Is it possible for life to spread across a galaxy as large as the Milky Way without technology? This question remains unanswered. The new study uses a simulation galaxy similar to the Milky Way to answer that question. They then looked at the model to determine how organic compounds might travel between its star systems.
Science's central question is likely How did life start? Unfortunately, there has been no satisfactory answer. The secondary question, which is easier to answer: Can life spread star by star? This is the theory of panspermia.
Panspermia is a key question in Earth's history. Scientists believe there wasn't enough time between the Earth becoming habitable and the appearance life. However, not all scientists believe that. There are many opinions on the subject. The question is: Was DNA-based life able to develop independently on Earth? Or did panspermia play an important role?
Panspermia is often about simple lifeforms moving between stars. But there's more serious talk about the movement of essential organic compounds that are necessary for life. Scientists have discovered some of these compounds on comets, as well as elsewhere in space. They are not always rare, we now know. These compounds can be moved from one solar system to another.
Raphael Gobat from Instituto de Fsica Valparaso in Chile is the lead author of the new study. The paper can be downloaded from arxiv.org.
Is panspermia possible? It seems possible in a solar system similar to ours. It is quite possible that meteorites from Mars have reached Earth. This is solid evidence. Why not use chemicals on or in rocks that can make the trip? Could spores make interstellar travel between star systems?
This was the question that the research team set out to address. The team used a simulated galaxies from MUGS (the McMaster Unbiased Galaxies Simulations). MUGS is a collection of 16 simulated galaxies that were created by researchers in 2000. Gobat et al. added GH16 to their modified galactic habitability modeling in 2016.
The galaxy they chose is g15784. It is slightly larger than the Milky Way, and has a history with quiescent mergers. It hasn't merged with any large objects in a very long time and is orbited by many spherical galaxyes.
This is the simulation galaxy g15784. The image shows two spheroidal galactic galaxies, one located above the galactic plane, and one located below. Image Credit: Gobat et al 2021.
The team calculated a level of habitability per star particle in the galaxy. This is the number of main sequence low mass stars that have terrestrial planets in their habitable zones. To do this, they used GH16. GH16 considers stellar metallicity, maximum and minimum mass, formation history, as well as the inner and outer ranges (HZ.) of its habitability area (HZ.).
They also looked at the impact of supernovae on habitability. The galaxy's galactic core is home to the largest concentration of people. There are more potential habitable planets in the galactic core, but there are also more supernovae. Each habitable planet is more likely to be destroyed by a supernova due to the higher density of stars within its core. According to the authors, the higher metallicity of the core can also decrease habitability. This makes the central region difficult for panspermia.
The group also examined the spiral arms at g15784. Although star density and supernova rates (SNR) are both high, they did not affect habitability as much as the bulge. They also examined the galactic disc and halo.
Three-panel paper figure showing a projected column at the z=0 position and a slice of 1 kpc width passing through the middle of g15784. The median value for natural habitability is shown at the top. The middle shows the number of possible cradles within the simulated galaxy. And the bottom shows the possible colonization targets. The magenta star indicates where the Sun would be in a Milky Way scenario. Image Credit: Gobat et al 2021.
Panspermia, although it is possible, was shown in the study. However, there is no easy answer. The median habitability of galactocentric radius increases with greater probability for panspermia, but the latter is not. This is due to the increased star density within the galactic bulge.
However, the central disk has a low panspermia chance. This is due to the higher supernova rates and lower escape fractions due to higher metallicity. Panspermia probabilities vary widely across the galaxy. Natural habitability is not affected by any changes in the galaxy.
The probability of panspermia was not related to the habitability of the received particle, according to the team. The simulations were of low resolution, so particle is a reference to a large number of stars.
They also found that panspermia was less effective than in situ prebiotic evolution. However, they don't know how to quantify it.
The authors conclude their work with a few caveats. First, it contains several factors we consider unknown constants (e.g. the capture fraction by target planets, relation between habitability, presence of life and speed of interstellar objects and absolute value of escape fractions of interstellar organic substances from source planets).
The researchers cautioned that, while real galaxies like the Milky way are dynamic and change constantly, their simulation galaxy is only a snapshot. These results are only applicable if panspermia's typical timescale is shorter than that of a galaxy.
The simulated galaxy is different from the Milky Way in other ways. There are other differences between the simulated galaxy and the Milky Way.
Two interstellar objects visited us recently: Oumuamua, and comet 2L/Borisov. We now know that objects travel between star systems. We are likely to have seen many more interstellar visitors than we were technologically able to see. We know that organic building blocks exist in space.
Although this doesn't prove that organic building blocks are capable of traveling between stars, it does suggest it is possible. This research has given us some insight into how likely it is and where it might occur.
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