The Solar System

FAS Astronomers Blog, Volume 31, Number 7.

We live in a solar system. So, just what is it? It is a star and all the objects and material that are gravitationally bound to the star. Until recently, the only example we knew of was our local group of planets orbiting the Sun. In 1995, Michel Mayor and Didier Queloz discovered 51 Pegasi b, a planet orbiting another star and the flood gates opened. Alien solar systems were found around many stars of the Milky Way. But, for now, let’s stay close to home and focus on our solar system.

The Sun

Most of the mass in the Solar System is contained in the Sun. The Sun is an “average” yellow dwarf star in the middle of the main sequence on the Hertzsprung-Russell diagram. There are many stars larger (e.g., blue and red giants) and smaller (e.g., red dwarfs) than the Sun.

The Sun, like all stars, is composed mostly of hydrogen. It is large enough that gravity creates so much pressure that the star “ignites”. Nuclear fusion in its core converts hydrogen to helium releasing a huge amount of energy, which we eventually see as light and heat. For more on the Sun along with the classification and life cycle of stars, see Stars and a previous article on The Sun.

The Planets

The Solar System officially includes eight planets. Most of us are familiar with them, and we learned their names at a very young age. Do you remember the mnemonic, “My Very Eager Mother Just Served Us Nine Pizzas?” Of course, today with the reclassification of Pluto, the Nine Pizzas have become Nachos.

• Mercury, Venus, Earth, and Mars are the small inner Solar System terrestrial planets.
• Jupiter and Saturn are the outer Solar System gas giants.
• Uranus and Neptune are the outer Solar System ice giants.

The terrestrial planets are small and rocky and formed inside the “frost line” where there was little gas or ice available. As these planets formed, heavier metallic material sunk to the core, while the remaining rocky material formed the crust and mantle.

The outer planets formed outside the “frost line” where gas and ice were plentiful. The gas giants (Jupiter and Saturn) are large and composed primarily of hydrogen gas. One can think of them as potential stars that were too small to ignite. The ice giants (Uranus and Neptune) are “mid-sized” composed of hydrogen, but with icy material (water, methane, and ammonia) in their interior.

The outer Solar System is home to many moons. In some ways I find the moons to be more interesting than the planets themselves. Although they are smaller and icier than the Earth, they are still more Earth like than the gas and icy outer planets. There are nineteen moons large enough to be spherical. Seven of these are larger than Pluto, and two (Ganymede and Titan) are larger than the planet Mercury. If they orbited the Sun directly, they would be considered planets or dwarf planets. To learn more, see a previous article about the Moons of the Solar System.

The Dwarf Planets

As of 2008, we now have five official dwarf planets, although not everyone is happy with this classification. A dwarf planet is an object like a planet (spherical and orbits the Sun), but it isn’t large enough to clear its neighborhood of other objects. As such, we find the dwarf planets either in the Asteroid belt or near the Kuiper belt.

• Ceres is the largest asteroid.
• Haumea (how-MAY-ah) is shaped like a football.
• Makemake (MAH-kay MAH-kay) has a reddish surface.
• Eris (Air-is) was once the 10^th planet and is almost the same size as Pluto.
• Pluto, of course, is everyone’s favorite ex-planet.

There are more “trans-Neptunian” objects that might be large enough to be classified as dwarf planets. The International Astronomical Union (IAU) has a rule that if an object has an absolute magnitude of 1 or brighter, they will consider it to potentially be a dwarf planet. So far, only those listed above meet that condition.

Astronomer Mike Brown has a website that keeps track of these objects far out in the Solar System. He assigns each a probability of being large enough to be spherical and eventually becoming a dwarf planet. As of May 2023, Brown concluded that there are another six “near certain” to be classified as dwarf planets (Gonggong/2007 OR₁₀, Quaoar, Sedna, Orcus, 2002 MS₄, and Salacia).

Exoplanets

Our solar system isn’t alone. There are alien planets and other solar systems out there in the cosmos. Officially the count (as of May 2023) of what we call extrasolar planets (exoplanets) is over 5,300. The number of other solar systems is over 4,000. All these exoplanets are within a few thousand light years from the Sun. Astronomers think that most stars have planets, therefore, there are likely billions of other planets in the Milky Way alone. The folks at NASA keep track of these, and you can learn more about exoplanets in a previous article.

Formation of the Solar System

It is thought that the Solar System formed from a cloud of dust and gas that was disturbed by a nearby supernova some 4 ½ billion years ago. Gravity took over and most of the material eventually coalesced into the Sun. The remaining material began spinning in a plane about the Sun. The dust and metals in the inner part of the spinning disk combined together to form planetesimals. The planetesimals joined together to form planets. In the same way, dust, ice, and gas in the outer Solar System formed the gas and ice giants. Material also began spinning in a plane about each of the outer planets and formed their moons.

Leftover material that didn’t coalesce into a planet or moon is found in the asteroid belt between Mars and Jupiter, the Kuiper belt beyond Neptune, and the Oort cloud almost a light year out from the Sun. Interestingly, the James Webb Space Telescope (JWST) just discovered three similar bands of material orbiting the star Fomalhaut.

Not all of the exoplanets orbiting other stars are found where they should be. For example, “Hot Jupiters”, such as 51 Pegasi b, have been discovered where we would expect to find only small rocky planets. The explanation for this comes from the theory of planetary migration.

In 2005, Hal Levison and others proposed the “Nice” model of planetary migration. The model is named for the French city Nice (Niece) where it was formulated. According to the model, the planets of our solar system were initially formed with circular orbits much closer to the Sun than they are today. Also, Neptune formed inside of the orbit of Uranus. After a few hundred million years, Jupiter and Saturn fell into a 2:1 resonance, where Jupiter completed two orbits for every one of Saturn’s orbits. The gravitational tug from this resonance pushed Neptune out away from the Sun and it passed the orbit of Uranus. It also carried Uranus farther out with it.

Neptune’s movement dislodged material in the Kuiper Belt and caused some of that material to fall into the inner Solar System. At the same time, Jupiter dislodged material in the asteroid belt that also fell inward. This inward movement of material is referred to as the Late Heavy Bombardment and created much of the cratering we see on the Moon today.

Find Out More

To find out more about the Solar System, see several previous articles about the planets and moons. You can find these by going to the FAS Astronomers Blog or searching for FAS37 followed by the name of the Planet or Dwarf Planet (e.g., “FAS37 Jupiter” or “FAS37 Eris”). We also have a table of facts about the planets and moons of the Solar System.