for studying panspermia would be a system with multiple Earth-type habitable planets, e.g. as moons of a gas giant or a Trojan configuration.

  • Adam September 14, 2008, 1:05

    Hi All

    Greg Egan’s latest book “Incandescence” takes cues from Lineweaver’s Galactic Habitable zone and has 14 independent panspermia waves filling the disk. Part of the story involves tracing the origins of a meteoroid with traces of DNA life near the Core.

  • Michael Spencer September 14, 2008, 7:41


    One of my favorite authors- Peter Hamilton- has the same ‘ear’ for introductory clauses. I don’t know where the two of you- and Peter’s editor- got that tin ear :-)

    By the way, I write a weekly column for the local paper. This morning when I logged in and was disappointed not to see a new piece from you I remembered just how hard it is to crank out columns. Some weeks I get three or more into the process, and some weeks- nothing, but you manage to crank them out fairly easily.


  • Administrator September 14, 2008, 8:23

    Michael, I do enjoy the regular posting process. Often just knowing I have one to do jogs the mind and gets things happening, so maybe I’m just a creature of habit. By the way, there will probably be no Centauri Dreams entry on Tuesday of this week as I get some needed obligations out of the way — things should get back to normal on Wednesday.

    I’ve written a number of weekly columns for various venues. What paper do you write for?

  • Athena September 14, 2008, 15:50

    If technologically feasible, it will be interesting to do a track-back: recreate the paths of the nearest star neighbors of the solar system since the time that life arose on earth and focus our life-searching efforts along those paths. Such life would be relatively easy to recognize, because it would be based on the same foundational principle as terrestrial life (carbon, water, nucleic acids as the genetic components).

  • Adam September 14, 2008, 22:28

    Hi Athena

    I think that’s one of the hoped for results of one of the big astrometry satellite missions coming up. Eventually the whole Galaxy will be possible to back-track, but not for a few decades yet.

  • Ron S September 15, 2008, 13:40

    Athena, even at a relative velocity of 10 km/s, that’s still 33 ly of linear distance per Myr. Life started round about 3.5 to 4 Gyr ago. This was also about 20 galactic rotations ago. The pot of stars has been thoroughly stirred.

    Having said that, it is possible, though I think the probability is low, that in cases where the motions of ourselves and a former companion stellar system both were along a galactic radius there would be some stability in our relative paths, even if not lasting for several Gyr, with our galactic orbital paths crossing from time to time. This is only a conjecture on my part.

    We may be better off looking for chemical signatures of other stars to see if the ‘dna’ is a close match to that of the Sun.

  • ljk September 17, 2008, 8:05

    Immigrant Sun: Our star could be far from where it started in
    Milky Way


    “A long-standing scientific belief holds that stars tend to hang out
    in the same general part of a galaxy where they originally formed.

    Some astrophysicists have recently questioned whether that is
    true, and now new simulations show that, at least in galaxies
    similar to our own Milky Way, stars such as the sun can migrate
    great distances.”

  • Ad Koppen October 12, 2008, 11:27

    Seeds from trees and plants has in my view the most chance to overcome the heat and pressure in a descent within a meteorite.
    I believe that in the future scientist will see that many planets with oxigen rich atmospheres homes trees and plants.
    Seeds can easily overcome very long time in wandering rocks and in the cold vacuum. So our extraterrestrial neighbours are already here for billions of years. They started the whole cycle of live.
    Bacteria are to delicate.

  • ljk November 6, 2008, 23:44

    Very cold ice films in laboratory reveal mysteries of universe.
    Could life have started in a lump of ice?


    “The universe is full of water, mostly in the form of very cold ice
    films deposited on interstellar dust particles, but until recently little
    was known about the detailed small scale structure.

    Now the latest quick freezing techniques coupled with sophisticated
    scanning electron microscopy techniques, are allowing physicists to
    create ice films in cold conditions similar to outer space and
    observe the detailed molecular organisation, yielding clues to
    fundamental questions including possibly the origin of life.”