Speaking of Hubble...

Archive: August 2011

Neptune’s Birthday?

August 19, 2011 by Frank Summers
These four Hubble images of Neptune, taken during the planet's 16-hour rotation, exhibit a full view of the planet.

These four Hubble images of Neptune, taken during the planet's 16-hour rotation, exhibit a full view of the planet.

On July 12, 2011, we celebrated what some have called “Neptune’s birthday.” Hubble released four pictures of the planet to commemorate Neptune’s first full orbit since its discovery on September 23, 1846. On that day, Johann Galle at the Berlin Observatory found Neptune based on the mathematical prediction of a French astronomer, Urbain Le Verrier. Because Neptune takes almost 165 years to complete its orbit around the Sun, a lot has happened during the Neptunian year since we learned of the planet’s existence.

However, calling this a “birthday” smacks of anthropomorphism and a human-centered view of the universe. Neptune has been around for billions of years, and no one can say when it was “born” to any precision better than many millions of years. In the grand scheme of things, does it really matter that we finally got around to noticing Neptune on that day? Plus, it is fun to note that Neptune had been seen before that day, but not recognized for the planet it is.

A few months earlier, in England, James Challis was actually searching for the new planet, based on the prediction of John Couch Adams, not Le Verrier. After the discovery was announced, it was uncovered that Challis had observed Neptune twice, but had not analyzed the data enough to find it. The national rivalries were strong in those days, and so was the outcry that England had “lost” the discovery to France and Germany. The full story of Neptune’s discovery makes for one of the best scientific soap operas of all time (try to find the out-of-print book “The Discovery of Neptune” by Morton Grosser).

Going back further, the earliest observations of Neptune date back to some of the earliest observations with a telescope. In December 1612 and January 1613, Galileo was following the motions of Jupiter and its four large moons, which he had discovered a couple years earlier. Neptune was in his field of study, and was noted by Galileo as seeming to move relative to a nearby star, yet not recognized as a planet.

All in all, anytime is a good time to release more Hubble pictures of Neptune. Since its the seasons last over 41 years each, we’re getting a drawn-out and detailed view of the emergence of summertime in Neptune’s southern hemisphere.

The Pluto Saga: New Moon

August 12, 2011 by Frank Summers
Four moons are seen orbiting Pluto, June 28, 2011.

Four moons are seen orbiting Pluto, June 28, 2011.

On July 20, 2011, NASA announced that Hubble had discovered another moon orbiting Pluto. This new moon, temporarily named P4, adds to Hubble’s previous discovery of the moons Nix and Hydra, announced in 2006. The press release called P4 the fourth moon around the dwarf planet Pluto. That description provoked a variety of responses.

First, some folks felt that the description devalued Pluto. If it has four moons, doesn’t that qualify Pluto as a planet? Remember that Pluto was called a planet when it was discovered in 1930. In 2006, the IAU changed its classification to dwarf planet. Today, most astronomers refer to Pluto as a Kuiper Belt Object (KBO).

The presence or absence of moons does not affect an object’s classification, as several types of objects have moons. While most planets have moons, note that Mercury and Venus have no moons. Other dwarf planets have moons, like Haumea, which has the two moons Hi’iaka and Namaka. Other KBOs have moons, such as 1998 WW31, tracked by Hubble in 2002. Even small asteroids can have moons, as evidenced by Ida and its moon Dactyl. From these examples, one can see that having moons is neither a necessary or sufficient condition to call an object a planet or a dwarf planet or a KBO or an asteroid.

Second, some other folks felt that the description devalued Charon. Because Charon has significant mass compared to Pluto, it is not really correct to say that Charon orbits Pluto. Instead they both orbit a point that, while closer to Pluto than Charon, is outside the volume of either (here is a rough illustration of their orbits). Many people consider Pluto/Charon to be a double dwarf planet. Hence, they consider P4 to be the third moon of Pluto/Charon.

My view goes just a bit further. I find the definition of dwarf planet to be entirely useless. It defines a category for which I see no purpose – it doesn’t add to our understanding of either the structure or development of the solar system. The proper classification is that Pluto/Charon is a double Kuiper Belt Object with three moons.

And that classification doesn’t mean Pluto is somehow considered worthless. I can’t wait until the New Horizons mission gets to Pluto/Charon in 2015. Three satellites around two KBOs makes for one interesting system to study.