Planets form by gathering dust, not from smaller bodies crashing into each other. Find out why this is both promising and thrilling for the search for life.
I remember my astronomy textbook in grade six. It had a big, black cover and it was very slim, like a coffee table book.
The cover had a bright, colour image of the solar system wrapped around its spine. Inside it was a profile of each planet in more vibrant colours.
We learned the names of all the planets, including Pluto which has since been kicked downstairs to the dwarf planet table. Most adults I know can list each planet’s name, and recognize them in pictures.
WE WEREN’T TOLD HOW THE PLANETS FORM IN THE FIRST PLACE
What we weren’t told back then was how the planets formed in the first place. Planetary formation is interesting in itself and it answers questions about our Earth and the more than 4.000 planets orbiting other suns that we keep discovering these days.
The story of how our planets formed begins about 4.5 billion years ago with a molecular cloud. That’s a dense cloud of gas and dust floating in between stars.
Astronomers call them molecular because molecules like hydrogen and helium form inside them. They also call them stellar nurseries because stars can form out of dense clumps of dust and gas inside them.
MOLECULAR CLOUDS ARE ALSO CALLED STELLAR NURSERIES
When a clump has enough of its own gravity, it collapses. In the case of our solar system, most of the collapsing mass was at the centre, which is where our sun arose.
The rest of the clump flattened out like a plate around the sun. We call this platter of gas and dust the proto-planetary disk or the solar nebula.
This is called the nebular hypothesis, and it goes back a long way. It started in the 18th century when Emanuel Swedenborg, Pierre-Simon Laplace and Immanuel Kant all came up with similar ideas.
NEBULAR HYPOTHESIS GOES BACK A LONG WAY
Yes, it was that Immanuel Kant, with all the deep, abstract insights I’ve always assumed are brilliant since I can’t decipher them. He also thought about the real world sometimes, as in his book Universal Natural History and Theory of the Heavens .
It’s been challenged here and there along the way. It was finally vindicated in the 70s by the theoretical work of Victor Safronov on how planets form and then confirmed in the 80s when astronomers discovered similar disks in other solar systems.
There was a lot of action in that early disk. Asteroids, moons and larger sized objects formed out of the dust and gas and also by colliding. That’s where new research published in the journal Science Advances last week by the Centre for Star and Planet Formation at the University of Copenhagen comes in.
CONSENSUS WAS THAT EARTH TOOK 20 MILLION YEARS TO FORM
Until now, scientists believed that the Earth and the other planets formed from random collisions between larger and larger objects in the proto-planetary disk. Since the chances of such a crash are low, the consensus until now has been that the Earth took about 20 million years to form.
The researchers, led by Associate Professor Martin Shiller, looked at extremely precise iron isotype measurements in various meteorites. In fact, these are the most accurate measurements ever published.
They found that a class of meteorites called CI chondrites had fascinating properties. We often use them to date the solar system because their chemical composition is similar to that of the early planetary system.
METEORITES CALLED CI CHONDRITES HAD FASCINATING PROPERTIES
In this new study, researchers found that, unlike any other class of meteorites, CI chondrites are practically identical to the Earth’s mantle. This coincidence contradicts the idea that the Earth and other planets formed from random impacts between large bodies.
The only time in the solar system’s history when there was enough CI floating around to make planets form was while the sun still had its proto-planetary disk. So, the earth’s iron core and its mantle must have formed by building up cosmic dust (scientists call this accretion) during that time.
As Dr. Shiller put it, “‘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.”
EARTH FORMED IN 5 MILLION YEARS INSTEAD OF 20 MILLION
This discovery has a few implications. Probably the biggest one is that, based on this new theory, the Earth formed in only 5 million years instead of roughly 20 million.
In turn, that means that planets form in ways that aren’t as random, rare or time-consuming as we thought. That explains why we’ve found that so many planets formed beyond our solar system. Astronomers call them exoplanets.
Scientists are pretty confident that water is essential for life. Professor Martin Bizzarro is the Centre Leader and a co-author of this paper on how planets form.
“INGREDIENTS OF LIFE MORE LIKELY ELSEWHERE IN the UNIVERSE”
He explained, “‘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.”
I think we all hope to find out if there’s life on other worlds, so this news is both promising and thrilling. It suggests that earth-like planets form with water included all the time.
Based on these ideas, if earthlike planets form this often, the universe could be teeming with life. Professor Bizarro wrapped up with this observation.
“planet formation happens everywhere”
“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.”
We always have more to learn if we dare to know.
University of Copenhagen
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