This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
It can come as a surprise just how uncertain we still are about the mechanisms and timelines for planetary origins. After all, with exploration of our own solar system and the discovery of thousands of worlds around distant suns we’ve assembled a remarkable overview of the workings of planetary systems and the possibilities for their origins.
At the same time, the generally accepted picture of how interstellar material is accumulated and processed to form new stars and new worlds is still built on hypotheses set up by the likes of Emanuel Swedenborg, Immanuel Kant (yes, that Kant), and Pierre-Simon Laplace in the 1700s. While these ideas seem to fit the gross features of planetary systems (whereby planets condense out of a gravitationally formed circumstellar disk of element-enriched interstellar gas and dust), lots of the details have proven to be awfully tough to figure out.
For instance, it’s still unclear how solid material agglomerates into planet-sized pieces. It has been thought that a lot of this process could be hierarchical, starting with tiny dust grains sticking together and building bigger and bigger bits that themselves collide and merge. But it turns out that might not be the sole route. Other options include processes that effectively skip the middle-stages, taking small pieces (dust or pebble-sized material) and having those come together en masse – drawn and dragged into newly-formed atmospheres for instance. And there is uncertainty about the degree to which rocky material crashes around in proto-planetary systems; either merging or splintering at it meets its contemporaries.
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In the past week or so there have been some very interesting reports of studies that all offer new potential insights.
The first of these actually comes from the outer solar system and the New Horizons mission’s flyby of the distant trans-Neptunian object in the Kuiper Belt now known as Arrokoth (a word relating to “sky” from the Powhatan language). In three research papers published in Science it is reported that this dumbbell-shaped 22 mile-long object seems to have formed via a gentle merger of its two main lobes, both of which represent very similar material. In other words, Arrokoth is a product of quite localized particle-cloud collapse of nebular material, and not so much a hierarchical assembly of a variety of matter from across its orbit.
The second piece of news comes from an analysis of the water content of Jupiter’s atmosphere from NASA’s orbiting Juno mission. Using microwave measurements to peer about 150 kilometers down into the gas giant’s gaseous envelope Juno finds that around Jupiter’s equator there seems to be about 0.25% water compared to the number of other molecules. This is more than previous measurements, made when the Galileo probe plunged into the atmosphere back in 1995, but also suggests that the water is not mixed uniformly in Jupiter's atmosphere.
Nonetheless, if this new estimate does reflect the global water content of Jupiter it hints that the proto-planetary pieces that originally helped form Jupiter were not the water-rich clathrate-hydrate chunks expected due to Jupiter’s distance from the Sun and location beyond the so-called “snow-line” where water ices can form.
Finally, the third item on the planet-formation mystery menu is a clue from the analysis of meteoritic material. In a study reported in Science Advances, researchers measured the abundance of a very specific isotope of iron, Iron-54, in a variety of meteorites; all primitive bodies dating back to the formation of the solar system some 4.5 billion years ago.
Only one grouping of meteorite material seems to match Earth’s own Iron-54 abundance, suggesting (bolstered by a variety of other evidence) that Earth may have formed rapidly in the young solar system, in about 5 million years. During this time a very specific part of the proto-planetary disk was flowing inwards (being accreted) towards the proto-sun, and that part would have supplied the material that built the bulk of the Earth. If Earth has formed over a more protracted period it should have included a wider range of stuff, with varying Iron-54 isotopic compositions, among other differences.
This also seems to be consistent with the idea of a “pebble-accretion” formation of the planet, with the rapid agglomeration of small particles onto some kind of asteroidal ‘seed’. As well supporting the notion that Earth’s volatiles (like water) were accumulated in these early stages, and less so in later ‘add-ons’ from other parts of the solar system.
There are implications for exoplanetary systems too. It might be that planet cores form rapidly, and that rocky worlds get their water early on, without relying on later, more variable scenarios to supply it. In other words, water-rich worlds could be more common.
Altogether a pretty impressive week for advances in understanding the intricacies of how the cosmos builds worlds.