The early solar system was donut-shaped, meteorite study suggests

a lumpy rough metallic rock
An iron meteorite from the UCLA Meteorite Gallery. (Image credit: UCLA Meteorite Gallery)

You're probably familiar with how the solar system looks today. There are eight officially recognized planets located more or less on the same plane, orbiting the sun. But have you ever given a thought to what it looked like billions of years ago? Things were very different back then. 

We used to think the early solar system looked a bit like a dartboard, with concentric rings of material orbiting our sun. But a team of researchers now suggests that the early solar system actually looked more like a donut. They've determined this from a rather unlikely source: iron meteorites.

Our solar system formed about 4.6 billion years ago, when a rotating cloud of dust and gas — the solar nebula — collapsed in on itself, forming the sun. But not all of that dust and gas became our star. The leftover material continued to spin around the sun in a chaotic mess, eventually condensing into planets and asteroids. This cosmic nursery is known as a protoplanetary disk. 

While we can't physically look back in time at the formation of our solar system, we can see other examples of protoplanetary disks elsewhere in the universe, and many of them showcase those concentric circles of material. And we originally thought the solar system might've looked like that, too.

But the UCLA researchers found clues in iron meteorites that indicate otherwise. Iron meteorites come from metallic cores of ancient asteroids that formed in the early years of the solar system, so they can give us insight into how the solar system formed. What caught the researchers' attention were refractory metals like iridium and platinum, which were abundant in meteorites from the outer disk of the early solar system. 

Related: Solar system planets, order and formation: A guide

That composition puzzled them. Those metals, which condense at high temperatures, should have formed closer to the sun, not in the cold reaches of the solar system. And if our solar system had a dartboard-like structure, these metals should not have been able to "jump" from ring to ring to end up in the outer disk. Thus, the researchers formed a new theory about the shape of the young solar system: It looked more like a donut, a shape that allowed refractory metals to move outward as the disk expanded. 

But then they encountered another issue. The gravity of the sun should have pulled these heavier metals back toward it over the last few billion years — yet it didn't. However, the team came up with a possible solution for that, too.

"Once Jupiter formed, it very likely opened a physical gap that trapped the iridium and platinum metals in the outer disk and prevented them from falling into the sun," planetary scientist Bidong Zhang, first author of a new study about the meteorite analysis, said in a statement

"These metals were later incorporated into asteroids that formed in the outer disk," added Zhang, who's a planetary scientist at the University of California, Los Angeles. "This explains why meteorites formed in the outer disk — carbonaceous chondrites and carbonaceous-type iron meteorites — have much higher iridium and platinum contents than their inner-disk peers."

And there you have it. Once upon a time, our solar system was a donut-shaped protoplanetary disk filled with heavy metal before slowly turning into the multiplanetary system it is today. 

The study was published online May 28 in the journal Proceedings of the National Academy of Sciences.

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Stefanie Waldek
Contributing writer

Space.com contributing writer Stefanie Waldek is a self-taught space nerd and aviation geek who is passionate about all things spaceflight and astronomy. With a background in travel and design journalism, as well as a Bachelor of Arts degree from New York University, she specializes in the budding space tourism industry and Earth-based astrotourism. In her free time, you can find her watching rocket launches or looking up at the stars, wondering what is out there. Learn more about her work at www.stefaniewaldek.com.

  • rod
    Interesting report and paper cited. This site presently shows 6796 exoplanets, the number keeps getting larger and larger now here, https://exoplanet.eu/home/
    An observation I make here about these numerous exoplanet systems documented now. Look at the mass found in the range 0 to 3 au from the parent stars. I found 4131 in that distance range, average mass 4.544 Jupiter, or 1444 earth masses. Compare to our solar system, we have much less mass in the solar system inside 3 au or closer to the Sun than many exoplanet systems identified now. There is much to chew on concerning disc simulations used to explain the population we see today.
    Reply
  • Unclear Engineer
    I think there is a lot of observational bias in the positions of exoplanets in the star systems we have the technology to partially observe. Big planets orbiting close to small stars are a lot easier to find than large planets far from large stars and small planets close to large stars.

    So, there really isn't statistically sufficient data yet, for the purpose of assigning probabilities to planets that could have water and plate tectonics and a protective magnetic field, etc. etc.
    Reply
  • Classical Motion
    A torus area indicates helical orbits, not elliptical orbits. I think our planets and their moons are in one turn helical orbits too. But our successful satellite elliptical orbits have blinded us to it. Check out the orbits of the ringlets.

    And I also believe because of the directional forces on the helix, they are self correcting self stabilizing orbits.

    These TWO orbits, one small stretched out rotation, forming the larger rotation. Two angular momentums. Locked angular momentums. A change in diameter in the small orbit, changes the length of the large orbit. A change in the large orbit, will cause a change in the small orbit. These are square changes, perpendicular changes, balance changes. Angular changes.

    I have read where all planets have a toroidal debris field around the Sun.

    Close radar inspection is needed, but will be hard to reference. Or find an orbit thru a debris field and look at the tracks.
    Reply
  • Unclear Engineer
    The 2-body gravitational dynamics make for a flat, elliptical orbit.

    But, put a lot of mass in a disk around a large mass body, and a smaller body orbiting the larger body within the disk of other matter will likely oscillate across the disk. That would produce something similar to a helical orbit, but not a round helix. It is the mass in the disk that is bringing the smaller body back and forth across the disk, We see this type of orbit for stars like our Sun around our galactic center
    Reply
  • Classical Motion
    We can only observe and measure 2D at long distance. And that's hard enough. Guessing size with angle is no good without distance. UN-measured unseen travel can change velocity and period. There is way too much play for an accurate guess.

    That's just my opinion, no one else.

    Let's stay local. Look at those ringlets. They have multiple turns of their trajectory around their planet. They seem to have the same pitch too. Have any measured the inner pitches against the outer pitches?

    Do all small ringlets on all the planets do this? Are the ringlets in plane? Or make their own plane? Is half the pitch above plane and half the pitch below? Planet pitch is.

    If I was studying gravity I'd be glued to this. And I would want to bust a portion of it up.......to see how the ringlets respond.

    I've heard some say that if it were disturbed the whole thing would come apart. It was suppose to be in perfect balance.

    My spitball is just the product of many others. But an early donut in any system doesn't surprise me. Nor is cause. And I have only my words to convince with. Can't prove a thing.
    Reply
  • Helio
    There is evidence supporting the hypothesis that the Sun formed, as do most stars, within a cluster. The embryonic cloud fragmented and formed many stars. Some clouds can form as many as one million new stars.

    The orbits of our planets, and another variable or two, limits that early cluster to be less than 3000 stars forming with the Sun. These stars, of course, are close to one another. And the more massive stars tend to form much quicker.

    So, I am curious if the extra heat needed with these results could be from a nearby (say 1000 AU) very hot early neighbor? It would not have stayed this close for millions of years, of course.
    Reply
  • Unclear Engineer
    Good point, Helio.

    Similarly, the potential for the rogue planet from one star system impacting the obits of planets in neighboring star systems seems much greater while the stars are close together.

    Do we have estimates for how long other stars were close to the Sun, starting about 4.6 billion years ago? Do we know what stars those were and where they are in today's observations from Earth? I suspect not. The Sun has been around the galactic center about 20 times (20 "galactic years") since the Sun formed, so I doubt we can extrapolate the "N-body problem" backwards enough without it becoming too unstable to be reliable.
    Reply
  • rod
    Some interesting comments in this thread. Today this site shows 6802 exoplanets now, https://exoplanet.eu/home/
    Just looking at those in the range 0 to 3 au from the host stars shows that many systems are filled with much more mass and larger exoplanet sizes than we see in our solar system from the Sun out to 3 au distance. 4133 confirmed 0 to 3 au from their parent star and you can look at the radii size too. 5.334 earth radii size is average compared to our solar system distribution. The NASA archive site, https://exoplanetarchive.ipac.caltech.edu/index.htm, 5678 exoplanets with 0 to 3 au count 5023, average radii size 5.94 and average distance 0.304 au. Basic data like this I feel should be clearly presented to the public when presenting information on habitable exoplanets and how many aliens could be buzzing around in the MW today :)
    Reply
  • Helio
    Rod, it looks like NASA may have more radii data than exoplanet.eu. Is this the case?
    Reply
  • rod
    Helio said:
    Rod, it looks like NASA may have more radii data than exoplanet.eu. Is this the case?
    Yes, the NASA site has multiple rows for each exoplanet showing different values for different parameters. Radius of earth is listed too and has more entries than radius jupiter.
    Reply