Ask Us

For info on the planets please see either NASA JPL's Welcome to the Planets or the University of Arizona's The Nine Planets Web sites.

There's an online orrery at Fourmilab. An orrery is something that shows the positions of the planets. Mechanical orreries existed before there were computers (and still exist), but now most are done in software.

Drs. Louis Barbier and Eric Christian, and Ms. Beth Barbier
(March 2000)

This is beyond our area of expertise, but we can refer you to some other websites:

• NASA page on "Why We Explore"
• NASA JPL page on Pluto
• NASA page on Sedna

This one addresses the issue from an educational viewpoint:

• Astronomy Education Review article "Teaching What a Planet Is"

I hope this helps.

Beth Barbier
(May 2007)

The New Horizons spacecraft, currently on its way to Pluto, will take about nine years to get there. But you can read more about this distance on the Cosmic Distance Scale site, where you will see that the Solar System goes well beyond the planets.

Beth Barbier and Eric Christian
(January 2010)

This isn't in our area of expertise, but I was able to find a website with a list of solar system scale models.

The first one on the list looks very good: How Big is the Solar System?

Beth Barbier
(August 2005)

A magazine such as Sky & Telescope will give you monthly sky maps that will include the locations of the planets. Or you can check Sky & Telescope online or any of several planetarium programs on the web, such as the Fourmilab site.

Dr. Eric Christian

Planets move because the gas cloud that originally formed the solar system was spinning. The gas that wasn't spinning fell into the proto-Sun, and the planets formed from gas and chunks that were spinning enough that they were in orbit. Once the planets formed and were in orbit about the Sun, there was no reason for them not to stay there (objects in motion tend to remain in motion). The expansion of the universe is essentially negligible on the scale of the solar system, that is, it appears to have only a very, very small affect on the solar system.

Dr. Eric Christian

The reason planets mostly form in the same plane is that the first step in the transformation from a gas cloud to a solar system is that the cloud collapses down to a disk. This happens because of conservation of angular momentum (the cloud originally has some spin), and random collisions tend to damp out the random velocities that fill up the spherical cloud. It's only after the disk forms that the densities get high enough and the collisions get frequent enough that planets can form.

Dr. Eric Christian
(February 2011)

Gravity from the Sun is what keeps the planets in orbit around the Sun, just as gravity from the Earth is what keeps the Moon and satellites and the space shuttle in orbit around the Earth. The reason the Moon doesn't hit the Earth (and the Earth and other planets don't hit the Sun) is that the Moon is moving fast enough to miss the Earth.

If you were to stand on Mt. Everest and throw a rock (and forget completely about air resistance), it would travel a certain distance before hitting the Earth. As you throw the rock faster, it travels farther. Eventually, if you throw the rock fast enough, it travels all the way around the Earth and hits you in the back. It is still falling towards the Earth, but the surface of the Earth (because the Earth is round) is falling away just as fast. The rock is now in orbit. Continue throwing it faster and faster and the orbit gets bigger (farther from the Earth) on the far side of the Earth, but still comes around to hit you in the back. Eventually you throw the rock at what is called the "escape velocity" and it breaks away from the gravity of the Earth and never falls back.

So there are three ranges of sideways velocity:

1. Slow enough so that the object falls down and hits
2. Fast enough so that the object is in orbit, but not too fast
3. Really fast so that the object escapes

All of the planets (and the Moon around the Earth) are in category 2. If this seems strange, it's not. When the Solar System was forming, everything that was in category 1 actually fell into the Sun, and everything that was in category 3 escaped from the Solar System, leaving only the stuff (planets, asteroids, comets, etc.) in category 2.

Dr. Eric Christian
(January 2002)

There is no physical law that states that when an explosion occurs all particles will be spinning in the same direction. Angular momentum (the physical measure of spinning) will be conserved, so if the original object was spinning (had angular momentum), the sum of the angular momentums of all the "shrapnel" will equal the original angular momentum, so more will be spinning in the original direction. But individual "shrapnel" can be spinning in the opposite direction.

In the case of Venus and Uranus, the most reasonable theory is that a massive collision while they were forming caused them to spin opposite (in the case of Venus) or at right angles (in the case of Uranus) to the other planets and the Sun.

Dr. Eric Christian
(March 2000)

If you go to the website and click on the words "Rotation Period" in the left column, you can get the answer:

Rotation Period (hours) - This is the time it takes for the planet to complete one rotation relative to the fixed background stars (not relative to the Sun) in hours. Negative numbers indicate retrograde (backwards relative to the Earth) rotation.

If you are asking why there is retrograde motion, we have already answered that question on our site. See Why Don't All the Planets Spin the Same Direction?

Dr. Eric Christian
(December 2003)

The orbits of the planets are ellipses, as discovered by Kepler. An ellipse is defined by its major axis (D) and the distance between the two foci (F). The eccentricity (e) is the ratio of F to D, or: e = F/D. For Mars, D = 4.556 x 10⁸ km, and e = 0.09.

For more information on the planets and their orbits, check out An Overview of the Solar System

Dr. Louis Barbier

Here are some references that should help:

1. QB51.3 .E43 M42 1998
   Astronomical algorithms / Jean Meeus.
   Meeus, Jean.

2. QB43.2 .M44 1997
   Mathematical astronomy morsels / Jean Meeus.
   Meeus, Jean.

3. Astronomical Algorithms
   Basic Source Toolbox
   Version 2.00
   Jeffrey Sax

Dr. Louis Barbier
(July 2001)

Kepler's Laws are:

• Planets move along ellipses, with the Sun at one focus.
• The line from the Sun to the planet covers equal areas in equal times.
• The square of the orbital period is proportional to the cube of the mean distance from the Sun.

Answering all your questions at once, Kepler's planetary laws can be derived from Newton's law of gravitation. Kepler organized the observations, but Newton came up with the underlying law. Kepler's laws can be applied to any so-called "two-body" system, where a smaller object revolves around a larger one. The constants are different, but the laws work.

More about Kepler's laws at From Stargazers to Starships and at the Kepler mission site.

Dr. Eric Christian and Beth Barbier

The energy comes from the orbital and rotational energy of the system. The first thing that will usually happen is that the rotation of the moon will slow down until it is locked, with one face always pointed towards the planet. The Earth's Moon and many other moons in the solar system have already reached this point. The next thing that happens is that the orbit of the moon gets larger and the planet's spinning slows down. Both have to happen at the same time in order to conserve angular momentum while draining energy from the system. Because of ocean tides on the Earth, the Moon is getting farther away and the Earth's day is getting longer. It is the same process.

Dr. Eric Christian
(January 2002)

No, the orbital speed of planets decreases the longer the distance. For circular orbits the "year" increases like the radius to the 3/2 power, and the velocity decreases proportional to the square root of the radius of the orbit.

Dr. Eric Christian
(December 2000)

You'd do it pretty much the way you do it on Earth, except you'd use enclosed hoppers, smelters, etc., and you would have to design your ore transport system a bit different. Just letting the ore fall onto conveyor belts won't work. I'll point out, however, that other planets and even moons will have plenty of gravity for you to work with. Mining meteoroids and asteroids might be done in near zero gravity, however.

Dr. Eric Christian

Venus is too hot for liquid water, but about 0.1% of the atmosphere is made up of water vapor. I got this information from "Welcome to the Planets" Venus page.

Dr. Eric Christian

The composition of Mars is similar to that of the Earth, with maybe a bit more iron and sulfur. For more information on Mars, I suggest you check the Nine Planets website.

Dr. Eric Christian

First, the overall composition (atmosphere, rock, everything) of the Earth and Mars are similar because they formed in a similar region of the proto-solar nebula. Second, the primitive atmosphere of the Earth was much higher in carbon dioxide, so there were probably times (billions of years ago) where the atmospheric composition of Mars and the Earth were similar. There is no evidence that atmosphere of Mars had at any point a composition that is similar to the present day Earth's, as far as I know.

Dr. Eric Christian
(May 2001)

Mars is red because there is a lot of iron oxide, also known as rust, in the rocks and sand that make up the surface.

Dr. Eric Christian

It was almost certainly Jupiter. There are many good magazines like Sky & Telescope that you can get from your local library or newstand that describe what's visible in a given month. A telescope is not needed; a lot can be done with a reasonable pair of binoculars.

Dr. Eric Christian

Jupiter has about 2.6 times Earth's gravity. So, in feet per second per second, the acceleration on Jupiter its 32 * 2.64 = 84.5 feet/sec/sec.

Dr. Louis Barbier

Your question is beyond our area of study, but you might want to check the following:

• The Great Red Spot at the Online Journey Through Astronomy
• Jupiter at The Nine Planets

Beth Barbier

The energy comes from the orbital and rotational energy of the system. The first thing that will usually happen is that the rotation of the moon will slow down until it is locked, with one face always pointed towards the planet. The Earth's Moon and many other moons in the solar system have already reached this point. The next thing that happens is that the orbit of the moon gets larger and the planet's spinning slows down. Both have to happen at the same time in order to conserve angular momentum while draining energy from the system. Because of ocean tides on the Earth, the Moon is getting farther away and the Earth's day is getting longer. It is the same process.

Dr. Eric Christian
(January 2002)

Neptune's orbit isn't that erratic: only Venus has an orbit that's more circular than Neptune's. It's Pluto that's the oddball, having an orbit that is more of an ellipse than any other planets. That ellipse has its perihelion (closest approach to the Sun) closer than Neptune's orbit, although its average distance is more. Pluto also has an orbit that makes a larger angle to the plane of the solar system (the ecliptic) than other planets. Although the orbits appear to cross, it is only because a drawing is only a two dimensional image of the orbits. In three dimensional space, the orbits look like two loops of string that are interlocked, but don't actually touch.

So, although Pluto is currently closer to the Sun than Neptune, the orbits don't cross in a way that will ever lead to a collision. There are some diagrams to help explain this at The Nine Planets.

Dr. Eric Christian

Using an orrery program (a program that charts the planets) is probably easier than calculating it from orbital elements. Depending upon what accuracy you need, it should be pretty easy to get. It's a little over 20 years (from January 1979 to February 11, 1999). The difference between sidereal years and calendar years is in the fifth decimal place, smaller than the effect of the eccentricy of Neptune's orbit (0.008).

Dr. Eric Christian

Pluto takes 248 years to orbit the Sun.

Dr. Eric Christian

All of the planets move closer and then farther from the Sun every half "year". The Earth moves closer by several million miles between aphelion (furthest position from the Sun) and perihelion (closest position). We've just observed Pluto during the half of its year that it's moving closer.

Dr. Eric Christian

There is no evidence that there is any planet in our solar system beyond the orbit of Pluto. You might want to read more about this topic at Phil Plait's Bad Astronomy web site.

Dr. Eric Christian and Ms. Beth Barbier

Shepherd moons keep ring edges sharp in the radial direction. They have nothing to do with the fact that rings form in planes. The reason rings, solar systems, and galactic disks form in planes is just a function of the original angular momentum of the system. You can look at the Nine Planets web site for more information on shepherd moons.

Dr. Eric Christian

I recently heard some curious discussion about all the planets in our solar system becoming aligned in the year 2004. There is speculation on this having an effect on the Earth's magnetic poles and perhaps the Earth actually fipping over on its axis. Can you shed some light on this -- will the planets' alignment create some forces on the Earth that affect weather, gravity, continent movements, etc? How often has this happened in recorded history, and if it has, were there any noted effects? What is the term for this phenomenon?

No, all nine planets are not going to be in a straight line on May 5th, 2000. Planetary alignments of one sort or another happen all the time and have for 5 billion years, and they have NO effect on the Earth, despite what some kooks and con artists say.

The gravitational attraction of anything other than the Sun and the Moon on the Earth is essentially negligible. The energy required to actually flip the spin axis of the Earth is huge, due to something called conservation of angular momentum. The magnetic field of the Earth does flip aperiodically, and actually we are long overdue, but the alignment of the planets will have nothing to do with it. The magnetic field flip probably has its origin in the turbulent magma dynamo that generates the magnetic field.

The term for this alignment phenomenon is "hoax".

Dr. Eric Christian

If you are talking about an "exact" line, it has probably never happened. Even having all 9 planets in a rough line is very infrequent (millions of years, depending upon how "rough" a line you're willing to take).

For a lot more information, you can check out Phil Plait's Bad Astronomy web site. You might also check out a popular astronomy magazine like Sky and Telescope, which covers visual positions of the planets. If you are interested in alignments in space, you can check an online Orrery like that at Fourmilab.

Dr. Eric Christian

Three of us worked together on this question - Drs. Charles Smith (part of the "Ask Us" team), William Kurth (University of Iowa), and Michael Desch (NASA GSFC).

This is a very good question that has significant implications. Some years back Dr. Smith had to worry about this because he was studying the solar wind in the vicinity of Saturn. He needed to exclude those periods when the Voyager spacecraft (the source of his data) passed through Jupiter's distant tail. Others (Drs. Kurth and Desch) have studied it specifically to understand Saturn's reaction to the Jovian tail.

Before we dive into this too far, let us mention the papers that discuss the properties of these distant tail observations:

• Behannon et al. (Journal of Geophysical Research, volume 86, page 8386, 1981)
• Kurth et al. (Journal of Geophysical Research, volume 86, page 8402, 1981)
• Kurth et al. (Journal of Geophysical Research, volume 87, page 10,373, 1982)

Your library can probably get these articles through interlibrary loan. They aren't necessarily easy to read, since they are the original source documents written by the scientists, but they get to the heart of the matter.

Planetary magnetospheres react to changing solar wind conditions. If the wind conditions are stationary, the magnetosphere adopts a quasi-stable configuration. It reacts periodically, in almost a rhythmic fashion, but is largely stable. When solar wind conditions change, the magnetosphere seeks out a new equilibrium, and this can lead to some dramatic changes.

When the pressure of the solar wind is reduced, the magnetosphere expands. When the pressure increases, the magnetosphere contracts. In the process, portions of the magnetosphere will break off and the entire structure reorganizes itself with dramatic results (the aurora, changing magnetic fields on the Earth's surface, etc.). Expansion may not be quite as dramatic as contraction, but it is often characterized by the absence of familiar activity.

Jupiter's tail represents a region of reduced solar wind pressure, so Saturn's magnetosphere would have expanded under these conditions.

One diagnostics of the expansion was the disappearance of Saturn's kilometric radiation. This is a radio signal produced by the acceleration of electrons in association with the planet's auroral activity. This signal vanished for several days during the times when Saturn was within Jupiter's distant tail. It's probably true that Saturn's magnetosphere expanded a good deal during this time, and this is part of the explanation for the disappearance of the kilometric radiation, but we have no independent measure of the magnetosphere's expansion since there wasn't a spacecraft in the right location at that time to measure the expanded magnetosphere. Along with that expansion comes a general relaxation of the processes that accelerate auroral electrons on Saturn.

A description of when the Saturnian kilometric radiation disappeared is available in Desch (Journal of Geophysical Research, volume 101, page 6904, 1983).

Looking at the reverse effect of hitting Saturn's magnetosphere with a dense, fast-moving solar transient, there is a recent paper by Prange et al. (Nature, volume 432, 78-81, 2004) that demonstrates a strong Saturnian aurora driven by a disturbance that struck Saturn's magnetosphere.

Drs. Charles Smith, William Kurth, and Michael Desch
(December 2004)

I'm not an expert on extra-solar planets, but I think I can point you in the right direction. There are about 15 good candidates for planets around stars other than the Sun. There is one, 51 Peg b (one of the first ones discovered), that is in the Pegasus constellation and has a "year" of a little more than four days. There's a good page on extra-solar planets at Scientific American.

Dr. Eric Christian and Beth Barbier

Harvard maintains a good web site devoted to the extrasolar planets. I would suggest you look at: The Extrasolar Planets Encyclopaedia.

Dr. Louis Barbier
(November 2001)

There are basically only two possibilities for stable planetary orbits in a binary star system. Either the stars are far apart and the planet is close enough to one that the other star is just a small perturbation on it's orbit around the close star. The other possibility is that the stars are really close together, and the planet orbits the two stars as if they were just one. Neither condition does what you want.

However, you don't need a binary star system to have "planet-wide seasons". If a planetary orbit is more elliptical than the Earth's (which is pretty circular), then you would get seasons as you moved closer and furthur from the star. At the extreme, you could have seasons like the comets have, a year or so of summer, followed by many years of extreme winter.

Dr. Eric Christian