Life, planets and the import/export business

Every now and then, a paper appears on astroph that has us thinking – is this serious? Or is this an imagination gone a tad wild? (Or is it April 1st?) For example, a friend at the IoA once wrote a proposal (for a graduate course) aiming to find extrasolar planets through bioluminescence. Rarely has an observing proposal been so entertaining.

So it was this week when this paper appeared on astroph. The proposal is as follows: “Genomic complexity” estimates that life on earth began 10 billion years ago (give or take three billion years). This is older than the solar system. A slight problem, perhaps. Suppose, perchance, that life didn’t develop on Earth. Suppose that, long before the solar system was formed, a planet was ejected from another planetary system. En route, this wandering planet (or rogue planet) forms life deep underground, shielded from cosmic rays, lethal radiation and freezing temperatures on the planet surface and warmed by radioactivity of the planet core.

Now suppose that the planet wanders into our solar system. The planet is most likely on an unbound orbit, and thus this will not be a long visit. (The paper estimates this number at around 43 years). But, bathed in the radiation of the sun and the solar wind, the surface of the planet may be stirred up enough to throw rocky shards off the guest planet. Should one of these shards reach a newly-formed Earth, and carry with it life-forms, then we would receive a population of organisms with a few billions of years of evolution behind them – they would be older than the Earth itself.

The central calculation of the paper is an estimation of the probability that a wandering planet would, by chance, wander into the planetary zone of another solar system. They estimate that “3 out of every 10000 free planets manage to enter some planetary system”. Thus:

if there are enough of these planets there will be enough encounters so that panspermia becomes an important mechanism in the propagation of life in the universe. The transit times of the guest planets are long enough so that by stellar wind or by asteroid impacts, the SLF [simple life forms] may be transferred from the guest planet to the host planetary system. The hypothesis of the free planets solves the problems presented by panspermia in meteorites, comets or the SLF alone because it do not have the problem of exposure to space.

What are we to make of this? Well, it’s certainly an interesting idea. There are a couple of points that should make us pause to think.

1. The entire article is written in comic sans.
2. I’m surprised that the prediction of “genomic complexity” for the age of life is so confidently predicted to be 10 Gyr, give or take 3 Gyr. I don’t know the field well enough to know whether this is a widely accepted figure.
3. The paper states that the planet, whilst wandering through interplanetary space, would have an effective temperature of 30K. Without abundant liquid water, one assumes that chemicals will be less mobile, and thus life would have an even harder time forming on the wandering planet than on Earth.
4. Also, during the planets interstellar voyage, it needs to catch up on 5 or so billion years of evolution. This is a problem. The 10 billion year “genomic complexity” clock presumably assumes rates of evolution as they are on Earth. But evolution surely must proceed slower on the wandering planet, because all chemical reactions proceed slower at lower temperatures. If the dependence on temperature were linear, then 5 billion years of reactions at 300K on Earth would take 50 billion years on the wanderer – much longer than the age of the universe.
5. How many wandering planets are produced during the formation of the average planetary system? I don’t think that anyone would even pretend to know the answer to that question. A quick calculation of the energetics reveals that it is not out of the question – accelerating an Earth-sized planet to escape velocity at the orbit of Jupiter would only require 1/30th of Jupiter’s orbital kinetic energy. Presumably,the mechanism for such an ejection is a gravitational scatter.
6. The “three in 10,000” figure is probably approximately correct, but we would need to fold in (inter alia) the probability that life survived the journey, the probability that life-bearing rocks would be ejected from the planet during its 40 year visit to our solar system, the probability that one of those rock would find earth and the probability that life-survives re-entry. Surely, only the simplest, smallest life-forms could survive such conditions. The question is then whether this gives life on Earth all the headstart it needs.

There is plenty more to say, but I’ll sum up. We’re a long way from a fully-formed theory, so calling the idea “viable” is a bit of a stretch. The astrophysics – wandering planets visiting other solar systems – is at least plausible. How one could test the idea … I have no idea.