By Sabine Stanley, Ph.D., Johns Hopkins University
About the formation of our solar system, we have some clues on how long it took the planetary nebula to contract into a disk and the gas to blow out of the disk. These clues come from looking at other protoplanetary nebulae around other proto-stars and assuming that our protoplanetary nebula worked much the same way. Let’s find out what these clues are and what they tell us.
Meteorites Are Important Markers of Time
When it comes to the formation of the planets, meteorites are an important marker of time.
Meteorites come from a variety of sources, like the Moon, Mars, and asteroids. Some of the oldest meteorites have come from planetesimals like the ones that formed the planets. The fact that we can determine their ages from radioisotope dating means that we can study their features to learn what was happening when they formed.
In this way, meteorites help provide us with a timeline for the formation of the planets by constraining various processes.
Learn more about how the solar system family is organized.
Chondrites Are the Oldest Meteorites in the Solar System
Let’s start with some of the oldest meteorites in the solar system: chondrites. These meteorites are very pristine, meaning they haven’t been melted or altered much since their formation. Most of the matrix of the chondrites is made of the original condensates in the protoplanetary disk.
But chondrites are named for an unusual feature they have: chondrules. These are small, round, rocky inclusions, about a micrometer to a centimeter in size. They formed as molten droplets in space before they were incorporated into the planetesimals that eventually brought us the chondrite meteorites.
These chondrules must have experienced some sort of flash heating event to become molten like this and then freeze in spherical blobs. We don’t know for sure what caused the heating. Proposed theories include collisions between molten planetesimals, shock heating, or even nebular lightning.
Learn more about how our Sun defines our solar system.
The Oldest Thing in the Solar System
Chondrite meteorites also contain the oldest things in the solar system: another important feature called calcium-aluminum-rich inclusions, or CAIs for short. These are fluffy minerals that were among the first condensates in the protoplanetary disk. Some even formed up to 3 million years before chondrules. The oldest CAIs ever found are in a chondrite named Allende.
When meteorites are found on Earth they are named after the location at where they were found, and Allende was found in Allende, Mexico. The CAIs in Allende are 4.567 billion years old. Since this is the oldest thing ever dated in the solar system, we mark this age as the time equals zero point for the formation of the solar system. Other chondrites have been dated to 4 million years later: 4.563 billion years old.
This tells us that the dust grew to planetesimal-sized within 4 million years after CAIs formed. These ages are determined from radioactive dating of isotopes in the meteorites.
For example, an isotope of uranium, uranium-238, decays into a specific isotope of lead, lead-206. And it does so at a specific rate we can measure. By determining how much of the uranium in the meteorite has decayed into lead, we can determine its age.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
Other isotope systems can tell us the timing of when bodies differentiated if they have affinities for metal over rock. For example, there’s hafnium which decays into tungsten. Hafnium-182 is lithophile, which means ‘rock-loving’. Tungsten-182, instead, is siderophile, which means ‘iron-loving’. Hafnium decays into tungsten with a half-life of about 9 million years.
This is great for determining whether a planetary body differentiates during the first tens of millions of years of its formation.
If an object differentiates early in the solar system, during just a few million years, then most of the hafnium hasn’t decayed yet. Since hafnium is lithophile, it preferentially stays with the rocky material forming the mantle and crust of the planetary body. It then later decays into tungsten, but differentiation has already happened, so the tungsten stays in the rocks.
If instead an object differentiates later, after tens of millions of years, then most of the hafnium has already decayed into tungsten. The tungsten will then preferentially go with the iron to the core of the planet. This will leave the outer rocky layer of the planet depleted in tungsten. So, the more tungsten we see in the mantle, the earlier it tells us that the differentiation occurred.
Learn more about comets, the Kuiper belt, and the Oort cloud.
What Do the Different Dating Techniques Tell Us?
Using hafnium-tungsten dating, we know that some planetesimals differentiated in less than 10 million years after the formation of CAIs. That’s really fast and it means that some of the planetesimals that were colliding to form planets were already differentiated. Hafnium-tungsten analysis for Earth also tells us that the Earth differentiated in the first 100 million years of the solar system.
Radioisotope dating of Moon rocks has also told us that the Moon was formed 4.5 billion years ago. That means that most of the terrestrial planets were formed within 100 million years after the beginning of the solar system.
Radioactive dating has also given us dates of other events in the solar system. The oldest meteorite we have from Mars is named Allan Hills 84001. It’s about 4.1 billion years old. Radiometric dating not only tells us its age, but also gives us a date when it was blasted off Mars by a meteor impact – about 16 million years ago. And the date it fell to Earth—only 13,000 years ago.
Learn more about how planets migrated in our early solar system.
The Sun and the Protoplanetary Disk Formed Around the Same Time
Meteorites also provide us with another important clue—that the Sun and the protoplanetary disk formed around the same time. The Sun has sort of the same composition as meteorites. That is, if you set aside the hydrogen and helium in the Sun, then the rest of the composition of the Sun is quite similar to the composition of the most pristine meteorites in the solar system, a class of chondrites known as C1- chondrites.
Let’s compare the composition of the Sun to the composition of C1-chondrites. Volatile elements like nitrogen, carbon, oxygen, and hydrogen are preferentially lost in chondrites over time, giving them lower concentrations in meteorites than in the Sun. Similarly, helium itself, and other light noble gases are also lost in meteorites because they preferentially escape when the meteorite parent bodies are heated.
On the other hand, we know that the Sun’s photosphere has preferentially lost lithium because the high temperatures in the Sun cause it to transmute into helium through collisions with protons at very high temperatures.
In a way, it seems like these exclusions suggest that the Sun is made of the same stuff as the oldest meteorites, except for all the most common elements.
The overlap of other elements is unmistakable, and we don’t know of another way for the Sun and meteorites to have these matching compositions of all the other elements. It looks like the Sun and oldest meteorites formed from the same material, at the same time. And meteorites tell us that the solar system is over 4.5 billion years old.
Common Questions about Meteorites and Chondrites
Meteorites come from a variety of sources, like the Moon, Mars, and asteroids. Some of the oldest meteorites we have come from planetesimals like the ones that formed the planets.
Chondrites are some of the oldest meteorites in the solar system. These meteorites are very pristine, meaning they haven’t been melted or altered much since their formation.
The meteorites found in Allende, Mexico were chondrite meteorites. Chondrite meteorites also contain the oldest things in the solar system called calcium-aluminum-rich inclusions, or CAIs for short. The CAIs in Allende are 4.567 billion years old, which is the oldest thing ever dated in the solar system.
Using radioisotope dating of Moon rocks, we have learned that the Moon was formed 4.5 billion years ago.