Posted on by Abby Cessna

Orbits of the Planets

Take a look at the Solar System from above, and you can see that the planets make nice circular orbits around the Sun. But dwarf planet’s Pluto’s orbit is very different. It’s highly elliptical, traveling around the Sun in a squashed circle. And Pluto’s orbit is highly inclined, traveling at an angle of 17-degrees. This strange orbit gives Pluto some unusual characteristics, sometimes bringing it within the orbit of Neptune. Credit: NASA

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Centuries ago, people believed that the Earth was the center of the Solar System. Slowly, that view was replaced with the heliocentric view. With that change came the realization that the planets orbit the Sun.

When Pluto was reclassified as a dwarf planet, Mercury became the planet with the most eccentric orbit. The eccentricity of an orbit is a measurement of how much the orbit deviates from a circular shape. If an orbit is a perfect circle, it has an eccentricity of zero, and that number increases with an increase in eccentricity. Mercury has an eccentricity of .21. Its orbit ranges from 46 million kilometers at the closest point to the Sun to 70 million kilometers at the farthest point. The closest point to the Sun in an orbit is called the perihelion, and the farthest point is the aphelion. Mercury is the fastest planet to orbit the Sun at approximately Earth 88 days.

Venus has the least eccentricity of any planet in our Solar System – eccentricity of .007 – with a nearly perfect circular orbit. Venus’ orbit ranges from 107 million kilometers at the perihelion to 109 million kilometers from the Sun. It takes 224.7 of our days to orbit the Sun. A day on Venus is actually longer than a year because the planet rotates so slowly. Seen from the Sun’s north pole, all of the planets rotate counter-clockwise, but Venus actually rotates clockwise; it is the only planet to do that.

Earth also has a very low eccentricity of .017. On average, the planet is about 150 million kilometers from the Sun, but it can range from 147 million kilometers to 152 million kilometers. It takes our planet roughly 365.256 days to orbit the Sun, which is the reason for leap years.

Mars has an eccentricity of .093 making it one of the most eccentric orbits in our Solar System. Mars perihelion is 207 million kilometers and its aphelion is 249 million kilometers from the Sun. Over time, Mars’ orbit has become more eccentric. It takes 687 Earth days to orbit the Sun.

Jupiter has an eccentricity of .048 with a perihelion of 741 million kilometers and an aphelion of 778 million kilometers. It takes 4331 Earth days – 11.86 of our years – for Jupiter to orbit the Sun.

Saturn has an eccentricity of .056. At its closest point, Saturn is 1.35 billion kilometers from the Sun, and 1.51 billion kilometers away at its farthest point. Depending on what position it is in its orbit, Saturn’s rings are fully visible or almost invisible. The planet takes 29.7 years to orbit the Sun. In fact, since it was discovered in 1610, Saturn has only orbited approximately 13 times. Earth has orbited the Sun almost 400 times since then.

Uranus has a perihelion of 2.75 billion kilometers and an aphelion of 3 billion kilometers from the Sun. Its eccentricity is .047. It takes Uranus 84.3 Earth years to orbit the Sun. Uranus is unique because it actually rotates on its side with an axial tilt of almost 99°.

Neptune’s eccentricity is .009, almost as low as Venus’. The planet has a perihelion of 4.45 billion kilometers and an aphelion of 4.55 billion kilometers. Since Pluto was reclassified as a dwarf planet, Neptune is the planet with an orbit farthest from the Sun.

Universe Today has articles on orbits of all the planets including Mercury and Mars.

There are a number of other sites, including one with animations of the orbits and what an orbit is.

Astronomy Cast has an episode on the orbit of the planets.

Posted on by Abby Cessna

What are the Sizes of the Planets?

Size of the planets compared.

It is often difficult to grasp just how large the planets actually are. There are a number of ways to measure a planet, including diameter, volume, and surface area.

Mercury is the smallest planet in our Solar System since Pluto was demoted to a dwarf planet. It has a diameter of 4,879 km, and a surface area of 17.48 x 107 km2, which is only about 11% of Earth’s surface area. Mercury’s volume is even smaller in comparison at 6.083 x 1010 km3, which is only 5.4% the volume of Earth.

Venus is similar in size to Earth, which earned it the title of Earth’s twin. Venus has a diameter of 12,100 km and a surface area of 4.6 x 108 km2. These measurements are 95% and 90% of Earth’s diameter and surface area respectively. With a volume of 9.38 x 1011 km3, Venus’ volume is 86% of  Earth’s.

Earth has a diameter of 12,742 km and a surface area of 5.1 x 108 km2. Its volume of 1.08 x 1012 km3 gives the planet the largest volume of any of the terrestrial planets.

Mars is also a small planet, the second smallest in our Solar System. Mars’ diameter is 6,792 km, only about 53% of Earth’s diameter. At only 28% of Earth’s surface area, Mars has a very small surface area of 1.45 x 108 km2. Mars’ volume of 1.63 x 1011 km3 is only 15% of Earth’s volume.

All of the gas giants are larger in size than the four inner planets. Jupiter is the largest planet in our Solar System. It has a diameter of 143,000 km, which is more than 11 times the size of Earth’s diameter. The numbers only get larger from there. Jupiter has a surface area of 6.22 x 1010 km2. That is 122 times greater than Earth’s surface area. Jupiter’s volume of 1.43 x 1015 km3 is an incredible number. You can fit 1321 Earths inside Jupiter.

Saturn is the second largest planet in our Solar System. It has a diameter of 120,536 km across the equator, and a surface area of 4.27 x 1010 km2. With a volume of 8.27 x 1014 km3, Saturn can hold 764 Earths inside.

Uranus has a diameter of 51,118 km and a surface area of 8.1 x 109 km2. Although Uranus is much smaller than Jupiter, it is still large. With a volume of 6.83 x 1013 km3, you could fit 63 Earths inside the gas giant.

Neptune is slightly smaller than Uranus, but still very large. The planet has a diameter of 49,500 km. You could fit 57.7 Earths inside Neptune, which has a volume of 6.25 x1013 km3.  Neptune has a surface area of 7.64 x 109 km2, which is 15 times Earth’s surface area.

We have written many interesting articles about the Solar Planets here at Universe Today. Here’s tWhat are the Different Masses of the Planets?, What Is The Atmosphere Like On Other Planets?, What is the Average Surface Temperatures of the Planets? and What are the Diameters of the Planets?

For more information, check out this website to learn all about the planets and this page from NASA to learn about the planets.

Astronomy Cast has an episode on each of the planets including Mercury.

Posted on by Abby Cessna

Weight on Other Planets

Planets and other objects in our Solar System. Credit: NASA.

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Many children, and even adults, dream of visiting other planets and wonder what it would be like to stand on another planet. For one thing, your weight would be different on another planet, depending on a number of factors including the mass of the planet and how far you are away from the center of the planet.

Before we start, it’s important to understand that the kilogram is actually a measurement of your mass. And your mass doesn’t change when you go anywhere in the Universe and experience different amounts of gravity. Your weight is best measured in newtons. But since your bathroom doesn’t measure your weight in newtons, we’ll use kilograms. This is what your bathroom scale would say if you stepped on another world.

Mercury is the smallest planet in our Solar System, but it is dense. Because Mercury is so small, it has very little gravity. If you weighed 68 kg on Earth, you would only weigh 25.7 kg on Mercury. 

Venus is very close to Earth in size and mass. Venus’ mass is roughly 90% of the mass of the Earth. Thus, it is no surprise that someone would weigh a similar amount on Venus. Someone who weighed 68 kg on Earth would weigh 61.6 kg on Venus.

Mars is quite a bit smaller than Earth with only 11% of our planet’s mass. Mars is larger than Mercury, but it is not as dense as the smaller planet. If you weighed 68 kg on Earth then you would weigh 25.6 kg on Mars. Since Pluto was demoted to a dwarf planet, Mars became the planet where you would weigh the least.

Jupiter is the largest planet in our Solar System with the most mass. Because of Jupiter’s mass, you would weigh more on that planet than on any other one in our Solar System. If you weighed 68 kg on Earth then you would weigh 160.7 kg on Jupiter, over twice your normal weight. That is if you could actually stand on Jupiter’s surface, which is impossible because it is a gas giant, and gas giants do not have solid surfaces.

Saturn is a gas giant best known for its planetary rings system. It is also the second biggest planet in our Solar System. Despite its mass though, the planet has a very low density and a lower gravity than Earth. If you weighed 68 kg on Earth, you would weigh 72.3 kg on Saturn.

Uranus is a gas giant without a solid surface. Although Uranus is larger in size than Neptune, it has less mass and therefore less gravity. You would only weigh 60.4 kg on Uranus, if you weighed 68 kg on Earth.

Neptune, the last planet in our Solar System, is a gas giant. If you weighed 68 kg on Earth, then you would weigh 76.5 kg on Neptune if you could stand on the planet’s surface.

Although the Moon is not a planet, it is one of the few objects that astronauts have actually visited. Because the Moon is so small, it has a low density and low gravity. If you weighed 68 kg on Earth, then you would only weigh 11.2 kg on the Moon.

Universe Today has a number of articles to check out including weight on the moon and mass of the planets.

If you are looking for more information then determine your weight on other planets and facts about the planets.

Astronomy Cast has an episode on gravity.

Posted on by Abby Cessna

Plane of the Ecliptic

Solar eclipse. Credit: NASA

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Plane of the ecliptic, also known as the ecliptic plane, is a phrase you will often hear in astronomy. A basic definition is that the plane of the ecliptic is the plane of the Earth’s orbit, but that does not mean much to most people. Space is a three-dimensional vacuum, which you can think of as a kind of pool with the planets suspended in it. The Earth orbits the Sun on a particular angle and its orbit is elliptical in shape. The orbit is often shown as an ellipse made of dotted lines with the Sun at its center. If you made this ellipse a solid surface and extended it infinitively, then you would have the plane of the ecliptic. Actually our entire Solar System can be thought of as flat because all of the planets’ orbits are near or on this plane.

The ecliptic plane is used as the main reference when describing the position of other celestial objects in our Solar System. The angle between the plane of the ecliptic and the plane of an orbit is called the inclination. Until it was stripped of its status as a planet, Pluto was the planet with the most extreme inclination – 17°. Mercury is the only other planet with a significant inclination of 7°. There is also a 7° inclination between the plane of the Sun’s equator and the ecliptic plane. There are other celestial bodies that have a much greater inclination than any of the planets, such as Eris with a 44° inclination or Pallas with a 34° inclination.

The ecliptic plane got its name from the fact that a solar eclipse can only happen when the Moon crosses this plane to block out the Sun. Our Moon crosses the ecliptic about twice a month. A solar eclipse occurs when a new Moon crosses the ecliptic, and a lunar eclipse occurs when a full Moon crosses it.

Seasons on Earth are caused by our planet’s axial tilt of 23.5°, which causes variations in the amount of sunlight different parts of the Earth receive. This goes for all the other planets too. For example, Uranus rotates on its side with an axial tilt of 97.8°, which results in extreme variations in its seasons. The eclipse is also home to the constellations of the zodiac. There are twelve constellations in the zodiac, which are important symbols in astrology and can also be found in the Chinese calendar.  Here’s a list of all the zodiac symbols.

Universe Today has a number of articles including Virgo one of the zodiac signs and axial tilt.

You should also check out these articles on the ecliptic plane and ecliptic facts for more information.

Do not forget to tune into Astronomy Cast’s episode about the planet’s orbits.

Reference:
NASA: The Path of the Sun, the Ecliptic

Posted on by Abby Cessna

Minor Planets

Main Belt Asteroids
Ceres, the recently promoted dwarf planet in the asteroid belt is still too small to be easily seen by Hubble credit: NASA/ESA/STScI

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Minor planet is a term used to refer to a celestial object – that is not a planet or comet – which orbits the Sun. Found in 1801, Ceres, also known as a dwarf planet, was the first minor planet discovered. The term minor planet has been in use since the 1800’s. Planetoids, asteroids, and minor planets have all been used interchangeably, but the situation became even more confusing when the International Astronomical Union (IAU) committee reclassified minor planets and comets into the new categories of dwarf planets and small solar system bodies. At the same time, the IAU created a new definition of what a planet is, and Pluto was reclassified as a dwarf planet. Hydrostatic equilibrium – the ability to maintain a roughly spherical shape – is what separates dwarf planets from the more irregularly shaped small solar system bodies. The names become even more confusing because the IAU still recognizes the use of the term minor planets.

Minor planets are extremely common with over 400,000 registered and thousands more found each month. Approximately 15,000 minor planets have been given official names while the rest are numbered. When asteroids were first discovered, they were named after characters from Greek and Roman mythology like Ceres was. At first, astronomers thought that the asteroids, especially Ceres and Pallas were actually planets. Astronomers also created symbols for the first asteroids found. There were symbols created for 14 asteroids and some of them were very complex like Victoria’s symbol, which looks like a plant with three leaves growing out of an off center starburst. Soon, astronomers ran out of mythological names and started christening asteroids after television characters, famous people, and relatives of discoverers. Most names were feminine, attesting to an unnamed  tradition. As the numbers ran into the thousands, scientists started using their pets as inspiration. After an asteroid was named 2309 Mr. Spock, pet’s names were banned. That did not stop the oddness though because names such as 9007 James Bond and 6402 Chesirecat have been suggested and actually accepted.

There are a number of different categories that minor planets fall into including asteroids, Trans-Neptunian objects, and centaurs. There  are various types of asteroids, although most of them can be found in the asteroid belt, which is the region of space between Mars and Jupiter. Trans-Neptunian objects are celestial bodies found orbiting beyond Neptune, and centaurs are celestial bodies with unstable orbits located between Jupiter and Neptune. The categories also overlap, making classifying things a nightmare. For example, Ceres is a dwarf planet and minor planet, additionally it can also be classified as an asteroid.

Universe Today has a number of articles including astronomers find new minor planet and why Pluto is no lone a planet.

You can also check out these articles on asteroids and the solar system.

Astronomy Cast has an episode on the asteroid belt you will want to listen to.

Reference:
Wikipedia

Posted on by Abby Cessna

Habitable Planet

Habitable zone

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The term “habitable planet” seems rather broad. Does it mean that it is habitable for humans? Is it merely capable of supporting some other form of life? Quite simply, planetary habitability refers to a planet’s ability to both develop and sustain life.

Unfortunately, scientists have had to base their calculations for a habitable planet on Earth’s characteristics and do some guesswork. Some of the factors that astronomers look at when evaluating a planet’s habitability are mass, surface characteristics, orbit, rotation, and geochemistry.

One of the most basic assumptions that astronomers make when searching for a habitable planet is that it has to be terrestrial. This means that the planet is composed mostly of rock and metal and has a solid surface. A gas giant on the other hand has no solid surface, which makes it an unlikely candidate for supporting life. Mass is also an important factor, because low mass planets have too little gravity to keep their atmosphere. They also do not have live volcanoes and other geologic activity, which helps temper the surface to support life, because they lose energy as a result of a small diameter. Planets with high orbital eccentricity – the irregularity of the orbit – have a greater fluctuation in surface temperatures because they are closer to the Sun at some points and much further away at other points in the orbit. In order to be habitable, a planet has to have a moderate rotation. If there is no axial tilt then there are no change of seasons, and if the axial tilt is too severe than the planet will have a difficult time achieving homeostasis – balance. Another assumption astronomers make when determining planetary habitability is that life on other planets will also be carbon-based. The four elements most important for life are oxygen, nitrogen, carbon, and hydrogen. With so many considerations, it is not surprising that scientists have a difficult time determining whether a planet can sustain life.

Astronomers are searching for habitable planets in other solar systems too. They have started by searching in the habitable zones of other solar systems. A habitable zone is the region in space with conditions most favorable for supporting life. Astronomers are unsure exactly what the extent of the habitable zone of our Solar System is. Earth is located in the center of it, but it may even extend as far as Mars, and it almost reaches Venus. The habitable zone and planetary habitability focus on carbon-based life, so they do not help predict other forms of life.

Universe Today has a number of articles you should take a look at including the habitable zone and number of habitable planets.

You should also check out habitable planets and habitable planets are common.

Astronomy Cast has an episode on the search for water on Mars, which tells why finding water is a clue to finding life.

Posted on by Abby Cessna

Discovery of the Planets

Sir William Herschel

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We actually only know the exact date of when a few of the planets were discovered. Five of the planets, not including Earth, have been known to exist for thousands of years  – Mercury, Mars, Venus, Jupiter, and Saturn. The Ancient Greeks and Romans wrote about the planets many centuries ago. Because the planets look like stars to the naked eye, that is where the term planets comes from. Because the planets move in the sky, they were termed wandering stars. The term planet comes from the Greek word for wanderer, “planetes.” Many ancient people thought that the planets were gods, so they gave them the names of their gods. All of the planets, except Earth have names of Roman deities.

The other three planets – Uranus, Neptune, and Pluto – were not discovered until at least the 1700’s. Pluto is no longer a planet since it was reclassified as a dwarf planet in 2006. It was known as the ninth planet for 70 years though, so its discovery will be included here. Uranus was discovered in 1781 by the famous astronomer Sir William Herschel, although that was not the first sighting of it. The planet had been sighted as early as 1690 by the English astronomer John Flamsteed. It was also sighted by Pierre Lemonier in the mid 1700’s. Sir Herschel at first thought that Uranus was a comet, but he noticed the irregularities early on and compared it to a planet in his notes.

Because Neptune cannot be seen without the help of a telescope, it was not discovered until after 1610, when Galileo created the telescope. Alexis Bouvard, a mathematician, saw that another planet had to be affecting Uranus’ orbit, so astronomers started looking for it. Two astronomers, John Couch Adams and Urbain Le Verrier, discovered Neptune independently or rather made the calculations and determined where Neptune could be found. The planet turned out to be 1° from Verrier’s calculations and 12° from Adams’. There was a dispute between France and England over who discovered the new planet because Adams and Verrier are from England and France respectively.

Pluto was the last planet discovered, although that distinction returned to Neptune when Pluto was reclassified as a dwarf planet. Pluto was discovered in 1930 by the astronomer Clyde Tombaugh. Many people had been searching for a ninth planet – the elusive planet X – for quite a while. Since Pluto was discovered near the calculated location of planet X, they thought the two planets were one and the same. Later, astronomers realized that there was no such planet X.

Universe Today has a number of articles on the planets including who discovered Neptune and the planets of our Solar System.

Check out these other articles including mathematical discovery of the planets and the planets.

Astronomy Cast has episodes on all of the planets including Mercury.

Posted on by Abby Cessna

Names of the Planets

Planets and other objects in our Solar System. Credit: NASA.

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You may recognize the names of the planets from your high school literature course or a history class. That is because many of the planets were first discovered by ancient civilizations, and so planets are named after their gods.

The Romans named Mercury after the messenger of the gods because it appears to move so quickly.

Venus was named after the Roman goddess of love because of its shining presence. The planet is the brightest object in the sky beside the Moon and the Sun. A number of other cultures also named Venus after their own gods or goddesses of love and war.

Earth is the only planet not named after a god. The name is based on Germanic and Old English words for “ground.”

Mars was named after the Roman god of war because of its red color, which reminded people of blood. Other civilizations also had names for the planet based on its color. The Egyptians called it “Her Desher,” which means “the red one.”

Jupiter was named after the king of the gods – Zeus by the Greeks and Jupiter by the Romans. Ancient civilizations most likely named this planet after the most powerful god because of its size. Jupiter is the largest and most massive planet in our Solar System.

Saturn was named after the father of the king of gods as well as being the god of agriculture and harvest. In mythology, Saturn had taken the position of king of the gods from his own father, Uranus, and then Jupiter overthrew him. Saturn is the last planet that can be seen from Earth without the aid of a telescope.

Uranus was not discovered until 1781 by Sir William Herschel, so it was not necessarily going to be named after a Roman god. In fact, Herschel named the planet “Georgium Sidus” in honor of George III who was King of England at the time. Others called the planet Herschel in honor of the astronomer who had discovered it. The name Uranus, which is the name of the Roman god who is the father of Jupiter, was suggested by the astronomer Johann Bode. That name was widely accepted in the mid 1800’s, and it fit in with the other planets, which all had names from mythology.

Neptune had been observed by a number of astronomers, but they believed it was a star. Two people, John Couch Adams and Urban Le Verrier, calculated the planet’s location. Johann Galle, the astronomer who discovered the planet using Verrier’s calculations, wanted to name the planet after Verrier. Many astronomers objected though, so it was named after Neptune the Roman god of the sea. The name was very fitting because the planet is a bright sea blue.

Universe Today has a number of articles on the planets including facts about the planets and the planets of the solar system.

If you are looking for more information on the planets take a look at the planets and interesting facts about the planets.

Astronomy Cast has episodes on all of the planets, so start with Mercury.

Posted on by Abby Cessna

Planetary Alignment

Planets and other objects in our Solar System. Credit: NASA.

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Thousands of years ago, men looked to the planets to guide them. Planetary alignments foretold momentous events of good and evil. In those days, planning wedding,s or even assassinations, according to the planets and stars was common in many cultures. Many people still look to the planets to tell the future. But what really is a planetary alignment and how does it happen?

There are two different ways that the term alignment is used referring to the planets in our Solar System. The first way is if you were standing on the Sun, all the planets would appear to be lined up in a row. This is what most people think of when the they hear the term planetary alignment. The other kind of alignment is if all the planets follow a straight line. In other words, it is as if a straight line can be drawn through the center of all the planets.

It is impossible for an exact alignment as viewed from the Sun – where the planets are superimposed – because of the differences in axial tilts of the planets. What we actually see in the sky is planetary configuration where the planets are in the same quadrant. It is very rare for all the planets to be within the exact same quadrant – approximately 90° of each other. Having all the planets within about 90° of each other only occurs about every 200 years, which is a long time for humans, although it is nothing for the universe. The planets do however line up loosely more often than that.

In 2000, five of the planets – Mars, Saturn, Jupiter, Mercury, and Venus were within 50° of each other. You may have heard a lot about future planetary alignments, including rumors about 12/21/2012 when some say the world is going to end, and there will be a planetary alignment. That is all it is, a rumor. There will be no planetary alignment in 2012. Many of the planets will align in a rather straight line in 2010. On June 13, 2010, Uranus, Jupiter, and Mercury will be lined up on one side of the Sun while Venus, Mars, and Saturn will be lined up on the other side. Earth will be perpendicular to the Sun at this time. In September of the year 2040, four of the planets – Saturn, Venus, Jupiter and Mars – will be roughly aligned when seen from Earth. Check out when other planetary alignments will happen, and then search for them in the sky.

Check out this site about planetary alignments and this site, which shows you the position of the planets for any date you enter.

Universe Today has a number of articles to read including galactic alignment and planets line up.

Astronomy Cast has an episode about planetary alignment.

Posted on by Abby Cessna

What is Interplanetary Space?

The heliosphere Credit: NASA/Feimer)

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The region of space within our Solar System is called interplanetary space, also known as interplanetary medium. Most people are so fascinated by the planets, Sun, and other celestial objects that they do not pay any attention to space. After all, there is nothing in outer space right? A common misconception is that outer space is a perfect vacuum, but there are actually particles in space including dust, cosmic rays, and burning plasma spread by solar winds. Particles in interplanetary space have a very low density, approximately 5 particles per cubic centimeter around Earth and the density decreases further from the Sun. The density of these particles is also affected by other factors including magnetic fields. The temperature of interplanetary medium is about 99,727°C.

Interplanetary space extends to the edge of the Solar System where it hits interstellar space and forms the heliosphere, which is a kind of magnetic bubble around our Solar System. The boundary between interplanetary space and interstellar space is known as the heliopause and is believed to be approximately 110 to 160 astronomical units (AU) from the Sun. The solar winds that blow from the Sun, and are part of the material in interplanetary space, flow all the way to the edge of the Solar System where they hit interstellar space. The magnetic particles in these solar winds interact with interstellar space and form the protective sphere.

The way that interplanetary space interacts with the planets depends on the nature of the planets’ magnetic fields. The Moon has no magnetic field, so the solar winds can bombard the satellite. Astronomers study rocks from Earth’s Moon to learn more about the effects of solar winds. So many particles have hit the Moon that it emits faint radiation. Some planets, including Earth, have their own magnetospheres where the planets’ magnetic fields override the Sun’s.  The Earth’s magnetic field deflects dangerous cosmic rays that would otherwise damage or kill life on Earth. Material leaking from the solar winds is responsible for auroras in our atmosphere. The most famous aurora is the Aurora Borealis, which appears in the sky and is only visible in the Northern Hemisphere.

Interplanetary medium also causes a number of phenomena including the zodiacal light, which appear as a faint broad band of light only seen before sunrise or after sunset. This light, brightest near the horizon, occurs when light bounces off dust particles in the interstellar medium near Earth. In addition to interplanetary space, there is also interstellar space, which is the space in a galaxy in between stars.

Universe Today has a number of articles on space including the heliosphere and zodiacal light.

Check out these articles from NASA on the heliosphere and sunspots leaking plasma into interplanetary space.

Astronomy Cast has an episode on the heliosphere and interstellar medium.

References:
NASA: Heliosphere
NASA Voyager: Interstellar Mission
What’s It Like Where Voyager Is?