Questions and Answers about Planets

1.a) How much would it cost to completely track all possible earth orbit crossing objects?

 There is no single answer to this question, because the answer depends  on the time span over which the mission is to be accomplished.  If you  want to track them "all" in a single year, then it would require an  enormous amount of money, and probably couldn't be accomplished with  existing technology in any event.  If you want to track them "all" over  the course of a century, then current efforts are more than adequate, objects.  At the 10-meter size level, we'll never find them "all", nor  do we need to, because most of them will burn up in the atmosphere; the  iron objects that don't will produce a small amount of local damage.

Dave Tholen

b) Why won't the government fund this?

 The government is providing some funding.  The million dollar figure you  mentioned is incorrect.  NASA's Near Earth Object program is currently  providing about 3.5 million dollars per year.  The reason it isn't  higher is most likely because some people think that the risk is small  enough that we don't need to find them all in a ten year span, as a  NASA committee had recommended.  We could spread out the effort over  fifty years, for example, with only a tiny chance of an impact in that  fifty year span that we might otherwise have been able to prevent had  we worked faster.  The more the effort is spread out, the less it costs  per year.  At the current funding levels, it will take about 20 years  to find all the asteroids larger than 1 km in diameter, which is the  size generally considered to be the threshold for a global disaster.

Dave Tholen

c) Is there anyone that would be willing to privately fund a program of this nature?

 If so, send them to me!  Seriously, I've heard of some people who are interested in providing  some funding, but nobody who is willing to put up enough money to fund  a significantly greater effort than the government-funded one. There are a couple things I can add. Perhaps you heard of the NEAR mission? "NEAR" stands for "Near-Earth Asteroid Rendezvous". This is a spacecraft that was sent to the asteroid Eros to study it up close. It arrived at Eros last February and took data for about a year. One of the reasons Eros was chosen as the destination for this mission is because its orbit is very slightly unstable, and so there is about a 5% chance that, 50 million years from now, it might crash into Earth. So the NEAR mission will provide information on the sort of asteroid we might someday want to be able to deflect. You can find out more about the NEAR mission at this URL:


The name of the NASA project looking for asteroids that might impact Earth is the "Spaceguard Survey".  You can find their web site here:


There is actually a privately-funded European effort to encourage similar surveys internationally. It's called the Spaceguard Foundation. You can find their home page (in English) here:


Dave Tholen

2. What is the difference between our moon’s name and other moons of other planets?

Actually, the word "Moon" is a proper name, when applied to the Earth's one and only natural satellite. An Old English word, it has come to mean in modern times any large, naturally formed body orbiting around a planet. This is a direct example of how a specific name can become a gerund for a complete class of similar objects in common usage - like the words "Xerox" and "Kleenex".

Dr. Casey Lisse

3. I was wondering if you have any thoughts about the arguments that are being made about Pluto not being a planet.  What would you classify it as?  Do you think that if we do ever reach out farther in the universe we will find other planets that are more mysterious than the ones we have traveled to so far?

My name is Zoe Leinhardt I am a graduate student in the Department of Astronomy at the University of Maryland. You have asked some excellent questions about planets. In general astronomical objects (asteroids, planets, stars, galaxies) are placed in categories based on what they look like (colors, brightness, size etc). As you have mentioned this technique of deciding what an object is by its physical characteristics gets us into trouble when there is no clear physical characteristic that can separate classes of objects. Pluto is large for a Kuiper Belt Object (more generically asteroids) and small for a planet. Until very recently astronomers couldn't observe much in the Kuiper Belt (telescopes just were not sensitive enough). But now we have detected another object almost as massive as Pluto but much farther away from the sun called Sedna. So now that we have confirmed that Pluto and Charon are members of the Kuiper Belt and that the Kuiper Belt has objects that are about the same mass it seems more logical to consider Pluto an asteroid instead of a planet.

I do think that we will find planets that have characteristics that we haven't thought of. When astronomers first started looking for planets outside of our own solar system we thought that we would find planets like our own Jupiter. Instead we discovered a lot of solar systems (~120) with planets the mass of Jupiter very close to their sun. Much closer than our own Jupiter is to our sun. I think that we still do not understand all the processes that occur during solar system formations so I do think that we will continue to be surprised with the planets that we find.

Zoe Leinhardt

4. I was wondering what is the diameter of the magnetic north pole?

The Earth's magnetic field, or geomagnetic field, is similar to the magnetic field of a bar magnet tilted at 11 degrees to the spin axis. However the field is not due to a real bar magnet in the Earth's interior. The origin of the magnetic field is the flow of molten or liquid outer core of the Earth.  The liquid core is so hot that the material is ionized (the electrons are stripped off the nucleus) and an electric current is produced when it flows. This current then produces a magnetic field.

The magnetic field produced by an electric current can be detected in a simple experiment. Take a piece of copper wire (with an insulated coating) and loop it around a wooden matchstick (for shape) and then connect the two ends to a battery through an electric resistor. When the current flows the looped wire behaves like a bar magnet aligned along the matchstick. One can use small paper clips or a compass to detect the magnetic field.

Now coming back to the Earth's magnetic field, because of the rotation around its geographical axis, the flow of the liquid core produces a current that is similar to the helical wire loop in the above experiment. This results in the similarity between the magnetic field of the Earth and that of a bar magnet.

Thus its not very useful to think in terms of magnetic north or south poles with fixed sizes.

PS.  The liquid core in the interior is so hot that any magnetic material (such as an iron bar magnet) will lose its magnetism.

Surja Sharma

5. I would like to ask you a question pertaining to the orbits of the outer most planets in our solar system.  What is the reason for the odd orbits of planet-like objects beyond Neptune? I am aware that two more planet-like objects have been discovered. Is it because they are so far from the gravitational pull of the sun that they go in and out of the orbits but are still caught by the pull of the sun?

You're on the right track about why the planet-like objects beyond Pluto have such odd orbits. Because they are so far away from the Sun, they feel a much smaller gravitational pull than they would feel if they were very close to the Sun like the Earth. If these planet-like bodies happen to pass close to one of the giant planets in our solar system (Jupiter, Saturn, Uranus, or Neptune), the planet-like body feels a larger gravitational pull from the giant planet than from the Sun.

When this happens, the smaller, planet-like body's orbit is changed.   Occasionally, the planet-like body is caught by the giant planet's gravity and begins to orbit the planet (we think this is how Triton became a moon of Neptune). The rest of the time, the giant planet's gravity acts like a slingshot and hurls the smaller planet-like body away from it in a random direction away from the Sun, resulting in the oddly shaped orbits we've seen. This idea of using the gravity from a close pass by a planet is how NASA has propelled some of its satellites faster than just using rockets!

Astronomers call the objects in the region just beyond Neptune "Kuiper Belt Objects" (KBOs) or "Trans-Neptunian Objects" (TNOs), depending on who you ask. Most of them orbit the Sun in more "normal" orbits, orbits in the plane of the solar system like Earth. We believe that the ones whose orbits are tilted relative to the plane of solar system used to have "normal" orbits, but happened to pass close to a giant planet and got thrown into the odd orbits we see today.

Matthew Knight

6. I was wondering if you could answer a question that I have had about extra solar planets.  What do you think would be discovered if we ever reached extra solar planets (e.g. for example atmosphere, composition, bacteria, etc.)?

About 110 extra solar planets have been discovered so far.  These planets are much more like Jupiter than like our Earth.  In order for them to be detectable, they must be massive planets. Jupiter is 318 times more massive than the Earth.  This is because the detection method depends on the gravitational effects of the planet on the star that it is orbiting. Massive planets produce effects that we can detect, but planets like Earth produce effects too small to measure.

The first extra solar planets detected were located very close to their stars.  Now we are beginning to find some systems with planets farther out, like Jupiter is in our Solar System.  Could there be Earth-type planets? Maybe in the systems that resemble ours.  If they exist and are located at a similar distance from the star, yes, they could resemble Earth.  We know that water existing as a liquid is pretty important both in the development of life and in sustaining it.  Planets with an atmosphere, liquid water, and a solid surface may have conditions conducive to life, bacteria. I am optimistic--if the conditions are right, simple life has a chance to evolve. There are some beautiful artistic depictions of the extra solar planets available on the web. Lynette Cook has done some nice work. Geoff Marcy's webpage is also great in representing the science in detecting extra solar planets.

Grace Deming

7. What is the possibility that we will be hit by a actual comet or asteroid. If this were to happen in the near future, could we count on the Government to inform us?

What is the possibility of being hit by an asteroid or comet?  While it is possible that an asteroid or comet could hit Earth, it is not very probable that we will actually be hit by one the size of a small asteroid or comet, which is between 1-10 km in diameter (or 0.62 - 6.2 miles).  Meteorites fall to Earth and they are fragments of asteroids, but they usually don't do much harm.  I believe once a dog was hit by a meteorite, and a woman was once but she only got a bruise. We know that asteroids have hit the Earth in the past. Sixty-five million years ago, an asteroid impact resulted in changes on Earth that lead to the extinction of the dinosaurs. But the probability of that happening is so low that I don't think anyone should worry about it.   But the thought makes a great Hollywood movie. So small objects and boulders do hit the Earth, but the atmosphere heats them up and they disintegrate rarely causing much damage at all. But the big ones are so rare that we don't have to worry about them. An asteroid that can do some real damage is expected to hit the Earth once every 100,000 years. The trouble is that we don't know which year that will happen, so we just can't worry about it.  Instead, we are trying to discover them and learn about what would happen if one were on a collision course with Earth.

I don't think you have to count on the government to let us know if there was an asteroid on collision course with Earth. Scientists like to share what they know, and I think they would tell you.  Scientists and the press will keep you informed. I know I wouldn't be able to keep it a secret if I found an asteroid or comet on collision course with Earth.

Lucy McFadden

8. Can you describe the pressure inside Jupiter, and whether nuclear fusion could be taking place, making Jupiter a brown dwarf or possible companion star to the sun.

To answer this question, we need to know what the "definitions" of a star, brown dwarf and a planet.

The definition of a star is pretty straightforward.  A star is an astronomical body that has sustained nuclear fusion in its core, where hydrogen is processed into helium.  When fusion occurs, it releases massive amounts of energy, causing the immediate environment to expand and the fusion process to shut down.  A star must have enough pressure in the core to overcome this expansion force, so that the fusion process can continue uninterrupted.  Because the pressure comes from the weight of the material around the core pressing inward, a star must have a very high mass in order to keep the fusion going.  Calculations show that fusion can only be sustained if the body has a mass greater than about a tenth of our sun's mass (actually 0.08 solar mass).

The definition of a brown dwarf is not quite as well defined.  It is a massive object that has not achieved sustained nuclear fusion.  So the upper limit is well defined at 0.08 solar masses - the point at which it would be a star.  There is no natural lower limit for a brown dwarf, though, so the line between a brown dwarf and a giant planet is fuzzier.  Astronomers have somewhat arbitrarily defined the dividing line at about 1/100th of a solar mass.

The definition of a planet is not well defined either.  There is no consistent definition that includes the nine objects that we define as planets, yet excludes all of the smaller bodies in the solar system.  For this discussion, however, we will accept that Jupiter qualifies as a planet in any proposed definition.

So, is Jupiter a star or brown dwarf?  It has a mass about 1/1000th of a solar mass.  This is well below the fusion limit, so it is clear that Jupiter has is not a star.  But it is also well below the limit that has been defined for a brown dwarf.  It would need to be about ten times larger than it is now to qualify as a brown dwarf.

So even though Jupiter does have a very high pressure in its core (by Earthly standards) it is still just a planet in orbit around a star.

Tony Farnham

9. How come helium was found on the earth after the sun? Why is helium so hard to find on Earth? I realize helium is an element with a very low atomic mass, but why does it have such a tendency to float?

Helium is an element known as a "noble gas".  The reason it's called that is that it doesn't form any chemical compounds (like a noble, who refuses to associate with commoners!).  In the 1800s, when people were discovering elements, they used chemical methods to separate and identify them, which is why it was tough to detect on Earth. It was detected on the Sun using a technique called spectroscopy: each element absorbs certain specific wavelengths of light, and people noticed that the Sun had some wavelengths absorbed that didn't correspond to any known element.  They wondered if this was a new element, and this spurred them to look for it on Earth.

The low atomic mass of helium causes it to float.  The reason is that gas on Earth moves in such a way as to make the pressure (force per area) as equal as it can.  That's why, when you see that a low pressure area is over your home town, you can expect winds and other nasty weather; that's the atmosphere's way of equalizing the pressure! The pressure turns out to depend on the temperature of the gas and the number of molecules per volume.  At roughly constant temperature, it is therefore the number of molecules per volume that matters. At the same pressure, then, the number of helium atoms per volume in a balloon is the same as the number of (much heavier) nitrogen and oxygen molecules in the surrounding atmosphere.  That means that a given volume of helium gas weighs much less than the same volume of air.  Now, the atmosphere has weight, and it balances that weight by having the pressure be greater near the ground than it is higher up (that's why you can have difficulty breathing at the top of a mountain; there's less air there!).  This balance, though, is in equilibrium for the air.  For the much lighter helium gas, the pressure force is the same but the gravitational force is less (because helium weighs less). Therefore, the helium balloon is pushed upward.

Cole Miller

10. In our astronomy class we watched a movie about the sun's expanding heat that will someday encompass the Earth. What are your thoughts on this matter? If this happens, which it has been predicted to do, it would destroy life on Earth. For some reason I have a hard time imagining humans will still exist in billions of years. If life does still exist, what do you think our decedents will do to stay in a safe zone out of the sun's expanding heat? I have briefly heard of some ideas about where humans could migrate to, in order to proceed with life. I am just curious as to what your thoughts are on this matter and any ideas you have to combat this future problem

We know enough about stars to say with certainty that life as we know it could not exist when the Sun becomes a red giant in about five billion years.  The luminosity of the Sun (i.e., the energy it emits in a given amount of time) will go way up, so all the oceans will boil and the rocks on the surface of the Earth will melt. You are therefore right that we'd be in trouble if we stayed.

However, I don't think there is reason for concern.  If we assume that our descendents are still around, they will have plenty of time to adjust by moving away.  Think of how much technology has improved over just the last hundred years, which is the blink of an eye compared to the billions of years it will take the Sun to change significantly. We have much more pressing concerns over shorter times, such as overpopulation and greenhouse warming that pose a greater threat because we can't easily adjust to them at our leisure.

Cole Miller

11. If we continue to harm this planet the way we are doing, will Earth eventually lose its inhabitants? Or maybe become a planet like Venus, with tons of greenhouse effect and poor soil?

My name is Meredith McCarthy, and I'm one of the graduate students in the Astronomy Department at Maryland.  Thanks so much for your question!  It's been forwarded to me since I share your concerns about the environment of Earth.  I'm not sure exactly what your background is, so if some of this is too easy for you, skip ahead to the rest of it.

First, here's a little background on the Greenhouse Effect.  This is a natural effect caused by the presence of certain types of gases, called greenhouse gases, in the atmosphere.  The greenhouse gas in largest quantity in our atmosphere is actually water vapor, although there are other important ones such as carbon dioxide and methane. Visible light from the Sun can penetrate our atmosphere without any problems, and this sunlight hits the surface of the Earth.  The Earth absorbs the sunlight and heats up.  Any object that has a temperature emits light in the infrared (this is what night-vision glasses look for), so the Earth’s surface then re-emits the energy it absorbed back out to the atmosphere in the infrared.  When this infrared light reaches the atmosphere, the greenhouse gases keep it from escaping the atmosphere.  The gases do this by absorbing this light and then reemitting it over and over, in all different directions.  Much of this energy is deflected back down to the Earths surface, or sent to other parts of the atmosphere.  Some does escape back into space. The overall effect of this is that these greenhouse gases force the Earth to retain heat it might otherwise let escape into space, so the overall temperature of the Earth rises.

Now, a little greenhouse effect is not a bad thing.  Without one on Earth, the planet would be much colder and not a fun place to live. But too much of a greenhouse effect, like on the planet Venus, is a very bad thing for life.  Once enough greenhouse gases get into an atmosphere, the natural cycles that the planets have for removing these gases cannot handle the quantity of the gases anymore.  Then gases begin building up and trapping more heat until we are left with the arid furnace of Venus.

What are these natural cycles I mentioned?  Well, for one thing, carbon dioxide dissolves easily in liquid water (that's how they get bubbles into your soda).  So the Eart's oceans can store some of the carbon dioxide temporarily.  While the carbon is stored in the ocean, it can be used by animals such as corals to build their shells. Also, trees help temporarily store carbon dioxide, but most of it is released at their death or during the winter.  Decaying animal and planet matter releases carbon dioxide, but if you can trap that material underground, the carbon is trapped in oil.  Burning this oil then releases the carbon.

So this is where the problem with carbon dioxide emissions from humans comes from. Scientists know that the more greenhouse gases in the atmosphere, the more greenhouse effect we will have.  And there are certainly more greenhouse gases building up in our atmosphere now.  The question becomes how much is too much?  And the unsatisfying answer is that we don't really know.  But we have in the last five years or so finally been able to observe real climate changes that are due to the warming of the planet.  And most people and scientists agree (outside of the US, where politics is heavily involved) that this warming is due to the gases we've put in the atmosphere.

Here are some of the observations that are helping us study global warming, and some of the problems that go along with those observations.

1. The polar ice caps and glaciers across the world are melting at alarming rates.  When they melt, the reflective ice layer is gone off the land and the darker earth is exposed.  Well dark colors retain heat more than light colors (just like how you want to wear white on a really hot day), so the darker land then gets even hotter.  This causes the ice that's still near it to melt even faster, so the whole process could runaway.  The more ice that melts, the more water that ends up in the ocean, so this could cause the ocean to rise fairly significantly.  Locations only a few meters above sea level could be in great danger of being flooded.

2. As the planet warms, so does the ocean.  The Earth relies on corals to remove carbon dioxide from its atmosphere.  But the corals are starting to lose their colorations and stop breeding because the water temperature is too hot.  If the corals die off, then the primary method to permanently remove carbon dioxide from the atmosphere will be lost.

3. Many animal species will be in greater danger of extinction if global warming continues.  Just for one example, polar bears rely on the ice sheets in the north to travel on in order to find their favorite food, seals.  If the ice sheets break up early or don’t form at all, the polar bears will be in big trouble.  The seals also rely on the ice sheets to serve as breeding grounds.  So the seals will be in danger as well.

These consequences and the many others that can result from global warming will certainly make life very difficult on this planet. Scientists want to get people to stop putting as many greenhouse gases into the air so that we don't push the safety systems of our planet so far that they break.  It's doubtful that we'll ever turn Earth into Venus since a lot of our carbon dioxide is pretty permanently stored in rocks.  However, we are going to cause ourselves a lot of trouble and endanger a lot of species of plants and animals if we don't stop.  Besides the one's I mentioned, many models also predict changes in seasons, rapid spread of disease, and more extreme weather as other consequences of global warming.

As with any scientific study, there are a lot of what ifs involved in climate study, since the systems are so complex.  So not everything we predict will certainly happen.  But more and more scientists are agreeing that there are real and very negative consequences to the overall environmental policy that humans have, and if we don't start doing something about it soon, it may be too late.

Meredith McCarthy

12. We have been talking in class about how the tsunami has effected the rotation of the earth, and I am interested to know how that happened, and what the long term effects will be regarding the length of our days. Will our calendar eventually be off?

This effect was mentioned in the media without really indicating how incredibly small it is.  Let me first explain why there was any effect at all.  The tsunami was caused by the slippage of part of the Earth's crust over another part.  That meant that a big amount of mass moved. Why does that have an effect?  Imagine standing and spinning on one foot with your arms outstretched.  If you bring your arms in quickly you'll speed up, and if you put them back out again you'll slow down. This is called conservation of angular momentum, and means also that when part of the Earth shifts (as it did during the earthquake that caused the tsunami), its spin changes.

But not by much!  According to the Earth's rotation was sped up just a little bit, so that the day is now 2.68 microseconds shorter.  That is a *tiny* amount. One day is 86,400 seconds long, so the change was only one part in 3.2 billion!  Put another way, assuming nothing else happens, it will take about 10 million years for this to make a one day difference.  In that time, of course, many other things will happen, including lots of other earthquakes that will make their own changes.

Therefore, it's a tiny effect.  The really impressive thing to me is that we have instruments sensitive enough to measure it!

Cole Miller

13. We were watching a movie about how our solar system has a safe or habitable zone, where Earth is located, and that is why there is life on our planet. So then could you measure the distance of where life could be in other star systems? If so then could you measure the approximate distance to where a planet would have to be to have life? If you could not measure the distance could you explain why you wouldn't be able to measure it? Further more, could there be a relation to the size in the star and where the habitable zone would lie?

Your intuition is exactly right. Yes, we can measure where the habitable zone is around other stars, and yes, the size and location of this zone depends on the size of the star.

Astronomers define the habitable zone as the region around a star where the temperature at the surface of a planet stays in the range where water is liquid. In our solar system, this is from roughly 0.9 AU to 1.4 AU (1 AU is the distance from the Sun to the Earth). The habitable zone depends on the luminosity (how much energy a star radiates per unit time) of the star. Luminosity is a function of the radius of the star squared and the temperature of the star to the fourth power (ie. L ~ R^2 * T^4). If a star is larger or hotter than the Sun, then it is more luminous and it's habitable zone must be farther away than in our solar system. If the star is smaller or cooler than the Sun, it's luminosity is lower and it's habitable zone must be closer than in our solar system.

One of the major goals in searching for planets around other stars is to find an Earth like planet orbiting in the habitable zone of a star, since this is the most likely place to find life. Astronomers have not yet found one, but this is mostly because our technology isn't quite good enough yet. There are a number of projects being designed and built right now which should be capable of finding such a planet. Hopefully, in the next few years we will find one!

Matthew Knight

14. Pluto was dropped from the planet list. What other changes have happened? Also, what about the theory that Jupiter was almost a star? And do you think we will colonize other planets?

Not only was Pluto reclassified as a dwarf planet, but did you know that Ceres, the largest Main Belt asteroid, was also classified as a dwarf planet?  I like to think of this as a promotion for Ceres.  Ironically, when Ceres was first discovered, it was classified as a planet, but as more asteroids were discovered, and we recognized their small size, they were given the designation: minor planet. This happened in the 19th century (the 1800s) and the same thing is happening now with respect to Pluto and the Kuiper Belt objects that are being discovered.  I find that interesting too. See the write up about this by my colleague, James Hilton.

  As for the history of the theory that Jupiter almost became a star, the fact is that Jupiter, Saturn and Neptune emit more thermal radiation than they absorb from the sunlight incident upon it, almost by a factor of 2 (the actual ratio of emitted to absorbed sunlight is 1.7 for Jupiter and 1.8 for Saturn and 2.6 for Neptune).  I think that someone thought that if the planet is emitting more than just incident solar energy, that if it were massive enough, it would be hot enough in the interior to start thermonuclear fusion and really radiate as a star.  But Jupiter isn~Rt big enough for that to happen. The temperature in Jupiter~Rs core is more than two orders of magnitude (100 times) cooler, than the 1,000,000 K required for thermonuclear fusion.

  As for the plan to colonize other planets, I have to say, that I am skeptical of it happening in your lifetime.  I say that, because it will cost so much to develop the technology to support humans off of the Earth.  And if I ask myself, if that is worth doing with tax payers~R money? I am not so sure.  But if we keep at it long enough, we will be able to do it eventually. I just don~Rt know how long it will take, and when will be the right time to have it happen.

    Lucy A. McFadden

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