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Most of the Milky Way's life-hosting planets may have formed early on

An artist's illustration of the potentially habitable exoplanet Kepler-1649c orbiting its host red dwarf star.
An artist's illustration of the potentially habitable exoplanet Kepler-1649c orbiting its host red dwarf star.
(Image: © NASA/Ames Research Center/Daniel Rutter)

Rocky planets that formed early in our Milky Way galaxy's 13.2-billion-year history have a better chance of supporting life than worlds that were later to the party, a new study suggests.

That's because the early formers are more likely to sport plate tectonics, magnetic fields and other features conducive to the development and persistence of life as we know it, researchers said.

"Plate tectonics is important for habitability, and it looks like the optimum conditions [for] plate tectonics existed for planets forming early in the galaxy’s lifespan, and may be unlikely to easily recur," study lead author Craig O'Neill, director of the Macquarie Planetary Research Centre at Macquarie University in Sydney, Australia, said in a statement. "For life, maybe that was as good as it gets."

Related: The Biggest Alien Planet Discoveries of 2019

O'Neill and his colleagues studied Milky Way exoplanets — worlds orbiting stars other than the sun.   

"Because of the great distances involved, we have a limited amount of information on these exoplanets, but we can understand some factors, such as position, temperature and some idea of the geochemistry of the exoplanets," O'Neill said. "This allows us to model how they develop."

The team plugged these parameters into planet-development simulations run using processors at the National Computational Infrastructure, which is based at Australian National University in Canberra. 

The researchers found that planets that form relatively early in their galaxy's history are more likely to develop plate tectonics, which serves as a built-in thermostat of sorts that keeps the surface cool enough for life as we know it to evolve. 

The lack of plate tectonics is bad news for life in multiple ways, O'Neill said.

"This doesn't just affect the surface temperature; this means that the core stays hot, which inhibits the development of a magnetic field," he said. "If there's no magnetic field, the planet is not shielded from solar radiation, and will tend to lose its atmosphere. So life becomes difficult to sustain. A planet needs to be lucky to have the right position and the right geochemistry at the right time if it's going to sustain life."

A planet's geochemistry is also affected by the era in which it formed, by the way, because the chemistry of galaxies changes over time. For example, later on, more heavier elements have been distributed through supernova explosions, and less lighter material (such as hydrogen or helium) may be available for forming stars and planets. 

The new results were presented last month at the Goldschmidt Virtual 2020 conference, which ran from June 21 through June 26. 

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  • rod
    A very interesting simulation and unexpected results. The source or statement cited in the article says this, "He commented: "Plate tectonics act as a kind of thermostat for the Earth creating the conditions which allow life to evolve. The Earth has a lot of iron in its core, and we had assumed that this would be necessary for tectonic development. However we found that even planets with little iron may develop plate tectonics if the timing is right. This was completely unexpected"., https://www.eurekalert.org/pub_releases/2020-06/gc-lit061920.php
    Commonly in exoplanet studies, rocky planets need to form around host stars with certain metal abundances but this new simulation calls that thinking into question by pushing those type of exoplanets back, earlier in MW history according to stellar evolution dating method where less metals should exist. I used this exoplanet site, http://exoplanet.eu/, listing 4281 confirmed that shows host star ages too. I ran MS SQL query for host stars between 6E+9 to 13.2E+9 years old, found 190 exoplanets listed. It would be interesting to see if any of the 190 exoplanets support the simulation model. Their distances range 6 to 5448 LY, average metallicity (Fe/H), 1.07 solar. Average exoplanet mass 3.76 Mjup, average host star mass 0.9235 solar.

    This report does call attention to a constraint for life on exoplanets, plate tectonics and a magnetic field.

    "Researchers have found that rocky exoplanets which formed early in the life of the galaxy seem to have had a greater chance of developing a magnetic field and plate tectonics than planets which formed later. As both these conditions are considered favourable to the development of life, this means that if life exists in the Galaxy, it may have developed earlier than later, and that planets formed more recently may have less chance of developing life."

    Defining constraints on models used in science is important--Rod
    Reply
  • Helio
    What I had expected to read was something specific to why later formation would directly affect plate tectonics?

    IIRC, only about 1% of cloud composition is dust, the rest being H or He, primarily. If we increase that 1% due to time and SN activity, say to 2%, how would that prevent plate tectonics? I would have guessed it would have helped, so what am I missing?

    Earth had the good fortune of encountering Theia. The result of that event gave us a larger than normal iron core and a hot one for greater convection and tectonic action, not to mention a large moon (plausibly two initially) that gave inclination stability important for climate control.

    Also, SN produce radioactive material, so later formation would produce hotter and more convective cores, hence more tectonic activity seems likely but this report states otherwise, but why?
    Reply
  • rod
    This new simulation and report may affect the Drake equation too and modifications that followed. Perhaps some are scrambling even as I post this note here :)
    Reply
  • Helio
    rod said:
    This new simulation and report may affect the Drake equation too and modifications that followed. Perhaps some are scrambling even as I post this note here :)
    :) The Drake Equation uses broad terms to give us the big picture, but the number of sub-variables for each category could be very numerous indeed. I only think of it as a broad guide since the unknowns are just too great at the moment. To me it's like the Copernican Principle when I know that Copernicus would not have approved of it except only in the broadest sense.
    Reply
  • Don M
    Admin said:
    Rocky planets that formed early in our Milky Way galaxy's 13.2-billion-year history have a better chance of supporting life than worlds that were later to the party, a new study suggests.

    Most of the Milky Way's life-hosting planets may have formed early on : Read more

    I wish this article had gone deeper, even in following up its title. For example,
    1. Why would early-formed exoplanets (of what type?) be more likely to develop plate tectonics?
    2. Why would a hot core inhibit creation of a magnetic field?
    Reply
  • Torbjorn Larsson
    Life evolved 4.5 billion years ago to the best of our dating, while we don't yet know when plate tectonic started. So while plate tectonics help biosphere productivity it may not be a proxy for life.

    There may be a correlation here, but I don't see how we would model and test that.
    Reply
  • Torbjorn Larsson
    Helio said:

    Earth had the good fortune of encountering Theia. The result of that event gave us a larger than normal iron core and a hot one for greater convection and tectonic action, not to mention a large moon (plausibly two initially) that gave inclination stability important for climate control.

    Correlation isn't causation. That Earth has a large moon and also life may be entirely unrelated.

    The axis stabilization and climate regime stabilization was erroneously modeled at first, modern models doesn't find much of a problem (but Earth behavior is more beneficial for complex life). E.g. both Venus and Mars are long term stable. And even the Faint Young Sun problem doesn't necessarily need tidal heating from the early nearby Moon:

    "Here we review the faint young Sun problem for Earth, highlighting the latest geological and geochemical constraints on the early Earth's atmosphere, and recent results from 3D global climate models and carbon cycle models. Based on these works, we argue that the faint young Sun problem for Earth has essentially been solved. Unfrozen Archean oceans were likely maintained by higher concentrations of CO2, consistent with the latest geological proxies, potentially helped by additional warming processes." http://astrobiology.com/2020/06/is-the-faint-young-sun-problem-for-earth-solved.html ]
    Reply
  • rod
    FYI, in thread #8, the source cited for Faint Young Sun and the arxiv report available does not mention the word *Moon*, ref the comment "And even the Faint Young Sun problem doesn't necessarily need tidal heating from the early nearby Moon:"

    Some comments appear to be jumping around. I do not see where astrobiology simulations for life arising via abiogenesis, the simulations show that the Earth today would be filled with life, complex life too - absence the Moon, extrapolated back at least 4 billion years or more. This comment does admit perhaps the Moon and tides are important though in thread #8. "(but Earth behavior is more beneficial for complex life).
    Reply
  • rod
    Torbjorn Larsson and Helio. Something that I like to point out about the giant impact model used with Theia and the proto-earth. It does not take much to show that the wrong mass for Theia or wrong impact angle and velocity, the proto-earth could be destroyed, Theia could be destroyed, and things completely rearranged from anything like we see today. If that happened - none of us would be here today reading this discusson. So Theia and the giant impact model, contain some fine tuned parameters, it seems :)---Rod
    Reply
  • Zoomer
    "Most of the Milky Way's life-hosting planets...".
    Just what life hosting planets are you referring to?
    The only ones I'm familiar with are on old StarTrek reruns.
    Thanks in advance.
    Reply