A black hole is a region of space where gravity is so strong that even light cannot escape. Because matter has been compressed into a small space, gravity is extremely strong. This can occur when a star dies. People cannot see black holes because no light can escape. They are unnoticed. Space telescopes equipped with specialised tools can aid in the discovery of black holes. Special tools can observe how stars in close proximity to black holes behave differently than other stars. Black holes can be large or small in size. Scientists believe the tiniest black holes are as small as one atom. These black holes are extremely small in size but have the mass of a large mountain. The amount of matter, or "stuff," in an object is defined as its mass. A "stellar" black hole is a different type of black hole. Its mass could be up to 20 times that of the sun. There could be a plethora of stellar mass black holes in Earth's galaxy. The Milky Way is the name given to Earth's galaxy. Scientists believe that the tiniest black holes formed when the universe first began. Stellar black holes form when the core of a massive star collapses in on itself. When this occurs, it results in a supernova. A supernova is a star that explodes, ejecting a portion of the star into space. Scientists believe that supermassive black holes formed at the same time as the galaxy in which they exist.

Earth is the third planet from the Sun and, in terms of size and mass, the fifth largest planet in the solar system. Its most notable feature is that its near-surface environments are the only places in the universe where life has been discovered. It is represented by the symbol. The name Earth derives from Old English and Germanic words for ground and earth in English, the international language of astronomy, and it is the only name for a planet in the solar system that does not come from Greco-Roman mythology.Since the 16th century Copernican revolution, when the Polish astronomer Nicolaus Copernicus proposed a Sun-centered model of the universe (see heliocentric system), enlightened thinkers have regarded Earth as a planet like the others in the solar system. Concurrent sea voyages provided practical proof that Earth is a globe, just as Galileo's use of his newly invented telescope in the early 17th century quickly demonstrated that various other planets are also globes. However, it wasn't until the dawn of the space age, when photographs from rockets and orbiting spacecraft first captured the dramatic curvature of Earth's horizon, that the concept of Earth as a roughly spherical planet, rather than a flat entity, was confirmed by direct human observation.When Apollo 8 carried astronauts around the Moon in December 1968, humans saw Earth for the first time as a complete orb floating in the inky blackness of space. Robotic space probes on their way to destinations beyond Earth, such as the Galileo and Near Earth Asteroid Rendezvous (NEAR) spacecraft in the 1990s, used their cameras to provide additional unique portraits of the planet.

Jupiter is the largest planet in the solar system and the fifth in terms of distance from the Sun. It is one of the brightest objects in the night sky, rivalled only by the Moon, Venus, and, on rare occasions, Mars. Jupiter is represented by the symbol ♃. When ancient astronomers named Jupiter after the Roman ruler of the gods and heavens (also known as Jove), they had no idea of the planet's true size, but the name is appropriate because Jupiter is larger than all the other planets combined. It takes nearly 12 Earth years to orbit the Sun and rotates once every 10 hours, which is more than twice as fast as Earth; its colourful cloud bands can be seen with a small telescope.It has a narrow ring system and 79 known moons, one larger than Mercury and three larger than Earth's Moon. Some astronomers believe Jupiter's moon Europa may be hiding an ocean of warm water—and possibly even life—below its icy crust.

Mercury is our solar system's smallest planet and the closest to the sun. The tiny planet has no moon and orbits the sun faster than any other planet, which is why the Romans named it after their swift-footed messenger god.Mercury was also known to the Sumerians at least 5,000 years ago. According to a site connected to NASA's MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) mission, it was frequently associated with Nabu, the god of writing. Mercury was also given distinct names for its appearance as a morning and evening star. However, Greek astronomers recognised that the two names referred to the same body, and Heraclitus, around 500 B.C., correctly assumed that both Mercury and Venus orbited the sun, rather than Earth.Mercury is the second densest planet after Earth, with a massive metallic core that spans 2,200 to 2,400 miles (3,600 to 3,800 kilometres), accounting for roughly 75% of the planet's diameter. Mercury's outer shell is only 300 to 400 miles (500 to 600 kilometres) thick. For years, scientists have been perplexed by the combination of its massive core and composition, which includes an abundance of volatile elements.Mercury is the second densest planet after Earth, with a massive metallic core that spans 2,200 to 2,400 miles (3,600 to 3,800 kilometres), or roughly 75% of the planet's diameter. Mercury's outer shell is only 300 to 400 miles (500 to 600 km) thick. Scientists have been perplexed for years by the combination of its massive core and composition, which includes an abundance of volatile elements.