Our Solar System: A Familiar Structure

The Solar System, which we are most accustomed to, consists of eight planets orbiting the Sun in a flat disk. The four closest planets—Mercury, Venus, Earth, and Mars—are classified as terrestrial or rocky planets. Beyond the Main Asteroid Belt, located past Mars, lie the four outer planets: two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune).

The distinction between the rocky inner planets and the gaseous outer ones can be attributed to the conditions during the Solar System’s formation approximately 4.6 billion years ago. The heavier elements, known as metals (like iron, nickel, and silicon), could withstand the intense heat (around 1,500 Kelvin) closer to the Sun due to their higher boiling points.

During this early phase, the gases present at these inner distances either evaporated due to solar radiation or were blown away by solar winds, leaving metals as the primary solidifying elements for the inner planets.

Conversely, lighter materials, including ices (such as frozen methane and water) and gases (predominantly hydrogen and helium), thrived in the cooler, distant regions. This abundance of lightweight ingredients explains why the outer planets are significantly larger than their rocky counterparts.

Beyond Neptune lies the Kuiper Belt, a vast region teeming with over 100,000 icy bodies at around 40 Kelvin, including Pluto, Makemake, and Eris. This belt is notably larger and more massive than the Main Asteroid Belt.

Farther still, we encounter the hypothesized Oort Cloud, a spherical shell of icy objects that envelops our Solar System and is thought to be the origin of comets.

Regarding orbital speeds, Earth, with a mass of approximately 5.98×10²? kg at a distance of 150 million km from the Sun, travels at an astonishing 108,000 km/h (66,500 miles/h) to complete its orbit in 365.25 days. Mercury, the innermost planet, holds the record for the fastest speed at 172,000 km/h (107,000 miles/h), while Neptune moves at the slowest pace of 19,500 km/h (12,000 miles/h).

Our Milky Way Galaxy: A Larger Structure

Our Solar System exists within the Milky Way Galaxy, a vast collection of stars (approximately 400 billion, including our Sun), gas, dust, and dark matter. This Galaxy, formed roughly 12 billion years ago, is held together by gravity, which also governs its dynamics.

Most of the stars, dust, and gas are concentrated in a flattened region known as the stellar disk, which has a mass about 50 billion times that of our Sun and spans 100,000 light years in diameter.

Stars are typically born in the spiral arms of the galaxy, which are regions rich in gas and dust. At a distance of about 27,000 light years from the Milky Way’s center, our Solar System resides in the Orion arm, orbiting the Galaxy at an impressive speed of 720,000 km/h (447,000 miles/h). Despite these remarkable velocities, a single orbit around the Milky Way takes about 240 million years.

At the center of the stellar disk lies a dense region filled with older stars, gas, and dust orbiting a supermassive black hole known as Sagittarius A*. This black hole is one of the densest objects in the Universe, with a gravitational field so strong that not even light can escape from it.

Surrounding the stellar disk is a spherical stellar halo, composed of older stars and globular clusters, which accounts for only about 1% of the total stellar mass of the Galaxy. Further out, a hypothesized dark matter halo, with a diameter of at least 600,000 light years, contains an unidentified type of matter that constitutes approximately 28% of all matter in the Universe.

When all forms of matter are considered, nearly 90% of the Milky Way’s mass can be attributed to the dark matter halo, bringing the total mass of the Galaxy to approximately 1.5 trillion solar masses. The existence of this halo, while not directly observed, is inferred from the behavior of stars and gas within the Galaxy.

Our Local Group: Part of a Larger Community

Zooming out further, we find ourselves within the Local Group, which consists of a collection of around 40 galaxies (with some estimates suggesting even more) divided into two main subgroups: the Milky Way and the Andromeda Galaxy.

These two spiral galaxies are currently on a collision course and are expected to merge in the next 3 to 5 billion years, as they are approximately 2.5 million light years apart and approaching each other at a speed of 432,000 km/h (268,000 miles/h).

The total mass of the Local Group is estimated to be at least 2.4 trillion solar masses (some studies suggest up to 3.7 trillion solar masses), spanning a region of space around 10 million light years across.

The three largest galaxies in the Local Group are Andromeda (with a diameter of 220,000 light years), the Milky Way (100,000 light years), and the Triangulum Galaxy (56,000 light years). In terms of mass, the ranking is Milky Way (1.5 trillion solar masses), Andromeda (800 billion solar masses), and Triangulum Galaxy (80 billion solar masses).

The remaining galaxies are smaller dwarf galaxies that orbit either the Milky Way or Andromeda, known as satellite galaxies. The closest of these to the Milky Way are the Canis Major Dwarf Galaxy and the Sagittarius Dwarf Elliptical Galaxy.

Our Virgo Supercluster: A Galactic Neighborhood

The Local Group is situated within the Virgo Supercluster, a massive structure comprising many galaxy groups and clusters. Roughly 90% of the 100 billion galaxies in the Universe are found in these large formations, which total about 10 million.

The Virgo Supercluster, classified as a "poor" supercluster due to its lower number of clusters, measures 98 million light years across and contains a mass of 1.5 quadrillion solar masses. It includes the Virgo Cluster as its primary cluster and numerous smaller groups like the Local Group.

Our Local Group is located on the outskirts of the Virgo Supercluster, approximately 53 million light years from the Virgo Cluster's center.

Some of our closest neighboring groups within the Virgo Supercluster include the Maffei 1 Group, M81 Group, and Sculptor Group, each containing several galaxies. The Virgo Cluster, home to between 1,300 and 2,000 galaxies, includes the supergiant elliptical M87, which made headlines for capturing the first-ever image of a black hole's shadow in 2019.

Additionally, our Local Group is moving towards the Virgo Cluster at a speed of 972,000 km/h (604,000 miles/h) in a phenomenon known as Virgocentric infall.

Our Laniakea Supercluster: A Cosmic Assembly

The Virgo Supercluster is part of a larger structure known as the Laniakea Supercluster, which comprises four major components: the Virgo Supercluster, the Hydra-Centaurus Supercluster, the Pavo-Indus Supercluster, and the Southern Supercluster.

Spanning 522 million light years and containing around 100 quadrillion solar masses, the Laniakea Supercluster is home to 100,000 galaxies and numerous groups and clusters. Notable clusters include the Norma Cluster and the Centaurus Cluster.

In addition to clusters, Laniakea encompasses several cosmic voids—regions of low galaxy density—such as the Local Void and the Sculptor Void.

Our motion towards the Virgo Cluster is complemented by an even faster trajectory toward the Great Attractor, the gravitational center of the Laniakea Supercluster located within the Centaurus Cluster.

However, it’s important to note that the Laniakea Supercluster is not a gravitationally bound entity; dark energy, which drives the Universe’s expansion, will eventually cause galaxies within it to drift apart.

Our Pisces-Cetus Supercluster Complex: A Broader Network

The Laniakea Supercluster is part of an even larger assembly known as the Pisces-Cetus Supercluster Complex, which includes five segments: the Laniakea Supercluster, the Pisces-Cetus Supercluster, the Perseus-Pegasus Chain, the Pegasus-Pisces Chain, and the Sculptor Region.

This complex is nearly 1 billion light years long and 163 million light years wide, forming a cosmic filament—a high-density structure of galaxies.

Superclusters are flanked by galactic voids, creating a sponge-like network of high-density regions across the Universe.

The Pisces-Cetus Supercluster Complex is interlinked with other large structures, including the South Pole Wall—a significant filament measuring 1.4 billion light years in length.

Our Cosmic Home: A Perspective

Much like Russian nesting dolls, our exploration has taken us from Earth through the Solar System, the Milky Way, the Local Group, the Virgo Supercluster, and the Laniakea Supercluster, culminating in the vast Pisces-Cetus Supercluster Complex.

This expansive perspective not only contextualizes our existence on Earth but also enriches our understanding of the myriad fascinating structures within the cosmos, many of which remain millions of light years away.

As we continue to explore the observable Universe, one wonders how long it will be before we discover an even larger cosmic "doll" encompassing our galactic home.