Start at center, with the sun. Our middle-aged star may be calmer than most, but otherwise it is not remarkable. Its planets, however, are a different story.
First, Mercury: More charred inner than a full-fledged planet, it probably lost its outer layers in a traumatic collision a long time 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, never lost layers; it just stopped growing. After Mars, we have a wide ring of remnants of rock, and then things change. Suddenly there is Jupiter, so large that it is practically a half-baked sun, containing the vast majority of the material left over from our star’s creation. In the past, there are three more enormous worlds – Saturn, Uranus and Neptune – forged by gas and ice. The four gas giants have almost nothing in common with the four rocky planets, despite being formed at about the same time, from the same substance, around the same star. The eight planets of the solar system present a puzzle: Why these?
Now look out past the sun, far beyond. Most of the stars contain their own planets. Astronomers have seen thousands of these distant star-and-planet systems. But oddly enough, they have so far found none resembling ours. So the puzzle has gotten harder: Why these, and why them?
The swell catalog of extrasolar planets, along with observations of distant, dusty planetary nurseries and even new data from our own solar system, no longer match classical theories of how planets are made. 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 explaining each planet around each star, or even the wildly divergent orbs orbiting our sun. “The laws of physics are the same everywhere, but the process of building planets is complicated enough for the system to become chaotic,” he said. Alessandro Morbidellia leading figure in planetary formation and migration theories and an astronomer at the Côte d’Azur Observatory in Nice, France.
Yet the results animate new research. In the midst of the chaos of world-building, patterns have emerged that have led astronomers toward powerful new ideas. Teams of scientists are working out the rules for collecting dust and pebbles and how planets move when they melt together. There is a heated debate about the time of each step, and about what factors determine the fate of a budding planet. In the context of these debates, some of the oldest questions people have asked ourselves are: How did we get here? Are there other places like here?
A star and its acolytes are born
Astronomers have been understanding the basic contours of the solar system for almost 300 years. The German philosopher Immanuel Kant, who, like many Enlightenment thinkers, dealt with astronomy, published in 1755 a theory that remains largely 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, was decomposed into its elemental base material,” he wrote.
We actually come from a diffuse cloud of gas and dust. Four and a half billion years ago, the cloud collapsed under its own gravity, forming a new star, probably shot by a passing star or by the shock wave of a supernova. It is how it went afterwards which we do not really understand.
As the sun lit up, excess gas swirled around it. Eventually, the planets were formed there. The classical model that explained this, known as the solar mass with minimum mass, imagined 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 to 1977, assumed that planets were formed where we see them today, beginning as small “planetesimals” and then incorporating all the material in their area as locusts devouring each leaf in a field.