Author: Kon
Reviewed by: Xiao

  The Solar System that humanity inhabits contains planets, moons, comets, and other celestial bodies of every size. The Sun itself is a thoroughly ordinary member of the Milky Way’s “family of stars.” This raises two questions:

  1. Is our existence unique?
    In more specific astronomical terms: are planetary systems like the Solar System common throughout the universe?
  2. How did the Solar System on which we depend come into being?

  Before humanity could search for exoplanets on a large scale, it was difficult to answer either question satisfactorily. Without a substantial set of comparison samples, the Solar System itself was our only model for understanding planetary systems. The two questions are also linked to some extent: once the mechanism of planetary-system formation is established, it can in turn predict whether such systems should be common. Historically, several theories have sought to explain “planet formation”:

  The first theory holds that planet formation is connected to the formation of a planet’s host star. Soon after the star forms, a residual “disk of gas and dust” remains around it, and planets form within that disk. This theory naturally implies that planets are relatively common.

  Other theories argue that planet formation is unrelated to star formation and is instead triggered by separate events. The “tidal stripping hypothesis,” for example, proposes that when two stars pass close to one another, tidal forces “strip” some gas from a star and that planets then form from the stripped gas. Even in a region as densely populated with stars as the Milky Way, however, the number density of stars in space is remarkably low. The tidal stripping hypothesis therefore implies that planets are exceedingly rare.

  Even before the search for exoplanets began, most people nevertheless tended to believe that planetary systems like the Solar System were common. Modern science seems to favor making anything connected with humanity as unspecial as possible. The Copernican Revolution and the Curtis–Shapley debate both drove humanity away from the “most central and exceptional” position and into increasingly inconspicuous places. This, however, leads into questions in the philosophy of science that lie beyond the scope of this article.

  Thanks to the launch of the Kepler space telescope, exoplanets began to be detected in large numbers after 2010. By 2026, more than 6,000 exoplanets had been confirmed. Current observations show that even the stars in the Sun’s immediate neighborhood host so many planets. Extrapolating to the whole Milky Way, a space artist’s depiction of the “hot Jupiter” 51 Pegasi-b (left) and its host star 51 Pegasi (right)—the first “hot Jupiter” discovered in human history—the number of stars within it is on the order of hundreds of billions. Seen in that context, planets are naturally very common.

  As for the second question, the mainstream theory of planet formation holds that a newly formed star does not draw in all the gas and dust around it. The remaining material forms a “protoplanetary disk” around the star. Within that disk, planets grow through collisions, beginning as micrometer-sized grains and becoming kilometer-scale planetesimals. Because a protoplanetary disk extends across a large region, matter is distributed differently at different distances from the Sun. The “snow line” marks the boundary beyond which the disk is cool enough for water and other volatile substances to condense into ice. Inside the snow line, planetesimals consist mostly of rock; outside it, most are mixtures of rock and ice. Beyond the snow line, the gravitational feeding zone is larger and icy solids are more abundant, so the solid cores of planetesimals “grow” extremely quickly. Once they reach a certain mass, they can rapidly capture the gas left in the protoplanetary disk and eventually become enormous gas planets. Inside the snow line, planetesimals have smaller feeding zones and fewer icy solids, leaving them with less massive solid cores. They therefore struggle to accrete large quantities of gas and usually must wait until the gas in the protoplanetary disk has gradually dispersed before growing through collisions with other planetesimals. Their increase in mass is consequently very limited, and their atmospheres are also extremely thin.

  This conclusion was originally developed in an effort to “reconstruct” the Solar System’s present distribution of planets: small, low-mass solid planets dominate the inner region, while large, high-mass gaseous planets dominate the outer region. Since the first “hot Jupiter”—a Jupiter-mass gas giant orbiting very close to its host star—was discovered in 1995, however, many more hot Jupiters have been found. Their discovery has called the original theory of planet formation into question to some extent. The discovery of the Kuiper Belt also seems to suggest that the Solar System’s actual formation history was far more dramatic than we had thought.

  The image below illustrates the evolution of a star and its planetary system. Under gravity, diffuse gas in space collapses to form a star. The gas and dust left around the newly formed star create a “protoplanetary disk,” in which planets then form. Late in its evolution, the star returns some of its material to interstellar space. If that residual gas gathers together, it may form a new star.

Diagram of the evolution of a star and its planetary system