What's really in our solar system?

Back in my school days, the solar system had nine planets and the edge of the solar system ended at Pluto. It turns out that Pluto is now considered a dwarf planet and the solar system is much larger and more complicated than anyone ever imagined.

A solar system is technically a star with all of the planets, comets, asteroids and whatever orbiting it. The only solar system we knew until recently was ours. Now we realize that solar systems exist all over our galaxy and planetary systems are probably common entities in the universe.

The reason why Pluto was demoted from being a planet is because astronomers have discovered many dwarf planets out there, including some that are larger than Pluto. This discovery changed everything we knew about our solar system. The last planet is Neptune, and beyond it is the so-called Kuiper belt. Dwarf planets like Pluto are considered trans-Neptunian objects.

The Sun is at the center of the solar system. It's a nearly million mile in diameter ball of hot gas that is fusing hydrogen into helium in its core. Obviously, the Sun is the largest and most massive object in the solar system. As you might know, the inner planets, Mercury, Venus, Earth and Mars are rocky planets. Mercury has no atmosphere, Venus and Earth have substantial atmospheres and Mars has a thin atmosphere. Earth is currently the only planet known to contain life.

Between Mars and Jupiter is the asteroid belt. This is region where planets that did form were busted up because of Jupiter's gravity. There are probably millions of objects in this region and some of them are large like Ceres, a dwarf planetoid with a diameter of just under 1,000 kilometers. Sometimes the asteroids in this region get knocked out of orbit and cross Earth's orbit. Occasionally, they enter Earth's atmosphere where they're called a meteor (shooting star) and some even hit the surface where they're called a meteorite. Meteorites are worth a lot of money, and some people hunt them for both a hobby and profit.

The gas giants lie beyond the asteroid belt and include Jupiter, the largest, followed by Saturn, Neptune and Uranus. These large planets have plenty of moons orbiting them. The gas giants are the big boys of the solar system and are responsible for keeping the orbits of all of the planets where they are supposed to be. They also help suck up asteroids that could strike Earth. Jupiter is our savior in this respect.

Beyond Neptune are many strange objects, including Centaurs (icy comet-like bodies), comets, trans-Neptunian dwarf planets and an assortment of ice chunks and rocks. The Kuiper belt extends out from the orbit of Neptune at 30 AU (an astronomical unit is equal to the distance from the Earth to the Sun) out to 50 AU. That's 4.6 trillion miles.

The main dwarf planets discovered so far are Pluto, Haumea, and Makemake. I like the last one's name. Some of these objects follow normal orbits around the Sun while others are more extreme. This odd behavior results in the Kuiper belt looking like a torus. Much of this is the fault of Neptune' gravitational pull and it's thought that some of the moons of Kuiper were captured out of this region.

Pluto has been recently studied by a NASA probe, New Horizons, and it revealed a lot of new and interesting data about this dwarf planet. The pictures of Pluto are breathtakingly detailed, and the amazing thing is that they were taken during a flyby. We now know that Pluto's radius is 1,187 kilometers. We also know more about Pluto's main moon, Charon, which has a radius of 606 kilometers, and it's obvious that Charon and Pluto both were created by an ancient collision. The mysteries about Pluto have only been intensified. Pluto has mountains and there are deposits of ice of varying colors. Pluto has blue sky, which means it has an atmosphere, but it and the red ice is caused by chemicals know as tholins. These are dye-like heteropolymers (formed from polymerization of more than one monomer) that have formed on Titan and other locations in space, but not on earth. They are formed by Ultraviolet irradiation of methane and ethane. There is a dark spot on Charon and icy patches on Nix and Hydra, two of Pluto's smaller moons. There are lots of mysteries there to keep astronomers busy for years.

But, this is not the end of the solar system. The solar system extends out much further, probably to as much as a lightyear or even farther. What's out in this far-flung region is theoretical. Obviously, some of the material this far out was stolen from another star, most likely from Proxima Centauri. Some astronomers call this region the Oort Cloud, a spherical cloud where trillions of comets orbit, some of which are in elliptical orbits that bring them into the inner solar system.

Where the solar system ends and interstellar space begins is open to question. It may be that the boundary extends much further than we think. Voyager I has reached this so-called boundary, but the data from it suggests that it's still not really at the proposed edge of the solar system, which is called the Heliosphere. This is where the Sun's solar wind bucks up against the particle streams from other stars. NASA announced that Voyager I had passed into interstellar space, but this is a problem because of the magnetic bubbles formed by solar wind interactions with cosmic rays. It's hard to tell where the edge of the Heliosphere is. Combined with the fact that it'll take Voyager I 300 years to get to the Oort cloud and then take 30,000 years to pass through it gives one an idea of how vast this region is. This is mind-boggling. It proves that we are only just learning about what the solar system is comprised of.

Thanks for reading.