Introduction and FAQ
The Nine Planets is a collection of information about our Solar System
intended for a general audience with little technical background.
No special expertise or knowledge is needed;
all technical and astronomical terms and proper names are
defined in the glossary.
The bulk of this material should be familiar to planetary scientists and
astronomers but they may find a few interesting tidbits, too.
This site consists of about pages, one page for each major body
in the Solar System. Each page has:
- a large picture of its object and usually several smaller thumbnail images (all linked to their full-size originals)
- some scientific and historical facts about it,
- if the object has satellites
then its page has a table of data on them and links to their pages,
- links to more images and
information about the object elsewhere on the Web, and
- a list of open issues for which we as yet have no answers.
To truly justify the title of "Multimedia Tour", I've also included:
- short sound clips from
(about 10 seconds or 180k each) for seven of the planets;
- sound clips of my mellifluous voice pronouncing some of the more unusual
- links to "movies" of a few objects.
There are also a few miscellaneous pages: on planetary science spacecraft, the
glossary, a list of some of the planetary images available elsewhere on the Net,
some bits of history, several pages of data and a special
plea for your support of the space program.
The pages of this document are organized in a hierarchy based on the
primary-satellite relationship. In addition, there are many hyperlinks enabling
the interactive viewer to jump around and view the pages in many ways. (If you
"get lost" you can always jump back to the table of contents.)
At the bottom of each page is a set of links to other related pages.
To visit the next body in an ordered traversal of the solar system choose the
link immediately to the right of the name of the current page. You can also go back
to the previous page, the "parent" page, the table of contents or to the
detailed data page.
I've chosen ten of the most interesting bodies and linked them into an
Express Tour. If you don't have time for the full
tour, don't miss these.
And if you want to read offline or just explore in more depth, visit The Nine Planets Bookstore or the "Hardcopy" links found at the right side of some pages.
Q: May I use your pictures for my class project?
A: Yes. For other uses, please see my copyright page.
Q: May I make a link to your site?
The New Solar System
Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
Print version of Phil Plait's excellent website. Get the straight story on many popular urban legends, myths and misconceptions. Great fun, too!
The Demon-Haunted World
Carl Sagan's plea for reason in an irrational world.
A: Yes. You may make links to the whole site or any of the individual pages.
Q: When will the planets next line up? Will there be a disaster?
A: Never. It
can't happen. And even if it did it would be no problem. The planets
are too small to have a significant gravitational effect on the Earth.
See the following for more information:
Phil Plait's Bad Astronomy: Review: Tomb Raider
Planets Alignments in 2000 - Griffith Observatory
Planets Alignments: Fact or Fiction?
An excellent article from the National Solar Observatory
Q: Is there a tenth planet (Planet X)? Will it destroy the Earth?
A: No. It is remotely possible that there might be a body out
there beyond Pluto that is large enough to be called a planet but we have
no evidence for such a thing nor any reason to suspect one might be there.
Careful searches have been made and nothing has been found. If there's a
large planet out there it must be very far out. (There are lots of smaller
bodies, though, see my page about the
For more interesting stories about this topic see the Hypothetical Planets page by Paul Schlyter.
Q: Is Pluto a really planet?
A: Yes. The official arbiter of such questions is the IAU and they have decided that Pluto should be classified as a planet. There are good arguments on both sides of this issue, however.
Q: What's the definition of "planet", anyway?
A: There is no good generally accepted definition of the term "planet".
As far as our own solar system is concerned there are good historical reasons to just define "planet" as "one of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto". Clearly, this isn't very scientific. But any more scientific definition that isn't totally arbitrary is likely to result in a different set of objects included as "planets" and thus confuse the traditional use of the word. And it just gets worse when we consider the planet-sized objects we've recently discovered orbiting other stars.
It's not really very important how we classify the various objects out there. What's important is to learn about their physical nature and their origins.
Here are a few links about this topic:
news report from UC Berkeley
IAU extra solar planet definition
Dr. Basri's links
Q: What about Sedna?
The newly discovered object 2003 VB12 aka "Sedna" has a diameter of about 1800 kilometers, slightly smaller than Pluto (2300km). (This number has a pretty large uncertainty, we are as yet unable to measure its diameter directly.) This is clearly not big enough to qualify as a "planet" by most definitions.
It is nevertheless a very interesting object.
For more information see here.
Q: Where can I buy calendars, large prints and posters with astronomical images?
A: Try online stores of
The Planetary Society
Astronomical Society of the Pacific.
Q: What's the mnemonic for remembering the order of the planets?
A: I know of two:
"My very excellent mother just sent us nine pizzas."
"Most voters earn money just showing up near polls!"
Q: What does the "e" in some of your numbers mean?
A: "times ten to the power".
So "1.2e3" means "1.2 times ten to the power 3" or "1200". Or you can think of it as moving the decimal place to the right the number of digits after the "e". You can see why we use this notation when the number after the "e" is large: "1.23e18" means "1230000000000000000".
Q: What's the bright thing I'm seeing in the sky?
A: The easiest way to figure out questions like this is with a
There are lots of good ones. Some of are are fancy and expensive and intended for serious
amateur astronomers; some are free; almost all will answer the basic questions.
My favorite is Starry Night (for both Mac and PC).
Q: How far away from Earth is Mars (or some other planet) right now?
A: That is difficult to answer without a computer. You cannot simply
subtract the average distance from the Sun of the two planets since it's only at one special time
when they're both in a line with the Sun. Fortunately, most
will do the trick.
Q: How do you know that the world is so old?
A: There are many lines of evidence all converging on an age for the whole universe
of 13.7 billion years
and 4.6 billion years for our solar system and the Earth.
A detailed answer is beyond the scope of this FAQ.
There's a nice introduction at
Q: Why are the planets round?
A: Think about what it would mean if a planet
were not round (spherical). That would mean that some places on
the surface are farther from the center than others (ie there would
be mountains). As we know mountains do exist on the planets. But
even the largest ones must be small compared to the radius of the planet
(the height of tallest mountain in the solar system, Olympus Mons on
Mars, is less than 1% of Mars's radius). Why? Because if it were
very much taller the rocks at the bottom would not be strong enough
to hold it up. They would bend like plastic. Rock is simply not
strong enough to support a mountain that is large compared to a
planet. But on a smaller body, like some of the smaller moons and
asteroids, the force of gravity is much weaker and so rock can
support relatively large "mountains".
Q: Why do all the planets orbit in (approximately) the same plane?
A: Because the Sun and the planets were originally condensed out of a
spinning nebula of gas and dust. As it collapesed, the cloud flattened into a disk with the
Sun at the center and the planets formed farther out.
But why did all the dust (and gas, the vast
majority of the mass) end up in a plane? Because if you start with a
rotating irregular blob, which is the usual case, then
collisions between the particles tend to average out the motions of
the particles. Thus the motions perpendicular to the spin equator get
zeroed out and the motions parallel to it get averaged to the general
Q: Who discovered Jupiter (Saturn, Mars, Mercury, Venus)?
A: Lots of people.
Jupiter (and the other "classical" planets) have been known since before the
beginning of history. This is not surprising since they are so bright and
easy to see
and so obviously different from the stars (since they move). See my
chronology page for more recent discoveries.
Q: Does our solar system have a name? Does our moon have a name? Does our sun have a name?
A: No. No. No. Sorry. They should. But they don't. At least not in English.
There are, of course, many words used to refer to the Sun and the Moon in
"Sol" and "Luna" are often thought of as proper names but they're really Latin, not English.
So far there hasn't been a need for anything more.
Maybe when we start living on other places besides the Earth....
Q: What's the deal with astrology?
A: It's simply nonsense, a way to separate fools from their money.
(The worst thing you can do to an astronomer is to call him an astrologer.)
To be fair, in the past astrology was a legitimate field of study. Some of the great
men of science, in particuler Johannes Kepler, were astrologers. But in modern times we have come to realize that the basic idea of astrology, that the positions of the planets influence life on Earth, is not true. But due to the vagaries of history some astrological terminology (ie the names of the constellations) has survived in astronomy and in popular culture ("What's your sign?").
Q: What about UFOs?
A: There's a huge amount written and talked about but don't believe everything you read. I have never seen any credible evidence for UFOs.
Q: My question isn't here...
A: If it's a question about terminology, check the glossary.
If it's a question about numbers, check the data.
Try Phil Plait's excellent site, Bad Astronomy. It's actually one of the best.
Other Solar System Info
There are several other collections of information about
the Solar System available on the World Wide Web:
- Views of The Solar System, by Calvin J. Hamilton formerly of the Los Alamos National Laboratory (now maintained at the Hawaiian Astronomical Society but still referred to here as "LANL")
- The Nine Planets - For Kids, a version of this site tailored for younger audiences
- Kids Astronomy, another nice astronomy site for a younger audience
- Universe Today - Space news from around the Internet, updated every weekday.
- StarChild, A learning center for young astronomers
- Welcome to the Planets from
Jet Propulsion Laboratory ("JPL"); direct from "the source"
- Exploring the Planets from the National Air and Space Museum
- The Solar System by Ken Edgett of Arizona State University ("ASU"); somewhat smaller than the above
- Sol Station
- Regional Planetary Image Facility at the Smithsonian Institution, Washington DC ("RPIF")
- The Solar System from the Royal Greenwich Observatory ("RGO")
- National Space Science Data Center Photo Gallery ("NSSDC") and a variety of information at the Planetary Sciences home page
- Virtual Solar System from the National Geographic Society
- NASA's Planetary Science Research Discoveries, readable but peer-reviewed articles on current research
- Planetary Tour Guide compiled by Gordon Johnston of NASA HQ.
- Browse the Solar System from USGS
- StarDate Guide To The Solar System from McDonald Observatory
- Planets and the Solar System, a resource list from SEDS
- Browse the Solar System (mostly data) from USGS Flagstaff
- Updates to Jay Pasachoff's textbook
- Class notes by Nick Strobel of Bakersfield College
- Class notes by Joseph Cain of Florida State University
- Our Solar System from NASA Spacelink
- Solar System Live,
the Interactive Orrery of the Web.
- The Celestial Times -- where to find the planets for the current month
- NASA's Twelve Year Planetary Ephemeris provides detailed and accurate geocentric positions
- Planetary Remote Sensing (part of the awesome Remote Sensing Tutorial by Nicholas M. Short Sr.)
- Planetary Tour Guide
- Windows to the Universe, from the University of Michigan (very nice)
- A Space Library, simulated views of the solar system, maps and more (from JPL)
Most of the inline pictures here come from these sources, especially the first two.
Without their efforts and the efforts of all the scientists and engineers at
NASA and JPL
this tour would not be possible.
Other Astronomical Pages
Where to go next
The full tour continues with the Overview
(or if you're in a hurry take the Express Tour).
The names at the bottom of each page provide access to the next world in the full
tour and a few other related pages; the icons provide access to the table of contents,
the pages of numerical data and this site's "home page".
Bill Arnett; last updated:
2004 Mar 16