What is beyond the horizon? Answering this question continually pushes human knowledge further, expanding our understanding of the natural world and our place within it. In 1990, the only planets and moons we knew of orbited the Sun in the same system with Earth. Since then our horizons have expanded to include thousands of other worlds orbiting other stars, called exoplanets, opening up a new field of scientific research and new potential for putting our cosmic experience in context. Are our Earth, our sun, our solar system unique? Are we alone?
Worlds of Difference
From our home on the Earth, we look out into the distances and strive to imagine the sort of world into which we were born. Today, we have reached far into space. Our immediate neighborhood we know rather intimately. But with increasing distance our knowledge fades. …The search will continue. The urge is older than history. It is not satisfied and will not be suppressed.
Our first reference for an alien world is the Moon, and beyond that the other planets, moons, and dwarf planets of our solar system. While each is unique and very different from the environment we know on Earth, they follow what we have come to regard as a logical organization: a single middle-aged yellow star orbited most closely by rocky planets (Mercury, Venus, Earth, Mars), with giant planets farther out (Jupiter, Saturn, Uranus, Neptune), plus broad fields of smaller bodies—the rocky Asteroid Belt and the icy Kuiper Belt, leftover material from the process of planet formation. Earth, the only known world with life, has vast liquid water oceans and a substantial atmosphere.
Spacecraft missions have shown us that the moons of our solar system are also greatly varied worlds. Jupiter’s Europa has an icy crust laced with fissures that release water vapor, while its moon Io is the most volcanically active world in the Solar System. Saturn’s moon Titan has large surface lakes of methane.
Since the first exoplanets were discovered orbiting a pulsar in 1992, astronomers have realized there are more and stranger varieties of worlds than we could have imagined, and many likely don’t fit into the neat plan we’ve come to recognize in our own system. For example, Proxima b, the closest exoplanet to us, is in the triple-star system Alpha Centauri. There are also Jupiter-sized gas planets orbiting very close to their star, huge rocky “super-Earths,” and “warm Neptunes.” Webb’s spectroscopic instruments will study the atmospheres of Solar System worlds and exoplanets, determining which elements are there and what they indicate about the world, including its potential to support life.
The diversity of other worlds is a result of the diversity of stars and the content of the nebulas from which they both form. As a nebula of gas and dust swirls around a young protostar, the building blocks of a planetary system can come together in many ways. The original material in the nebula determines the composition of the star(s), planets, moons, and asteroids that may form as complex processes transform tiny specks of dust into great worlds.
Webb’s ability to observe as far as the mid-infrared portion of the light spectrum allows astronomers to study planet formation in dusty protoplanetary disks that block visible light, providing new clues to the enduring mysteries of how the universe works, and how our own home system was formed.
Brown dwarf formation is a specific mystery astronomers look forward to investigating with Webb. Brown dwarfs defy easy definition, having properties of both planets and stars. Like a planet, brown dwarfs can have atmospheres with thick clouds, but they have masses many times the size of the largest planet in our solar system, Jupiter. However, they are not massive enough to generate their own light like a star. Yet some brown dwarfs have been found in a binary relationship with a star. These unique qualities makes the formation process of brown dwarfs uncertain—is it like a star, by the contraction of gas? Or like a planet, by the accretion of material in a protoplanetary disk? The answers will shed light not only on the ambiguous nature of brown dwarfs, but on the formation of stars and planets as well.