Chapter 144: Celestial Body

Uranus is the seventh planet from the Sun, with a diameter of 51,118 kilometers. Its volume is approximately sixty-five times that of Earth, ranking third among the nine traditional planets after Jupiter and Saturn. Uranus’s atmosphere consists of 83 percent hydrogen, 15 percent helium, 2 percent methane, and small amounts of acetylene and other hydrocarbons.

Methane in the upper atmosphere absorbs red light, giving Uranus its blue-green appearance. The atmosphere gathers into cloud bands at fixed latitudes, resembling the vivid stripes that girdle Jupiter and Saturn. The average temperature of Uranus’s cloud layers is minus 193 degrees Celsius. Its mass is approximately 8.6810 × 10²⁵ kilograms, or 14.63 times the mass of Earth. It has a relatively low density of only 1.24 grams per cubic centimeter—74.7 percent of Neptune’s density.

Neptune is the eighth planet from the Sun, with a diameter of 49,532 kilometers. Its orbit lies at an average distance of 4.5 billion kilometers from the Sun, and it takes 165 years to complete one revolution. Since its discovery in 1846, Neptune has not yet completed a single full orbit. Its diameter is similar to Uranus’s, while its mass is slightly greater. The principal atmospheric components of both Neptune and Uranus are hydrogen and helium, and their internal structures are also remarkably alike. For this reason, Neptune and Uranus are often described as twin brothers.

Neptune has the strongest winds in the Solar System, with measured speeds reaching 2,100 kilometers per hour. The temperature at the tops of its clouds is minus 218 degrees Celsius, making it one of the coldest regions in the Solar System. The temperature at Neptune’s core is approximately 7,000 degrees Celsius, comparable to the surface of the Sun. Neptune was discovered on September 23, 1846, and is the only planet discovered through mathematical prediction rather than through a planned observational search.

Pluto lies in the inner region of the Kuiper Belt beyond Neptune and is the largest known object in that belt. Its diameter is approximately 2,370 ± 20 kilometers, about 18.5 percent of Earth’s diameter. On August 24, 2006, the General Assembly of the International Astronomical Union voted to cease classifying Pluto as one of the traditional nine planets and instead designate it a dwarf planet.

The resolution adopted by the assembly defined a planet as a celestial body that orbits the Sun, possesses sufficient self-gravity to overcome the forces of rigidity and assume a nearly spherical shape, and has cleared the region around its orbit of other objects. Of the traditional nine planets in the Solar System, only Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune meet these requirements.

Because Pluto’s orbit intersects Neptune’s, it does not satisfy the new definition of a planet and was therefore automatically reclassified as a dwarf planet. Pluto’s surface temperature ranges roughly from minus 238 to minus 228 degrees Celsius.

Pluto is composed of approximately 70 percent rock and 30 percent a mixture of water ice. The brighter regions of its surface may be covered with solid nitrogen, along with small amounts of solid methane and carbon monoxide. The darker regions may consist of basic organic substances or products of photochemical reactions triggered by cosmic rays.

Pluto’s atmosphere is composed mainly of nitrogen, with small amounts of carbon monoxide and methane. It is extremely tenuous, and the atmospheric pressure at the surface is only a few micropascals.

Earth is the third planet from the Sun and the home of humanity. Although it is an ordinary planet in the Solar System, it is unique in many respects.

For example, it is the only known planet in the Solar System whose surface is predominantly covered by water, and the only world currently known to harbor life. Its mass is 5.9742 × 10²⁴ kilograms, and its surface temperature ranges from minus 30 to plus 45 degrees Celsius. British researchers reported in the journal Astrobiology that, barring events such as asteroid impacts capable of radically altering the environment, Earth may remain habitable for humans for approximately another 1.75 billion years. Human-caused climate change, however, could shorten that span.

No one knew whether they could win such a protracted war without adequate preparation, so they needed to learn more about the planet beneath their feet and the galaxy in which it lay. Hua Feng understood this perfectly. As one of the most diligent among the trainees, he dared not relax for even a moment.

Comets are small bodies in the Solar System made of dust and ice that travel around the Sun.

Scientists have analyzed the chemical residues of comets with probes and discovered that they are composed primarily of ammonia, methane, hydrogen sulfide, hydrogen cyanide, and formaldehyde. They concluded that the smell of a comet would be a combination of rotten eggs, horse urine, alcohol, and bitter almonds.

The Solar System is also enclosed by an immense Oort Cloud. Countless blocks of ice, snowballs, and fragments of rock are scattered throughout it. Some are drawn inward by the Sun’s gravity and enter the inner Solar System; these become comets. As the ice, snow, and rock fragments enter the inner reaches of the system, their surfaces begin to vaporize under the force of the solar wind.

That is why comets trail long tails, which grow longer and more distinct as they approach the Sun. Interplanetary space within the Solar System is not a vacuum. It is filled with all kinds of particles, radiation, gases, and dust.

The Kuiper Belt is a theoretically predicted ring extending from roughly 50 to 500 astronomical units from the Sun, near the outermost reaches of the Solar System, and is believed to be the source of short-period comets. It is a vast ring of icy debris beyond Neptune’s orbit, encircling the outer edge of the Solar System.

The Diversity of Matter

A red giant is the stage a star enters after passing through its long period of maturity—the main-sequence phase—and beginning its old age. It is called a giant because of its enormous size.

During the giant phase, a star’s volume may expand by as much as a billion times. It is called a red giant because, as the star rapidly expands, its outer surface moves farther from its center, causing its temperature to fall and its light to shift increasingly toward the red end of the spectrum. Although its temperature decreases, the red giant becomes extraordinarily luminous because of its immense size. Once formed, a red giant proceeds toward the next stage of stellar evolution: the white dwarf.

A white dwarf is a star of low luminosity, high density, and high temperature. It is so named because it appears white and is relatively small in size. White dwarfs are highly unusual celestial bodies: they are small and dim, yet massive and extraordinarily dense. They represent the final evolutionary stage of low- and intermediate-mass stars.

Toward the end of the red-giant stage, a star’s core ceases to generate energy because its temperature and pressure are insufficient, or because nuclear fusion has reached the iron stage. The gravity of the stellar envelope then compresses the star into a highly dense object. A typical stable white dwarf has approximately half the mass of the Sun and is only slightly larger than Earth.

Its density is surpassed only by that of neutron stars and quark stars. If a white dwarf’s mass exceeds 1.4 times the mass of the Sun, the electrostatic repulsion between atomic nuclei can no longer withstand gravity. Electrons are forced into the nuclei, forming a neutron star. Atoms consist of nuclei and electrons, with nearly all an atom’s mass concentrated in its nucleus. Under immense pressure, the electrons break free from the nuclei and become free electrons.

This gas of free electrons occupies the gaps between atomic nuclei as fully as possible, greatly increasing the amount of matter contained within a given volume and causing the density to rise enormously.

In simple terms, the atomic nuclei are then “immersed” in electrons, a condition commonly called degeneracy. Most stellar cores generate energy by fusing hydrogen nuclei, converting mass into energy and producing light and heat. Once the hydrogen fuel within a star has been exhausted, helium fusion begins, producing heavier elements such as carbon and oxygen. For stars similar to the Sun, this phase is relatively brief and ultimately produces a white dwarf composed of carbon and oxygen. If its mass exceeds 1.4 times that of the Sun, a Type Ia supernova explosion occurs.

Since the 1960s, astronomers have also discovered a type of object outside the Milky Way that appears as a star-like point of light, yet actually possesses the luminosity and mass of a galaxy. These objects are called quasars, and thousands of them have now been identified.

A supernova is one stage in the evolution of a star. A supernova explosion is a violent detonation experienced by certain stars near the end of their evolution. It is generally believed that stars with masses below roughly nine times that of the Sun cannot produce supernovae after undergoing gravitational collapse.

When a massive star reaches the late stages of its evolution and can no longer generate new energy internally, its immense gravity causes the entire star to collapse rapidly toward the center, compressing the central material into a neutron state and forming a neutron star. The infalling outer layers then strike this hard “neutron core” and rebound, triggering an explosion. This is a supernova. When the mass is even greater, the center may instead form a black hole. The energy released during a supernova explosion is equivalent to the energy our Sun would produce by burning for ninety billion years.

The study of supernovae has profound implications for humanity’s own fate. If a supernova were to explode very close to Earth—an interval generally regarded by the international astronomical community as within one hundred light-years—it could have a significant impact on Earth’s biosphere. Such an event is called a nearby supernova. Some studies suggest that the mass extinction at the end of the Ordovician Period may have been caused by a nearby supernova, which led to the disappearance of nearly 60 percent of Earth’s marine life at the time.

A pulsar is the remnant left after a star explodes as a supernova. Once the explosion is over, only a “core” remains, usually no more than a few dozen kilometers across. It rotates at tremendous speed, with some pulsars completing as many as 714 rotations per second. As it spins, its magnetic field generates powerful radio waves that radiate outward. A pulsar is like a lighthouse in the cosmos, continuously sending electromagnetic waves into space. These waves arrive intermittently and with extraordinary regularity.

Because of this remarkable regularity, pulsars are regarded as the most precise clocks in the universe.