The precursor of our planet, the proto-Earth, formed within a time span
of approximately five million years, shows a new study from the Centre for Star
and Planet Formation (StarPlan) at the Globe Institute at the University of
Copenhagen.
On an astronomical
scale, this is extremely fast, the researchers explain.
If you compare
the solar system’s estimated 4.6 billion years of existence with a 24-hour
period, the new results indicate that the proto-Earth formed in what
corresponds to about a minute and a half.
Thus, the
results from StarPlan break with the traditional theory that the proto-Earth
formed by random collisions between larger and larger planetary bodies
throughout several tens of millions of years — equivalent to about 5-15 minutes
out of the above-mentioned fictional 24 hours of formation.
Instead, the new
results support a more recent, alternative theory about the formation of
planets through the accretion of cosmic dust. The study’s lead author,
Associate Professor Martin Schiller, explains it as follows:
“The other idea
is that we start from dust, essentially. Millimetre-sized objects, all coming
together, raining down on the growing body and making the planet in one go,” he
says, adding:
“Not only is
this implication of the rapid formation of the Earth interesting for our solar
system. It is also interesting to assess how likely it is for planets to form
somewhere else in the galaxy.”
The bulk composition of the solar
system
The key to the
new finding came in the form of the most precise measurements of iron isotopes
that have so far been published scientifically.
By studying the
isotopic mixture of the metallic element in different meteorites, the
researchers found only one type of meteoritic material with a composition
similar to Earth: The so-called CI chondrites.
The researchers
behind the study describe the dust in this fragile type of meteorite as our
best equivalent to the bulk composition of the solar system itself. It was dust
like this combined with gas that was funnelled via a circumstellar accretion
disk onto the growing Sun.
This process
lasted about five million years and our planets were made from material in this
disk. Now, the researchers estimate that the proto-Earth’s ferrous core also
formed already during this period, removing early accreted iron from the
mantle.
Two different iron compositions
Other
meteorites, for example from Mars, tell us that at the beginning the iron
isotopic composition of material contributing to the growing Earth was
different. Most likely due to thermal processing of dust close to the young
sun, the researchers from StarPlan explain.
After our solar
system’s first few hundred thousands of years it became cold enough for
unprocessed CI dust from further out in the system to enter the accretion
region of the proto-Earth.
“This added CI
dust overprinted the iron composition in the Earth’s mantle, which is only
possible if most of the previous iron was already removed into the core. That
is why the core formation must have happened early,” Martin Schiller explains.
“If the Earth’s
formation was a random process where you just smashed bodies together, you
would never be able to compare the iron composition of the Earth to only one
type of meteorite. You would get a mixture of everything,” he adds.
More planets, more water, perhaps
more life
Based on the
evidence for the theory that planets form through the accretion of cosmic dust,
the researchers believe that the same process may occur elsewhere in the
universe.
This means that
also other planets may likely form much faster than if they grow solely from
random collisions between objects in space.
This assumption
is corroborated by the thousands of exoplanets — planets in other galaxies —
that astronomers have discovered since the mid-nineties, explains Centre Leader
and co-author of the study, Professor Martin Bizzarro:
“Now we know
that planet formation happens everywhere. That we have generic mechanisms that
work and make planetary systems. When we understand these mechanisms in our own
solar system, we might make similar inferences about other planetary systems in
the galaxy. Including at which point and how often water is accreted,” he says,
adding:
“If the theory
of early planetary accretion really is correct, water is likely just a
by-product of the formation of a planet like the Earth — making the ingredients
of life, as we know it, more likely to be found elsewhere in the universe.”
Journal article: https://advances.sciencemag.org/content/6/7/eaay7604
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