Astronomers radically rethink the making of the planets

Start at the middle, with the sun. Our middle-aged star may be calmer than most, but it’s otherwise inconspicuous. However, its planets are a different story.

First, Mercury: More charred inner than full-fledged planet, it probably lost its outer layers in a traumatic collision long ago. Next comes Venus and Earth, twins in some respects, though strangely enough only one is fertile. Then there is Mars, another small world, one that, unlike Mercury, has never lost layers; it just stopped growing. After Mars we have a wide ring of remaining rocks, and then things move. Suddenly there is Jupiter, so large that it is virtually a half-baked sun, containing the vast majority of the material left over from our star’s creation. Past these are three more enormous worlds — Saturn, Uranus, and Neptune — forged from gas and ice. The four gas giants have almost nothing in common with the four rocky planets, despite the fact that at about the same time, they formed from the same thing, around the same star. The solar system’s eight planets offer a puzzle: Why this?

Now look past the sun, far beyond. Most of the stars house their own planets. Astronomers have observed thousands of these distant star-and-planet systems. But strangely enough, so far they have not found anyone who looks like us at a distance. So the puzzle has become more difficult: Why this, and why the?

The swell catalog of outer solar planets, along with observations of distant, dusty planetary nurseries and even new data from our own solar system, no longer agree with classical theories about how planets are made. Planetary scientists, forced to abandon decades-old models, now realize that there may not be a great unified theory of world creation – no single story explaining each planet around each star, or even the wildly diverse spheres orbiting our sun not. “The laws of physics are the same everywhere, but the process of building planets is complicated enough that the system becomes chaotic,” says Alessandro Morbidelli, a leading figure in planetary formation and migration theories and an astronomer at the Côte d’Azur. observatory in Nice, France.

Yet the findings encourage new research. Amidst the chaos of world-building, patterns emerged that led astronomers to powerful new ideas. Teams of researchers work out the rules of dust and rock composition and how planets move once they fuse. Fierce debate rages over the timing of each step, and over what factors determine an emerging planet’s fate. At the merger of these debates are some of the oldest questions people have asked ourselves: How did we get here? Is there anything else like this?

A star and its acolytes are born

Astronomers have understood the basic outlines of the solar system’s origins for nearly 300 years. The German philosopher Immanuel Kant, who, like many Enlightenment thinkers, gambled with astronomy, published in 1755 a theory that remained fairly correct. “All the matter that makes up the spheres that belong to our solar system, all the planets and comets, at the origin of all things, is broken down into its elementary basic material,” he wrote.

Indeed, we come from a diffuse cloud of gas and dust. Four and a half billion years ago, probably by a passing star or by the shock wave of a supernova, the cloud collapsed under its own gravity to form a new star. This is how things went down after that that we do not really understand.

As soon as the sun caught fire, excess gas revolved around it. Eventually the planets formed there. The classic model that explained it, known as the minimum-mass solar nebula, envisioned a basic “protoplanetary disk” filled with just enough hydrogen, helium, and heavier elements to make the observed planets and asteroid belts. The model, which dates from 1977, assumed that planets were formed where we see them today, starting as small “planetesimals”, and then including all the material in their area like locusts devouring each leaf in a field.