Humanity has long gazed at the night sky, driven by a desire to comprehend its place within the vast cosmos. This curiosity has propelled centuries of exploration and discovery, gradually unveiling a cosmic address far more intricate than imagined. From our home planet to the edge of the observable universe, each step reveals an increasingly complex cosmic geography, challenging previous understandings of our location.
Our Home Planet and Solar System
Our immediate cosmic neighborhood begins with Earth, a planet orbiting the Sun within the Solar System. This system formed approximately 4.6 billion years ago from a rotating cloud of gas and dust, with the Sun at its center, accounting for over 99% of the system’s total mass. Eight major planets orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. These are divided into inner rocky planets and outer gas and ice giants.
Beyond the planets, the Solar System includes numerous smaller celestial bodies. The asteroid belt, a region primarily composed of rocky remnants, lies between the orbits of Mars and Jupiter. Farther out, beyond Neptune, is the Kuiper Belt, a vast disc of icy objects and dwarf planets like Pluto, Eris, and Ceres. The outermost theoretical boundary is the Oort Cloud, a spherical shell of icy bodies extending tens of thousands of astronomical units from the Sun.
Within the Milky Way Galaxy
Our Solar System is nestled within a much larger structure, the Milky Way Galaxy, which is classified as a barred spiral galaxy. This immense galaxy spans approximately 100,000 light-years in diameter and is about 1,000 light-years thick in its disk. It is estimated to contain between 100 billion and 400 billion stars.
The Solar System is located within the Milky Way’s disk, on the inner edge of a smaller spiral arm known as the Orion Arm. This position places our Solar System roughly 26,000 to 27,000 light-years from the galactic center. The Milky Way’s structure includes a central bulge, a dense region of older stars about 10,000 light-years in diameter, and a surrounding halo of stars and dark matter. At the very heart of the galaxy lies a supermassive black hole, known as Sagittarius A. Our Solar System completes one orbit around the galactic center approximately every 230 to 250 million years.
Our Galactic Neighborhood and Beyond
The Milky Way is not isolated in space; it is part of a gravitationally bound collection of galaxies known as the Local Group. This group has a diameter of roughly 10 million light-years and includes over 50 galaxies. The two largest members are our own Milky Way and the Andromeda Galaxy, with the Triangulum Galaxy being the third largest. The Andromeda Galaxy is currently about 2.5 million light-years away from the Milky Way.
The Local Group is part of an even larger structure called the Laniakea Supercluster. This supercluster spans approximately 500 million light-years in diameter and encompasses around 100,000 galaxies. Within Laniakea, galaxies are not randomly distributed but are organized into filaments, flowing towards a region known as the Great Attractor.
The Observable Universe and Cosmic Web
At the grandest scale, our cosmic address extends to the observable universe, which represents the portion of the universe from which light has had sufficient time to reach Earth since the Big Bang. This limit is due to the finite speed of light and the universe’s estimated age of 13.8 billion years. However, because the universe has been continuously expanding, objects whose light we observe from 13.8 billion years ago are now much farther away.
Current estimates place the diameter of the observable universe at approximately 93 billion light-years. On these immense scales, galaxies and superclusters are not uniformly distributed but form a vast, interconnected network often referred to as the “cosmic web.” This web consists of dense filaments of galaxies interspersed with enormous, relatively empty regions called voids. Our Laniakea Supercluster is merely a tiny component within this sprawling, intricate cosmic structure.