All the planets

It is great to focus on the planets! To get us started on this journey, we should first establish what exactly holds this entire cosmic neighborhood together.

What is a Solar System?

At its simplest, our solar system is a single star—the Sun—and every single object that is bound to it by . This cosmic web includes the eight planets you are excited to explore, along with their moons, dwarf planets, asteroids, and comets. Everything in the system is constantly falling around the Sun in a orbital dance, kept in line by the Sun's immense mass.

Continue and list the eight planets

Let's take a look at the cosmic lineup! Our solar system features exactly eight planets, which we can split into two very distinct neighborhoods based on what they are made of and how close they are to the Sun.

The Eight Planets

First, we have the , which are the four closest to the Sun: Mercury, Venus, Earth, and Mars. These are rocky worlds with solid surfaces you could actually walk on. Past Mars, we cross an astronomical boundary to find the giant planets. First are the gas giants, Jupiter and Saturn, which are massive ball-shaped worlds made mostly of hydrogen and helium gas. Finally, on the outer chilly edges of the system, we find the ice giants, Uranus and Neptune, which contain heavier elements like water, ammonia, and methane frozen under immense pressure.

A horizontal 2D map showing the order of the eight planets from the Sun, grouped by physical category.

I have sketched out a quick visual map of this planetary order so you can see how they line up.

Next, we should look at how this whole family came to be in the first place. Ready to see how a messy cloud of dust built these eight worlds?

Continue to how the solar system formed

It is mind-blowing to realize that everything we see today—from our oceans to Saturn's rings—began as a quiet, dark cloud of cosmic dust.

The Solar Nebula Hypothesis

About 4.6 billion years ago, a giant interstellar cloud of gas and dust, known as a solar nebula, began to collapse under its own weight, likely triggered by the shockwave of a nearby exploding star. As this cloud collapsed, it began to spin faster and flatten out, much like a ball of pizza dough flattening into a disk when tossed in the air. At the center of this spinning disk, material piled up and grew incredibly hot and dense, eventually sparking nuclear fusion to create our Sun. Meanwhile, the outer parts of the disk cooled down, allowing tiny dust grains to stick together, eventually forming the planets.

Three consecutive visual stages showing the formation of a solar system from a collapsing dust cloud to a spinning protoplanetary disk and planetary accretion.

I have created an interactive visualization of these three formation stages so you can watch how a chaotic cloud transforms into a structured disk.

Next, we can explore how those tiny dust grains in the disk actually managed to build the rocky inner planets.

Explain time: how AD and BC intersect and what happens at year 0.

That is a classic question. When we talk about cosmic timescales like the 4.6 billion years it took for our Solar System to form, we rely on scientific dating, but our everyday calendar has a very different starting point.

Understanding BC and AD

Our calendar is split into two main eras: BC (Before Christ) and AD (Anno Domini, meaning "In the year of our Lord"). These eras function like a number line in mathematics, but with a surprising twist.

Instead of counting forward forever, the BC era counts backward as you go further into the past. For example, 500 BC happened before 250 BC. Once you reach the transition point, the direction flips, and the AD era counts forward into the future.

To make this calendar system easier to use, historians often use the modern equivalents BCE (Before Common Era) and CE (Common Era), which align with the exact same years but use secular terminology. This global standard was originally structured by a 6th-century monk named , who was trying to calculate the correct dates for Easter.

You might assume that this transition happens at a "Year 0," but in our calendar system, there is no Year 0.

The timeline jumps directly from December 31, 1 BC to January 1, 1 AD. This is because Roman numerals (which were used when these calendar systems were first designed) did not have a symbol or concept for zero.

Now that we have cleared up how we track time on Earth, let's get back to cosmic history. Are you ready to see how a swirling disk of dust grains collided to build the rocky inner planets?

Can you show me a visual of this timeline?

Here is a visual timeline of how our calendars align! Because there is no Year 0, the timeline jumps directly from 1 BC to 1 AD.

A horizontal timeline showing the sequence of years from 3 BC directly to 1 AD, with a vertical marker highlighting the absence of a year zero.

Now that we have cleared up our earthly calendars, let's turn back to the cosmic clock to see how our worlds were made.

Inside the spinning protoplanetary disk, the baby Sun was surrounded by swirling dust and gas. Over millions of years, these tiny, microscopic dust grains began to collide. Because they were moving at similar speeds in the same direction, they did not smash each other to pieces; instead, they stuck together, gradually forming larger and larger clumps in a process called .