Planets and other objects in our Solar System. Credit: NASA.
Mercury is the closest planet to the Sun but not the hottest. That distinction goes to Venus. The planet was named after the Roman messenger of the gods because it orbits the Sun so quickly. Mercury is a small, grayish planet that is often said to resemble the Earth’s Moon.
Venus, the second planet from the Sun, is the hottest planet because its atmosphere tends to trap heat. Named after the Roman goddess of beauty, Venus is the brightest planet. In fact, the only celestial body that is brighter is the Moon. Venus is around the same size as Earth with similar gravity, causing it to be referred to as Earth’s twin.
Earth is the third planet from the Sun. It is the only planet where life has been confirmed to exist. Roughly two-thirds of Earth’s surface is covered with oceans, and so far Earth is the only place where liquid water is known to exist.
Mars was named after the Roman god of war because of its red color, which is caused by rust in the rocks on the surface. Since it is the closest planet to Earth, people have long wondered if life could exist on Mars. Although no life has been discovered so far, some people still think that there may be life on Mars.
Jupiter, a gas giant, is the largest planet in this solar system. It was named after the Roman king of the gods, probably because of its size. Jupiter has 63 moons, one of which, Ganymede, is the solar system’s largest moon. Jupiter is also home to an enormous storm, the Great Red Spot, which has been raging for over two hundred years.
Saturn, the sixth planet from the sun, was named after the Roman god of agriculture and harvest, Saturnus. It is also a gas giant and therefore does not have a solid surface. One distinctive feature of the planet is its rings, which are composed of small pieces of rock and ice.
Uranus, the third largest planet, is also a gas giant. One interesting fact is that its moons were named after characters from works of literature by Shakespeare and Alexander Pope. Uranus orbits very slowly; it takes the planet 84 years to circle the sun.
Neptune is the furthest planet from the Sun. It was named after the Roman god of the sea; this is not surprising because it is bright blue, reminding one of a beautiful ocean. Neptune has four rings, although they are difficult to see. When Pluto was reclassified as a dwarf planet, Neptune became the eighth and last planet in the solar system.
Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA
When you look up into the night sky, it seems like you can see a lot of stars. There are about 2,500 stars visible to the naked eye at any one point in time on the Earth, and 5,800-8,000 total visible stars (i.e. that can be spotted with the aid of binoculars or a telescope). But this is a very tiny fraction of the stars the Milky Way is thought to have!
So the question is, then, exactly how many stars are in the Milky Way Galaxy? Astronomers estimate that there are 100 billion to 400 billion stars contained within our galaxy, though some estimate claim there may be as many as a trillion. The reason for the disparity is because we have a hard time viewing the galaxy, and there’s only so many stars we can be sure are there.
Structure of the Milky Way:
Why can we only see so few of these stars? Well, for starters, our Solar System is located within the disk of the Milky Way, which is a barred spiral galaxy approximately 100,000 light years across. In addition, we are about 30,000 light years from the galactic center, which means there is a lot of distance – and a LOT of stars – between us and the other side of the galaxy.
Artist’s impression of the Milky Way Galaxy. Credit: NASA
To complicate matter further, when astronomers look out at all of these stars, even closer ones that are relatively bright can be washed out by the light of brighter stars behind them. And then there are the faint stars that are at a significant distance from us, but which elude conventional detection because their light source is drowned out by brighter stars or star clusters in their vicinity.
The furthest stars that you can see with your naked eye (with a couple of exceptions) are about 1000 light years away. There are quite a few bright stars in the Milky Way, but clouds of dust and gas – especially those that lie at the galactic center – block visible light. This cloud, which appears as a dim glowing band arching across the night sky – is where our galaxy gets the “milky” in its name from.
It is also the reason why we can only really see the stars in our vicinity, and why those on the other side of the galaxy are hidden from us. To put it all in perspective, imagine you are standing in a very large, very crowded room, and are stuck in the far corner. If someone were to ask you, “how many people are there in here?”, you would have a hard time giving them an accurate figure.
Now imagine that someone brings in a smoke machine and begins filling the center of the room with a thick haze. Not only does it become difficult to see clearly more than a few meters in front of you, but objects on the other side of the room are entirely obscured. Basically, your inability to rise above the crowd and count heads means that you are stuck either making guesses, or estimating based on those that you can see.
A mosaic of the images covering the entire sky as observed by the Wide-field Infrared Survey Explorer (WISE), part of its All-Sky Data Release. Credit: NASA/JPL
Imaging Methods:
Infrared (heat-sensitive) cameras like the Cosmic Background Explorer (aka. COBE) can see through the gas and dust because infrared light travels through it. And there’s also the Spitzer Space Telescope, an infrared space observatory launched by NASA in 2003; the Wide-field Infrared Survey Explorer (WISE), deployed in 2009; and the Herschel Space Observatory, a European Space Agency mission with important NASA participation.
All of these telescopes have been deployed over the past few years for the purpose of examining the universe in the infrared wavelength, so that astronomers will be able to detect stars that might have otherwise gone unnoticed. To give you a sense of what this might look like, check out the infrared image below, which was taken by COBE on Jan. 30th, 2000.
However, given that we still can’t seem them all, astronomers are forced to calculate the likely number of stars in the Milky Way based on a number of observable phenomena. They begin by observing the orbit of stars in the Milky Way’s disk to obtain the orbital velocity and rotational period of the Milky Way itself.
Estimates:
From what they have observed, astronomers have estimated that the galaxy’s rotational period (i.e. how long it takes to complete a single rotation) is apparently 225-250 million years at the position of the Sun. This means that the Milky Way as a whole is moving at a velocity of approximately 600 km per second, with respect to extragalactic frames of reference.
Infrared image of the Milky Way taken by NASA’s Spitzer Space Telescope. Credit: NASA/JPL-Caltech
Then, after determining the mass (and subtracting out the halo of dark matter that makes up over 90% of the mass of the Milky Way), astronomers use surveys of the masses and types of stars in the galaxy to come up with an average mass. From all of this, they have obtained the estimate of 200-400 billion stars, though (as stated already) some believe there’s more.
Someday, our imaging techniques may become sophisticated enough that are able to spot every single star through the dust and particles that permeate our galaxy. Or perhaps will be able to send out space probes that will be able to take pictures of the Milky Way from Galactic north – i.e. the spot directly above the center of the Milky Way.
Until that time, estimates and a great deal of math are our only recourse for knowing exactly how crowded our local neighborhood is!
We have written many great articles on the Milky Way here at Universe Today. For example, here are 10 Facts About the Milky Way, as well as articles that answer other important questions.
Astronomy Cast did a podcast all about the Milky Way, and the Students for the Exploration and Development of Space (SEDS) have plenty of information about the Milky Way here.
And if you’re up for counting a few of the stars, check out this mosaic from NASA’s Astronomy Picture of the Day. For a more in-depth explanation on the subject, go to How the Milky Way Galaxy Works.
Saturn is sometimes called the ”Jewel of the Solar System” because its ring system looks like a crown. The rings are well known, but often the question ”what are Saturn’s rings made of” arises. Those rings are made up of dust, rock, and ice accumulated from passing comets, meteorite impacts on Saturn’s moons, and the planet’s gravity pulling material from the moons. Some of the material in the ring system are as small as grains of sand, others are larger than tall buildings, while a few are up to a kilometer across. Deepening the mystery about the moons is the fact that each ring orbits at a different speed around the planet.
Saturn is not the only planet with a ring system. All of the gas giants have rings, in fact. Saturn’s rings stand out because they are the largest and most vivid. The rings have a thickness of up to one kilometer and they span up to 482,000 km from the center of the planet.
The rings are named in alphabetical order according to when they were discovered. That makes it a little confusing when listing them in order from the planet. Below is a list of the main rings and gaps between them along with distances from the center of the planet and their widths.
The D ring is closest to the planet. It is at a distance of 66,970 – 74,490 km and has a width of 7,500 km.
C ring is at a distance of 74,490 – 91,980 km and has a width of 17,500 km.
Columbo Gap is at a distance of 77,800 km and has a width of 100 km.
Maxwell Gap is at a distance of 87,500 km and has a width of 270 km.
Bond Gap is at a distance of 88,690 – 88,720 km and has a width of 30 km.
Dawes Gap is at a distance of 90,200 – 90,220 km and has a width 20 km.
B ring is at a distance of 91,980 – 117,580 km with a width: 25,500 km.
The Cassini Division sits at a distance of 117,500 – 122,050 km and has a width of 4,700 km.
Huygens gap starts at 117,680 km and has a width of 285 km – 440 km.
The Herschel Gap is at a distance of 118,183 – 118,285 km with a width of 102 km.
Russell Gap is at a distance of 118,597 – 118,630 km and has a width of 33 km.
Jeffreys Gap sits at a distance of 118,931 – 118,969 km with a width of 38 km.
Kuiper Gap ranges from 119,403 -119,406 km giving it a width of 3 km.
Leplace Gap is at a distance of 119,848 – 120,086 km and a width of 238 km.
Bessel Gap is at 120,305 – 120,318 km with a width of 10 km.
Barnard Gap is at a distance of 120,305 – 120,318 km giving it a width of 3 km.
A ring is at a distance of 122,050 – 136,770 km with a width of 14,600 km.
Encke Gap sits between 133,570-133,895 km for a width of 325 km.
Keeler Gap is at a distance of 136,530-136,565 km with a width of 35 km.
The Roche Division is at 136,770 – 139,380 km for a width 2600 km.
F ring is begins at 140,224 km, but debate remains as to whether it is 30 or 500 km in width.
G ring is between 166,000 – 174,000 km and has a width of 8,000 km.
Finally, we get to the E ring. It is between 180,000 – 480,000 km giving it a width of 300,000 km.
As you can see, a great deal of observation has been dedicated to understanding and defining Saturn’s rings. Hopefully, having the answer to ”what are Saturn’s rings made of” will inspire you to look more deeply into the topic.
We have written many articles about Saturn for Universe Today. Here’s an article about the orbit of Saturn, and here’s an article about the temperature of Saturn.
