On July 14, NASA’s “New Horizons” spacecraft will zip past distant Pluto, 7,000 miles above its icy surface, while traveling at 25,000 mph. New Horizons will complete a saga that started 85 years ago this month in Flagstaff, Arizona. On Feb. 18, 1930, Clyde Tombaugh, a 23-year-old Kansas farm-boy-turned-astronomer, found Pluto by comparing telescopic photos of the same region of the night sky taken a week apart: Stars stay still, planets move.
As a planet, Pluto was always a bit suspect. Instead of moving roughly in the same plane as of that of Earth — the ecliptic — and the other planets, Pluto orbits at a steeply inclined 17 degrees. And unlike the eight “true” planets which swing around the sun in more-or-less well-mannered circles, Pluto’s 248-year orbit is highly eccentric, swinging crazily between 30 and 50 times Earth’s distance from the sun. (For 20 years of each orbit, Pluto is actually closer to the sun than Neptune, with which it shares a 2:3 resonance; for every two orbits Pluto makes around the Sun, Neptune makes three.)
Another oddity about Pluto is that it’s really a binary object, since its main satellite, Charon, is relatively large, half the size of Pluto. Other than Earth and Pluto, the moons of the solar system are tiny compared to their parent bodies. Pluto and Charon, just 1,200 miles apart, are locked in orbit like a pair of waltzing dancers, with the same hemispheres always facing each other. Pluto itself is tiny, just two-thousandths the size of Earth.
Despite these anomalies, astronomers accepted Pluto as the ninth planet until 1992, when they started to question its classification. That’s the year that sky watchers spotted a second body orbiting beyond Neptune, and since then they’ve found over 100 objects wandering those far reaches. Finally, in a controversial vote in 2006, the International Astronomical Union officially downgraded Pluto from “planet” to “dwarf planet.”
The trans-Neptunian neighborhood is the outermost region of the sun’s gravitational domain. Like Caesar’s Gaul, the solar system can be divided into three parts. The middle region is the home of the gas giants Jupiter, Saturn, Uranus and Neptune, while the innermost is our own — the realm of the rocky planets Mercury, Venus, Earth and Mars. The region beyond Neptune holds particular interest for astronomers, not only because of Pluto and a handful of other dwarf planets, but because that’s where innumerable “Kuiper Belt Objects” (KBOs) are found. KBOs — basically big lumps of frozen methane, ammonia and water — are comprised of 5-billion-year-old virgin material that could help us understand the early history of the solar system. The plan is for New Horizons to visit one or more KBOs after its Pluto flyby.
Before then, in addition to taking detailed photos, the 1,000-pound spacecraft will look for evidence of a subsurface ocean on Pluto, similar to those on four moons of Jupiter and Saturn (Europa, Ganymede, Enceladus, Titan). The presence of liquid water beneath Europa’s icy surface is particularly intriguing to astronomers, who speculate on the possibility of finding microscopic life there. The discovery of a liquid water ocean on Pluto would similarly pose the question, Could life have arisen there? Too bad Tombaugh, who died in 1997, won’t be around to complete the round of discovery he initiated on that February morning 85 years ago.
Barry Evans (barryevans9@yahoo.com) finds it ironic that when New Horizons was launched, Pluto was officially designated a planet.
This article appears in 35 Days in the Forum.
In my opinion, there’s nothing to argue about here. I just wonder why some people are constantly trying to “denigrate” the 9th planet?
Pluto is still a true planet. There is absolutely no reason why the IAU definition, which constitutes one view in an ongoing debate, should be given privileged status as the “official definition” when the reality is it is just one of many currently in use.
Pluto is not suspect as a planet. Why should an object have to orbit in the same plane as Earth to be considered a planet? That type of thinking goes back to the pre-Copernican view in which Earth was seen to have a privileged position. The ecliptic is not even the path of the Sun; it is the path the Earth takes around the Sun. Mercury also has an elliptical orbit that is inclined to the ecliptic. Does this mean Mercury isn’t a planet?
Pluto’s orbit may be eccentric, but it is stable. Pluto will not crash into the Sun or into any other planets. Significantly, astronomers have discovered many giant exoplanets with orbits around their stars that are far more eccentric than Pluto’s around the Sun. Does that mean these giant objects are not planets? Some hot Jupiters have close orbits that are not even stable, meaning they will eventually fall into their stars.
Many astronomers and planetary scientists never stopped viewing Pluto as planet. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists but other types of astronomers. Their decision was immediately opposed in a formal petition of hundreds of professional astronomers led by New Horizons Principal Investigator Dr. Alan Stern. Ironically, Stern is the person who first coined the term “dwarf planet,” but he did so intending to designate a third class of planets in addition to terrestrials and jovians, not to designate non-planets.
Stern and like-minded planetary scientists prefer an alternate definition, the geophysical planet definition, according to which a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet. The only size that matters is that the object is big enough and massive enough to be squeezed into a round or nearly round shape, a state known as hydrostatic equilibrium. Pluto well exceeds that threshold, as does its moon Charon. Since Pluto-Charon orbit a barycenter outside of Pluto itself, that makes them a binary planet system.
The author seems to have a sense of glee at the demotion of Pluto. Rubbing it in, as it were. All with old arguments and without any historical account of how the vote in 2006 went down. Only 232 or so members of the 10,000 strong IAU voted to demote Pluto in Prague on the last day of the General Assembly. Members who the Executive Committee and their lackeys knew were for Pluto’s demotion were asked to stay for the surprise vote. The fix was in. Pro-Pluto speakers who happened to be at the session were cut off in mid-sentence. I watched the entire session on video.
Dwarf stars like our sun are still considered stars. Dwarf galaxies are still considered galaxies. Dwarf planets should be a subcategory of planets, like rogue planets should be. The definition of planets needs a redux.
The writer, instead of being excited about 2015 being the year of the dwarf planet, viz., Ceres, Charon, and Pluto, seems to gloat in the old hackneys arguments of Pluto haters who claim to have killed the planet and have stooped so low as to actually behead a Disney doll of Pluto.
The demotion of Pluto has nothing to do with Science. It was political, and it will not last. I am Ceres.
of course I respect what they have decided about Pluto but in my heart Pluto remains a planet in its own right.