In Depth

Introduction

The planetary system we call home is located in an outer spiral arm of the Milky Way galaxy.

Our solar system consists of our starthe Sunand everything bound to it by gravity – the planets MercuryVenusEarthMarsJupiterSaturnUranusand Neptune; dwarf planets such as Pluto; dozens of moons; and millions of asteroidscometsand meteoroids.

Beyond our own solar systemthere are more planets than stars in the night sky. So farwe have discovered thousands of planetary systems orbiting other stars in the Milky Waywith more planets being found. Most of the hundreds of billions of stars in our galaxy are thought to have planets of their ownand the Milky Way is but one of perhaps 100 billion galaxies in the universe.

While our planet is in some ways a mere speck in the vast cosmoswe have a lot of company out there. It seems that we live in a universe packed with planets – a web of countless stars accompanied by families of objectsperhaps some with life of their own.

Namesake

There are many planetary systems like ours in the universewith planets orbiting a host star. Our planetary system is called “the solar system” because we use the word “solar” to describe things related to our starafter the Latin word for Sun"solis."

Size and Distance

Our solar system extends much farther than the eight planets that orbit the Sun. The solar system also includes the Kuiper Belt that lies past Neptune's orbit. This is a sparsely occupied ring of icy bodiesalmost all smaller than the most popular Kuiper Belt Object – dwarf planet Pluto.

NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 142015. Credit: NASA/JHUAPL/SwRI | Full caption and image

Beyond the fringes of the Kuiper Belt is the Oort Cloud. This giant spherical shell surrounds our solar system. It has never been directly observedbut its existence is predicted based on mathematical models and observations of comets that likely originate there.

The Oort Cloud is made of icy pieces of space debris - some bigger than mountains – orbiting our Sun as far as 1.6 light-years away. This shell of material is thickextending from 5,000 astronomical units to 100,000 astronomical units. One astronomical unit (or AU) is the distance from the Sun to Earthor about 93 million miles (150 million kilometers). The Oort Cloud is the boundary of the Sun's gravitational influencewhere orbiting objects can turn around and return closer to our Sun.

The Sun's heliosphere doesn't extend quite as far. The heliosphere is the bubble created by the solar wind – a stream of electrically charged gas blowing outward from the Sun in all directions. The boundary where the solar wind is abruptly slowed by pressure from interstellar gases is called the termination shock. This edge occurs between 80-100 astronomical units.

Two NASA spacecraft launched in 1977 have crossed the termination shock: Voyager 1 in 2004 and Voyager 2 in 2007. Voyager 1 went interstellar in 2012 and Voyager 2 joined it in 2018. But it will be many thousands of years before the two Voyagers exit the Oort Cloud.

Moons

There are more than 200 known moons in our solar system and several more awaiting confirmation of discovery. Of the eight planetsMercury and Venus are the only ones with no moons. The giant planets Jupiter and Saturn lead our solar system’s moon counts. In some waysthe swarms of moons around these worlds resemble mini versions of our solar system. Plutosmaller than our own moonhas five moons in its orbitincluding the Charona moon so large it makes Pluto wobble. Even tiny asteroids can have moons. In 2017scientists found asteroid 3122 Florence had two tiny moons.

These six narrow-angle color images were made from the first-ever 'portrait' of the solar system taken by Voyager 1which was more than 4 billion miles from Earth and about 32 degrees above the ecliptic. Credit: NASA Planetary Photojournal

Formation

Our solar system formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsedpossibly due to the shockwave of a nearby exploding starcalled a supernova. When this dust cloud collapsedit formed a solar nebula – a spinningswirling disk of material.

At the centergravity pulled more and more material in. Eventuallythe pressure in the core was so great that hydrogen atoms began to combine and form heliumreleasing a tremendous amount of energy. With thatour Sun was bornand it eventually amassed more than 99% of the available matter.

Matter farther out in the disk was also clumping together. These clumps smashed into one anotherforming larger and larger objects. Some of them grew big enough for their gravity to shape them into spheresbecoming planetsdwarf planetsand large moons. In other casesplanets did not form: the asteroid belt is made of bits and pieces of the early solar system that could never quite come together into a planet. Other smaller leftover pieces became asteroidscometsmeteoroidsand smallirregular moons.

Structure

The order and arrangement of the planets and other bodies in our solar system is due to the way the solar system formed. Nearest to the Sunonly rocky material could withstand the heat when the solar system was young. For this reasonthe first four planets – MercuryVenusEarthand Mars – are terrestrial planets. They are all small with solidrocky surfaces.

Meanwhilematerials we are used to seeing as iceliquidor gas settled in the outer regions of the young solar system. Gravity pulled these materials togetherand that is where we find gas giants Jupiter and Saturnand the ice giants Uranus and Neptune.