Author: Jerry Coffey

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If you give most people enough time to think, they can come up with the names of all of the planets. Most will still throw in Pluto, despite its demotion. What those same people will have difficulty with is the Solar System order. It can be difficult to remember which planet is where. The current Solar System order is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The best thing to do is come up with a handy mnemonic. The most common one taught in school used to be My Very Educated Mother Just Served Us Nine Pizzas, then Pluto lost the prestige of planetary status. Most educators have cast about for another, just as handy mnemonic device. This one should work just fine: My Very Educated Mother Just Served Us Noodles. Of course, both devices assume that everyone knows that the Sun is at the center of our Solar System.

Here are a few details about each planet that might be of interest to you.

Mercury has a tenuous atmosphere, so, despite being the planet closest to the Sun, it is unable to retain the heat it is exposed to. The temperature ranges by a few hundred degrees Celsius each Mercurian day.

Venus has a thick atmosphere and an average surface temperature of 460 degrees Celsius. If you were standing on Venus, you would choke on the high amounts of carbon dioxide as your skin dissolved in the sulfuric acid rain.

Earth needs no introduction or details.

Mars is perhaps the most studied planet besides Earth. It has a nearly nonexistent atmosphere, so it is a cold world. Temperatures are about -140 Celsius in the winter. At the height of summer you could not comfortably wear shorts.

Jupiter is 2.5 times as massive as all of the other planets in the Solar System combined. Jupiter has 63 recognized moons, but more are thought to be in orbit. That accounts for about 1/3 of the moons in our Solar System.

Saturn is a contradiction. It is the second largest planet, yet it has a very low density. It would float if you had enough water to put it in. There are 60 acknowledged moons orbiting Saturn.

Uranus is tilted like crazy. All planets are slightly tilted on their axis, but Uranus is tilting at 98 degrees.

Neptune, is last, but not least. It orbits an average of 4.5 billion km from the Sun. It was discovered in 1846, making it the most recent recognized planet to be discovered.

And there you have the Solar System order, a way to remember it, and a few interesting facts about each planet. We encourage you to explore the NASA website to get more information.

Here’s an article from Universe Today that goes into more detail for each of the planets in the Solar System.

Here’s an article from the BBC with several suggestions down in the comments. And here are some more.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.

Reference:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys

[/caption]The largest asteroid in the Solar System is 4 Vesta. Ceres is much more massive, but has been promoted to dwarf planet status, leaving Vesta the largest asteroid. Ceres and Vesta will be orbited and studied by the Dawn spacecraft.

Vesta was first discovered on March 29, 1807 by Heinrich Wilhelm Olbers. The asteroid measures 578 km by 458 km and has a mass of 2.67 x 10²⁰ kg. It has a magnitude of +5.4 to +8.5 and can be easily observed with binoculars on a clear night. It has been seen with the unaided eye on several occasions. Vesta rotates on its axis every 5.342 hours and has an axial tilt of 29º. Temperatures on the surface range from a frigid -188ºC (85 K) to -18ºC (255 K). Hubble images have revealed ancient lava flows. This is a direct contradiction of the belief that asteroids are simple cold, dead rocks floating in space. There is a gigantic impact basin so deep that it exposes the asteroid’s mantle at the South pole. The mantle is thought to be 10 km below the asteroid’s surface.

Several NASA scientists have concluded that Vesta is the parent body of many meteorites. That means that we have parts of only five celestial bodies here on Earth: Earth(obviously), the Moon, Mars, Vesta, and the comet Wild 2. Vesta is the parent body of the eucrite meteorite group. The group formed approximately 4.56 billion years ago. Many of them metamorphosed to temperatures up to 800° C and were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface, but presumably escaped later brecciation. The cumulate eucrites formed at a depth where metamorphism may have persisted for an undetermined amount of time. These meteorites may have originated from the large impact at the south pole of the asteroid.

The Dawn mission is designed to be the first spacecraft to orbit two non-Earth objects. It arrived in orbit around Vesta on July 15, 2011. It will study the largest asteroid in the Solar System for about a year before leaving orbit for Ceres in 2012. Vesta was chosen as a destination because of its unique qualities. It accounts for 9% of the mass in the main asteroid belt and it is an evolved object(has a mantle, core, and crust). NASA scientists fully expect to make several interesting discoveries from the study of Vesta. Be sure to check back later for updates.

Here’s an article about how Vesta formed fast and early in the Solar System, and some Hubble images of the asteroid.

Here’s more on Vesta from Solar Views, and some images from NASA.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.

Sources:
http://research.jsc.nasa.gov/PDF/Ares-6.pdf
http://www.nasa.gov/multimedia/podcasting/jpl-cassini-20080428.html
http://www.nasa.gov/mission_pages/dawn/news/dawn20110716.html
http://www.nasa.gov/mission_pages/dawn/news/dawn20110329.html

Defining the diameter of the Solar System is a matter of perspective and characterization. You can look at the Solar System’s diameter as ending at the aphelion of the orbit of the farthest planet, the edge of the heliosphere, or ending at the farthest observable object. To cover all of the objective bases, we will look at all three.

Looking at the aphelion(according to NASA figures) of the orbit of the farthest acknowledged planet, Neptune, the Solar System would have a radius of 4.545 billion km and a 9.09 billion km diameter. This diameter could change if the dwarf planet Eris is promoted after further study.

Sedna is three times farther away from Earth than Pluto, making it the most distant observable object known in the solar system. It is 143.73 billion km from the Sun, thus giving the Solar System a diameter of 287.46 billion km. Now, that is a lot of zeros, so let’s simplify it into astronomical units. 1 AU(distance from the Earth to the Sun) equals 149,597,870.691 km. Based on that figure, Sedna is nearly 960.78 AU from the Sun and the Solar System is 1,921.56 AU in diameter.

A third way to look at the diameter of the Solar System is to assume that it ends at the edge of the heliosphere. The heliosphere is often described as a bubble where the solar wind pushes against the interstellar medium and edge of where the Sun’s gravitational forces are stronger than those of other stars. The heliopause is the term given as the edge of that influence, where the solar wind is stopped and the gravitational force of our Sun fades. That occurs at about 90 AU, giving the Solar System a diameter of 180 AU. If the Sun’s influence ends here, how could Sedna be considered part of the Solar System, you may wonder. While it is beyond the heliopause at aphelion, it falls back within it at perihelion(around 76 AU).

Those determinations of the diameter of the Solar System may seem about as clear as mud, but they give you an idea of what scientists are trying to place a definitive value on. The distances involved are mind boggling and there are too many unknowns to place a absolute figure. Perhaps, an exact number will be determinable as the Voyager probes continue their outward journey.

Here’s an article on Universe Today about the closest star to Earth, and another about how long it would take to travel to the closest star.

Here’s an article from the Physics Factbook about the diameter of the Solar System, and a cool way to visualize it using the Earth as a peppercorn.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.

References:
Neptune Fact Sheet
NASA: Planet-Like Body Discovered at Fringes of Our Solar System
NASA Science: Heliophysics
Wikipedia

The answer to ‘how many stars are in the Solar System’ is pretty straightforward, or is it? There is only one star that has ever been observed in our solar system, but some scientists have theorized that there is a second star out beyond the Oort Cloud that only comes close enough to be observed every 32 million years. That length of time between observational periods would explain why a human has never proven its existence.

As scientists explore our galaxy, it seems that ours is a somewhat unique solar system in many ways. Most do not have as many orbiting bodies and very few are single star systems. A majority have at least two stars(binary). A system could theoretically have an unlimited amount of stars. Systems with up to six stars have been observed.

Now, a little more about the theoretical companion star within our our solar system. The other star would have to be a red or brown dwarf and has been given the name Nemesis. In 1984, a pair of scientists, Raup & Sepkoski, claimed that mass extinctions, like the one that killed the dinosaurs, occur every 32 million years. The most widely held theory for the demise of dinosaurs is an asteroid or cometary impact, so the length of time would suggest that some mechanism is needed to disturb the comets in the Oort Cloud every 32 million years. Richard Muller, among others, hypothesized that a companion that orbits the Sun in that period could be the explanation. To prove their theory, Muller and a few colleagues embarked on a search for Nemesis. The team ran into this hurdle immediately; ‘Every star of the correct spectral type and magnitude must be scrutinized. … We are currently scrutinizing 3098 fields, which we believe contain all possible red dwarf candidates in the northern hemisphere.’ With nearly 3,100 possibilities in the Northern Hemisphere alone and a limited number of clear observational days, it is easy to see how daunting this task is.

Just to be clear, there is no evidence of any kind that makes scholars think that there is a companion star in our Solar System. It is a theory based solely on a need to explain the periodic mass extinctions that our planet has experienced. So, the only answer to ‘how many stars are in the Solar System’ that can be proven through observation is one…the Sun.

Here’s an article about a possible Planet X, and how it could disrupt the Solar System (and how it probably doesn’t exist), and an article about how multiple star systems come together.

Here’s Wikipedia’s entry on Nemesis, and another answer to the question from NASA.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.

References:
NASA Ask an Astrophysicist
Nineplanets.org
Wikipedia

How old is the Solar System? That is a question that cuts to the heart of it all. By studying several things, mostly meteorites, and using radioactive dating techniques, specifically looking at daughter isotopes, scientists have determined that the Solar System is 4.6 billion years old. Well, give or take a few million years. That age can be extended to most of the objects and material in the Solar System.

The United States Geological Survey(USGS) website has a lot of indepth material about how the age of the Solar System was determined. The basics of it are that all material radioactively decays into a stable isotope. Some elements decay within nanoseconds while others have projected half-lives of over 100 billion years. The USGS based their study on minerals that naturally occur in rocks and have half-lives of 700 million to 100 billion years. These dating techniques, known as radiometric dating, are firmly grounded in physics and are used to measure the last time that the rock being dated was either melted or disturbed sufficiently to re-homogenize its radioactive elements. This techniques returned an approximate age for meteorites of 4.6 billion years and Earth bound rocks around 4.3 billion years. The USGS admits that they were unable to find any rock that had not been altered by the Earths tectonic plates, so the age of the Earth could be refined in the future.

When the gasses of the early solar nebula began to cool, the first materials to condense into solid particles were rich in calcium and aluminum. Eventually solid particles of different elements clumped together to form the common building blocks of comets, asteroids, and planets. Astronomers have long thought that some of the Solar System’s oldest asteroids should be more enriched in calcium and aluminum, but, none had been identified until recently. The the Allende meteorite of 1969 was the first to show inclusions that were extremely rich in calcium and aluminum. It took 40 years for the spectra of the inclusions to be discovered and then extrapolates to very old asteroids still in orbit around the Sun. Astronomer Jessica Sunshine and colleagues made this discovery with the support of NASA and the National Science Foundation

Additionally, the Universe is thought to have been created about 13.7 billion years ago. Measuring two long-lived radioactive elements in meteorites, uranium-238 and thorium-232, has placed the age of the Milky Way at in the same time frame. From these measurements, it appears that large scale structures like galaxies formed relatively quickly after the Big Bang.

Here’s an article from Universe Today that gives more information about the radioactive dating process of studying meteorites, and another article about how the solar nebula probably lasted about 2 million years.

Here’s a great article from the USGS that explains how the dating process works, and a great series from UC San Diego.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.

References:
U.S. Geological Survey
NASA: How Old is the Universe?
NASA Earth Guide: Age of the Solar System

It takes Saturn 10,832 Earth days to complete one orbit around the Sun. That means the answer to ”how long is a year on Saturn” is 29.7 Earth years. The length of Saturn’s year is a direct effect of its orbital distance from the Sun. Saturn orbits at an average of 1.43 billion km, or 9.58 AU, from the Sun.

Knowing how long a year is on Saturn might make one wonder if the planet experiences seasons like we do here on Earth. Yes, Saturn experiences seasons. Saturn has an axial tilt of 26.73 degrees, allowing different hemisphere to experience varying levels of sunlight. Of course, the seasons only go from cold to a whole lot colder. Also, the seasons last nearly 30 times longer because of the length of the planet’s year. Can you imagine a seven year summer that never reaches higher than -23 C?

The length of a day on Saturn is 10.656 hours. While that number seems to be pretty precise, it took a lot of study to arrive at that figure. There is no way to observe the planet’s surface region, so a way had to be found to estimate the planets rotational speed. Scientists first turned to radio emissions for an estimate, then observation by space craft. They then found that the rotational period varied by as much as 1% over the span of a week. The current stated length of a day on Saturn is an average from all observations.

Saturn’s movement through its orbit occasionally causes its rings to disappear. The phenomenon is called ”ring plane crossing”. Ring plane crossings occur when the tilt of the planet and its position in its orbit combine to allow a side-on view of the rings. The rings seem to disappear, but, without the glare from the rings, the planet’s moons are more easily observed. Also, these crossings are the best time to see Saturn’s blue north pole.

29.7 Earth years is the answer to ”how long is a year on Saturn”, but it leads to many other questions about our mysterious neighbor. Direct observation is the answer, but there have only been four missions to visit the planet as of today(October, 2011). The Casinni-Huygens mission is currently in orbit sending data on a regular basis. Hopefully, it will expand our knowledge of Saturn beyond expectations.

Here’s an article that discusses how Saturn’s rings can seem to disappear, and here’s how long a day is on Saturn.

Here’s a great photo collage of Saturn’s rings seen at various angles to the Earth, and some general Saturn facts.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

Sources:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Saturn
http://www2.jpl.nasa.gov/saturn/faq.html#what