To Infinity and Beyond: A Journey of Cosmic Discovery

Exploring the solar system was made possible by innovations after the discovery and acceptance that Earth was not at its center. In particular, the science of spectroscopy—which can “determine motions, temperatures, rotation rates, and especially the chemical properties of objects” (86). Spectroscopy utilizes light, the range of spectra, to make its measurements. Its application to astronomy created the word “astrophysics” in the 19th century. In the 20th century, technology was developed that allowed for the launch of probes and the collecting of physical samples; thus, knowledge of the solar system continues to expand.

The solar system—the sun and all eight planets (plus their various moons and other space debris, like asteroids)—was formed at once. The sun of this particular solar system was born out of the death of another star 4.5 billion years ago. The sun’s thermonuclear core is the sole thing that keeps it from collapsing in on itself; eventually, it, too, will die—but not for about another 5 billion years. The sun itself possesses its own atmosphere, as does the Earth; however, its atmosphere consists of solar wind with solar flares and intense heat coming off the surface. The sun also produces coronal mass ejections (CMEs), which can be quite dangerous to the planets in the solar system. Should a large CME happen today, it could destroy satellites and the Earth’s power grid (wiping out cell service, debit cards, and airplanes), causing mass chaos. The Parker Solar Probe was sent in 2021 to investigate activity on the sun to understand these events better. The planets that orbit the sun are the winners in what was, billions of years ago, a chaotic competition for survival—in this case, the coalescence of matter without violent interference.

The planet closest to the sun, Mercury, is heir to “the worst reputation” (94), according to the authors. It is considered a bad omen: Mercury in retrograde is believed to be bad luck. Before Copernicus proposed the idea of a heliocentric solar system, Earth was thought to hold the fixed, central position in the solar system; from that point of view, Mercury occasionally appeared to be moving backward across the sky. While this has been disproven, Mercury in retrograde still looms large in astrology. The least explored of all the celestial bodies in the solar system, Mercury presents challenges to probes and landers: The speed of its orbit and low gravity make it difficult to calculate a precise landing. Using “a cosmic slingshot” (101), wherein a spacecraft makes use of a planet’s orbital speed to push it along helped the MESSENGER mission in 2004. This is also how the Voyager probes made it to interstellar space. This mission discovered water ice on Mercury in its four years of orbit around the planet.

Venus, another inner terrestrial planet (Mercury, Venus, Earth, and Mars), appears as one of the brightest objects in the night sky. During Galileo’s observations of Venus, he concluded that the planets revolved around the sun, confirming Copernicus’s claims. Venus’s orbit—specifically its transit in front of the sun (from Earth’s point of view)—also allowed 17th-century scientists to approximate “the distance from Earth to the sun, the size of Venus, and the size of the entire solar system out to Saturn” (106). When more sophisticated technology arrived in the 18th century, these measurements were refined and confirmed. When Venus was finally explored via probe and lander, hopes that it would be habitable were dashed: The atmosphere is mostly carbon dioxide. This is because the greenhouse effect operates in high gear on Venus; when all its surface water evaporated, it became enveloped in vapors that trap even more heat. This is what led astrophysicist Carl Sagan to express his concerns, back in the 1980s, about the growing greenhouse effect on Earth from the burning of fossil fuels.

The Earth is dependent on the moon for its tidal systems and the stability of its tilted axis. Theories about how the moon was formed were once abundant, but after the lunar landing collected numerous samples, it was determined that the moon’s “composition is strikingly similar to Earth’s crust” (117). This spawned the giant-impact hypothesis: The moon was formed after a large planetoid or asteroid struck the Earth, knocking loose debris that would, because of gravity, eventually coalesce into the moon. With the passage of cosmic time (measured in billions of years), Earth’s rotation has slowed—in part because of the tidal forces imposed by the moon—and the moon is moving farther away from Earth. The authors debunk the notion that the moon’s tidal forces impact human behavior.

Mars was instrumental in proving that orbits are elliptical, not perfectly circular, as was believed throughout the ancient and medieval worlds. Observing Mars allowed Johannes Kepler, a 17th-century astronomer, to understand how orbits work and to devise his laws of orbits, areas, and periods, which govern the motion of the planets. In addition, Mars has provided generations with the hope that extraterrestrial life exists on another planet within the solar system since Mars is the most Earth-like planet. Due to a translation error, the idea that Mars possessed canals—rather than channels—also took hold before the Mariner missions debunked the idea in the late 1960s. Mariner 4 gave scientists a better idea of what Mars itself was actually like: “thin atmosphere, freezing temperatures, and weak magnetic field” (129). Mars has no protection from the solar elements, so it is unlikely that intelligent life ever flourished there. Nevertheless, Mars has also sparked hopes of terraforming—adapting the planet to support human life. The many obstacles to such a plan include political ideologies, technological difficulties, and ethical quandaries. The authors suggest that these energies are better used to preserve the Earth.

Between the four rocky inner planets and the four outer planets lies the asteroid belt, a region filled with space debris, including asteroids, a dwarf planet, and various other bits of matter that never coalesced into planets. Early in the history of astronomy, some objects in the asteroid belt were considered planets; at one time, scientists believed that the solar system contained 11 planets. Asteroids can pose significant threats to Earth (such as the impact that killed the dinosaurs), and the authors address how a large asteroid could be diverted from its path should it be heading toward Earth. Rather than sending retrained oil rig workers to plant a nuclear bomb on the asteroid as in the movie Armageddon, the authors suggest simpler but potentially effective solutions, from spray painting the asteroid white to reflect the sun and thereby redirect the asteroid’s path to launching jets that would use the force of gravity to nudge it in a different direction. While the idea of an asteroid striking the Earth seems like fodder for science fiction films, it is a viable threat.

Jupiter and Saturn, beyond the asteroid belt, are considered the gas giants of the solar system. Jupiter, named for the king of the Roman gods, is an inadvertent protector of Earth as its gravity pulls in objects that otherwise might crash into the Earth. Its Great Red Spot—a gigantic storm that’s twice as wide as the Earth—has spurred scientific curiosity for centuries. Nobody knows when the storm began or when it will end. Jupiter also boasts the largest ocean in the solar system—though it is made of liquid hydrogen—which contributes to the planet’s strong magnetic field. The other gas giant, Saturn, is known for its rings, which are made of space debris, ice, and minuscule moons. Saturn consists mostly of hydrogen; as the authors put it, “the whole planet could float in a basin of water” (151). One of Saturn’s moons, Titan, contains a geography much like Earth’s with rivers and lakes (of methane) carving canyons into its surface. The authors speculate that a different kind of lifeform might be supported by such liquid methane. There has also been speculation as to whether three of Jupiter’s moons—Ganymede, Enceladus, and Europa—might support life since they have underground oceans.

The ice giants, Uranus and Neptune, are so far from Earth—billions (not millions) of miles away—that they “still await orbiters and landers of their own” (156). Made up of water ice but too far from the sun to support life, the two planets are known only by the Voyager missions as they sailed past them. Uranus completes its orbit around the sun on its side, probably due to an ancient collision with another planet. Neptune is scoured by massive winds of up to 1,000 miles per hour. While not much is known about either of these planets, most astronomers agree that these kinds of planets represent most of the exoplanets in the galaxy.

Finally, Pluto is no longer considered a proper planet within the solar system but rather a dwarf planet. Once named Planet X, early astronomers thought that its existence explained anomalies in Neptune’s orbit. As the Voyager probes left the solar system, they sent back images confirming that Pluto was too small to cause those deviations and too small to be a planet. Instead, it was part of another kind of asteroid belt, named the Kuiper belt: a mass of space debris at the edge of our solar system that contains not only asteroids and other matter but also dwarf planets. The New Horizons probe, launched in 2006, sent back pictures of a mountainous terrain and a partially frozen ocean on Pluto in 2015. It joined Voyagers 1 and 2, along with Pioneers 10 and 11, to leave the solar system for interstellar space.