Some time back, I had a post about dark matter.  So, I thought that I’d say a bit more about matter.  First of all, just what is matter?  After all, if we are going to talk about dark matter, we should have some idea what non-dark matter is, right?  This is going to be a bit more technical post than most that I have done.  Long time readers will note that I once in a while toss in things like this.

Ages ago, in a physical science class in high school, I learned a definition of matter that I still hear bandied about.  That definition went something like:  Matter is something that has mass and takes up space.  Well, that’s cute.  So, what is mass, and what is space?  After all, if you are going to use a definition, then the terms used in that definition should also be understood.  What I find, though, is that my physics students, much less the non majors, don’t understand the terms matter, mass, and space.  Heck, a number of physicists seem unclear on the concepts, too.

Already, in a previous post, I talked about space not really being empty.  This is a clue that there is something more going on in space than simply dimensional measure.  In cosmology, we learn that space can expand.  Two galaxies can each be sitting stationary in space, and yet be getting farther away from each other because the space between them is expanding.  This is hard to understand if you are stuck with the idea that space is simply a dimensional measure.  It may be more appropriate to think of space as an interaction realm.  Things can interact with one another, and the realm and range of that interaction is space.  This is still not quite right, but I don’t think that anyone really understands space completely, and this way of looking at it is a bit better than most people do.  The dimensions (length, width, height) are simply a way of quantifying how much space there is.  Space itself is something beyond just the dimensions. 

Now, this throws a monkey wrench into our definition of matter.   Implicit in the definition is that two separate pieces of matter can not occupy the same space.  But, if two objects do not interact, what keeps them from occupying the same space?  After all, if they don’t interact at all, they for all practical purposes can’t occupy the same space, since one object’s space is not the same as the other object’s space.  That’s a tough concept, I know.  Furthermore, it is not at all clear that some particles, such as electrons, actually take up any space at all, at least in terms of dimensions.  Electrons, and some other particles, are essentially point particles.  Now, we can discuss the interaction range of an electron with a probability distribution described by a wave function, but that isn’t really the same as saying that the electron is occupying space.  Now, there does exist a limit to how small a piece of space we can describe in physics, something called the Planck Length, which is about 1.6x10^-35 meters.  At this distance scale, the normal rules of physics start to break down, and something new is needed.  So, we can’t really say if such point particles are really point particles, or just tiny things the size of the Planck Length, but for all practical purposes, the statements are equivalent.

But, no one argues that electrons are not matter.  So, we are left with matter being something that has mass.  But, don’t be mislead.  Matter is not mass.  These are separate concepts.  However, mass is a property of matter.  All matter has mass.  But, doesn’t everything have mass?  No.  Photons, particles of light, don’t have mass, and neither do gravitons, the particles postulated to carry the gravitational force.  So, that makes photons and gravitons, and numerous other particles not matter.

So, let’s talk about mass for a moment.  What is mass?  Actually, this is a deep question.  If mass is that makes something matter, then we should understand what we mean by the term mass.  There are two basic types of properties of matter that we call mass.  This means that there are two separate masses.  These are the Inertial Mass and the Gravitational Mass.  These are different properties, but interestingly, they have the same value. 

The inertial mass is that property at the heart of Newton’s Second Law of Motion: 

F = m a.  Or, to put in another way, m = F/a.  You apply a force to some object, and it accelerates.  The ration of the applied force to the acceleration is the object’s inertial mass.  The more mass it has, the harder it is for the force to accelerate it.  Once moving, though, it takes force to slow it down or to stop it.  Again the ratio of the force to the deceleration is the mass, and this quantity is the very same as the mass that was determined in speeding the object up in the first place.  There are various reasons to expect this to be the case, but we don’t want to get even more technical than this is already.

Gravitational mass, though, is a different beast altogether.  Two pieces of matter interact with one another through the gravitational force.  They may also interact via the electromagnetic force, the weak force, or the strong force.  These four forces are the only forces in nature that we know about.  Everything that you can conceptualize as a force is really one or more of these forces at work (most commonly experienced forces in the macroscopic world other than gravity are electromagnetic in nature).  Now, not all pieces of matter interact via all of these forces.  Some don’t interact at all through one or more of these forces, but all interact via the gravitational force.  The magnitude of the interaction between matter objects depends upon a quantity associated with the objects called their gravitational masses.  The magnitude of the interaction is given by Newton’s Universal Law of Gravitation:  F = (G m₁ m₂) / r².  The m’s are the masses, the r is the separation of the masses, and G is a quantity known as the universal gravitational constant.  Its value is determined by the strength properties of the gravitational force and the particular choice of units that we select.  People who work on this sort of thing even talk about two types of gravitation mass:  active mass and passive mass.  The passive mass is the mass that interacts with a gravitational field, and the active mass is the mass that makes the gravitational field.  There are excellent reasons that the two are the same thing, but again we won’t go into that.

Now, the interesting thing about the inertial mass and the gravitational mass is that they seem to be the exact same quantity.  It isn’t altogether clear why they should be the same quantity, after all the gravitational mass is related to the gravitational field, and unless you are dealing with a gravitational force, then the mass has nothing to do with the force, so why should acceleration be dependent upon it?  There is actually still work being done on this to try to explain why things work this way.  But, as I said, it is not clear why this must be the case, so theorists have discussed what might happen if the two were different, even by a very tiny bit.  The things that would happen if the gravitational mass and the inertial mass are not the same quantity are called the Nordtvedt Effect, after the theorist who proposed it.  So far, every experiment to detect the Nordtvedt effect has failed to yield any measure of it.  If the Nordtvedt effect did exist, it would be most apparent in very large masses, such as those of galaxies or galaxy clusters.  So, this is the realm of the astrophysicist.  Again, we see absolutely no evidence of any Nordtvedt effect.  So, the matter goes back to the theorists to explain why the inertial mass and the gravitational mass must be the same.  One theory that I have heard is that perhaps a body moving through space interacts with the virtual particles there (see my earlier space post) and that might cause the quantities to be the same.  I don’t know.  That seems to perhaps be stretching it, but I am not a theorist.

Interestingly, Einstein has shown us that matter and energy are interrelated.  His famous equation E = mc² shows that energy and matter can be interchanged.  Photons, which are not matter, can become matter.  In fact, high energy physicists sometimes quote the mass of a particle as how much energy it takes to make one.  For example, they may way that an electron has a mass of 511 keV, meaning that it takes 511,000 electron volts of energy to make an electron.  However, a problem does arise when you create matter from energy.  There seems to be a conservation law in effect.  Matter has some property other than just mass.   Some particles of matter also have electric charge.  The conservation laws of charge tell us that with any interaction, you have the same electric charge before the interaction that you did after the interaction.  So, if you make a proton, having charge +e, out of a photon, which has no charge, then you have a problem.  Likewise, if you made an electron, having charge –e, from an uncharged photon, you have a problem.  So, you might suggest that you simply need an energetic enough photon to make both and electron and a proton, thus their charges of –e and +e balance each other out.  Well, that is correct, their charges would balance out, and if conservation of charge were the only thing going on, that would be fine.  But, there is more that goes on here.  As it turns out, you have to conserve all kinds of other properties besides electric charge.  Furthermore, it appears that there is some other hidden variable that must be conserved, so you can’t just balance a proton with an electron.  The only thing that can balance the equation for these other properties for a proton is something exactly opposite of a proton, or to balance an electron would require something exactly opposite of an electron.  As it turns out, such things exist.  There are particles exactly opposite protons that we call antiprotons.  They have the same mass, the same spin, the same everything else, except that they have charge –e.  Likewise, there exist anti-electrons (which we call positrons), which are just like electrons, but with charge +e.  These antiparticles, we call antimatter.  You can even have a positron encircling an antiproton to make antihydrogen.  Having mass, you’d think that we’d just call these antiparticles matter.  Well, the sort of are matter, in that they do interact with matter particles and other antiparticles with gravitational forces.  However, there is something different about them, because if a particle and its corresponding antiparticle meet, they annihilate each other, and both are converted into pure energy.  Likewise, energy, say from photons, for example, always creates a particle and its corresponding antiparticle if you make matter out of that energy.  There have been all sorts of interesting ideas about what antiparticles really are, including a suggestion that they might be particles moving backwards in time.  So, a creation or annihilation event is simply the particle changing direction in its time path.

In an antimatter particle is the same as a particle, except for opposite charge, what about electrically neutral particles?  Particles such as neutrinos, actually seem to act as their own antiparticles.  That can add a whole layer of weirdness to this.  But, neutrons are not the same thing as antineutrons.  It turns out that neutrons are actually composed of smaller things, called quarks, and those quarks have electric charge.  The quarks composing a neutron simply have charge that adds up to zero.  So, antineutrons are composed of antiquarks, with each antiquark being the same as its corresponding quark, but with opposite charge.

It turns out that subatomic particles have a property that we call spin.  The name comes from the fact that a number of their behavioral characteristics can be accounted for by pretending that they actually are little spinning things.  Really, they aren’t.  They just have angular momentum.  Don’t ask me how they can do that if they are point particles and don’t really spin, like an electron.  It turns out that there are only certain spins that a particle can have, and an exchange of spin always occurs in units of a fundamental unit of angular momentum.  Particles can be classified by their spin behavior.  Particles that always have angular momentum that is an even multiple of this fundamental angular momentum are called bosons.  All the particles that mediate, or transmit, force between things are bosons.  However, particles that have spins that are always odd multiples of ½ of the fundamental angular momentum (such as +1/2, or -1/2), are called fermions.  Electrons, protons, and neutrons are fermions.  Particle physicists generally only consider fermions to be matter, even if some of the bosons have mass.  Fermions have an interesting property, one that without which the universe as we know it would not function the way that it does, and we would not be here.  That property is that no two fermions that can interact can be quite identical in every way.  They must have one property or another (angular momentum, spin, energy, etc) different from one another.  This is called the Pauli Exclusion Principle.  Without it, chemistry, nuclear physics, and all sorts of other things would not work.  You would not have stars, planets, galaxies, or people.

Fermions, themselves, can be grouped into two categories:  haydrons and leptons.  Haydrons are particles that can experience the strong force, and leptons do not experience the strong force.  Neutrons and Protons are haydrons, and electrons and neutrinos are leptons.  Haydrons are actually composed of smaller particles called quarks.  Haydrons can be classified as two types, baryons (composed of three quarks, and are fermions) and mesons (composed of two quarks, and are bosons).  As I indicated, many would not include mesons in matter, though I rather think of them that way.  Quarks are the only particles that interact using all four fundamental forces.  Interestingly, the current models suggest that haydrons are composed of a sea of quarks of all types, with only two or three of these quarks (called valence quarks) lending their properties to the haydron.  That they can be composed of a sea of quarks, allows one haydron to transform into another one by a change of valence quarks.  Thus, though neutrons have zero charge, the approximate mass of a neutron and proton combined, and they decay into a proton, electron, and neutrino if left alone for a while, they are most definitely NOT a combination of electron and proton, but are entirely separate particles from each. 

Dark matter, whatever it is, is not simply matter that is not lit up.  It is composed of something that does not interact via the electromagnetic force, and thus we are unable to detect it except by its gravitational effects (light is electromagnetic in character).  This suggests that it is most definitely not made of haydrons, because haydrons are composed of quarks, which do have electric charge and will interact with the electromagnetic force.

Ah, the joys of subatomic physics.  I get to confuse my general physics students with this stuff at the end of their second semester class.  I hope that I haven’t confused you too much.

-Astroprof

In the northern sky this time of year is the constellation Ursa Major. For those of us in the northern hemisphere, it is high in the northern sky, and up pretty much all night.  A bit south of the equator is is low in the northern sky, and up only for a while during the night.  If you are too far south, then you don't see it at all. 

Ursa Major is Latin for "Big Bear," and that is what this constellation is supposed to be:  a big bear.  However, I, like most people that I know, look at Ursa Major and immediately think mainly of the seven brightest stars in the constellation.  These stars make an unmistakable pattern in the sky that is one of the first patterns that anyone learns, and it is one of the few that most everyone that I meet knows.  Here in North America, these stars are almost universally known as the Big Dipper.  Four stars make a box shape that is the bowl of the dipper, and the remaining three are its handle.  However, this is only part of the whole constellation of the bear.  In particular, it is the back end of the bear.  Unofficial groupings of stars like these are called asterisms, rather than constellations.  The term constellation only refers to 88 official constellations recognized by astronomers.  The official list was decided in 1933 by the International Astronomical Union.  66 of these date back to the Romans.  Ursa Major is one of those 66.  However, all of the constellations, not just the 66 Roman ones, are named in Latin.  Though in the past, constellations were mental pictures made of the stars, modern astronomers don't think of constellations as patterns in the sky, anymore.  Instead, we think of the constellations as regions of the sky.  The entire sky is divided up into the 88 official constellations.  Now, before this time, every group of astronomers had their own set of constellations, most being the same, but some not.  So, this cleared up a lot of confusion.  Also, all sorts of cultures around the world have made constellations out of stars in the sky.  They are generally not the same, and sometimes not even the same groups of stars that make up the constellations.  There are lots of ways of taking the same set of stars and imagining pictures out of them!

As I said, the Big Dipper is only part of Ursa Major.  This group of seven stars is really very easy to spot, and so many groups of people over the years have made it a constellation of its own.  For example, many of the people of sub-Saharan Africa regarded these seven stars as a Drinking Gourd.  In parts of Africa, a hollowed gourd could be used to scoop water for drinking.  These areas of Africa where these stars were considered a gourd were where many people were taken to be slaves in the Americas, a sad note in our history.  These Africans brought with them many aspects of their culture, including some of their lore.  Much of that was lost, but the seven stars of the Drinking Gourd were so easy to spot and recognize, that it was no wonder that they remained.  As I said, even today, this is one of the first patterns that people learn, and one that sticks with them.  Well, the youngsters learning the stars from their elders had never seen a gourd.  However, they had seen metal dippers used to scoop water from barrels to drink.  So, the Drinking Gourd became the Big Dipper.

But, from the tale, it is obviously going to be an American asterism.  Indeed, in parts of Europe those seven stars had been seen as a Wagon or a Cart.  The four stars making the bowl of the dipper were the actual vehicle, with the other three being the handle with which it is pulled.  Other parts of Europe, particularly near Britain, saw this as the Plow, or more correctly, I suppose, the Plough (the British spelling).  The four stars that we regard as the bowl of the dipper were the blade of the plough and the other three were it's handle.  Think of a horse-drawn plow to get the picture of the plough.  (Now if that isn't a weird looking sentence, with both the American and British spellings of a word in it!  Probably those of you with a better literary sense than I are cringing right now!)

But, as I said, this part of the sky has been seen since ancient times as a bear.  How the Romans got a bear out of the stars of Ursa Major is sort of interesting.  What is more interesting is that these stars also seem to be a bear in a number of North American Indian nations.  This is baffling to me, since there is nothing even bear-like in these stars!  I seldom see a bear, even though I know that one is there.  The bowl of the dipper is the hind quarters of the bear.  His hind legs extend from what we'd think of the bottom of the bowl, and his front extends well past the front of the bowl.  The handle of the dipper is often portrayed as the bear's tail.  Huh?  Bears don't have long tails. Well, the Romans were notoriously anatomically inaccurate with their constellations.  H. A. Rey (author of Curious George) wrote a book some years back in which he formed new, easier to see patterns of the constellations.  He turned the bear around, and made the handle the nose of the bear.  It really looks more like a bear that way.  (Actually, Rey's book is a fantastic introduction to the sky for anyone wanting to learn enough to be able to go out and recognize some of the things that you see.)

But what do we make of the American Indians seeing a bear in the same set of non bear-like stars?  There have been several theories.  One idea is that this implies some interaction between Europe and the Americas.  We know that the Vikings made it to the American mainland, so perhaps they brought stories of the sky with them.  Or perhaps the earliest post Columbus explorers and settlers brought European constellations with them.  Native Americans could have picked up on these stars being a bear then, and then they could have incorporated that into their own tales.  Most of these early Europeans coming to America didn't really care what the people already here thought.  By the time that people started to take not of Native American star lore, the idea of these stars being a bear might have already become part of that lore.  And, naturally, there is always the way out there weird ideas that people come up with, like a group of aliens that look like bears visted both Europe and North America from stars in that part of the sky.  OK, I just had to toss that one into the mix.  As hard as it is to believe, I have actually heard that!  Of course, I've heard lots of crazy things.  (I might do a blog entry sometime on some of the wilder things that I've heard.  Hmm, I haven't done one like that in a while...).  But, trying to figure out how these stars got to be a bear in North America is a bit beyond my expertise.  I'll leave it to others.

Not all Native Americans have the same bear stories for these stars.  One that I like is a story that has it being a bear pretty much like the Roman one, with a long tail.  According to this tale, the bear was watching a fox one winter day.  This was before bears hibernated in the winter, and when they had long fluffy tails.  The bear observed that the fox would carefully walk out onto a frozen lake, and then use his teeth and claws to chip a hole in the ice.  He would then lower his long fluffy fox tail into the water.  A fish would come along and nibble on the tail thinking that it were some sort of strange weed.  The fox would quickly pull his tail out of the water, and the fish would come along with it.  The fox then would eat the fish.  Well, the bear saw all of this and thought to himself, "If Fox can do this, then so can I, for I am Bear, and I am much better than Fox."  So the bear went out onto the ice and punched a hole in the ice.  He then lowered his long fluffy tail into the water.  Sure enough fish came to nibble.  But, the bear thought to himself, "I am much bigger and hungrier than Fox.  One fish won't do for me.  I had better leave my tail in the water until several fish come along.  Then, I'll pull it out and eat them all!"  Well, soon, the bear lost feeling in his tail.  When he tried to sit up to find out what was wrong, he found to his horror that the water had frozen back, and now his tail was trapped in the ice!  He pulled and pulled, until finally he pulled loose, but his tail had come off and was still embedded in ice.  That is why bears don't have tails, and after this horrible incident, bears now hibernate all winter so that they don't have to go through this again.  (Of course, I am not sure how they could, since they don't have tails any more.)  Anyway, this tale tells a moral lesson to youngsters:  don't get greedy.  Or perhaps, it just tells them not to sit in freezing water.

Not all Native Americans had the same stories.  Another tale is that of three Indians hunting a bear.  The stars of the Dipper's bowl are the bear.  The other three are the braves hunting the bear.  As the sky rotates through the night, the stars of the bear always leads the hunters.  They are always chasing the bear.  But, every year in the Fall, they get close enough to the bear to wound him with a spear.  The bear then bleeds, and his blood spills from the sky onto the trees, turning the leaves red and orange.  But the bear always escapes, and the braves always keep after him.  Of course, the relative spacing and positions of the stars don't really change, but I guess that might take the fun out of the story.

And, while I am at it, I may as well relay a bear tale told to me by a park ranger with the Texas Parks and Wildlife Department.  This has nothing to do with stars, but it appealed to my warped sense of humor.  You see, my ranger friend was telling me of a friend of his who was a park ranger up in Colorado.  This Colorado ranger frequently led park visitors on a nature hike through the park.  Well, this park had bears in it.  These bears really didn't want much to deal with people, and if they heard people coming then they'd just walk away, so you never saw them.  However, you didn't want to walk up on one of them and surprise it.  So, they issued little bells to the hikers.  They made noise as they walked, and the bears would hear that and go somewhere else.  I guess that the nature walk must have focused on plants, since pretty much any wild animal would go somewhere else when a group of tourists came through.  The plants were stuck.  They couldn't run away.  Anyway, as they were doing their tour, they came upon some bear scat (a polite way of referring to the material that had passed through their digestive track).  Anyway, the ranger pointed it out, and told the visitors that this was bear scat, and it looked fairly fresh, so a bear must have been in that immediate area not long ago.  Well, as often happens with my students, there just had to be one of the tourists who would catch some notion and not let it go, making a pest of himself.  Well, on this tour, there was this one guy who wouldn't let the ranger get away with just saying that it was bear scat so a bear had been there.  He wanted to know what kind of bear.  Well, the ranger explained that you couldn't really tell from the scat.  There were two main types of bears in the park, though, black bears and grizzly bears, so it was probably one of them.  Well, the tourist wouldn't let it go.  He insisted that the ranger tell him what kind of bear it was.  The ranger patiently explained that both grizzly bears and black bears eat pretty much the same things, so you can't tell from the scat what kind of bear it was.  The size of the scat might give a clue, but that wasn't reliable either.  You simply can't tell from the scat what kind of bear left it.  Well, the tourist still wouldn't let it go, and the ranger really wanted to get on with the tour and not stand around discussing bear droppings!  So, he finally gave in and said that he'd investigate.  So, he took as stick and poked at the scat, stirred it up a bit, and then pronounced that it was a black bear.  As expected, the tourist still wasn't satisfied, and he wanted to know how the ranger knew that it was black bear and not grizzly bear.  "Well, " the ranger said.  "It is easy.  There are no bells in it."

-Astroprof

For a long time, astronomers have believed that planets form as a consequence of star formation.  The accretion disk feeding the protostar becomes a disk of material orbiting the young star.  This disk, called a proplyd, is the site of forming planets.  Otto Struve even proposed that planets are a natural consequence of star formation, not just an occasional thing.  So, an intensive search was on to try to find planets around other stars.  Several claims of extrasolar planets came forth, but were eventually disproved.  Finally,  in 1995, Michel Mayor and Didier Queloz, of Switzerland’s Geneva Observatory, presented the first conclusive evidence of an extrasolar planet orbiting the star 51 Pegasii.  But, this wasn’t really the first extrasolar planet found.  It was merely the first normal extrasolar planet.  What was an utter and complete shock to astronomers, and really blew my socks off, was the discovery in 1992 of two planets around the pulsar PSR 1257+12.  Now, let me explain why this was so shocking.

First, let’s discuss pulsars.  When a massive star, about eight or so times the mass of the Sun or more, dies, it does so in an amazingly violent manner.  The core of this star collapses from something about the size of the Earth to an object about the size of a large city.  This is an object so compressed that the atoms themselves are destroyed, and even most of the protons are transformed into neutrons.  In fact, most of the object is neutrons, so we call it a neutron star.  The energy released in forming this object then blows the rest of the star into space in a monstrous explosion.  This explosion, called a supernova is unbelievably violent.  A supernova momentary is so bright that it is as bright as an entire galaxy worth of stars.  The radiation and blast is pretty much enough to blow up anything nearby.  About all that can survive is the core of another star.  I might say more about supernovae in a later post.  For now, we are interested in the thing left behind:  the neutron star.  This neutron star has a massive magnetic field, and this magnetic field will likely be offset somewhat from the rotational axis, like so many rotating bodies with magnetic fields.  The magnetic field’s poles then serve as radiation sources, as radiation and particles stream outward.  As the rotating neutron star spins, these beams sweep around the sky.  If you happen to be somewhere in space where these beam sweep, then you will see a flash of radiation (mostly radio) at regular intervals.  This is a pulsar.  In other words, a pulsar is a neutron star whose rotation carries these beams of radiation past Earth on a regular basis.

So, a pulsar is a neutron star.  A neutron star is the byproduct of a supernova.  A supernova is an amazingly powerful and violent explosion.  Such an explosion should totally disrupt any planets that might be orbiting a star.  So what the heck is a planet doing orbiting a neutron star????  Well, to make it even more confusing, Aleksander Wolszczan and Dale Frail, the discoverers of the pulsar planets around PSR 1257+12 announced the discover of two planets orbiting the pulsar!  Adding even more confusion, in 1994, the same team of astronomers announced a third planet around this pulsar.  Then, in 2002, they announced yet another object orbiting PSR 1257+12.  All of these objects were found by the tiny gravitational effect that they have on the pulsar.  As the objects orbit the pulsar, they cause it to shift back and forth slightly.  The pulsar pulses at very regular intervals.  But, if the pulsar moves back and forth by a little bit, careful measurements can detect the tiny shift in arrival times of the pulses.  These pulses move at the speed of light.  The speed of light is FAST, but is it not infinite.  So, if the pulsar is slightly farther away, then the pulses arrive slightly late, and if it is slightly closer, then they arrive earlier.  Carefully studying the timing of these pulses shows the slight regular shift back and forth, revealing the presence of the planets.

The third planet found is actually the closest to the pulsar, and has a mass of only 2% that of Earth (about double that of our Moon).  Such a tiny mass makes for a tiny shift in the pulsar, making it a very difficult thing to detect.  The second and third planets are the ones first discovered.  These planets are similar in size, the second one being 4.3 times the mass of the Earth, and the third one being 3.9 times the mass of the Earth.  They are big enough and orbit close enough together that they have effects on each other’s orbits as the pass (the second planets orbits once every 66.5 days, and the third one once every 98.2 days).  This provides proof positive that they are really there, and that they are not an artifact of measurement.  The fourth object is really tough to call a planet, since it is a truly tiny thing, only 0.04% the mass of the Earth.  That makes it even smaller the Pluto, probably closer to the size of a large asteroid like Ceres.  This fourth object is also much farther out than the others.   It is 2.7 AU from the pulsar, whereas the other planets are located at 0.19 AU, 0.36 AU, and 0.47 AU (AU stands for Astronomical Unit, the average distance between the Earth and the Sun).  It is possible the this fourth object is the larger of a host of similar things located at this greater distance from the pulsar.  If so, it may signify the presence of an asteroid belt or a comet cloud, similar to our Kuiper belt.

Finding one object would be remarkable, as it is hard to imagine how such an object could survive a supernova explosion.  Four, or more, is just plain bizarre.  So, is this simply an anomaly?  After all, the universe is pretty big, and there are a lot of things in the universe.  So much so, in fact, that if something can happen, even if it is highly improbable, there are enough chances for it to happen that somewhere it likely will happen.  So, is PSR 1257+12 such an anomaly?  Perhaps.  But, perhaps not.

The problem with that interpretation is that another pulsar planet was found associated with pulsar PSR B1620-26 by Stephen Thorsett in 1993.  This one is another weird system.  It turns out that pulsar PSR B1620-26 was already known to be a binary system consisting of a pulsar and a white dwarf orbiting one another.  A white dwarf is what you get when a star smaller than 8 solar masses dies.  It is the collapsed core of a star, but it is only collapsed to about the size of the Earth.  The outer parts of the star are pushed off into space, but in a rather gentle manner compared with a supernova.  Thorsett’s finding was of a third body, about 2.5 times the mass of Jupiter, orbiting the pair of stellar remnants.  It is believed that this strange system results from the merger of two prior stellar systems, one of a neutron star and ordinary star, and a second system consisting of an ordinary star with a gas giant planet.  When the stellar systems passed close to one another, the original companion of the neutron star was ejected, and the new star and neutron star settled into orbit around one another while the gas giant went into orbit around the pair of them.  Eventually, the new star died and formed a white dwarf.

But, what about PSR 1257+12?  The same thing didn’t happen there.  Several theories have been proposed.  One is that perhaps the neutron star “stole” the planets from a passing normal star system.  But, that can’t really work, since it would be virtually impossible to steal a whole planetary system!  Another proposal has been that the original star had originally had several gas giant planets with large rocky cores.  The supernova stripped these gas giants of all but their cores, and these cores then spiraled close to the neutron star.  That might work for one or two such objects.  However, it doesn’t explain the smallest objects like the one found in 2002.   It definitely doesn’t explain how a belt of such things might exist, which is what this smallest object might signify.  This leads to the possibility that these are secondary planets, formed out of fallback from the debris cloud created by the supernova.

The suggestion that pulsar planets might be formed as secondary planets is strengthened by the announcement just a couple months ago by a team of astronomers let by MIT’s Deepto Chakrabarty of a dusty disk of material, similar to a proplyd, surrounding the pulsar 4U 0142+61.  This disk is believed to hold about 10 or so times the mass of the Earth worth of material.  If left alone, it may form one or more planets. 

So, this then leads to a new question:  How common are pulsar planets?  They are hard to detect, because they will only be detectable if they orbit neutron stars whose radiation beams pass by Earth.  But, most of the neutron stars will have radiation beams not too far from their rotational poles, so most motion of the pulsar due to planets will be side-to-side, which will not produce any detectable shift in pulse times.  Only back-and-forth motion is readily detectable, and that only if the beams of radiation originate from points on the neutron star fairly far from the poles.  So, this limits the number of possible cases where we could even hope to detect planets.  That we’ve found planets this way with PSR 1257+12 is amazing.  PSR B1620-26 is another strange case.  But, 4U 0142+61 is interesting because the dust disk was not detected by the shift of the pulsars, but rather by using the orbiting Spitzer infrared telescope.  That we have found this disk suggests that perhaps planets may form fairly regularly around pulsars.  Clearly, this is an area that needs lots more work.

-Astroprof