What Is The Solar System?
The Planets
Mercury
The closest planet to the sun, Mercury is only a bit larger than Earth's moon. Its day side is scorched by the sun and can reach 840 degrees Fahrenheit (450 Celsius), but on the night side, temperatures drop to hundreds of degrees below freezing. Mercury has virtually no atmosphere to absorb meteor impacts, so its surface is pockmarked with craters, just like the moon. Over its four-year mission, NASA's MESSENGER spacecraft has revealed views of the planet that have challenged astronomers' expectations.
Discovery: Known to the ancients and visible to the naked eye
Named for: Messenger of the Roman gods
Diameter: 3,031 miles (4,878 km)
Orbit: 88 Earth days
Day: 58.6 Earth days
Venus
The second planet from the sun, Venus is terribly hot, even hotter than Mercury. The atmosphere is toxic. The pressure at the surface would crush and kill you. Scientists describe Venus’ situation as a runaway greenhouse effect. Its size and structure are similar to Earth, Venus' thick, toxic atmosphere traps heat in a runaway "greenhouse effect." Oddly, Venus spins slowly in the opposite direction of most planets.
The Greeks believed Venus was two different objects — one in the morning sky and another in the evening. Because it is often brighter than any other object in the sky — except for the sun and moon — Venus has generated many UFO reports.
Discovery: Known to the ancients and visible to the naked eye
Named for: Roman goddess of love and beauty
Diameter: 7,521 miles (12,104 km)
Orbit: 225 Earth days
Day: 241 Earth days
Earth
The third planet from the sun, Earth is a waterworld, with two-thirds of the planet covered by ocean. It’s the only world known to harbor life. Earth’s atmosphere is rich in life-sustaining nitrogen and oxygen. Earth's surface rotates about its axis at 1,532 feet per second (467 meters per second) — slightly more than 1,000 mph (1,600 kph) — at the equator. The planet zips around the sun at more than 18 miles per second (29 km per second).
Diameter: 7,926 miles (12,760 km)
Orbit: 365.24 days
Day: 23 hours, 56 minutes
Mars
The fourth planet from the sun, is a cold, dusty place. The dust, an iron oxide, gives the planet its reddish cast. Mars shares similarities with Earth: It is rocky, has mountains and valleys, and storm systems ranging from localized tornado-like dust devils to planet-engulfing dust storms. It snows on Mars. And Mars harbors water ice. Scientists think it was once wet and warm, though today it’s cold and desert-like.
Mars' atmosphere is too thin for liquid water to exist on the surface for any length of time. Scientists think ancient Mars would have had the conditions to support life, and there is hope that signs of past life — possibly even present biology — may exist on the Red Planet.
Discovery: Known to the ancients and visible to the naked eye
Named for: Roman god of war
Diameter: 4,217 miles (6,787 km)
Orbit: 687 Earth days
Day: Just more than one Earth day (24 hours, 37 minutes)
Jupiter
The fifth planet from the sun, Jupiter is huge and is the most massive planet in our solar system. It’s a mostly gaseous world, mostly hydrogen and helium. Its swirling clouds are colourful due to different types of trace gases. A big feature is the Great Red Spot, a giant storm which has raged for hundreds of years. Jupiter has a strong magnetic field, and with dozens of moons, it looks a bit like a miniature solar system.
Discovery: Known to the ancients and visible to the naked eye
Named for: Ruler of the Roman gods
Diameter: 86,881 miles (139,822 km)
Orbit: 11.9 Earth years
Day: 9.8 Earth hours
Saturn
The sixth planet from the sun is known most for its rings. When Galileo Galilei first studied Saturn in the early 1600s, he thought it was an object with three parts. Not knowing he was seeing a planet with rings, the stumped astronomer entered a small drawing — a symbol with one large circle and two smaller ones — in his notebook, as a noun in a sentence describing his discovery. More than 40 years later, Christiaan Huygens proposed that they were rings. The rings are made of ice and rock. Scientists are not yet sure how they formed. The gaseous planet is mostly hydrogen and helium. It has numerous moons.
Discovery: Known to the ancients and visible to the naked eye
Named for: Roman god of agriculture
Diameter: 74,900 miles (120,500 km)
Orbit: 29.5 Earth years
Day: About 10.5 Earth hours
Uranus
The seventh planet from the sun, Uranus is an oddball. It’s the only giant planet whose equator is nearly at right angles to its orbit — it basically orbits on its side. Astronomers think the planet collided with some other planet-size object long ago, causing the tilt. The tilt causes extreme seasons that last 20-plus years, and the sun beats down on one pole or the other for 84 Earth-years. Uranus is about the same size as Neptune. Methane in the atmosphere gives Uranus its blue-green tint. It has numerous moons and faint rings.
Discovery: 1781 by William Herschel (was thought previously to be a star)
Named for: Personification of heaven in ancient myth
Diameter: 31,763 miles (51,120 km)
Orbit: 84 Earth years
Day: 18 Earth hours
Neptune
The eighth planet from the sun, Neptune is known for strong winds — sometimes faster than the speed of sound. Neptune is far out and cold. The planet is more than 30 times as far from the sun as Earth. It has a rocky core. Neptune was the first planet to be predicted to exist by using mathematics, before it was detected. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest some other might be exerting a gravitational tug. German astronomer Johann Galle used calculations to help find Neptune in a telescope. Neptune is about 17 times as massive as Earth.
Discovery: 1846
Named for: Roman god of water
Diameter: 30,775 miles (49,530 km)
Orbit: 165 Earth years
Day: 19 Earth hours
So that’s a basic overview of the planets in our solar system. Now we’re going to talk about stars and the stars that are closest to us.
What Is A Star?
75% of the matter in the Universe is hydrogen and 23% is helium; these are the amounts left over from the Big Bang. These elements exist in large stable clouds of cold molecular gas. At some point a gravitational disturbance, like a supernova explosion or a galaxy collision will cause a cloud of gas to collapse, beginning the process of star formation.
As the gas collects together, it heats up. Conservation of momentum from the movement of all the particles in the cloud causes the whole cloud to begin spinning. Most of the mass collects in the center, but the rapid rotation of the cloud causes it to flatten out into a protoplanetary disk. It’s out of this disk that planets will eventually form, but that’s another story.
The protostar at the heart of the cloud heats up from the gravitational collapse of all the hydrogen and helium, and over the course of about 100,000 years, it gets hotter and hotter becoming a T Tauri star. Finally after about 100 million years of collapse, temperatures and pressures at its core become sufficient that nuclear fusion can ignite. From this point on, the object is a star.
Nuclear fusion is what defines a star, but they can vary in mass. And the different amounts of mass give a star its properties. The least massive star possible is about 75 times the mass of Jupiter. In other words, if you could find 74 more Jupiters and mash them together, you’d get a star. The most massive star possible is still an issue of scientific disagreement, but it’s thought to be about 150 times the mass of the Sun. More than that, and the star just can’t hold itself together.
The least massive stars are red dwarf stars, and will consume small amounts over tremendous periods of time. Astronomers have calculated that there are red dwarf stars that could live 10 trillion years. They put out a fraction of the energy released by the Sun. The largest supergiant stars, on the other hand, have very short lives. A star like Eta Carinae, with 150 times the mass of the Sun is emitting more than 1 million times as much energy as the Sun. It has probably only lasted a few million years and will soon detonate as a powerful supernova; destroying itself completely.
Most stars are in the main sequence phase of their lives, where they’re doing hydrogen fusion in their cores. Once this hydrogen runs out, and only helium is left in the core, the stars have to burn something else. The largest stars can continue fusing heavier and heavier elements until they can’t sustain fusion any more. The smallest stars eject their outer layers and become white dwarf stars, while the more massive stars have much more violent ends, become neutron stars and even black holes.
The twelve closest stars to our solar system in order of distance are: Proxima Centauri, Rigil Kentaurus, Barnard's Star, Wolf 359, Lalande 21185, Luyten 726-8A and B, Sirius A and B, Ross 154, Ross 248, Epsilon Eridani.
Proxima Centauri
The closest star to our own solar system is called Proxima Centauri. It will not always be closest, since stars do move through space. Proxima Centauri is the third star in the Alpha Centauri star system, also known as Alpha Centauri C.
- Distance: 4.2 light-years
Rigil Kentaurus
The second closest star is a tie between the sister stars of Proxima Centauri. Alpha Centauri A and B make up the other two stars of the triple star system Alpha Centauri.
- Distance: 4.3 light-years
Barnard's Star
This is a faint red dwarf star, discovered in 1916 by E. E. Barnard. Recent efforts to discover planets around Barnard's Star have failed.
- Distance: 5.9 light-years
Wolf 359
This star is known to many as the location of a famous battle on Star Trek the Next generation. Wolf 359 is a red dwarf. It is so small that if it were to replace our Sun, an observer on Earth would need a telescope to see it clearly.
- Distance: 7.7 light-years
Lalande 21185
While it is the fifth closest star to our own Sun, Lalande 21185 is about three times too faint to be seen with the naked eye. You'd need a good telescope to pick it out of the night sky.
- Distance: 8.26 light-years
Luyten 726-8A and B
Discovered by Willem Jacob Luyten (1899-1994), both Luyten 726-8A 726-8B are red dwarfs and too faint to be seen with the naked eye.
- Distance: 8.73 light-years
Sirius A and B
Sirius, also known as the Dog Star, is the brightest star in the night sky. It has a companion called Sirius B, which is a white dwarf.
- Distance: 8.6 light-years
Ross 154
Ross 154 appears to be a flare star, which means that it can increase its brightness by a factor of 10 or more before reverting to its normal state, a process which takes only a few minutes.
- Distance: 9.693 light-years
Ross 248
While it is now the ninth closest star to our solar system, around the year 38000AD, the red dwarf Ross 248 will take the place of Proxima Centauri as the closest star to us.
- Distance: 10.32 light-years
Epsilon Eridani
Epsilon Eridani is among the closest stars known to have a planet, Epsilon Eridani b. It is the third closest star that is viewable without a telescope.
- Distance: 10.5 light-years