The Solar System: Our Home in the Universe
Have you ever wondered where we are in space? Well, from an astronomy point of view, we’re on one of the arms of a spiral galaxy called the Milky Way, and we live in a group of objects that orbit a star. We’re talking about the Solar System!
Our galaxy is medium-to-large in size and is part of what’s known as the Local Group or cosmic neighborhood. This group has three main spiral galaxies: Andromeda, the Triangulum Galaxy, and our own Milky Way. There are also about 40 smaller galaxies, many of which act like satellite galaxies orbiting one of the three main ones. Examples of these are the Large and Small Magellanic Clouds, Ursa Minor Dwarf, and the Sculptor Dwarf.
Among the hundreds of billions of stars in the Milky Way is our Sun, a yellow star of type G, class V. It’s huge, with a diameter of about 1,500,000 kilometers, and its surface temperature is around 5700 degrees Kelvin. The Sun’s gravity holds together a bunch of objects that travel around it in paths that can sometimes be very oval-shaped. This system includes planets, moons, asteroids of different sizes, dwarf planets, comets, cosmic dust, and gas. All of this stuff doesn’t even make up 0.15% of all the matter in the Solar System – the other 99.86% is in the Sun!
Position of the Solar System in the Milky Way.
What is a Planet?
We can think of a planet as a big ball of stuff in space that is round because of its own gravity. But it can’t make its own light like a star because it’s not big enough for the special reactions that happen inside stars (which need temperatures around 10 million degrees Kelvin).
Planets usually travel around a star that pulls on them with gravity, and they move in elliptical (oval-shaped) paths. In our Solar System, we know that this movement follows Newton’s laws of motion and some other rules described by Johannes Kepler, who figured out the math behind how planets move. These rules are:
- Kepler’s First Law: All planets move in elliptical orbits around the Sun, with the Sun at one of the focal points of the ellipse.
- Kepler’s Second Law: A line connecting a planet and the Sun sweeps out equal areas during equal intervals of time.
- Kepler’s Third Law: The square of a planet’s orbital period is directly proportional to the cube of the semi-major axis of its elliptical orbit.
Over the years, there’s been some debate about exactly what counts as a planet. Right now, the International Astronomical Union says that an object has to meet four conditions to be called a planet:
- It must orbit a star.
- It must have enough mass for its own gravity to make it round.
- It must have cleared the area around its orbit of smaller objects (this means it’s the dominant gravitational force in its orbit).
- It must not produce its own light.
These rules mean that some well-known objects, like Pluto, are now considered dwarf planets along with other objects far away from the Sun, like Eris, Haumea, Makemake, and Sedna.
Surface of Pluto. Now considered a dwarf planet.
Geology of the Planets
Let’s talk about the planets starting with the closest to the Sun and moving outwards:
- Mercury: This planet is about 2500 km in radius and spins very slowly. Because of this slow spin, it barely has a magnetic field. Spacecraft that have visited Mercury show a surface that looks a lot like the Moon, with craters of different ages and dark, flat areas. Mercury doesn’t have an atmosphere or any moons, and it’s very dense, which suggests that its core (the center) is large compared to the rest of the planet and is probably made of iron.
- Venus: This planet is very similar to Earth in size and density. The main difference is its atmosphere, which is mostly made of CO2. This creates a huge greenhouse effect, making the daytime temperatures hotter than 400 degrees Celsius! Geologically, Venus is very active, and its surface is made of basalt rock that is young (geologically speaking), with no formations older than 800 million years. This suggests that Venus went through a big, planet-wide event a relatively short time ago in its history. Its magnetic field is very weak because it spins slowly, and it even spins in the opposite direction to most other planets in the Solar System!
- Earth: Our home planet is definitely the most complex in terms of geology and life, and describing it in detail would take more than one article! In general, we can say that it’s a slightly flattened sphere at the poles with an atmosphere rich in nitrogen, a crust made of silicates (rock-forming minerals), a mantle made of peridotite (another type of rock), and a core mostly made of nickel and iron. Earth spins on its axis once every 24 hours and orbits the Sun at a speed of about 29.5 kilometers per second, taking 365.25 days to go around once. It has one big natural satellite, the Moon, which has a radius of about 1700 km. The Moon orbits Earth in about 27.32 days (sidereal month) if we look at the stars, or about 29.5 days (synodic month) if we look at the Sun. Because the Moon doesn’t have an atmosphere and has low gravity, its surface has distinct features like maria (large, dark plains), mountain ranges, and craters.
- Mars: Mars has an atmosphere rich in CO2 but with much less pressure than Earth’s. This means that wind plays a big role in shaping the planet, creating structures similar to sandy deserts on Earth. The surface is covered in regolith (loose rock and dust), which has a thin coating of siltstone and goethite (iron-containing minerals) that gives the planet its characteristic reddish-brown color. Notable features on Mars include large shield volcanoes and huge valleys like Valles Marineris, as well as mysterious grooves that look like they were formed by flowing liquids in the past. Mars has two small moons called Phobos and Deimos.
- Jupiter: This is the biggest planet in our Solar System – it’s 2.5 times bigger than all the other planets combined! However, it’s also much less dense. One of its most distinctive features is its bands of color, which are caused by its very active atmosphere driven by the planet’s own heat from gravitational contraction. Jupiter is mostly made of hydrogen, and its center is thought to be a rocky core about 10-20 times the size of Earth. It also has strong winds parallel to its equator, reaching speeds of up to 600 km/h, and big atmospheric storms like the Great Red Spot, which is a giant swirling storm. Scientists have found up to 92 natural satellites (moons) orbiting Jupiter! Some of these moons are quite large, even planet-sized, like Io, Ganymede, Europa, and Callisto.
- Saturn: Saturn is the second-largest planet in the Solar System and has an atmosphere similar to Jupiter’s, but with more methane and ammonia. It also has bands of high and low clouds, but they are much less noticeable than Jupiter’s, with most of the atmospheric activity happening near the equator. There are turbulences, convection cells, east-west currents, and strong winds near the equator that can reach speeds of up to 1800 km/h! Saturn is famous for its beautiful rings, which are made of chunks of ice mixed with dust and mineral fragments. It also has a large number of natural satellites. Some of the well-known ones are Tethys, Dione, Rhea, and Titan.
- Uranus: Based on the data we have, scientists think Uranus is a planet with a solid or fluid rocky core made of silicates and iron, and an upper layer (mantle) made of water, methane, and ammonia. It also has an atmosphere rich in hydrogen and helium. Uranus has 9 rings that are darker than Saturn’s because they aren’t covered in ice, and it has 15 known natural satellites. Some of these are Miranda, Umbriel, Titania, and Oberon.
- Neptune: Neptune has a similar composition to Uranus and is slightly larger in size. It also has a ring system, but the rings are much fainter than the others we’ve talked about. Neptune has 14 known moons. The largest of these is Triton.
Sources
- Gribbin J (2011). Biography of the Universe.
- Anguita F. (1988) “Origin and history of the Earth”
- Space.com
