Sunday, January 3, 2010

The Death of Planets

The Death of Planets.  No, not the planets themselves.  The idea or concept of what we have recently called planets is doomed, with the demotion of Pluto to a dwarf planet being the most recent step in a process that began with Copernicus.  Then end game will be the realization that the concept of a planet as a meaningful astronomical class of objects is antiquated and archaic.  The concept of planet is no longer compatible with what we know about the Universe.  The bickering by a minority of astronomers about the definition of a planet, and which has achieved tremendous exposure in the public arena, is as pointless as the flat Earthers arguing about whether the flat Earth is rectangular or circular. 

Since humans first looked up at the night sky, they noted that there were some objects, often brighter than the rest that did not follow the other objects in their regular movement across the sky.  The ancient Greeks labeled these objects as “The Wanderers”, or as we know them, planets.  Thousands of years ago, the segregation of these special objects from the rest of the celestial chaff made sense; they were clearly different than all the lights in the night sky, and there was no mistaking what was a planet and what was a star.

       The path of Mars against the fixed stars as viewed from Earth. Click on Image to see animated version.  (Image Source: Unknown) 

Then came Nicolaus Copernicus (19 February 1473 – 24 May 1543) and Galilei Galileo (15 February 1564 – 8 January 1642).  In less than a century, the crystalline spheres upon which the stars rode and the Ptolemic epicycles that described the motion of the planets, came crashing down.  The wanderers were in orbit about the Sun, not the Earth, and they were not just points of light, but other worlds.  Some like Jupiter, had moons, just like Earth.  Luna, the Earth’s moon, had mountains and craters.  And there was more, much more.  The skies were crowded with stars that could not be seen with the naked eye, but which revealed themselves through magnification (this includes nebula and galaxies which could not be easily distinguished from individual stars back then).  The origin of the “milkiness” of  the Milky Way became obvious—it was composed of stars, packed so closely together in such great density that they appeared as a hazy cloud. 

Until 1781, the number of known planets remained at six, all of which could be observed by the naked eye without the aid of a telescope (Mercury, Venus, Earth, Mars, Jupiter, and Saturn).  Sir William Herschel discovered Uranus in 1781.  Neptune was discovered by a Ph.D. student, Johann Galle in 1846, based on predictions provided by Urbain Le Verrier, which explained the small orbital perturbations of Uranus.  The search for a ninth planet (at the time needed to explain additional perturbations in the orbits of Neptune and Uranus, but no longer needed now that the masses of the previous eight planets are better constrained) was ended in 1930 with the discovery of Pluto by Claude Tombaugh.  Pluto was a bit of an odd planet.  It was very small, but more importantly, it orbited in a plane about the Sun that was significantly different than all the other planets.  Neither Uranus, Neptune nor Pluto can be seen without a telescope.

The orbits of the classical planets about the Sun.  All but pluto orbit in nearly the same ecliptic plane. (Image Source: Unknown)

Oddly enough, what we now call an asteroid named Ceres, was discovered in 1801 by Giuseppi Piazzi.  Like the other known planets at the time, it was a wanderer, in orbit about the Sun between Mars and Jupiter.  By all accounts Ceres was a planet, and although not known at the time, it was spheroidal like a planet.    Indeed, it was considered a planet at the time of its discovery.  Within a year or two, the inner Solar System began to get a lot more crowded.  Pallas, Juno, and Vesta were discovered, also in orbit about the Sun between Mars and Jupiter.  It was not long before dozens and dozens of these objects were found.  By the turn of the century, there were hundreds.

The known objects in the inner Solar System, including asteroids (white) and trojans (green).  Hundreds of additional objects are discovered every year.  (Image credit: Unknown; data source: Minor Planet Data Center).

For no other reason than their location between Mars and Jupiter, these objects which had all the same orbital properties of the classical planets, were put into a new class of objects, called asteroids, as suggested by Sir Herschel (the discoverer of Uranus).  However, for many years, the terms planets and asteroids were used interchangeably in the scientific literature.  By the time thousands of the objects had been discovered, the need to distinguish within the literature the classical planets from these minor planets led to the acceptance of the term asteroid (as well as minor planet), which has remained to this day.  Importantly, at the time, the distinction between asteroids and planets was merely one of convenience. 

Asteroid and former planet Ceres, as viewed through the Hubble Space Telescope by my friend and colleague J. Parker at Southwest Research Institute.  Ceres is massive enough that its shape is spheroidal.  (Image credit:  NASA)

For more than a hundred years after the discovery of the first asteroid, Ceres, the known objects in the Solar System consisted of the Sun, the classical planets and their moons, the asteroids, and the occasional comet.  Then, suddenly, the population of the Solar System once again exploded.

Dave Jewitt and his former graduate student Jan Luu found the first Kuiper Belt Object (KBO) in 1992.  KBOs are found outward from the orbit of Neptune to ~55 AU.  Pluto is within this region and is one of the  largest KBOs.  Thousands of these objects are now known to exist, including Eris, which is thought to be slightly larger than Pluto, and Quaoar, Makemake, Haumea, and Ixion, all of which are at least half as large as Pluto.  Pluto is not alone in the outer Solar System, nor is it unique. 

 Known Outer Solar System Objects consist mainly of KBOs. (Image source: Unknown; Data obtained from Minor Planet Center).

Beyond the Kuiper Belt there has been theorized to exist the Oort cloud, filled with additional icy debris left over from the early formation of the solar system.  It is from the Oort cloud that comets may originate.

We now know (thanks to increasingly powerful telescopes and spacecraft exploration) that objects in the Solar System come in a remarkable variety of shapes, sizes and compositions.  The classical planets are all spheroidal.  The inner planets and asteroids are rocky.  The outer planets are gaseous or liquid metal.  The KBOs are icy (probably mostly methane and water ice).   The largest of the asteroids and KBOs are also spheroidal; this an inescable consequence of physics—at some point an object becomes large enough that its self gravity causes it to collapse upon itself and reaches so-called hydrostatic equilibrium.   At the very small end of the size distribution, we know there exist small grains of dust.  Thus, the Solar System is populated by objects smaller than a dust grain and as large as the Sun with a continuum of objects in between.

Asteroids can occasionally present a hazard to the Earth (just ask the dinosaurs).  Recent efforts to survey the asteroid belt for those objects that may pose a future hazard has resulted in a very well characterized size distribution.  As the size of asteroids decreases, the number of asteroids increases.  In other words, there are just a few very large asteroids (Ceres, Vesta), and thousands upon thousands of the very small (under 100 m).  

The size distribution of asteroids surveyed by the Sloan Telescope.  The population number is normalized the population of 10 km objects.  The asteroid population is a continuum, as is the population of objects in the Solar System.  (Image credit:  Unknown).

If the origin of the classical planets and their moons, the asteroids, the KBOs, and the uncountable dust grains were different, these different origins might serve as a means by which to establish a scientific categorization.   However, the origin of all these objects is the same.  All the objects are the result of countless collisions and gravitational collapse of a protoplanetary disk.   It is the collision and accretion that results in the nearly log-normal size continuum of objects in the Solar System.  Asteroids are just the bits of rubble left over that were not accreted by larger objects.   The asteroids themselves may very well be conglomerations of smaller bits of dust and rock rather than the more solid monolithic structures of the science fiction genera.  KBOs are the bits of rubble in the Outer Solar System that were not accreted.

So here we are today.  We inherited the term planet, originally used to describe the wandering nature of objects against the more predictable background stars.  Millennia ago, we were aware of only a handful of these objects.  In time we found that the wanderers were not only distinct from stars in their journey across the sky, but that they were not stars at all; they really were in a class by themselves.  The idea of planet made sense.  Then, we found that there were more than just a handful of planets.  First there were dozens, then hundreds, then thousands, then tens of thousands!  It still makes sense to distinguish these objects from stars, as they are quite clearly different (for example, there is no nuclear fusion and they are not at the center of the Solar System). 

What does not make scientific sense is to further scientifically categorize the range of solar system objects.  Doing so inherently requires defining arbitrary defining lines in a size continuum.  While such dividing lines can certainly be legislated by such bodies as the International Astronomical Union (IAU), they have no meaningful basis, and such an exercise merely brings to light the hubris of man trying to sort nature into boxes and bins.

In time, the concept of a planet will find its way to the dust bins of astronomical rubbish, atop celestial spheres and the geocentric model of the Solar System.  Astronomers of the future will recognize the continuum of objects in our Solar System and in extra solar systems, too.  Any distinction between objects will be one of convenience rather than of scientific purpose, just as asteroids were once distinguished from planets solely for convenience.  Historians of science will study the transition of the Classical Planetary Model to the Modern Solar System Continuum Model, and children of the future will chuckle at the silly argument made over whether Pluto is a planet, just as we all chuckle over the argument of a circular or rectangular flat Earth.

The concept of a planet is dead.