About 4.6 billion years ago, the celestial clouds collapsed, paving the way for the formation of the solar system. After that, a nebula with a strong gravitational pull was formed, and the birth of the sun began. But subsequent details leave room for debate as to which planet formed first.
So what do we know about which planets appeared earlier and which developed later?
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“It’s complicated. There’s no easy answer to measuring time in space,” Michael Meyer, director of astronomy at the University of Michigan, told Live Science. “That’s the hardest thing in astronomy.”
The most commonly accepted explanation for how the eight planets in our solar system formed is called accretion. This is when small particles of gas and dust collide and stick to each other, gaining gravity and growing over time.
According to NASA, one common theory that posits that accretion occurred is that large planets first began to form far from the Sun. As they grew larger, they moved outward, making room for terrestrial rocky planets to form. These formations occurred close to the Sun and after millions of years (a relatively short period in astronomical terms).
“To form a gas giant, you need enough gas to form Jupiter, and that puts a hard upper limit on how long it takes for a gas giant to form,” Meyer says. “If the whole process doesn’t start early enough and the gas disappears, you can’t create gas giant planets. That’s why we think gas giant planets formed in the first place.”
However, a competing theory called the streaming instability model offers a different explanation. This theory could allow a completely different sequence to occur by allowing planets to gather mass more spontaneously.
“I would argue that perhaps terrestrial planets formed first and then giant planet formation stopped when the gas ran out,” Kau Borlina, assistant professor of planetary science at Purdue University, told Live Science. “After that, it just becomes a chaotic buildup.”
determine the age of a planet
Scientists are still debating which theory best explains the formation of the solar system. But even the idea of a planet’s age is not simple.
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“There are two different ways of thinking about the age of planets,” MIT planetary scientist Gaia Stuckey de Kee told Live Science. Instead of dating a planet based on its origin, some scientists focus their attention on its surface.
“What I think about age is more the age of the surface, because sometimes the surface is very old, like a preserved surface, and sometimes the surface is very young, when something is still happening, like active plate tectonics,” Stuckey-de-Key said.
One of the methods scientists use to determine the age of a planet is to manually count the craters on its surface. From this perspective, Earth is considered the youngest planet because its surface is continuously changing, with Venus and Mars coming next, she said.
Unfortunately, due to the limitations of current planetary dating methods, scientists can only estimate the age of each planet. And because even a small error can span millions of years in the history of the universe, researchers are still working to collect data to assemble a more accurate timeline.
“If we want to get a complete picture of when and how planets form, I think samples are an important part of that,” Borlina said. “And some places are a little more difficult to get samples than ours, but we currently have samples stored on Mars waiting for us to bring them back.”
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