In the 18th century, Kant and Laplace proposed that the Sun and its planets formed from a vast, spinning nebula (interstellar cloud) that flattened out as it was pulled together by gravity. The Sun formed at the center of the nebula, and the planets and other small bodies formed in an extended “disk." Although this Nebula Hypothesis explained various observational features of the Solar System, it didn’t make any predictions that could be tested at the time.
Fast-forward a couple of hundred years. When I began graduate school in the early 1980s, scientists who studied the planets in our Solar System (planetary scientists) and those who studied “the rest of the Universe” (astronomers) didn’t talk to each other much. In the late 1970s, planetary scientist Tom Gehrels had proposed a Protostars and Planets project to bring these communities together to study how stars and planets form. Naysayers wagered the initiative wouldn’t succeed because astronomers and planetary scientists were too far apart to even speak the same language, but the discoveries of the long-sought-after flattened nebulae around young stars (circumstellar disks) and of exoplanets orbiting other stars provided strong incentives for these communities to learn to work together! After all, how else could progress be made in understanding how vast interstellar clouds could eventually form stars with orbiting worlds, some of which might have active geology, and perhaps even harbor life?
Since the mid-1990s, the number of known exoplanets has increased almost exponentially. In fact, it looks like virtually every star may have planets! The diversity of exoplanetary systems is far beyond what scientists imagined a couple of decades ago, based on the architecture of our Solar System and what we thought we understood about how planets form. In tandem with the observations, sophisticated computer models of planet formation indicate that the planets in our Solar System probably did not form precisely in their present-day positions, but rather reflect significant interactions between the forming planets and the Sun’s circumstellar disk.
Consider the implications. We now understand our Sun to be one among about 400 billion stars in the Milky Way Galaxy, which in turn is one among hundreds of billions of galaxies in the observable part of the Universe. The number of stars is far greater than the number of grains of sand on all Earth’s beaches. At present, Earth is the only planet on which we know life exists, but only two decades ago, we knew of only one planetary system. The incredible diversity of exoplanetary systems suggests that, if anything, we have been far too conservative in thinking about what life might be like elsewhere. Science fiction writers are surely smiling!
Written by Dr. Grace Wolf-Chase, astronomer, Adler Planetarium and the University of Chicago.