Category Space

All of the planets in the Solar System formed from the same cloud of debris. The inner planets have solid cores of iron, surrounded by rocky mantles, topped with a very thin silicate crust. The Gas Giants have solid cores of rock and ice, but these are much smaller in proportion to those of the inner planets. Jupiter and Saturn are made of hydrogen and helium, which becomes denser towards their centres. Uranus and Neptune both have mantles of icy water, methane and ammonia.

Astronomers think the giants first formed as rocky and icy planets similar to terrestrial planets. However, the size of the cores allowed these planets (particularly Jupiter and Saturn) to grab hydrogen and helium out of the gas cloud from which the sun was condensing, before the sun formed and blew most of the gas away. 

Since Uranus and Neptune are smaller and have bigger orbits, it was harder for them to collect hydrogen and helium as efficiently as Jupiter and Saturn. This likely explains why they are smaller than those two planets. On a percentage basis, their atmospheres are more “polluted” with heavier elements such as methane and ammonia because they are so much smaller.

Scientists have discovered thousands of exoplanets. Many of these happen to be “hot Jupiters,” or massive gas giants that are extremely close to their parent stars. (Rocky worlds are more abundant in the universe, according to estimates from Kepler.) Scientists speculate that large planets may have moved back and forth in their orbits before settling into their current configuration. But how much they moved is still a subject of debate.

There are dozens of moons around the giant planets. Many formed at the same time as their parent planets, which is implied if the planets rotate in the same direction as the planet close to the equator (such as the huge Jovian moons Io, Europa, Ganymede and Callisto.) But there are exceptions. 

One moon of Neptune, Triton, orbits the planet opposite to the direction Neptune spins — implying that Triton was captured, perhaps by Neptune’s once larger atmosphere, as it passed by. And there are many tiny moons in the solar system that rotate far from the equator of their planets, implying that they were also snagged by the immense gravitational pull.

Our solar system formed from the force of an exploding star. When some stars reach the end of their lives, they can explode into a supernova, sending shockwaves of energy deep into space. Roughly 4.5 billion years ago, a shock-wave from a supernova, travelling at 30 million kilometres (19 million miles) per hour, hit a cloud of ice, dust and gas. The force of the impact caused the cloud to flatten and rotate. From this spinning disc, our Solar System began to form.

The most widely accepted scientific explanation for the formation of the Solar System is called the Solar Nebular Model. According to this model, the entire Solar System formed around 4.5 billion years ago from the gravitational collapse of a small fraction of a giant molecular cloud, also known as a nebula. 

A disturbance, most likely a nearby supernova, caused a giant cloud of gas and dust floating in space to contract and begin to collapse on itself. Most of the gas collected in the center to form a gaseous sphere that would eventually become the Sun. As more gas was drawn inward by the force of gravity, friction and pressure caused this sphere, called a protostar, to become hot and start to glow. 

As the nebula continued to contract, conservation of angular momentum caused it to spin faster. It flattened out into a protoplanetary disk, with the hot, dense protostar in the center. Over millions of years, all eight planets formed by accretion from this disk. In other words, gravity pulled the disk into many clumps of gas and dust. These clumps stuck together and grew larger and larger, turning into planetesimals. The planetesimals further coalesced to eventually form planets, with comets and asteroids being the leftovers. Gravitational interaction with the planets caused them to be grouped into distinct regions such as the asteroid belt and Kuiper belt. 

Due to their higher boiling points only metals and silicates could exist in the warm inner solar system, and these would form the rocky planets of Mercury, Venus, Earth, and Mars. Since metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. It is thought that as many as 100 small protoplanets used to exist in the inner solar system, but they eventually collided and merged to create the four inner planets we know today. 

The gas giants (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line where icy compounds can remain solid. The gas and ice that formed the Jovian planets was more abundant within the protoplanetary disk, allowing them to become massive enough to gain large atmospheres of hydrogen and helium and grow to mammoth proportions. Uranus and Neptune are thought to have formed closer to the Sun, and then migrated out to their current orbits. 

Throughout all this, the infant Sun continued to grow hotter. Once the temperature and pressure at the core was high enough, thermonuclear fusion of hydrogen began, and the Sun became a fully-fledged main-sequence star. Solar wind swept away the remaining gas and dust leftover from the protoplanetary disk into interstellar space, ending the growth of the planets. This entire process of solar system formation happened within several hundred million years and was finished by around 4.5 billion years ago. 

Planets and moons are just a few of the objects orbiting the Sun. Astronomers already know of thousands of large rocky bodies called asteroids (shown right), and icy objects called comets. Millions of smaller rocks, called meteoroids, also orbit the Sun.

More than 150 moons orbit worlds in our solar system. Known as natural satellites, they orbit planets, dwarf planets, asteroids, and other debris. Among the planets, moons are more common in the outer reaches of the solar system. Mercury and Venus are moon-free, Mars has two small moons, and Earth has just one. Meanwhile, Jupiter and Saturn have dozens, and Uranus and Neptune each have more than 10. Even though it’s relatively small, Pluto has five moons, one of which is so close to Pluto in size that some astronomers argue Pluto and this moon, Charon, are a binary system.

Too small to be called planets, asteroids are rocky chunks that also orbit our sun along with the space rocks known as meteoroids. Tens of thousands of asteroids are gathered in the belt that lies between the orbits of Mars and Jupiter. Comets, on the other hand, live inside the Kuiper Belt and even farther out in our solar system in a distant region called the Oort cloud.

The solar system is enveloped by a huge bubble called the heliosphere. Made of charged particles generated by the sun, the heliosphere shields planets and other objects from high-speed interstellar particles known as cosmic rays. Within the heliosphere, some of the planets are wrapped in their own bubbles—called magnetospheres—that protect them from the most harmful forms of solar radiation. Earth has a very strong magnetosphere, while Mars and Venus have none at all.

Most of the major planets also have atmospheres. Earth’s is composed mainly of nitrogen and oxygen—key for sustaining life. The atmospheres on terrestrial Venus and Mars are mostly carbon dioxide, while the thick atmospheres of Jupiter, Saturn, Uranus, and Neptune are made primarily of hydrogen and helium. Mercury doesn’t have an atmosphere at all. Instead scientists refer to its extremely thin covering of oxygen, hydrogen, sodium, helium, and potassium as an exosphere.

Moons can have atmospheres, too, but Saturn’s largest moon, Titan, is the only one known to have a thick atmosphere, which is made mostly of nitrogen.

The planets in the Solar System form two very different groups — inner and outer. The inner planets, often called terrestrial planets, are composed mainly of rock and metal, with solid surfaces, no rings and few satellites. The outer planets, called Jovian or Gas Giants, are much larger than their inner neighbours. They are composed primarily of hydrogen and helium, have very deep atmospheres, rings and lots of satellites.

Our solar system consists of many planets, one of which is Earth. The total number of planets is eight although there have been disagreements to this statement with some saying there are more than eight (the opponents of the theory that Pluto is not a planet). Whatever the case, when we talk about planets we divide them into two groups; inner planets and outer planets. This classification is relative to the planets’ position with respect to the Sun. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. We shall now make clear which of these are inner planets and which are outer planets and what actually differentiates them.

Inner planets are those planets which are closest to the sun and include the first four planets (Mercury, Venus, Earth and Mars) in order of increasing distance from the Sun. Mercury is the closest, followed by Venus, Earth and then Mars. Outer planets are those which are further away from the Sun and include the next four planets in order of increasing distance from the Sun (Jupiter, Saturn, Uranus and Neptune), with Neptune being the furthest.

The inner planets are made up of rock and metal and are therefore solid. These planets move slowly as they are considered to be heavy. They have an average diameter of about 13000 km as they are small planets. On the other hand, the outer planets are said to be made of gases and they are not really solid. The gases which make them up are Hydrogen and Helium; huge balloons floating in the space are considered as giant gas planets by people and they have an average diameter of about 48000 km.

Furthermore, the inner planets are warmer than outer planets simply due to the fact that they are closer to the Sun. Outer planets are composed of lighter elements such as gases and inner planets are composed of heavy elements such as iron. Inner planets have fewer moons, small, silicate surface, nickel-iron core, higher density and rotate more slowly compared to outer planets. Outer planets have a greater number of moons, no solid part; rotate faster, have a lower density as well as rings in some cases (Jupiter and Saturn). Outer planets are significantly bigger than the inner planets as Jupiter is measured to be 88846 miles in diameter and Mercury is measured to be 3031 miles in diameter.

There is significant difference between the rotation and the orbit of the two types of planets. For example, for Jupiter it would take 9 hours and 55 minutes for a day to complete (or to complete one rotation) and on Venus it would take 234 hours for a day to complete. (The time period of a day is that compared to the standard 24 hour day on Earth.) The inner planets take lesser time to orbit the Sun whereas the planets which are far away need more time as they have to cover more ground. For example, Jupiter takes 164 Earth years to complete one orbit!

An object’s ORBIT is the path it takes around another, more massive object in space. Each of the nine planets in the Solar System is held in orbit by the Sun’s gravitational pull. However, the planets do not orbit the Sun in circular paths but in elliptical (oval) ones. Orbit lengths, and the orbital period (the time it takes a planet to complete one orbit) increase with successively distant planets.

Orbit is the path of a body as it moves under the influence of a second body. An example is the path of a planet or comet as it moves around the Sun. Planets and satellites that orbit other bodies trace out a path called an ellipse. An ellipse is a closed curve of oval shape wherein the sum of the distances from any point on the curve to two internal focal points is constant. In everyday life you probably just call this an oval. As shown in the picture below, an ellipse has a major axis and a minor axis.

The major axis is always at least as long as or longer than the minor axis. When both the major and minor axes are the same length, this is a special case of an ellipse we commonly call a circle. Therefore, orbiting bodies can also trace out a circular path. Although a circle is a special type of ellipse, people commonly refer to satellite and planetary orbits as either circular or elliptical. The orbital period is the time to complete one full orbit.

After ten years of work, Kepler discovered the relationship between the time it takes a planet to orbit the Sun and its distance from the Sun. Kepler’s third law says that the square of the orbital period of a planet is directly proportional to the cube of the average distance of the planet from the Sun. Mathematically, this is given by the ratio T^2/r^3 and applies to all planets. The practical application of Kepler’s third law is to calculate the radius of a planet’s orbit by observation of that planet’s orbital period.