Solar System

What is a star??

A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity. Nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements. The Sun is the closest star to Earth. A star’s color relies on its temperature: hotter stars emit bluer light and cooler stars emit redder light.  

What is a planet??

According to IAU, an object must meet three criteria in order to be classified as a planet.

1.       First, it must orbit the Star.

2.       Second, it must be big enough for gravity to squash it into a round ball.

3.       And third, it must have cleared other objects out of the way in its orbital neighborhood.

What is a galaxy??

A galaxy is a collection of stars held together by their mutual galaxy. In other words, all the stars in a galaxy are kept together by the gravity of all the other stars (as well as the invisible, mysterious dark matter). The best estimates suggest that the Milky Way contains about 500 thousand million stars.

What is a comet??

Comets are roving time capsules, carrying primitive debris from long, long ago.

What is a satellite??

A natural satellite, or moon, is a celestial body that orbits another body, e.g. a planet, which is called its primary. There are 173 known natural satellites orbiting planets in the Solar System, as well as at least eight orbiting IAU-listed dwarf planets.

What is a meteor??

A meteor or "shooting star" is the passage of a meteoroid or micrometeoroid into the Earth's atmosphere, incandescent from air friction and shedding glowing material in its wake sufficiently to create a visible streak of light. Meteoroids become meteors   when they interact with a planet's atmosphere and cause a streak of light in the sky.

A list of planet with number of satellite, distance from sun, a year and a day measurement is given below:

No.	Planet	Satellite	Distance	Orbit	Day
1	Mercury	0	57  million km	88 days	58.6 days
2	Venus	0	108 million km	225 days	241 days
3	Earth	1	150 million km	365.24 days	23 hours, 56 minutes
4	Mars	2	228 million km	687 days	24 hours,  37 minutes
5	Jupiter	63	779 million km	11.9 years	9.8  hours
6	Saturn	60	1.43 billion  km	29.5  years	10.5 hours
7	Uranus	27	2.88 billion  km	84  years	18 hours
8	Neptune	13	4.50 billion  km	165 years	19  hours

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 F (450 C), 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.

Discovery: Known to the ancients and visible to the naked eye

Named for: Messenger of the Roman gods

Venus

The second (2nd) planet from the sun, Venus is terribly hot. 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, and 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

Earth

The third (3rd) planet from the sun, Earth is a water world, 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 — slightly more than 1,000 mph — at the equator. The planet zips around the sun at more than 18 miles per second.

Mars

The fourth (4th) 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

Jupiter

The fifth (5th) 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 colorful 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

Saturn

The sixth (6th) 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

Uranus

The seventh (7th) 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-sized object long ago, causing the tilt. The tilt causes extreme seasons that last 20+ 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

Neptune

The eighth (8th) 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 math, 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

Pluto

It is unlike other planets in many respects. It is smaller than our moon. Its orbit carries inside the orbit of Neptune and the way out beyond that orbit. From 1979 until early 1999, Pluto had actually been the eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune's path and once again became the solar system's most distant planet — until it was demoted to dwarf planet status. Pluto will stay beyond Neptune for 228 years. Pluto’s orbit is tilted to the main plane of the solar system — where the other planets orbit — by 17.1 degrees. It’s a cold, rocky world with only a very ephemeral atmosphere.

Discovery: 1930 by Clyde Tombaugh

Named for: Roman god of the underworld, Hades

The problem for Pluto

The problem for Pluto is the fact that its orbit is in the Kuiper Belt along with 43 other known Kuiper Belt Objects (KBOs). There are possibly billions of objects in the Kuiper Belt that have not been cataloged yet. Scientists have even found 8 KBOs between Neptune and Pluto. Some scientists view the new definition as unclear. Exactly how much does Pluto have to "clear" from its neighborhood to be considered a planet? And how much has Pluto already influenced its own neighborhood since the planet formed? These and other questions have been raised in response to the IAU's definition of a planet.

Consider this: Pluto crosses into Neptune's orbit, but Neptune is still classified a planet. This is because of the orbits of Pluto and Neptune and that they never get closer to each other than 17AU (AU=distance from Earth to the Sun). Pluto may cross orbits with many other Kuiper Belt Objects, but how close do these objects get to Pluto? How close to objects have to get to Pluto to be considered "in" Pluto's neighborhood? Diagram of the planet orbits in our solar system, including Pluto, distinctly shows the crossover of Neptune's and Pluto's orbits.

sources:
wikipedia.com
nasa.gov

space.com