Most of us can name all eight planets in the solar system. You might even be able to describe the features of each. But do you know what constitutes a planet? What makes it different from a moon or sun?
It seems like a relatively simple question. After all, you’re standing on one right now! But you might be surprised to know the definition of “planet” has changed numerous times over the years. Since the ancient Greeks first coined the term over 2,000 years ago, the definition has been probed, prodded, and pricked. In fact, the lack of consensus among astronomers has seen the number of planets in our solar system fluctuate from a high of sixteen to a low of six.
In the 21st century, the discovery of several new worlds at the edges of our solar system prompted scientists to start sizing up what was and wasn’t a planet. After much debate, a new definition was coined by the International Astronomical Union in 2006 with four distinct prerequisites for planethood.
A planet must:
Unfortunately, Pluto — the “ninth planet” in our solar system —failed to meet the fourth condition and was demoted to a “dwarf planet.”
Poor little Pluto. The ice-cold outlier must endure a 248-year-long orbit around the sun, only to be kicked out of the planet club! But the blow to Pluto was a blessing in disguise for many astronomers who have since been on the hunt for more dwarf planets.
A dwarf planet must:
Since the re-classification of Pluto, astronomers have identified new dwarf planets, including Ceres, Eris, Haumea, and Makemake. But, that’s only the tip of the iceberg. According to NASA, there may be more than a hundred dwarf planets waiting to be discovered, especially beyond the Kuiper Belt, which signifies the beginning of our solar system’s third zone, known as “vast realm of ice worlds.”
Our solar system consists of three zones. The inner zone closest to the Sun is home to rocky planets, known as terrestrial planets due to their resemblance to Earth. While the outer zone contains Jupiter, Saturn, Uranus, and Neptune — the gas giants. And beyond here, we know as “vast realm of ice worlds.”
The furthest planet from the sun is Neptune. With an oval-shaped orbit, the planet stays an average 3 billion miles away from the sun. That’s roughly 30 x Earth’s distance from the Sun, making it completely invisible to the naked eye. It’s so far away it takes 165 years for Neptune to orbit the sun!
However, the most distant ‘dwarf planet’ (currently recognized by the IAU) is Eris—a staggering 68 x Earth’s distance from the Sun. Discovered in 2003, Eris lies well beyond the Kuiper Belt in some 6 billion miles from the sun. At this distance, it takes sunlight over five hours to reach Eris, which makes the planet extremely cold with temperatures dropping as low as -405 degrees Fahrenheit.
Do you know which planet is closest to the sun? The answer is Mercury. At the closest point of only 29 million miles, Mercury gets nearer to the sun than any other planet during its elliptical orbit. As a comparison, Earth—the third closest planet to the sun after Venus—orbits the sun at an average distance of 92 million miles.
Due to its proximity to the sun, it only takes Mercury 88 days to complete its orbit. Can you work how many times a year you would celebrate your birthday on Mercury? There’s something else special about Mercury too—surprisingly it’s not the hottest planet in our solar system, but it is the smallest!
Since the discovery of the telescope, astronomers have been fascinated by the sheer size of the solar system and its planets, both big and small. When we talk about the size of a planet, we can use several different measurements.
Firstly, we can measure the planet’s mass. Mass refers to how much matter an object contains and its basic unit of measurement is kilograms. You may have, at times, confused weight for mass. The second is volume, which refers to how much space an object takes up. We can also look at the planet’s total surface area, circumference, and diameter.
If you’re wondering what is the biggest planet; look no further than Jupiter, the largest planet in our solar system by a wide margin. Its mass, volume, and total surface area far exceed that of any other planet. In fact, the giant’s mass is more than 300 times that of Earth, and its diameter stretches some 88,000 miles. To put things in perspective, if Jupiter were a basketball, Earth would be a grape!
Have you ever wanted to visit the planets of our solar system? The team at AstroReality are fascinated by our planets too, and we’ve created the Solar System Mini featuring nine 3D models and an Augmented Reality (AR) app. Now you can explore the solar system in intricate detail! Each model is 30mm in diameter and color-printed with precision accuracy.