How did our solar system form? What’s in it? A brief tour of our stellar neighborhood.
Formation of the solar system
The birth of the sun kicked off the formation of our solar system. This likely happened in a giant cold cloud of gas and dust – mainly made of Helium and Hydrogen. An exploding star nearby caused a pocket of this dust to start spinning, creating a whirling vortex.
As gravity drew material into the center of this vortex, it became denser and hotter. A protostar was formed – an early stage in the formation of a star. This grew to the point where fusion kicked off, and the star started burning hydrogen – creating huge amounts of energy in the process.
While the star was young, there was a brief 10-million-year window when all the planets could form out of the leftover gas and dust. This dust and gas formed into a disk around the young sun known as an accretion disk.
During the brief window when planets were forming, this became a protoplanetary disk. Clumps of dust gathered near the star and grew to form planetary embryos, which then collided in a chaotic way to form the rocky planets. The gas and ice giants formed in the cold space further away from the sun. At the same time, rocky and icy cores formed and attracted huge amounts of the leftover hydrogen, helium, and other gases such as methane.
Our sun
Our sun may be only one of 100 billion stars in the Milky Way, but it’s ours. It’s our closest star and provides the power for all life on Earth.
At 1.4 million km across, our sun sits around the middle of the range of possible star sizes. But because so many of the stars in the universe are small red dwarfs, it’s among the top 10% in terms of size.
In the sun’s core, hydrogen ions fuse to form helium, releasing energy. This takes place on a huge scale – the energy released is equivalent to 400 million megatons of nuclear bombs every second.
The constantly moving ions in the center of the sun create magnetic fields. These magnetic fields, in turn, create sunspots and can create solar flares. They release plasma and magnetism from the sun, known as coronal mass ejections. The energy and debris released create the stunning light shows we call aurorae. However, they also have the potential to disrupt electricity networks, cause mass blackouts, and damage satellites.
Orbits
Most objects in the solar system orbit the sun. Kepler’s laws describe the journey of objects around the sun in ellipses, with the sun at one focus of the ellipse. The planets’ orbits are nearly circular. Some other objects, such as comets, follow highly elliptical orbits – tracing paths around the sun like very squashed circles.
Most of the planets also have their own orbiting bodies in the shape of moons.
All the planets move in the same direction around the sun – anti-clockwise if you were to look down from the North pole.
They are also, within a few degrees of variation, on the same orbital plane. This is no coincidence – it resulted from how the solar system was first formed.
As the cloud of gas and dust that became the solar system collapsed, it was spinning round – it had angular momentum.
Particles approached from all directions, but as they collided, they canceled each other out – until only particles moving in the same direction remained. As the cloud spun, it flattened into the disk, which formed the planets. This flattening accounts for the similarity in the planets’ orbital planes.
Mercury
Mercury is the closest planet to the sun and very different from the other terrestrial planets of Venus, Earth, and Mars. In fact, estimates suggest it’s more metallic than rocky: being made up of 70% metallic and 30% silicate material. This explains why it is very dense – the second densest object in the solar system after Earth.
Most of Mercury seems to be an iron core, and several theories have been proposed to explain this. One suggestion is that it was originally larger, but an impact event ripped the lighter materials away from the planet’s surface, leaving just the heavier core behind. Another is that the young, very hot sun vaporized the lighter materials on the surface.
Mercury doesn’t have an atmosphere but a much less dense equivalent known as an exosphere. With the sun close by, it can reach enormous temperatures of 800℉ (430˚C). Amazingly, water ice has been found in the bottom of deep craters at Mercury’s poles – known as ‘cold traps’.
Earth and its siblings – Venus and Mars
Today, Venus and Mars are both extreme and inhospitable places. Venus has been compared to Hell with clouds of sulfuric acid and extremely hot surface temperatures, hot enough to melt lead at 900℉ (475˚C). On the other hand, Mars is a cold, desolate desert.
However, we think Venus and Mars used to look remarkably like Earth as they are 3 terrestrial, rocky planets orbiting close to the sun. We have evidence Mars had liquid water, and some believe Venus did too – although climate modeling has cast doubt on this claim.
So what happened? With its thick atmosphere, a runaway greenhouse effect is most likely responsible for the extremely inhospitable environment on Venus.
Mars lost internal heat quickly because of its small size. As this internal heat was lost, so too was the magnetism that the heated core provided. Without this magnetic field, solar winds whipped away Mars’s atmosphere, thinning it and decreasing its protective power. A runaway cooling process resulted in the desolate planet we see today.
Jovian planets
Traveling further from the sun, the planets begin to look very different. The Jovian planets, also known as the giant planets, are between 5 times and 30 times further away from the sun than Earth.
The gas giants of Jupiter and Saturn are made up mostly of hydrogen and helium. They have dense cores of rocks and ice, surrounded by liquid metallic hydrogen. In the case of Jupiter, this liquid hydrogen probably makes up most of the planet.
Uranus and Neptune are almost twins in their make-up.
Both of them are, in large part, frozen water, ammonia, and methane – giving them the moniker of ice giants.
None of the giant planets have defined solid surfaces. Their surfaces are instead mostly liquids, gases, and ices. They are also enormous. The largest by far is Jupiter – which is 11 times the diameter of Earth. Saturn is 9 times the size of Earth, and both Neptune and Uranus are about 4 times wider than Earth.
Outer regions
The Jovian planets are not alone in the outer region of the solar system. They all have moons keeping them company – Saturn likely has 82 moons in its orbit. The outer region also contains comets, dust, and other debris.
Just beyond the orbit of Neptune is the Kuiper belt, a donut-shaped region filled with icy objects. The Kuiper belt is a source of comets and the home of the dwarf planet Pluto. Several other dwarf planets have been found there, too, including Eris, Makemake, and Haumea.
Around 2000 Kuiper belt objects have been cataloged so far, but there is likely much more to discover. Overlapping the Kuiper belt but reaching much further out is the scattered disk, which is sparsely scattered with small, icy bodies.
The Oort cloud is a theoretical sphere of icy fragments at the edge of the sun’s gravitational influence. While we don’t have any direct observations of the Oort cloud, it’s thought to be the source of long period comets that take hundreds of years to orbit the sun.
Interesting moons
Moons, otherwise known as natural satellites, are bodies in the solar system that orbit a planet, dwarf planet, or even another small body instead of the sun. We know of more than 200, and it seems likely more are out there.
Titan, a moon of Saturn, is bigger than Mercury. Unusually for a moon, it also has a thick atmosphere. It rains methane and ethane on Titan, and this collects in seas, which were observed by the Cassini mission.
The largest moon in the solar system, Ganymede, unsurprisingly orbits the largest planet Jupiter. It is around ¾ the size of Mars. It’s also the only moon we know of with a magnetic field.
Io, another moon of Jupiter, is the most volcanic body in our solar system because of the huge gravitational pull from Jupiter, causing the surface to bulge. Volcanoes on Io blast molten rock up to 190 miles (306 kilometers) into the air.
Our strange moon and the impact theory
Our moon – the moon that orbits the Earth – is the 5th biggest moon in the solar system. It’s about a quarter of the size of the Earth, which makes it by far the largest moon in the solar system relative to the body it orbits. Studying lunar rocks suggests that the Moon’s composition is very similar to Earth’s, but not quite the same.
Several theories about the origin of the moon have been suggested, but today, one holds the most sway. It involves a collision relatively late during the Earth’s formation. A glancing impact from a planet around the size of Mars known as Theia blasted debris from the Earth. This debris was then gathered into the Moon. This is known as the giant impact theory.
Impact events have been suggested to explain other eccentricities in the solar system, such as Uranus being essentially tilted on its side and Saturn’s spinning in the opposite direction to its orbit.
Small bodies
Rocky leftovers from the formation of the solar system fly through space as asteroids. Most of these asteroids can be found in the asteroid belt – a donut-shaped region between Mars and Jupiter.
Further out, the Kuiper belt contains small icy bodies. It’s thought to be one of the sources of comets in the solar system, along with the theoretical Oort cloud.
Comets are space snowballs – icy bodies made of frozen gas, dust, and rocks orbiting the sun.
They heat up and release gases as they get closer to the sun. These gases can create stunning tails that trail the comets for millions of miles.
Near Earth Objects (NEOs) are those small bodies in space whose orbits bring them close to Earth. If that object is bigger than 460 feet (140 meters) across, and its orbit crosses Earth’s orbit, it is a potentially hazardous object (PHO). PHOs and NEOs are usually asteroids but can be comets. Either way, when we look at the skies, we watch carefully for anything coming too close for comfort.
