Our Cosmic Address: Where is Earth Situated in the Universe?

Simone Lilavois is a NYC high school student passionate about the Cosmos. When not at swim practice, she can be found teaching herself astro/quantum/theoretical physics or pondering the nature of existence.

When measuring distances on Earth, we use inches, feet, yards, meters, miles, kilometers, and so on. However, when we try to describe the placements of planetary or stellar bodies in space relative to Earth, it becomes impractical to use the same units. Instead, when speaking in terms of the cosmos, we use significantly larger units of measurement to quantify vast distances. The smallest of these common units is the Astronomical Unit (AU), which describes the distance between the Earth and the Sun. One AU equates to about 150 million kilometers, 93 million miles, or around 8 light minutes.

For example, it is easier to describe Saturn’s relative distance to us in AU rather than kilometers. On average, Saturn orbits 1,427,000,000 km away from Earth; however, it is more practical to say that Saturn orbits at a mean distance of 9.5 AU.

The second way in which we describe distances within the universe is by the time it takes light to travel to that given distance. The speed of light is a universal constant and the fastest speed at which anything can move — it is a cosmic speed limit. Light travels through a vacuum at exactly 299,792,458 meters (983,571,056 feet) per second, which equates to approximately 186,282 miles per second. It is commonly denoted as c = 3 x 10⁸ m/s in calculations, where ‘c’ represents the speed of light. Within solar systems and galaxies, the measurements used are light minutes, light hours, light months or light years (1 light year = 63,241 AU). When describing the astronomical distances between galaxies, superclusters, and even the universe at large, we use mega-light years, (MLY) to describe distances by every million light years, and giga-light years, (GLY) to describe distances every billion light years.

For example, the light from the Moon takes 1.3 seconds to reach us, meaning it is 1.3 light-seconds away. The light from the sun takes 8 minutes to reach us, so we say it is 8 light minutes away. On a larger scale, the Andromeda galaxy, a neighboring galaxy to the Milky Way, is about 2.537 MLY away.

Andromeda Galaxy Cropped Image. Credit:
NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler.

The Inner and Outer Solar System

Using these units of measurements, we can begin to understand our place in the cosmos on a larger scale. To start, humans inhabit the planet Earth. Earth resides in our solar system, orbiting our host star, the Sun, along with seven other planets, and many dwarf planets, moons, comets, and asteroids. Our solar system can be divided into two groups: the inner solar system and the outer solar system. Within the inner solar system resides the terrestrial planets, Mercury, Venus, Earth, and Mars. The inner and outer solar system are divided by the asteroid belt, a ring of solid, irregularly shaped bodies between Mars and Jupiter. Its closest point is approximately 1.2 AU from Earth and its width spans 92 million miles, or just under 1 AU.

The outer solar system includes the gas and ice giants, Jupiter, Saturn, Uranus, and Neptune. Neptune is the farthest planet away from Earth in the solar system, at a distance of 30 AU or about 4 light-hours. However, it isn’t the farthest planetary body within the solar system. Beyond the orbit of Neptune lie several dwarf planets. There are certain requirements that a body must meet to be designated as a planet. Those which do not meet these requirements are considered dwarf planets.

The dwarf planets. From left to right: Pluto-Charon, Eris, Haumea, Makemake and Ceres. Credit: NASA

The three requirements are as follows:

The planetary body must orbit a star
The planetary body must have the adequate amount of mass so that its self-gravity allows for the achievement of hydrostatic equilibrium, meaning nearly spherical in shape
The planetary body must have sufficient gravity so that it has cleared the surrounding debris from its orbit

Dwarf planets within our solar system do orbit the sun and many are roughly round in shape. However, they often have comets, asteroids, or other large bodies such as other dwarf planets near them, disqualifying them as planets and demoting their rank to dwarf planets. The International Astronomical Union officially recognizes five dwarf planets in the solar system: Pluto, the most commonly known, along with Ceres, Makemake, Haumea, and Eris.

The Kuiper Belt

Several dwarf planets orbit beyond the outer solar system within the Kuiper Belt, also referred to as the Edgeworth–Kuiper Belt, a bagel-shaped region of icy objects. It’s predicted there are millions of such objects, aptly referred to as Kuiper Belt objects, (KBOs) or trans-Neptunian objects (TNOs). Except for Ceres, which inhabits the main Asteroid Belt, all the dwarf planets mentioned above are located within the Kuiper Belt. The belt stretches from around 30 AU, just beyond Neptune’s orbit, to 50 AU from the Sun, and is about 20 times as wide and up to 200 times as massive as the asteroid belt farther inside the solar system.

Both the Asteroid Belt and the Kuiper Belt are full of ancient remnants from the solar system’s early history and could provide valuable insights into the process of planetary formation, and hence the formation of Earth.

The Heliosphere

The next landmark within the solar system is the heliosphere, the region in which the Sun’s effects can be felt. The Sun constantly emits charged particles, also known as solar winds. The heliosphere forms a shell around the solar system, stretching 123 AU, until reaching the “termination shock.” It is at this point where the motion of solar wind particles is significantly slowed due to the outside pressure of the interstellar medium (ISM). Further out lies the heliosheath, the region between the termination shock and the heliopause. The heliopause is considered one of the outermost edges of the solar system. After passing the boundary of the heliopause, it is considered deep space, commonly referred to as the ISM. Once crossing this boundary, the effects from the Sun can no longer be felt, as the strength of the interstellar medium outweighs that of the solar wind particles.

The overall geometry of the heliosphere is similar to a comet. It is roughly spherical on one side and has a long trailing tail on the other, known as the heliotail.

The Oort Cloud

While some consider the heliopause to be the edge of the solar system, there is one more boundary beyond it. Last lies the Oort Cloud, marking the true outermost edge of the Solar System. While the trajectory of the planets, moons, asteroids, dwarf planets, and KBOs follow similar flat disk orbitals around the Sun, it is believed that the Oort Cloud is a giant spherical bubble surrounding the entire solar system.

It’s estimated that the inner edge of the Oort Cloud is between 2,000 and 5,000 AU from the Sun, while the outer edge is predicted to be between 10,000 and 100,000 AU from the Sun — 100,000 AU is approximately 1.5 light years. For some perspective of these vast distances, NASA’s Voyager 1 travels at around 1 million miles per day. The spacecraft will not reach the inner edge of the Oort Cloud for another 300 years and the outer edge for approximately 30,000 years, according to NASA.

The Oort Cloud is composed of billions and speculated even trillions of pieces of space debris. A disturbance to the orbit of one of the icy bodies within the Oort Cloud can knock it out of its normal trajectory and trigger a long fall toward our Sun. A recent example of this is C/2013 A1 Siding Spring, which passed by Mars in 2014 and survived potential disintegration by the Sun. However, it will not return to our solar system for about 740,000 years.

While scientists have not directly observed one of the icy worlds within the Oort Cloud, its existence is widely accepted within the scientific community, as it fits with observations of long-period comets in the planetary region of the solar system and provides an explanation for these comets’ origins.

This artist’s concept puts distances between areas in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. Credit: NASA / JPL-Caltech

Local Interstellar Cloud

Our solar system itself is moving through space and is currently within a very low density cloud, called the Local Interstellar Cloud (LIC). This cloud, also known as the Local Fluff, is composed of hydrogen and helium atoms and spans about 30 light years across. The LIC resides within an even larger gaseous cloud referred to as the Local Bubble, which is nearly ten-fold bigger, with an expected diameter of 300 light years. The existence of these sparsely dense gaseous regions can be attributed to 10–20 million old supernovae. According to a Journal of Physics: Conference Series paper published in March 2020, it is expected that the solar system may exit the LIC within the next 2,000 years.

The Orion Arm

The Local Bubble resides within a minor spiral arm of the galaxy, called the Orion Arm, or the Orion–Cygnus Arm. It is approximately 3,500 light-years across and 10,000 light-years in length. Within the arm, our solar system is close to its inner rim, approximately 26,000 light-years from the Galactic Center. The Orion Arm is located between two primary spiral arms: the Sagittarius Arm, which is closer towards the Galactic Center, and the Perseus Arm, which is closer to the outer edge of the galaxy. The Orion Arm is often called the Orion Spur, in an attempt to classify the region as less of a “full arm” and more of a collection of dust and gas lying between more massive arms.

Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA’s Spitzer Space Telescope, scientists have discovered that the Milky Way’s elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms. This annotated artist’s concept illustrates the new view of the Milky Way, along with other findings presented at the 212th American Astronomical Society meeting in St. Louis, Mo. The galaxy’s two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms. The major arms consist of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity. The artist’s concept also includes a new spiral arm, called the “Far-3 kiloparsec arm,” discovered via a radio-telescope survey of gas in the Milky Way. This arm is shorter than the two major arms and lies along the bar of the galaxy. Our sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms.

Milky Way galaxy’s arms. Source: NASA

The Milky Way Galaxy

These spiral arms come together to form the Milky Way Galaxy, an elegant display of the sheer strength of gravity over astronomical distances. It is estimated the galaxy contains between 100 and 400 billion stars, spanning 100,000 light-years across. In the past, it was believed that our galaxy was composed of four major arms: Scutum-Centaurus, Perseus, Norma, and Sagittarius. However, both the Norma and Sagittarius arms have been demoted to minor arms and are primarily filled with “gas and pockets of star-forming activity,” according to NASA. On the other hand, the major arms possess higher densities of a range of aged stars.

The Local Galactic Group

The Milky Way galaxy is not unique. Together with other neighbouring galaxies, both large and small, it makes up our Local Group. Credit:
ESO/Andrew Z. Colvin/N. Bartmann

Zooming out a little further, it can be seen that the Milky Way is just one of an estimated 30–50 galaxies located in the Local Galactic Group. It spans approximately 10 million light years in diameter. There are three main galaxies within the Local Group, the most prominent being the Andromeda Galaxy (M31), followed by the Milky Way and the Triangulum Galaxy. The majority of the galaxies within the Local Group are dwarf galaxies that are satellites to the Andromeda or Milky Way galaxies. Satellite galaxies are gravitationally bound to their host galaxy in the same way that the Moon is bound to the Earth, or the Earth is bound to the Sun.

The Virgo Supercluster

A supercluster is a vast group of already-clustered-galaxies or galaxy groups and are among the largest known structures in the universe. Our Local Galactic Group appears to be at the edge of the Virgo Supercluster, revolving around its center at approximately 400 km/s. The supercluster has a diameter of 110 MLY and a mass of about 10¹⁵ times that of the Sun.

The Laniakea Supercluster

It was believed that the Virgo Supercluster was the extent of our galaxy’s interconnectivity with other galactic structures. It wasn’t until 2014 that scientists realized the Virgo Supercluster is a mere fraction of a larger web of connecting galaxies, now referred to as the Laniakea Supercluster. The word is Hawaiian and translates to “immense heaven.” This Super(super)cluster is 520 million light-years wide, composed of about 300–500 smaller clusters, amounting to 100,000 galaxies.

The Local Group’s location within the Virgo Supercluster as part of the larger Laniakea Supercluster. Credit: Andrew Z. Colvin, Wikipedia Commons

The Laniakea Supercluster is just one of many throughout the observable universe, whose elegant geometry is revealed through connecting threads of galaxies over astronomical distances, unintelligible to our human minds. Together, the structures these galaxies form compose the web of the universe, a vast network of planets and stars which allow us to determine our place within the cosmos.

I suppose we could write our cosmic address as:

Humanity
Earth, 3rd Planet Inner Solar System, Orion Arm
Milky Way Galaxy, Virgo Supercluster, Laniakea Supercluster