Do you have a question about the Planets or Astronomy that you have been wanting to ask, but haven't?
Well, what are you waiting for!?
E-mail your questions to us at: planets@ssec.wisc.edu

Disclaimer: The answers are provided to satisfy your curiosity only. Notification of errors or other comments on the answers will be appreciated.

May 13, 1996
Questions from Ms. Ruth Paulson's Third Grade students
Cottage Grove Elementary School

1. How did the planets all start?

We believe that the planets formed by condensation of gas from the planetary nebula surrounding the sun onto small planetesimals which then grew by accretion and by collision with other similar objects.

2. How did the rings form (sic) around the planets?

The rings most likely formed by the break-up of small moons and other objects in the vicinity of the planets. The material in the rings is constantly colliding, forming larger objects and breaking up. The rings are threfore not static, but are continuously changing.

3. How did the Sun form?

4. Have you ever gone in space?

No. It would be fun to see the earth from space myself, but I am content to watch images of the earth taken by unmanned spacecraft.

5. Is the sun bigger than all the planets combined together?

Yes, quite a bit bigger. The diameter of the sun is about 9 times that of Jupiter. The combined mass of all the planets is only about 5% of that of the sun.

6. How did they name the planets?

Six planets were known to the ancients and different cultures/civilizations named them differently. The remaining three planets have been discovered after the advent of telescopes some three hundred years ago, and were named by the people who discovered them.

7. What is the difference between a comet and a shooting star?

A shooting star is not really a star, it is so named because the streak created by a particle of dust entering into the earth's atmosphere from space shines as bright as a star as it burns up due to the friction caused by the atmosphere, causing it to burn up. A comet on the other hand is a much larger object, made up of water ice, dust and other organic molecules.

8. Which is faster- shooting star or a comet?

The shooting star slows down as it enters the atmosphere. Before it enters the atmosphere, the comet and the particle have comparable velocities with respect to the earth. A comet moves very slowly when it is farthest from the sun, at that point some dust particles are undoubtedly moving faster with respect to the earth than the comet, but the when the comet is closest to the sun, the opposite would be generally true.

9. How come people think there is life in outer space, but haven't seen them?

It truly would be singular that in this vast universe, there is only one planet in only one solar system that life formed. We know that the basic building blocks of life, the organic molecules, are found in many different parts of the universe. In fact, it is likely that the seeds of life may have been brought to earth by comets. It is likely that conditions conducive to life may exist at some point or another on other planets at some point in time. Given the vastness of the universe, it is not surprising that we have not yet detected life elsewhere, in any form. After all, the optical telescope is not even a millenium old yet. Search for life has only recently begun. Some day, someone somewhere may quite discover a lifeform in the universe.

10. How come Pluto goes in a different orbit?

Each planet moves in its own orbit that can be described by its circularity, inclination of the plane of the orbit and orientation of the orbit. We really do not know why Pluto's orbit is so different from those of other planets. The answer is undoubtedly related to the origin and evolution of Pluto. Did it form with the rest of the planets and have a collision with another object, possibly creating its moon, Charon? Or, did both Pluto and Charon get captured into orbit around the Sun from their origins in the Kuiper belt? Until we learn more about Pluto and Charon, we won't know.

11. How may moons are there in our solar system?

If we count moons larger than say 100 km, then the number of known moons around the planets is 60. However, there may still be some undiscovered moons, particularly smaller ones. Note that all of the particles that make up the rings of Saturn could also be considered moons of Saturn. In that case there would be very many moons indeed. Rings around Jupiter, Neptune and Uranus appear to be made up of smaller particles rather than many boulder sized chunks.

12. Is Pluto a planet?

Yes, because it goes around the Sun in its own orbit, and also has a moon. The questions about Pluto not being a planet were raised by some scientists in view of the fact that it is small and appears to be different from the gaseous planets found far away from the sun. But, nobody ever defined what a planet should be!

13. Is Earth's moon bigger than Pluto?

Yes, the earth's moon, and the four Galilean moons of Jupiter are larger than Pluto. Pluto's radius is slightly less than 1,200 km.

14. Are there other planets besides our solar system?

Yes, and at least six have been discovered so far.

15. What planet has the biggest moon?

The largest moon is Ganymede (surface), although if you measure to the top of the atmosphere, Saturn's moon Titan would win that title.

16. If you combined all our solar system moons together, could you make a planet?

In theory yes. After all, amy moons are already larger than Mercury and Pluto, two smallest planets!

17. How come the Sun is the only star close to us?

Don't know. There are stars called binary stars, which are a pair of stars revolving about a common point (their center of mass). However, so far no planets around such pairs of stars have been found, but there appears to be no reason why they could not exist.

18. Does space ever end?

How would we ever know if we have reached the end of "space"?

19. Does it rain in space?

Depends on what you call rain. On earth it implies drops of water falling down. In space you have a "rain" of dust particles, but it does not "fall" down anywhere until the dust particles hit an object.

20. Why is Mars called the Red Planet?

Because it looks red to the naked eye when it is closest to the earth. But so does Mercury.

21. Is there anything bigger than the Sun?

Yes, other stars. Tail of a comet.

22.

23. What would happen if there were a big explosion on the Sun?

Explosions happen on the Sun all the time. Usually these are called eruptions and result in solar flares- a stream of very energetic charged particles that travels in space and occasionally reaches the earth.

24. Why is Earth one of only planets without toxic gases?

This is one of the questions that we are trying to find an answer to, on other planets. Undoubtedly the formation of life changed the atmosphere on the earth. The early atmosphere on the earth was not very oxygen rich. Different planets evolved differently and that process resulted in different compositions of planetary atmospheres. If Venus had remained cool, it is quite likely that its atmosphere would have been similar to that on the earth, especially if life had evolved on Venus.

25. What would happen if moon crashed into Earth and made Earth leave orbit, and then Earth crashed onto another planet and eventually into the Sun?

What do you think?

26. What do you do in your job?

Quest for knowledge about the weather on different planets forms the focus of my job. To achieve it, I have to write proposals to request funding to undertake the kind of data gathering and analysis that will allow me to pursue the research. This process requires knowing what other scientists are discovering and sharing information with them via personal contacts and conferences. There is thus some travel involved. Gathering of planetary data is done either via unmanned spacecraft missions or from earth based telescopes. Working on a space mission requires a long and tedious process that consists of a proposal to pursue a research interest or an instrument/experiment, many years of planning and refining the experiment, actual gathering of data once the spacecraft is launched, analysis of the data once they become available, announcing the results at conferences, workshops as well as presenting the work to the general public as opportunities arise. These include public lectures, school visits, appearances on local or national news shows, etc.

27. Why is the Milky Way galaxy called that?

It is a poetic term. Other cultures have different names for the Milky way, but typically it means a "river" because that is how it appears to the eye from a dark place at night.

28. How many stars are there? What is the smallest star?

29. Have you ever built anything for a space ship?

No, but the place where I work, has built several instruments that have gone to Venus and Jupiter and in orbit around the earth.

30. How did gravity get formed?

Gravity is an innate property of matter. As to why an object with a mass has that property, we do not know.

31. How come sun isn't a shooting star?

Because a shooting star is not really a star, whereas the Sun is a star, that creates its own energy by nuclear reactions in its interior.

32. How many solar systems in our universe are there?

We don't know yet. Only recently have we acquired the ability to detect stars with planets around them, and about half a dozen have been discovered so far. It is expected that many more will be discovered in the future.

33. If all the planets left their orbits and started to fall down (if there was gravity), would they hit anything?

First of all, how will the planets "leave" their orbits? Planets are "bound" to their orbits by gravitational force. Only when the gravitational force suddenly changes would the plaents budge from their orbits. This could happen for example, if the sun were to suddenly explode and turn into a supernova, thereby changing the mass distribution and hence the gravitational force. However, by then, the planets would be surrounded in a hot gas envelope created. Another way the planets could change their orbits is by collision with a massive object, e.g. an earth crossing asteroid. Would the planets survive such a collision though?

34. How does the Sun get energy?

Under the intense pressures encountered in the interior of the Sun, two hydrogen atoms combine to form a helium atom. This process releases a large amount of energy and is called nuclear fusion. This is the basic source of the energy produced by the Sun.

35. How did the red spot on Jupiter get there?

We don't really know. At least when observed from above, the Great Red Spot (GRS) as it is more commonly known, appears similar to a giant hurricane like storm. But what created it and why it has persisted so long are matters of speculation. If it is a hurricane like storm, then it is likely that it is powered by the same or a similar process that drives a hurricane- release of latent heat due to condensation of a condensible material- water vapor on earth. This is why hurricanes die out when they move over land- they are cut off from the source of water- the ocean surface. On Jupiter, since there is no solid surface, the "storm" always has a source of the condensible material, and hence it can survive for a long time. But why does it move ever so slowly with respect to the interior of the planet?

36. How close do fireballs come to earth?

I don't know which fireballs you are referring to.

37. Is your job interesting?

Yes, I enjoy my job tremondously.

April 3, 1996
More questions from Patti Ann Rose
Newbury Park

1) When I look at the pictures on your web page I see a round circle with sorta stripes on it. I know that if they weren't great pictures of something you wouldn't take the trouble to put them up.
What do these pictures tell you that I can't see? You also noted with each picture the passes and (is it Bezel) filter number.

The pictures of Jupiter on the web page show the latitudinal bands of clouds on Jupiter. There is something lost in converting the image data to a format that is compatible with the web browsers (gif files), so the images may not show up the same way on all computers that people use as they do on the one where we make them (more graphics memory you have, the better the picture will look, to a certain degree). These particular pictures were taken with an intent to image the Galileo probe entry site, and that is one reason they are special. In terms of "absolute" picture quality, they are not the best, but certainly the best under the conditions in which Jupiter had to be observed (in daylight, very close to the sun). With more processing, we have learnt from these and other pictures that the probe entered an area which was devoid of clouds compared to other places, and would look redder than the rest of the planet. I will try to put some processed images on the web.

The "passes" associated with each image indicate the range of wavelengths that the optical filter lets through while blocking other colors (wavelengths). The wavelengths are expressed in nanometers (1 nanometer equals 10 raised to the power (-9) meters). The range of wavelengths that the filter lets in is called the "bandpass". Thus, the 750 nm filter is one that lets in wavelengths centered around 750 nm while blocking others. These are just beyond the "red" color that the human eyes can see, and are called near infrared wavelengths.

The images such as these are being used to aid in the interpretation of the Galileo Probe results, particularly about the abundance of water vapor. We know for example that in the near infrared wavelengths we observed more radiation compared to the rest of the planet from the region where the probe entered, indicating that the clouds were "thinner" or absent in the upper atmosphere of Jupiter, letting the more intense (warmer) radiation to reach us without being absorbed (due to the absence of clouds). There appears to be a connection between this and the relatively dry atmosphere that the probe instruments observed.

2a) You also talk extensively of the light that reaches us and the time it took to travel that far. Aren't we looking at the universe using radio waves and other such things.

3) What kinds of things do we learn from these other frequencies? (Note. These are follow up questions on the answer to an earlier question below.)

All light and radio waves are electromagnetic radiation, and hence essentially similar in their behavior. The only difference is their frequency or wavelength (the two are related to the speed of these waves). All electromagnetic waves travel at the same speed in vacuum. If the medium is different, then their speed is decreased somewhat depending on the properties of the medium and the frequency or the wavelength. This is what causes the illusion of a drinking straw to appear to bend when placed in a glass- the effect is termed refraction of light. The same thing happens to radio waves when the pass through the atmosphere-- they travel along a curved path due to bending caused by refraction. This effect allowed us to measure the "density" of atmospheres of other planets and derive the profile of temperature as a function of height when the Voyager spacecraft passed behind the planets as seen from the earth. This technique is called "radio occultation".

Hello! I'm Wei San from Malaysia

Could you tell me anything about terraforming Mars?

Terraforming Mars implies altering the Martian environment for possible humans and other life forms to survive on Mars. Some research has been done regarding how it can be accomplished. For an illustrative paper (and references therein), click here

Do you think this is possible in the near future?

How near is "near"? Five years, or 50 years? In 5-20 years, the answer is probably no, given the amount of time required for planning, funding, etc. Beyond 20 years, yes, it appears to be feasible in principle, although it certainly would take a long time.

How can we do it?

By following the lessons learned in how we have impacted the earth's environment, and what we have learned about the other planets.

March 14, 1996
From Patti Ann Rosem
Newbury Park

One of the guys I work with says that astronomers are beginning to see so far that they think they have found the end of the universe. Is that true?  

Let's re-phrase your question... By end of the universe you do not imply the "end" of the universe in the sense that the universe is about to cease to exist, but that you can see the "end" or the "edge" of the universe. The sense that "end" or "edge" evoke a physical limit on the size of the universe. In this sense, no, the edge of the universe has not been found, nor is it the intent to find it. Let me explain. The concept of space and time are not truly separable when you start thinking in terms of how fast we can learn about an event given that "light" is the information carrier that brings the knowledge of that event to us, and that it travels at a constant speed in vacuum.

What your colleague is referring to is that astronomers now believe that they have detected light from galaxies that emanated when the universe was young. In other words, they are so far away that the light emitted by them is just now arriving at earth. However, this interpretation is based on several assumptions about what we think we know about the origins of the universe. Briefly, there is the fundamental assumption that the universe originated in a big bang, sending matter expanding outwards. A further assumption based on some interpretation of the data is that depending on the brightness of certain class of stars, a relationship between the "intrinsic" brightness of a star and its distance from us, exists. Rather than go into those assumptions, I would like to point you to an article by George Johnson that appeared in the Sunday, March 10 1996 edition of the New York Times, Week In Review Section (Page E4, National Edition) that explains it well.

March 12, 1996
Questions from 8th Grade Science Classes
Longfellow Junior High School
Wauwatosa, Wisconsin

1. Have any probes been sent to the sun to make observations?

Yes. the Ulysses Spacecraft was launched some years ago to study the sun. As initially planned, the mission was to have two spacecraft, one from NASA and one from ESA, the European Space Agency. They were to approach the sun over the poles, the NASA spacecraft to fly over one pole, and the ESA spacecraft, travelling in the opposite direction, the other pole. This unusual trajectories were to have been achieved by first travelling to Jupiter, and then using Jupiter's gravity to swing the spacecraft out of the plane of the ecliptic, towards the sun. As it turned out, lack of US funding eliminated the US spacecraft. The remaining Ulysses spacecraft was launched by NASA for ESA and was tracked by JPL. It obtained data about the magnetosphere around Jupiter once it was in the vicinity before being swung on towards the sun. You may learn about the results of this mission from the JPL homepage by clicking on the highlighted hyperlink here.

2. If Jupiter were to become a star, could we still survive on earth?

What an interesting question. A solar system with two stars. Assuming for the moment that it is possible to have another star in our solar system (would we still call our environment the solar system?!), why not? After all, at Jupiter's orbit, the new sun would be roughly five times away than our original energy star, so its energy falling on the earth would be about twenty five times less than that from the first sun (can you tell why?) Its effect would be comparable to adding about an hours worth of extra starshine at the equinox. In the cold northern climates, that would be quite welcome!

There are other issues to keep in mind though. For Jupiter to become a start, it would have to have more mass, comparable to that of the Sun. If that happens (can you think of how?), it is likely that the orbit of the earth would be altered, since the orbital motion is about a center of mass, which in this case would be determined by the two stars. You can see what consequences this will have. Perhaps you can think about it and come up with some answers that you may want to ask about later?

3. What causes a planet to rotate?

This is a very good question that may not have an ultimate answer, but there is an explanation. The reason a planet rotates is due to its origins from the nebula from which it formed. This nebula had to be rotating. You probably have heard about the gyroscope and how it works- conservation of angular momentum. Angular momentum is determined by the rate of rotation and the mass of the object and its distribution as a function of distance from the axis of rotation. This is the principle behind an ice-skater spinning up when the arms are brought close to the body, or slowing down when the arms are extended. Except for dissipation and action of other torques, angular momentum is held constant. Thus the rotation of the gases and dust from which a planet formed causes the planet to keep on rotating, to conserve the initial angular momentum.

The planet's rotation is of course changed by events both inside and outside of the planet. For example, it has been recently shown that the use of dams has changed the distribution of stored water on earth at different latitudes, sufficiently to change the length of the day! The difference is small, but has been detectable.

But, why is the angular momentum conserved? And why was the nebula rotating if the universe began in a big bang?

4. Why does Uranus appear to tilt on its side?

Because it is tilted!

5. Could the asteroid belt be the remains of "dead planets"

What are "dead planets?"

6. Why do planets revolve in elliptical orbits instead of circular orbits?

A circle is a special case of an ellipse, whose eccentricity is zero. So a circular orbit is an elliptical orbit with zero eccentricity. So your question really is "why is the eccentricity not zero for all the orbits?" The answer to this lies in the laws that govern the "motion under the action of a central force." In other words, when a body is moving under the influence of a force that has a definite "source" or a point of origin. For planets, or any object for that matter, this force is the gravitational force. The solution of the possible trajectories includes the general conic sections - hyperbola, parabola, and the ellipse. Only the ellipse is a closed trajectory. All planets around the sun move in closed trajectories, with the central force at one of the focii of the ellipse. This is very close to the location of the sun. In reality, the planets move about the center of mass of the planets and the sun, called the barycenter. The barycenter is close to the sun center, but not quite located at it, since the sun is so massive compared to all the planets combined.

J. Kneisel
Longfellow Middle School
Wauwatosa, Wisconsin

Is there a planet X ?

The belief that a planet may exist beyond the orbit of Pluto was based on the difficulty in modelling the orbital motions of the outer planets, mainly Uranus and Neptune. The reasoning was that a massive object or a planet beyond Pluto would exert enough gravitational attraction to disturb the motions of these planets. Uranus and Neptune themselves were discovered in the last two hundred years in this manner, along with Pluto in this century. However, since 1989 there has been a significant change. The Voyager 2 fly-by of the outer planets, from Jupiter to Neptune, improved our knowledge of the masses of these planets significantly. This knowledge, coupled with the observational errors in the positions of the Uranus, Neptune and Pluto since their discovery, show that their orbital motions can now be modelled well within the observational uncertainties. There is thus no reason to expect a large, massive planet, often called planet X. Since the orbital periods of Neptune and Pluto are approximately 164 and 248 years respectively, we still have not observed them to go around the sun since their discovery.

David Clarkson
South Africa

Can you please answer two questions that I have regarding the planet Jupiter. I have heard from different unscientific sources the "theory" that Jupiter may be a dormant Sun, or at least it consists of gases that are needed in the composition of a Sun.

Question 1 : Is the above science-fiction or is there any truth in it? If there is any truth in it can you please give a brief explanation.

The wording of your question is such that it could be ambiguously interpreted. Let me explain how. If by dormant, you imply that at some future point Jupiter could turn into a "sun", i.e. a star that radiates a lot of energy and has nuclear processes going inside of it (namely conversion of hydrogen into helium that produces much of the energy of the sun), then the answer is no.

If you interpret the question to mean that Jupiter is radiating more energy than it receives from the sun, then yes, Jupiter has this star-like quality. This excess energy is believed to come from the primordial heat of formation of the planet itself from the hot nebula from which the planet was formed. A small fraction may be due to ongoing gravitational compression.

One of the theories of its formation is that the planet formed from the solar nebula, and hence its composition is believed to be sun-like, which is valid to a large degree. Jupiter is mostly hydrogen and helium with trace amounts of water, ammonia, etc. The other theory is that Jupiter accreted by bombardment of comets onto a planetesimal (in which case the composition need not be solar like), and then accreted the gas surrounding it (solar composition). The precise abundance of trace gases is thus one means of distinguishing between these two hypotheses.

Question 2 : Does Jupiter have a solid core or is it made up entirely of gases?

Jupiter is believed to have a very small rocky core at its center, extending to about t 10,000 km from the center, or about 15% of its equatorial cloud top radius. Beyond this is a thick layer of metallic hydrogen and helium that contains most of the mass of Jupiter. The boundary between the metallic state and the liquid/gas phase is believed to be approximately 54,000 km from the center. The last layer that extends as much as 17,000 km, is the liquid/gas layer, It is at the opt of this layer that the visible clouds are seen, at a distance of about 71,400 km at the equator from the center. Jupiter is quite oblate, i.e.,. its polar radius is smaller than the equatorial radius by approximately 5,000 km, primarily because of centripetal acceleration due to its rotation rate (once every 9h 55min).