The Inside Scoop: A Look at Our Solar System's Inner Planets


The inner solar system is the place our planet calls home. We are one of the lucky four planets that get to be closer to the Sun and in our case the luckiest…since we got the goldilocks spot. Not too hot and not too cold-just right for life. While the other planets around us may not have life (that we know of now and as we know it to exist), they still are very interesting cosmic neighbors to explore.

Our inner solar system, otherwise know as the terrestrial planets, is comprised of four planets and only three moons total - Mercury, Venus, Earth with the Moon, and Mars with Phobos and Deimos. Unlike bodies in our outer solar system such as the gas giants and ice giants, the terrestrial planets are all made out of rocky material and are much smaller in size in comparison to their farther out planet counterparts.



Mercury is often overlooked when we talk about the planets in our solar system. It is understandable, since Mercury doesn’t have many mysteries on its surface. Not to say that we know everything about our smallest planet, but we know a lot about it partially because it close in relative terms, illuminated well by the Sun, and is very similar to our Moon. Mercury is only a little bigger than our Moon and is very close to the Sun at a distance of 36 million miles (58 million kilometers). With the temperatures of the surface ranging from 800 degrees Fahrenheit to -290 degrees Fahrenheit and a very thin atmosphere, it is highly unlikely that Mercury could support life as we know it, and that is one reason it hasn’t been visited as frequently (10 Things to Know About Mercury). In fact, only two spacecrafts have visited Mercury, Mariner 10 and MESSANGER. As I type this, two more spacecrafts are on their way to Mercury that were launched on October 20, 2018. These two Mercury orbiters are part of a joint ESA and JAXA mission called BepiColombo that is aiming to look at the surface and interior of Mercury to better understand the planet’s environment, the early solar system, and the evolution of near-star exoplanets (BepiColombo Blasts Off To Investigate Mercury’s Mysteries).


While Mars gets all the attention for being the nearest cosmic neighbor that we want to visit, Venus is also our next door neighbor. Venus has begged our attention for millennia since it is often one of the brightest objects in the night sky. It also peaked the interest of past astronomers such as Galileo since along with its brightness, Venus (and Mercury) goes through noticeable phases like our Moon. The planet was named after the Roman goddess of love and beauty, and has carried mythologies with it throughout history. However, scientists have realized that however lovely the myths, Venus is not a planet to be messed with. Venus is about the size of Earth and is covered in a thick atmosphere that is comprised of mostly carbon dioxide and nitrogen. The clouds that engulf the planet are made of sulfuric acid that has come from the planets long history of volcanic activity (Sulfuric Acid Clouds on Venus). Even though Mercury is closer to the Sun, Venus wins the prize for being the hottest planet with surface temperatures that can go beyond 880 degrees Fahrenheit (Venus: By the Numbers). Because of this lead-melting temperature and its acidic atmosphere, Venus has not been visited by our spacecrafts very often. Since the first Venera missions sent by the Soviets in 1961, we have had approximately eleven missions sent to explore it (Lunar and Planetary Science: Venus). During Venera 13 in 1982, the spacecraft was able to land on the surface of Venus and capture 14 images while it survived for two hours and seven minutes (Venera 13 on Venus-Colorized).


Ah, our home planet. People can often forget that Earth falls under the category of planet. That may seem silly to say…but pose pose the question, “what is your favorite planet” and you will rarely get the answer Earth. This is because we seem to separate our planet into its own category since to us it is familar and home to life as we know it. Other planets in our solar system are still mysteries to us. To be fair, Earth is a special place to us and it makes sense that we would internally think of it on a different level than the other planets. Earth has an abundance of water, geologic activity, dynamic weather, and life. We have every right to have a special place in our hearts for it, but Earth is still a planet. The interesting thing is for how much we like to explore other planets, there is still much we don’t know about our own and so some of the interplanetary missions we send off are working towards helping us understand our own home. Along with that, it is also forgotten that NASA doesn’t just focus on space, but on aeronautics and our atmosphere. There are dozens of NASA missions that are done centered around studying and observing Earth. A whole list of the Earth-based missions that NASA has done can be found here. They aim to learn more about subjects like our climate, gravity, land and vegetation, hurricanes, water cycle, the Sun’s influence on Earth and more.


Mars has been the terrestrial planet of interest for a long time. We have sent mission after mission there and there is a big push to get humans to Mars in the near future. If you would like to learn more about some of the missions we have sent to the red planet, you can find all the details in my previous blog post, Martian Exploration. It is understandable that we have our sights on Mars since out of all our options of places to go, it seems to be the most welcoming. Unlike Mercury, Mars is farther away from the Sun to even out the temperature a touch and is larger in size. Unlike Venus, Mars doesn’t have a thick, acidic, and super hot atmosphere that would toast and sizzle us alive. And unlike both of them, Mars has moons…which admittedly isn’t necessary but is cool. Yes, in comparison to the other terrestrial contenders, Mars does take the cake being the most hospitable planet to visit, but that also does not mean it is inviting. Mars has an atmosphere that is one hundred times thinner than Earth’s and is composed of elements that we can’t breathe. While we have found that Mars has water ice and possibly liquid water under the surface, we humans are pretty depended on H2O and would need more than that to survive. Mars is also a lot smaller than Earth, being only one third the size. The red planet gets its nickname from its distinct reddish-orange appearance which comes from the soil containing iron oxide, otherwise known as rust.

Katrina Sletten