Eight is Enough?

Creative Commons License photo credit: CommandZed

Two years ago this month, the International Astronomical Union adopted a new definition of ‘planet’ which excludes Pluto. Not only do I, as Planetarium Astronomer, continue to get questions about Pluto’s ‘demotion’, but scientists themselves continue to debate it. Right now (August 14-16, 2008), a conference called “The Great Planet Debate:Science as Process” is underway at the John’s Hopkins University Applied Physics Laboratory in Laurel, Maryland. The saga of Pluto and of the definition of ‘planet’ offers some insight into our solar system and into how science works.

northern tier sky
Creative Commons License photo credit: truello

The definition of ‘planet’ has changed before. Ancients looked at the sky and saw that certain ‘stars’ in the sky changed position, while most stars seemed to form the same patterns all of the time. The Ancient Greeks called the moving stars ‘planetes‘, or wanderers–this is the origin of the word. The Moon, too, appears near different stars each night. The Sun’s apparent motion is less obvious, since we don’t see the Sun and stars at the same time. Careful observers, however, can see that different stars rise and set with the Sun at different times of year. The full list of ‘planetes’, then, included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Astrologers still use this archaic definition of planet).

Thanks to Copernicus and Galileo, people began to realize that the Sun, not the Earth, was the center of the solar system. The definition of ‘planet’ changed from ‘object which moves against the background stars’ to ‘object in orbit around the Sun’. The Sun and Moon, which had been planets, no longer were.

The position of Uranus, discovered in 1781, seemed to fit a pattern described by astronomers Johann Titius and Johann Bode. That same ‘Titius-Bode rule’ also predicted a planet between Mars and Jupiter, so when Giuseppe Piazza discovered Ceres at just the right distance in 1801, it was considered a planet. By 1807, four new ‘planets’ had been found between Mars and Jupiter (Ceres, Pallas, Juno, and Vesta). By the middle of that century, however, dozens of these new objects were being discovered; up to 100 had been found by 1868. It thus became clear that astronomers had in fact found a new category of solar system object. Astronomers adopted the term ‘asteroid‘, which William Herschel had recommended in 1802; ‘planet’ was redefined to exclude very small objects that occur in bunches. This is how science works; we must constantly revise even long standing definitions as we learn more about the universe around us.

In the late 19th century, astronomers noticed that Uranus and Neptune seemed to deviate ever so slightly from their predicted positions, suggesting that another planet was perturbing them. in 1906, Percival Lowell started a project to find the culprit, which he called ‘Planet X’. In 1930, Clyde W. Tombaugh located Pluto in sky photographs he took at Lowell Observatory in Arizona. It soon became apparent, however, that Pluto was not massive enough to influence the orbits of Uranus or Neptune. Throughout the mid 20th century, astronomers continued to revise Pluto’s estimated size downwards. From 1985 to 1990, Pluto’s equator was edge on to us, such that we saw its moon Charon pass directly in front of and behind Pluto’s disk. This allowed scientists to measure Pluto’s diameter more precisely, proving that it had not been the Planet X that Percival Lowell sought. Pluto’s diameter is just under 2400 km, a little less than the distance from the Rio Grande to the US/Canadian border. Pluto’s discovery, it turns out, was an accident.

In addition to small size, Pluto has an unusual orbit. Planetary orbits are ellipses rather than perfect circles. The eccentricity of an ellipse indicates how ‘out-of-round’ it is on a scale from 0 (perfect circle) to 1 (parabola–far end at infinity). Pluto’s orbit has an eccentricity of about 0.25, much greater than that of planets such as Earth (0.01) or Venus (0.007). The planets orbit nearly (but not exactly) in the same plane; Mercury‘s orbit, inclined by 7 degrees, is the most ‘out of line’. Pluto’s orbit, however, is inclined by 17 degrees.

Released to Public: Solar System Montage (NASA)

Behold: a pluto-less solar system.
Creative Commons License photo credit: pingnews.com

We divide the planets of our solar system into two categories: the inner planets (Mercury, Venus, Earth, and Mars) which are made mostly of rock, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) which are gas giants with no solid surface. Pluto, however, fits in neither of these categories, as it is made of ice and rock (by some estimates, it’s 70% rock and 30% ice; by others, it’s about 50/50).

With its small size and abnormal orbit and composition, Pluto was always a misfit. Textbooks noted how Pluto fit in with neither the rocky inner planets nor the gas giants in the outer solar system. Still, Pluto remained a ‘planet’ because we knew of nothing else like it. There was simply no good term for what Pluto is.

That began to change in 1992, when astronomers began finding Kuiper Belt objects. The Kuiper Belt is a group of small bodies similar to the asteroid belt. Kuiper Belt objects (KBOs), however, orbit beyond Neptune’s orbit. Also, the Kuiper Belt occupies more space and contains more mass than does the asteroid belt. Finally, while asteroids are made mostly of rock, KBOs are largely composed of ice, including frozen ammonia and methane as well as water–just like Pluto. In addition to the Kuiper Belt proper, there is a scattered disc of objects thought to have been perturbed by Neptune and placed in highly eccentric orbits. Objects in the Kuiper Belt, scattered disc, and the much more distant Oort Cloud are together called Trans-Neptunian Objects (TNOs)

With the discovery of more and more KBOs, astronomers began to wonder if Pluto might fit better in this new category. Not only was the composition similar, but there is even a group of KBOs called plutinos, with orbits similar to Pluto’s. In the Kuiper Belt and the scattered disc, astronomers began to find objects approaching Pluto’s size, including Makemake, Quaoar, and Sedna.

Pluto can't get no respect
Pluto takes advantage of the wildly (?)
popular LOLcats to plead its case
with mankind.
Creative Commons License photo credit: the mad LOLscientist

To call Pluto a planet, but not these others, seemed arbitrary.

Finally, in 2005, a team of astronomers located Eris, which is slightly bigger than Pluto. Clearly, Eris and Pluto are the same kind of thing; either both are planets or both are not. If they both are planets, however, then should we include Quaoar et al., above? We have only just begun to explore and understand the Kuiper Belt and the scattered disc. Might we eventually find dozens of new ‘planets’ like Eris? Hundreds? Thousands?

This is what led the International Astronomical Union to reconsider the definition of ‘planet’ two Augusts ago. The IAU decided it was simpler to limit the number of planets to eight (Mercury through Neptune) and classify Pluto (and Eris, Quaoar, et al.) among the Trans-Neptunian objects. A new term, “dwarf planet,” includes the biggest asteroids and TNOs–those big enough to have assumed a spheroid shape. Still, other astronomers remain dissatisfied, hence the discussion going on in Maryland now.

There are two things we must keep in mind if we’re wondering when the Pluto question will be ‘resolved.’ First, decisions and conclusions of scientists are not holy edicts to be obeyed and never questioned. Quite the contrary, all such conclusions are provisional, pending new discoveries and better information. Any new decision reached this weekend is likely to be revised when the IAU meets again in 2009, and again in 2015 when the New Horizons mission arrives at Pluto. If it were any other way, science could not function.

Secondly, all categories which help us organize and understand things in our minds (including ‘planet’) are pure human inventions that only roughly correspond to nature. Although we need to categorize the things we see, nature does not; no matter how we classify objects, nature presents us with borderline cases that challenge us. Pluto is the same thing today as it was in 2005 or even before it was discovered in 1930. We need to distinguish our need for neat categories from our need to explore and describe nature.

Proud to be a space cadet? Learn more about astronomy:
Dust off your telescope – or visit the George Observatory – to see what’s in the night sky this month.
Ten billion trillion trillion carats – the universe has great taste in diamonds
If it blew a hole in your roof, you’re on the right track – how do you tell a rock might be a meteorite?

Meteorites and Meteor-wrongs

My role as Planetarium Astronomer includes answering astronomy questions from the public over the phone, by email, and in person.  Thus, it is up to me to examine meteorite samples brought in by the public.  Or, I should say, “meteor-wrongs,” as none of the samples brought in since 1996 (when I began doing this) have actually been meteorites. 

First, let’s define some terms.  A rock which is about to enter the Earth’s atmosphere is a meteoroid.  Someone who happens to see it as it is falling, and thus sees a streak of light in the sky, sees a meteor.  Once the rock has landed, it is a meteorite.  Most meteors that we see burn up completely in the atmosphere and therefore never land as meteorites.  A meteorite, then, is a rock which originated in outer space.

which is a meteorite?

Can you tell which of these is a meteorite?

If you have a sample you believe came from outer space, here are 4 simple tests you can do at home.  Note that passing these four tests will not guarantee that your sample is a meteorite; they serve primarily to eliminate ‘meteor-wrongs.’

1) Is the sample heavy for its size?  Meteorites are denser than Earth rocks; they have more mass per volume.  A meteorite will be heavier than an Earth rock of the same size.

2) Does the sample attract a magnet?  Most meteorites found and brought in are iron meteorites.  Even the stony meteorites, which are more common but rarely reported because they superficially resemble Earth rocks, have some iron in them.  A meteorite sample, then, should attract a magnet.  Any magnet, including the ones on your fridge, will suffice for this test.

3) Is there a dark fusion crust? Upon entry into our atmosphere, a meteorite acquires a thin ‘fusion crust’ because its surface melts under the heat of entry.   This crust is black when the meteorite is freshly fallen but may turn brownish due to weathering and rust.  Bright colored or silvery samples are not meteorites.

4) Does the sample have bubble holes?  Many volcanic rocks on Earth have these holes, which form when a bubble of gas or steam expands as the rock solidifies.  A meteorite, however, is never fully molten (only the surface melts on entry into the atmosphere).  Thus, a meteorite sample is a solid hunk, without tiny holes or perforations. 


The many large holes in this rock
are a big clue that it is not a meteorite.

So, which of the four is a meteorite? If you go back to the first photo in this post, you should be able to see holes in the top two samples – so those are out. And the bottom right is bright and silvery = not a meteorite. So, the winner is the smallest of all four, in the bottom left.  

For more information, surf to: http://meteorite-identification.com/ ,  http://meteorite.fr/ (the site is in English, too), or http://meteorites.wustl.edu/meteorwrongs/meteorwrongs.htm

Sky Walking: Astronaut Style


Thomas D. Jones, PhD is a veteran NASA astronaut, scientist, speaker, author, and consultant. He holds a doctorate in planetary sciences, and in more than eleven years with NASA, flew on four space shuttle missions to Earth orbit. In 2001, Dr. Jones led three spacewalks to install the centerpiece of the International Space Station, the American Destiny laboratory. He has been privileged to spend fifty-three days working and living in space.

He’s visiting the Houston Museum of Natural Science for  public lecture on May 5 and he was kind enough to give us a preview:

In Sky Walking: An Astronaut’s Memoir, I take readers along for an “inside-the-spacesuit” ride on each of my four space shuttle missions. My most recent was a demanding construction flight to the International Space Station. During the second of three spacewalks outside shuttle Atlantis, I moved carefully along the silvery hull of the Station’s Destiny science lab, hovering by my fingertips some 220 miles above the luminous Earth below.

Creative Commons License photo credit: pingnews.com

My spacewalking partner, Bob Curbeam, and I worked side-by-side on Destiny’s hull, installing the mechanical and electrical foundation for the Station’s robot arm, Canadarm II. We were interrupted by an exuberant call from German astronaut Gerhard Thiele in Mission Control: the robot spacecraft “Near-Earth Asteroid Rendezvous-Shoemaker” (NEAR-Shoemaker) had just landed on asteroid 433 Eros, the first time a machine from Earth had touched down on one of these mountain-sized remnants of the ancient solar system. Falling around Earth beneath the black sky and blazing sun, I tried to imagine what it might be like to walk Eros’ alien surface, held so lightly by its tenuous gravity that an easy leap would toss me aloft for hours. But a hundred million miles away, NEAR/Shoemaker was there, alive and transmitting. How long until an astronaut explorer might follow?

Near-Earth asteroids like Eros should be our next destination beyond the Moon. Their ancient rocks and resources will be key to our efforts to understand and tap the wealth of the solar system. Just as important, astronauts and their robot probes will gather the knowledge we need to keep Eros’ rogue cousins from someday threatening our civilization with a catastrophic impact. We now have the capability to intercept these objects and halt a cosmic force that has often changed the course of evolution on Earth. To survive as a species, we must do so. Only by “Sky Walking” can we ensure that we humans don’t go the way of the dinosaurs.

More details here, and here

Jones is a Distinguished Graduate of the U.S. Air Force Academy. He has engineered intelligence-gathering systems for the CIA, and helped develop advanced mission concepts to explore the solar system prior to joining NASA’s astronaut corps. He writes frequently about space exploration and aviation history in magazines such as Air and Space Smithsonian, Aerospace America, and Popular Mechanics. Tom’s current book is Sky Walking: An Astronaut’s Memoir, published in 2006 by Smithsonian Books-Collins.

Science Doesn’t Sleep (4.17.08)


Creative Commons License photo credit: antjeverena

So here’s what went down since you logged off.

You can stop building your asteroid impact shelter – that German kid got it wrong. NASA stands by their estimate of the asteroid Apophis’ chance of colliding with Earth, also denying they ever admitted an error. 1 in 450, 1 in 45,000 – it still seems like “way too likely” to me.

Robot alert! The Carnegi Science Center (those crazy kids that brought us the Robot Hall of Fame) is developing an exhibit called Roboworld, that will “will emphasize three aspects of artificial robotic behavior: sensing, thinking and acting.” Sadly, “taking over the world” is not on the list of behaviors to be featured.

We love nature – but it doesn’t love us back. Humans are more harmful to coral reef than the fallout from an atomic bomb.

Houston, we just keep stacking up the honors. In addition to being named the country’s fattest city, it turns out that Harris County is number one for something else – carbon dioxide emissions.

Are you planning anything for Earth Day? You can check out what people around the world are doing, get some suggestions from Google or, oddly enough, the government.