start with center, with the sun. Our middle-aged star may be calmer than most, but otherwise unremarkable. However, its planets are another matter.
First, Mercury: It has more guts than a full-fledged planet, and it likely lost its outer layers in a traumatic collision long ago. Next are Venus and Earth, twins in some ways, but oddly only one is fertile. And then there’s Mars, another tiny world that, unlike Mercury, never lost layers; it just stopped growing. After Mars, we had a large circle of remaining rocks, and then things changed. Suddenly there was Jupiter, so large that it was almost a half-baked sun, containing the vast majority of the material left over from the creation of our star. There used to be three larger worlds – Saturn, Uranus and Neptune – made of gas and ice. The four gas giants have almost nothing in common with the four rocky planets, even though they formed around the same star at about the same time, from the same material. The eight planets of the solar system present a conundrum: Why these?
Now look out, beyond the sun, into the distance. Most stars have their own planets. Astronomers have discovered thousands of these distant star and planetary systems. But strangely, they haven’t found anything similar to ours so far. So the puzzle gets harder: why these, why those?
The expanding catalog of exoplanets, along with observations of distant, dusty planetary nurseries, and even new data from our own solar system, no longer fit the classical theory of how planets formed. Planetary scientists, forced to abandon decades-old models, now realize that there may not be a grand unified theory of world creation — no single story that explains every planet around every star, or even the vastly different orbits around our sun sphere. “The laws of physics are the same everywhere, but the process of building planets is so complex that the system becomes chaotic,” said Alessandro Mobi, a leading figure in the theory of planet formation and migration and an astronomer at the Côte d’Azur Observatory Alessandro Morbidelli said. Nice, France.
Astronomer Alessandro Morbidelli of the Côte d’Azur Observatory in Nice, France, has devised an influential theory about planet formation and migration.Photo: Mattia Balsamini/GEO Germany
Still, the findings are motivating new research. In the chaos of world-building, patterns have emerged, leading astronomers toward powerful new ideas. The research team is studying the rules for dust and pebble assembly, and how planets move after mergers. A heated debate rages over the timing of each step and what factors determine the fate of a budding planet. At the heart of these debates are some of the oldest questions humanity has asked itself: How did we get here? Is there any other place like here?
A star and its followers are born
For nearly 300 years, astronomers have known the basic outlines of the origin of the solar system. Like many Enlightenment thinkers who dabbled in astronomy, German philosopher Emmanuel Kant published a theory in 1755 that is still quite correct. “All the matter that makes up the spheres of our solar system, all the planets and comets, is broken down into its basic elementary matter at the origin of all things,” he wrote.
In fact, we come from a diffuse cloud of gas and dust. 4.5 billion years ago, possibly propelled by the shock wave of a passing star or a supernova, the cloud collapsed under its own gravity to form a new star. That’s how things turned out later, we don’t really understand.
Once the sun ignites, excess gas swirls around it. Eventually, planets formed there. The classic model to explain this is called the Minimum Mass Solar Nebula, which envisions a basic “protoplanetary disk” filled with enough hydrogen, helium and heavier elements to form the observed planets and asteroid belts . The model, dating back to 1977, assumed planets formed where we see them today, starting with small “asteroids” and then combining all the matter in their region, like a locust eating every leaf in a field.
