Seeking Solace and Sciencewoman asked questions pertaining to the definition of a planet (as in the new object discovered beyond Pluto) --- a topic that came up on Tom’s page, too.  Fly Girl asked me for a beginner’s topic for those of my readers who aren't astronomers.  So, I figured that a good topic might be defining a planet.  However, this will be a long post, I am afraid.

Defining a planet seems a simple task.  After all, we all know what a planet is, right?  Well, maybe it isn’t so simple.  So, I looked in a dictionary, and it said that a planet is “a large non-luminous body orbiting the Sun, as in the nine planets of the Solar System.”  Great.  We define a planet as something that is a planet.  That is real useful.  So, how do we know if the new object announced last summer is a planet or not based on this definition?

So, let’s go back.  The word planet comes to us from the Greek, and it means “wanderer.”  The ancients noticed that most of the stars stayed put, and so they made patterns in the sky that stayed the same for as long as anyone could remember.  There were five exceptions, though.  These stars appeared to wander through the constellations.  They were Mercury, Venus, Mars, Jupiter, and Saturn.  These were the planets.  Sometimes included on the list is the Moon, since it is also in different constellations on different days.  Even the Sun moves around in the sky, sometimes being high in the sky, and sometimes low.  So, that gives seven objects that move around.  We have seven days of the week.  Saturn’s day, the Sun’s day, the Moon’s day, and so forth.  The others are named after non-Roman gods, but in Latin, French, or Spanish, we see that Tuesday is Mars’ day, Wednesday is Mercury’s day, Thursday is Jupiter’s day, and Friday is Venus’ day.

OK, this is nice, but doesn’t help our quandary.  Copernicus’s idea that the planets and the Earth all orbit the Sun led Giordano Bruno to presume that meant that Earth was a planet and that the planets were worlds like Earth.  When Hershel discovered Uranus, there was little doubt that he had found a new planet.  But, comets had been known since ancient times, and no one really argued that they counted as planets.  Then, Ceres, Vesta, and other small objects were found with orbits between those of Mars and Jupiter.  They became popularly called “asteroids”, though astronomers refer to them as “minor planets.”  The largest asteroid is only a tiny fraction of the size of a planet … not quite a thousand miles across, so they seemed to be surely a different class of object from a planet.  Then Pluto was discovered.  At first, it was mistakenly thought to be larger than Mars, and thus clearly a planet.  Eventually, though, we found it only a bit larger than the largest asteroid.  But, it was on the list by that point, and there was nothing between the size of the largest asteroid and Pluto.  Then, we found other Kuiper Belt Objects (see my previous two posts).  Some of these were nearly half the size of Pluto.  This raised a question as to how big does something have to be to be a planet.  Eventually, we found objects such as Quaoar, which were over half the size of Pluto, and then Sedna, nearly 80% the size of Pluto.  Finally, we found an object, provisionally designated 2003UB313 which is actually a bit larger than Pluto.  So, is it a planet?  Is Pluto a planet?  Hmm.  We are back to our lousy definition of a planet.

So, perhaps we can look at another definition.  Since asteroids are too small to be planets, then perhaps size is the key.  But what size do we use?  Is it fair to pick an arbitrary size, say Pluto sized or larger, so that Pluto and 2003UB313 are planets, but Sedna isn’t?  What about an object 97% the size of Pluto, and the same composition and structure.  Is it a planet?  How about 93% the size of Pluto, or 88%, or 75%?  Where do we draw the line?  What about saying that a planet has to be bigger than our Moon to count?  That leaves both Pluto and 2003UB313 off the list?  Is it fair to pick an arbitrary size and make that the definition?  Probably not.

OK, then perhaps some other size should be picked. The larger an object is, the more gravity it has.  Small objects have low gravity.  With low enough gravity, then the tidal forces in the Solar System will pull an object apart.  If we define a planet as something whose on gravity holds it together, then the Solar System has several million planets, including all the asteroids and comets.  OK, that is too generous.  Most of these small objects, are irregular in shape.  It takes a certain amount of gravity to pull protrusions down and to make an object more or less spherical.  This process is called isostasy.  So, do we define a planet as being large enough for isostasy to cause the body to be spherical?  Using this definition, nor only are Pluto and 2003UB313 planets, but so are Sedna, Quaoar, Ceres, etc.  There are well over a dozen such objects.

To make it even more confusing, what do we do with the Moon, or the major moons of Jupiter and Saturn?  They are larger than Pluto, and they have been pulled into spherical shapes.  Furthermore, Io and Europa have volcanism, there is rain on Titan (granted its methane, not water), Triton has geysers, and there is something that looks a lot like plate tectonics going on with Ganymede and perhaps Enceladus.  Titan and Ganymede are even larger than the planet Mercury, and Callisto and Triton are only slightly smaller.  Do these count?  Is it fair that they meet all criteria for a planet except that they orbit another planet?  How about Charon, Pluto’s moon?  It is over half the size of Pluto?  Is this a planet and moon, or a binary planet?  Furthermore, our own Moon is only slightly smaller than Mercury.  It would surely count as a planet if it were in orbit around the Sun.  Why does the Earth-Moon system count as a planet and moon rather than a double planet?  If Pluto counts because of its size, then the Moon should, too, right?  The Moon is still significantly larger than 2003UB313.

So, perhaps another definition should be used.  We know that Earth is rocky, as are Mercury, Venus, and Mars.  Pluto, though, is composed mostly of ices (we think), with rock being a small percentage of the planet.  So perhaps, we should say that a planet is a large chunk of rock.  Well, that still leaves the Moon on the list, but leaves Jupiter, Saturn, Uranus, and Neptune (the four largest objects in the Solar System other than the Sun, itself) off of the list!  These bodies are called gas giants because hydrogen and helium compose a major part of the planet’s mass.  In fact, these planets don’t even have solid surfaces.  The deeper you get into the planet, the higher the pressure, until hydrogen is compressed into a liquid.  Hmm. Maybe, some other definition would help.

How about we consider how planets form?  There are two models, nebular condensation and accretion.  Under nebular condensation, the planets start forming when the Sun is forming.  The final stages of the Sun’s formation consist of a vast disk of material swirling around feeding the forming Sun.  This is called an accretion disk.  If planets are forming in the disk, it is a proplyd.  Nebular condensation says that instabilities cause portions of the disk to get thick, and then the gravity of those thicker patches pulls in more material until you have a planet.  The accretion model says that small pieces of material in the proplyd come together to form small bodies loosely held together.  Some of these run into each other, forming larger objects called planetessimals.  The planetessimals collide to form planets.  Close to the forming Sun, it is too warm for ice to be stable, so the small objects are mostly rocky.  Farther from the Sun, you get a mixture of rock and ice.  Water is very common in the universe, so there would be perhaps more stuff in the outer Solar System to make planets with, and so they would be bigger.  If the planet is big enough, then it has the gravity to hold onto hydrogen and helium, the two most common elements in the universe.  Cool.  This model seems to fit what we see.  However, when you work through the mathematics, you see that Jupiter and Saturn are too big for this model.  So perhaps Jupiter and Saturn formed through nebular condensation and the Uranus, Neptune, and the inner planets formed through accretion.  Doing a bit more computation, you find that as Uranus and Neptune get bigger, their gravity interacts with one another, and they migrate farther outward in the Solar System, throwing vast amounts of material and smaller bodies even farther out, where they are too thinly distributed to ever come together to form anything larger.  This populates the Kuiper Belt.  Studies of asteroids and comets to date show that many, if not most, of these bodies are very loose collections of material, barely held together.  This fits with the accretion model.  However, some asteroids seem to come from larger bodies, perhaps as large as planetessimals that collided destroying one another.  Under this model, Pluto and 2003UB313 would simply be a couple of these icy things tossed out by the migration of Uranus and Neptune. 

So, if accretion is how you define a planet, then Jupiter and Saturn are again off the list, but you might argue that Ceres and perhaps Vesta (two very large asteroids)  should be on the list.  We don’t really know the structure of these bodies, but they may be closer to planetessimals than aggregates like Ida, Mathilda. or Eros, three asteroids that we have recently sent unmanned spacecraft to study.  Even more problematic is that the moon systems of Jupiter and Saturn probably formed from accretion during the formation of those bodies, so that means that Titan, Ganymede, and several others should be on the list of planets.  If you say that nebular condensation makes a planet, then only Jupiter and Saturn make the list!    Then, you have another issue.  Jupiter basically has the same composition as the Sun.  If it were much larger, then Jupiter would have enough mass to compress its interior to the point that nuclear fusion starts and it would be a star.  But, if a star has too little mass, it won’t fuse hydrogen.  Such a failed star is called a brown dwarf.  At what point do you differentiate a small brown dwarf from a very large planet?  We have found planets and brown dwarfs around other stars.  Nearly 150 planets have been found around other stars, and many dozens of brown dwarfs have been identified.  So far, we have found planets no more than tens of times the mass of Jupiter, and brown dwarfs nearly 100 times the mass of Jupiter, but we can easily imagine objects in between in size.  Are they planets or brown dwarfs?

So, have I cleared things up?  Yeah, right.  Sadly, astronomy, one of the oldest sciences, has never really developed a definition of “planet.”  We are working on this, and hope to have one soon, but whatever definition is picked will surely upset somebody.

Personally, I don’t think of Pluto as really a planet.  It is a giant comet nucleus-type-thingy.  That leaves 8 other objects that are clearly planets.  But, I tend to think of the Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton as being planetary in nature, due to their size and structure.  Triton may be a bit of a stretch, since, though it is nearly the size of Mercury, it seems to be a giant version of Pluto itself.  It has a bizarre orbit around Neptune, too, indicating that it may be a captured object.

So, you asked.  By the way, this will be part of my first lecture tomorrow in my planetary astronomy class.  I wish that I could have had a better answer for y’all, but unfortunately the term “planet” seems to be an artificial designation that does not really apply in nature.  There is a continuum of objects in the Solar System from very small to very large.  In other star systems, objects even larger still exist.  Even the dividing line between planets and stars begins to get hazy. 

-Astroprof 

Looking at my last post, it seemed to ramble a bit.  It is interesting that I seem to be able to write much more coherently about astronomy topics than I do about such intangible things as I did on the last post.  Hmm.  Perhaps that is why I went into the sciences in the first place.  On the other hand, it could have just been a response to the turn of conversation that happened over dinner with a group of friends earlier.  Some topics just leave me sort of on edge for some time thereafter.

Six years ago, a Delta rocket lifted off from the Cape Canaveral Air Force Station (located adjacent to the Kennedy Space Center, in Florida) carrying the Stardust space probe.  The goal of the Stardust mission is to bring back to Earth a sample of cometary material.  Using a gravitational boost from Earth, the spacecraft was in a position to capture this material  from Comet Wild-2 two years ago.  It is now returning to Earth in a matter of hours. 

The rocket used to launch Stardust did not have the capability of launching it directly to the comet, so it launched it into an orbit that brought it past Earth again three years after launch.  As it passed Earth, its orbit was such that Earth’s gravity pulled on the spacecraft speeding it up.  This had the effect of putting the spacecraft into an orbit that carried it farther from the Sun, just as if a much larger rocket had been used to launch it.  This sort of gravitational boost is sometimes called the “slingshot effect,” and it has been used for numerous other space missions, including Mariner-10 to Mercury, and both Voyager missions to the outer Solar System.

Comets are composed of ices such as water ice, dry ice, frozen methane, frozen ammonia, various organic molecules and silicates, and some rock.  Comets are interesting objects because they form in the outermost portions of the Solar System, where the cold keeps them in a deep freeze until something disturbs their orbits so that they fall closer to the Sun.  The frozen gasses and ices then melt (or sublimate, actually) and spew out, where they are pushed away from the Sun to give a pretty (but often dim) blue tail.  The dust is also spewed out, and it curves away from the comet to give a bright yellow tail.   Since the comets are effectively in a deep freeze since their formation billions of years ago, they are expected to hold clues to the formation of the Solar System.  It has also been speculated that the organic material of comets hitting Earth early in its history provided the seeds that were needed for life to begin on our planet.

Stardust is the first mission to return material from a comet to study in laboratories here on Earth.  Two years ago, on January 2, 2004, it was in position to collect samples of Comet Wild-2.  Actually the comet was moving much faster than the spacecraft, so it was positioned in the path of the comet, and the comet overshot the spacecraft, passing within 150 miles of Stardust.  The spacecraft had deployed a collector looking like an overgrown tennis racket to collect some of the dust and gas particles spewed out by the comet.  Interestingly, the same collector had been used previously to capture some other interesting samples from space.  The back side of the collector was deployed to collect samples of an interstellar particle stream that is passing through the Solar System.  This interstellar stream was discovered shortly before the completion of the Stardust craft, and its orbit was to take it through the stream.  The samples collected at that time will be the very first samples ever to return to the Earth from outside of the Solar System, and astronomers await a study of the interstellar particles as much as they do the comet particles.

The sample return capsule will enter Earth’s atmosphere and will land near Salt Lake City, in Utah.  This is a similar sample return as that used by the ill-fated Genesis spacecraft which collected samples of the solar wind to return to Earth.  After reentry, though, the parachutes did not open on the sample return capsule, and the capsule plunged to Earth, breaking open, and contaminating the samples.  It is believed that this was an isolated incident, and that the same thing will not happen to the Stardust sample return capsule.

I could keep going on, but I don’t want to bore you with all the details.  I did, however, think that it might be interesting to blog a bit about this historic event.

Good luck Stardust!

-Astroprof

I’ve been neglecting the blog for the last couple of days.  This week is the week of meetings before classes begin, so I’ve been pretty busy.  There is always so much to do!

A few days ago, Tom wrote about an Italian plan to construct an observatory on the surface of the Moon.  I thought that I’d say a few things about this.

The idea of putting an observatory on the surface of the Moon dates back to a suggestion to do just this made by Lyman Spitzer in 1946.   The idea was to get a telescope outside of Earth’s atmosphere.  Only a small fraction of the light from distant objects makes it through our atmosphere.  A telescope outside of the atmosphere would be able to collect data using these wavelengths of light.  Furthermore, even the light that does pass through the atmosphere is distorted and attenuated, and a telescope outside of Earth’s atmosphere would not have to deal with these problems, either.  Spitzer regarded the Moon as a logical place for a telescope.  Remember that the way data was collected in the middle of the 20^th Century was by astronomers working at a telescope reading dials or changing film plates.  Automated computerized instruments were not technologically possible, and even science fiction writers seldom dreamed of such things.  So, Spitzer presumed that any such telescope must be at a manned observatory.  A moonbase would seem a logical place for such an observatory.  Orbiting telescopes seemed totally out of the question as any observations would require time exposures, necessitating keeping a telescope trained on an object for an extended time.  At that time, it seemed impossible for an orbiting satellite to maintain such stability.

However, the Hubble Telescope proves that it is indeed possible to have an automated instrument on an orbiting satellite.  The technology that made HST is now even more advanced, so any new space telescope could be made even better.  A telescope in low orbit would be much cheaper than one on the Moon, so why would anyone today still think of a lunar observatory?  Well, one advantage is that it would be easier to build a telescope on the Moon than to build it in orbit.  This means that such a telescope would not be limited to the maximum size that a complete instrument could be launched into orbit.  The individual pieces could be launched and then assembled on site.  This would permit a larger telescope.  Construction of the International Space Station, though, has allowed some perfection of weightless construction techniques, so perhaps this is less of a concern than it used to be.

So, besides cost, what else might make a lunar observatory less attractive?  Well, there are two major limitations for a lunar observatory.  First, a lunar surface observatory would have half of the sky blocked off by the Moon itself.  An orbiting telescope can swing and point in any direction, so more of the sky could be viewed at any time.  A much more serious problem, though, would be moon dust.  This is actually a serious issue.  The moon dust picks up a static charge from the solar wind, and it sticks to everything.  The Apollo astronauts had a terrible time with moon dust getting all over the place and it was very hard to clean up, since it stuck to everything.  Even worse, you don’t have to stir up the moon dust in order for it to be a problem.  The static charges cause the moon dust particles to repel each other.  Dust particles then levitate above the surface of the Moon up to a height of several meters.  So, the telescope’s mirror would gradually become coated in a dust that would be very hard to get rid of.  This would seem to make putting an observatory on the Moon a problem.  Now, if we ever go back to the Moon and set up a permanent, or semi-permanent, base of operations, then it may not be such a bad idea to have some astronomy capability at the moon base, but that should not be the primary goal of such a base.

So, I’ve given some of the problems with a lunar observatory.  What might be unique advantages over orbiting platforms?  For Lyman Spitzer, the stability of the lunar surface was a primary advantage.  As we have seen, though, modern technology makes that much less of an issue.  However, one of the disadvantages of a lunar observatory can be made into an advantage.  Remember that the Moon itself blocks half of the sky as seen from the surface of the Moon.  The Moon always keeps one side towards the Earth, so the other side is always facing away from the Earth.  A telescope on that side of the Moon is shielded from Earth.  Now, we don’t really worry about sunlight reflected from Earth, because in space there is no atmosphere to scatter light, so that is not an issue.  You just look away from the Sun and Earth, and the sky is black.  There is no advantage to put the telescope on the far side of the Moon if you are just interesting in blocking light from Earth.  However, there is an advantage to the far side of the Moon if you want to block radio signals.  Humans produce an enormous amount of radio signals, and these can (and do) interfere with radio astronomy.  This interference is only getting worse.  A radio telescope on the far side of the Moon would have all of this manmade radio noise blocked.

So, it would seem that the surface of the Moon would not have any advantages for situating any but a radio telescope on the lunar far side.  This makes the Italian proposal, if it is true, somewhat surprising and confusing.  For those who know me, you’ll find that this is a reversal of my position on a lunar observatory from 10 years ago, when I argued that the Moon would be a great place for an observatory.  The technology, though, has advanced to the point that an orbital telescope would likely be much better, with the exception of perhaps a lunar radio telescope, or perhaps a VERY large telescope that would be too large to manage as an orbital instrument.  This is not the Italian proposal, though.

As a footnote, though, it is interesting to note that there actually has been a telescope on the Moon.  In 1972, Apollo 16 astronauts set up a small 3 inch ultraviolet telescope which was used to observe several celestial objects.  To date, this was the only astronomical telescope to be used on the Moon, and it was used for such a short period of time that moon dust was not a major issue.

-Astroprof