The fourth planet from the Sun is Mars. Mars, a terrestrial inner planet, orbits the Sun a single time every 686 Earth days. Once every 1.88 years, Mars completes a single orbit around the Sun.
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Inner Planets and Outer Planets
The four planets closest to the Sun are Mercury, Venus, Earth, and Mars. They are “Inner Planets.”
Jupiter, Saturn, Uranus, and Neptune are the four planets furthest from the Sun. They are “Outer Planets.”
Planets are also separated into different categories depending on their composition.
The four inner planets, Mercury, Venus, Earth, and Mars, are Terrestrial planets.
- Rotate slower than Gas giants
- Smaller diameter than Gas giants
- Fewer moons
- Lack of rings
- Rocky surface, iron, and solid (sometimes liquid) surface
- Nitrogen and carbon dioxide atmosphere
- A dense core that’s composed of iron and silicate
- Denser (overall) than Gas Giants
- Astronomers estimate more than ten billion terrestrial planets exist across our galaxy.
Gas Giants (AKA Jovian planets)
The four outer planets, Jupiter, Saturn, Uranus, and Neptune, are Gas Giant planets. The Gas Giants are also known as the Jovian planets. The Jovian planets are visually similar to Jupiter, so they’re grouped under the naming umbrella of Jovian, meaning “similar to Jupiter.”
- Larger than Terrestrial planets
- Further from the Sun
- More rings
- More moons
- Gaseous atmospheres; Hydrogen and helium.
- Cores contain rock, metal, and hydrogen. The center has a higher density than the Terrestrial planet’s core.
- Less dense (overall) than Terrestrial planets
|Planet Name||Planet type||Composition||Distance from Sun (km)||Distance from Sun (miles)||Days to orbit sun||Diameter (km)||Years to orbit sun||Full rotation (days)|
The chart above breaks out the planets for easy comparison. Mars’s diameter places it as the second smallest, or third largest, Inner/Terrestrial planet.
Fun fact: There’s a large gap between the diameter of the four Terrestrial planets and the four Gas Giants.
- Earth has the largest Terrestrial planet diameter at 12,756 km.
- Neptune has the smallest Gas Giant planet diameter of 49,528 km.
- Why aren’t any planets in the diameter gap between Earth and Neptune?
- Scientists believe there are planets in other galaxies with diameters that fall between Earth’s and Neptune’s diameter.
- The data leads astronomers to conclude that planets may have begun with similar atmospheres but split into different-sized planets during their formation.
- Some planets lost their atmospheres and remained smaller-sized planets.
- Other planets retained their atmospheres (primarily hydrogen) and continued to grow into Gas Giants.
Mars orbit time
How long does it take Mars to orbit the sun/ Mars will complete a single orbit around the Sun every 686 Earth days. Once every 1.88 years, Mars will begin a new orbit around the Sun.
Planet Orbit speed
Why do planets have different orbit times around the Sun? Let’s examine three planetary motion laws.
Kepler’s Second Law-The Law of equal areas
Based on Kepler’s second law, we know that the closer an object resides in its position to the Sun, the faster it will travel. The further away a planet resides from the Sun, the slower the planet will travel.
- The Sun is the gravity source. The closer the planet is to the Sun, the faster it will travel.
- Mercury, the closest planet to the Sun, orbits the Sun once every 88 days.
- Neptune, the furthest planet from the Sun, orbits the Sun once every 165 years.
Kepler’s Third Law-The Law of Equal Areas
Based on Kepler’s Third Law, we know that the further away a planet resides from the Sun, the longer it will take the planet to orbit it.
- Being further from the Sun, there’s less gravity on the planet from the Sun to “speed it up.”
- The further away a planet resides from the Sun, the longer it will take to orbit around the Sun.
|Planet Name||Orbital eccentricity||km per second||Miles per second|
The chart above demonstrates Kepler’s Third Law. Mercury, the closest planet to the Sun, travels approximately 30 miles per second (48 kps). Neptune, the furthest planet from the Sun, is casually poking along at a mere 3 miles per second (5.4 kps.)
Keplers First Law-The Law of orbits
The planets orbit around the Sun in an ellipse. The planets do not circle the Sun in a perfect circle.
Ancient Greeks believed in uniform circular planetary orbits. Johannes Kepler, a German astronomer, discovered the first law of planetary motions in 1605 when he studied the orbital path of Mars. (The first law is now known as Kepler’s First Law.)
Mathematics allowed Kepler to calculate a planet’s elliptical path around the Sun. A new age of Astronomy began.
Mars’ Elliptical path
All planets have an elliptical orbit around the Sun. Some Orbital eccentricity values are high, and others are lower.
- An orbital eccentricity value of zero is perfectly circular, shaped like a quarter or a dime.
- An eccentricity value of 1 is flattened and elongated, shaped like an egg
- Over time, millennia, Mars’s orbital eccentricity has been increasing and becoming more elongated.
- Eventually, Mar’s orbital eccentricity will return to close to zero.
|Planet Name||Orbital eccentricity|
Mar’s high eccentricity value of .093 means it travels around the Sun in a very elliptical path. Since Mars has a rotation value of 1.03, it has weather “seasons” just like Earth. See the higher orbital eccentricity values playing out in the chart below.
|Season||Earth Days||Percentage||Mars Days (Sol)||Percentage|
Earth, with a low orbital eccentricity value, has weather seasons whose durations are approximately equal.
Mars, with a high orbital eccentricity value, has a longer spring and summer season and shorter autumn and winter seasons.
How Long Does it Take Mars to Orbit the Sun? Wrap up!
How long does it take Mars to orbit the Sun? It takes 686 Earth days for Mars to orbit the Sun once. As Mars’ elliptical path further elongates, the time it takes Mars to orbit the Sun will increase.