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New Zealand's Southern Alps offer an amazing view of the Milky Way, our galaxy, which also houses our solar system. Image by Mike Mackinven / Getty Images.Our planet Earth is part of a solar system made up of eight planets that orbit a massive, glowing star known as the Sun. For centuries, astronomers have observed how these planets move across the sky in a regular pattern, with some planets speeding up and others even seeming to move in reverse.
Before we get ahead of ourselves, let's rewind and explore how this solar system began its journey.
Origins of Our Solar System
About 4.6 billion years ago, the young solar system began to form from a vast cloud of gas and dust, known as the solar nebula. The collapse of the nebula, possibly triggered by a nearby supernova, set off a spinning motion due to the conservation of angular momentum.
At the heart of this swirling cloud, a protostar emerged, growing increasingly hot and dense. As the surrounding material began to stick together via accretion, tiny dust particles collided and formed larger chunks known as planetesimals. These planetesimals merged and collided, eventually forming protoplanets that continued to grow in size and mass.
Meanwhile, the Sun continued to grow brighter and larger as it accumulated more matter, eventually becoming the most powerful entity in the solar system. It contained the vast majority of the system’s mass — more than all the planets, asteroids, and comets combined.
As the protoplanets accumulated more material, their interiors heated up, leading to differentiation. Denser materials sank toward their cores, while lighter materials rose to the surface. This process ultimately led to the creation of the rocky terrestrial planets (we'll explore those in more detail shortly).
The Sun's powerful radiation and solar wind swept away the remaining gas and dust, but only up to a certain distance. Farther out, where the temperature was lower, gas and ice could remain in their gaseous forms, leading to the creation of gas giants such as Jupiter and Saturn. Even farther out, ice giants like Uranus and Neptune developed their atmospheres and icy layers.
Understanding the Sun's Role
The Sun, a medium-sized star in the grand scheme of the universe, is still far larger than any planet in our solar system. It spans a diameter of 1,392,000 kilometers (864,949 miles), while Earth's diameter is only 12,756 kilometers (7,926 miles) — meaning over one million Earths could fit inside the Sun.
The Sun's immense mass generates a tremendous gravitational pull, which is strong enough to keep all the planets in our solar system in their respective orbits. Even the dwarf planet Pluto, which lies a vast 6 billion kilometers (3,728,227,153 miles) away, remains in orbit thanks to the Sun's gravitational force.
The Planets of Our Solar System
Every planet in our solar system has its own distinct characteristics, but there are also common features they share. For instance, all planets have a north and a south pole, which are located at the planet's ends, at its center.
A planet's axis is an imaginary line running through its center, linking the north and south poles. The imaginary line encircling the planet at its widest point, much like your waist, is called the equator. While all planets rotate on their axes, the speed of their rotation varies — some planets spin rapidly, while others rotate more slowly. The duration it takes for a planet to complete one full rotation on its axis is known as its rotation period.
As each planet spins on its axis, it also orbits the Sun. The time it takes for a planet to make a full orbit around the Sun is referred to as its year. The path the planet traces around the Sun is known as its orbit.
The asteroid belt, situated between Mars and Jupiter, also serves to divide our solar system into the inner and outer regions. Here’s a brief overview of the eight planets, listed in order of their distance from the Sun.
Terrestrial Planets
The inner solar system is made up of four rocky planets — Mercury, Venus, Earth, and Mars — all situated nearest to the Sun. These planets feature solid surfaces, varied terrains, and the possibility of secondary atmospheres.
- Mercury is the smallest planet and the closest to the Sun.
- Venus is shrouded in a thick, toxic atmosphere, making it the hottest planet.
- Earth is the only known planet capable of supporting life, with a breathable atmosphere, liquid water, and a protective magnetic field.
- Mars has a thin atmosphere and a barren, rugged surface.
Giant Planets
Jupiter, Saturn, Uranus, and Neptune are the four giant planets in our solar system, collectively known as Jovian planets. These outer planets are composed primarily of hydrogen and helium and feature rings, thick atmospheres, and numerous moons. The gas giants, Jupiter and Saturn, lack solid surfaces and are much larger than Earth's terrestrial planets. Uranus and Neptune are classified as ice giants.
- Jupiter, named after the Roman god, is the largest planet in our solar system.
- Saturn is famous for its vast ring system and has more moons than any other planet combined.
- Uranus is the only planet in the solar system whose equator is nearly perpendicular to its orbit.
- Neptune is the only major planet that cannot be seen with the naked eye from Earth without the aid of a telescope.
Thanks to data gathered by the Hubble Space Telescope, we've learned that gas giants are not unique to our solar system; some exoplanets beyond our own share similar traits.
Dwarf Planets (Including Pluto)
While we often focus on the sun and planets when thinking about our solar system, numerous other objects also orbit the sun along with Earth and its planetary companions. These include moons (some of which have their own moons), comets, meteors, asteroids, space dust, and the widely debated dwarf planets.
In 2005, scientists discovered a distant rock and ice body they named Eris. Its larger size than Pluto and its greater distance from the sun sparked philosophical questions about what truly qualifies as a planet. Was Eris the solar system's tenth planet? If not, why could Pluto be classified as a planet, but Eris was not?
In 2006, the International Astronomical Union (IAU) established that to be considered a planet, an object must meet certain criteria:
- The object must directly orbit the sun — this excludes Earth's moon, which orbits Earth, not the sun.
- It must be large enough to have a spherical shape, which results from its own gravitational pull.
- It must have "cleared its neighborhood," meaning it is the dominant object in its orbit.
The third criterion is what led to Pluto being reclassified from the 9th planet to a dwarf planet, joining Eris, Makemake, Ceres, Haumea, and Orcus.
Beyond Our Solar System
Beyond Neptune's orbit lies the Kuiper Belt, which is home to icy bodies like Pluto and other Kuiper Belt objects. The solar system stretches far beyond the planets, with the Oort Cloud — a massive collection of icy objects — marking its outermost edge.
Beyond the Oort Cloud, we reach the heliopause, which defines the boundary between our solar system and the vast emptiness of interstellar space, where there are only a few gas molecules and dust particles per cubic centimeter.
If you observe the solar system closely, you'll notice that some planets — particularly Venus and Mercury — appear to move in reverse across the sky. These planets aren't actually moving backward, but instead, they seem to due to their changing position relative to Earth. It's similar to what happens when you're driving and pass another car on the highway; the car you're passing seems to move backward, but it's simply because your car is overtaking it. This strange backward motion is known as "retrograde motion." A planet can also experience "retrograde rotation," meaning it spins in the opposite direction of its orbit. In contrast, most planets in our solar system have "prograde rotation," which means they rotate in the same direction as their orbit.
