The Solar system begins to settle down

kw: book reviews, nonfiction, astronomy, astronomers, memoirs, planets

I grew up with nine planets, like nearly everyone alive today. I didn't learn any fancy mnemonic, but just learned the names right off: Mercury-Venus-Earth-Mars-Jupiter-Saturn-Uranus-Neptune-Pluto. From time to time over the years, a new estimate of the size of Pluto was announced. Once thought to be a very dark object the size of the Moon, it gradually shrank (and its albedo increased), finally settling down to a size of about 1,400 miles; the current figure from images by the Hubble Space Telescope is 1,430 miles or 2,302 km.

In recent years, the discovery of large objects (but smaller than Pluto) in the Kuiper Belt and beyond precipitated a discussion of Pluto's status: Is it a planet or not? (The Kuiper Belt is like the asteroid belt, but composed of icy bodies that are farther from the Sun than Neptune.)

As of mid-2006, Pluto is definitely not a planet; that is, it is defined as a "dwarf planet", a designation that always merits the quote marks, because semantically, "dwarf planet" means "planet of a smaller size" rather than "not a planet at all".

This image from the Wikimedia Commons shows the status of the "dwarf planets" found beyond Neptune as of 2008. Six of these were found by Mike Brown, who was also instrumental in the decision to demote Pluto, though he disagrees with "dwarf planet" at least as much as I do. His memoir How I Killed Pluto and Why It Had It Coming is a delightful romp through the life of a young astronomer who set out to "find a planet", and was known for just about a year as the first person to find a planet since the 1930s. Bigger than Pluto, and 2.5 times farther from the Sun, the body he provisionally called Xena was called the Tenth Planet for those heady months, until the IAU decided to define "planet" for the first time. It took a few tries to get it right.

The current definition, settled on as the book was being written, includes these factors:

• A planet circles the Sun, not any lesser body in the Solar System.
• A planet is large enough to have gravitationally settled into a sphere.
• A planet gravitationally dominates its orbit, sweeping it clear of other bodies.

There is a bit of a quibble about Trojan asteroids in Jupiter's orbit, and similar bodies in the L4 and L5 regions of other planets, but the fact remains that Pluto shares its orbital region with several bodies of similar size, so it is demoted from planet status based on the third factor.

The book tells two stories. One is the search for planets beyond Neptune, and the other is the courtship and marriage of the author to his wife Diane, and the early years of their daughter's life. Both stories reveal a human side of an astronomer's life that is seldom seen. Astronomy is by its nature a night job, though this has abated somewhat now that professional telescopes are computer driven and gather their images automatically. But if an astronomer is not staying up all night tending a telescope, the midnight oil still gets burned: studying the images, using the information they convey to locate earlier images that may reveal more facts about the object of interest, and tons of book work eat up huge amounts of time. A night's observations can require months to digest and analyze, and long hours are the norm if a scientist doesn't want to get scooped by someone else willing to work longer hours.

Xena eventually got renamed Eris, and a satellite around it was named Dysnomia. Eris is the goddess of discord, and the body is fittingly named for the squabbles that erupted once its discovery was announced. Since about 2008 the larger Pluto-like bodies have been called Plutoids, and a few hundred other Kuiper Belt objects (KBO's) are called Plutinos.

The author conjurs a fantastic image of an alien approaching the Solar System, and noticing first the four Giant planets, led by Jupiter. Then four smaller bodies are seen from closer in, that lie between Jupiter and the Sun (Earth is the third, but the alien doesn't know that). Finally, two bands composed of multitudes of bodies are discerned, one inwards of Jupiter, the asteroid belt, and one outwards of Neptune, the Kuiper belt (plus an outer region called the scattered disk). We then can realize that the eight planets, four big and four small, dominate the System; a few rounded bodies are the largest members of the two belts, and it remains to be seen whether the Kuiper belt plus scattered disk will yield up an even larger body, or several. Eris may have a big brother or two waiting to be found.

Solar System astronomy is healthy and exciting.

P.S. What it takes to find a new Plutoid.

1. A telescope with at least a meter aperture (Mike Brown used a 48-inch Schmidt at Palomar, first with film, later with digital sensors).
   
2. A wide-angle camera with 50-100 megapixels of sensor. This combination can record bodies of Magnitude 18-24 (Xena/Eris was M17 when found).
3. A half year to a year of observing time, mostly the half-month each month when the moon is the darkest and out of the way.
4. Fast computer power to compare the millions of points on one day's images with the next.
5. Gallons of midnight oil, so to speak, because the computer produces "possibles", which an expert human has to verify into a shorter list of "probables", and make extra observations to nail them down.
6. Time reserved on larger instruments, including the Hubble, if possible, for that nailing down process.
7. An accommodating spouse; you'll be traveling a lot.

P.P.S. Magnitudes. Take the time to read This Wikipedia article. Briefly, bright stars have small numbers as their Magnitude; "First magnitude" refers to bright stars. The dimmest stars visible to a young person in a dark sky have Magnitude 6 or 6.5. Magnitude is the negative logarithm of brightness, to the base of the fifth root of 100 (about 2.5), so a magnitude difference of 1 is a brightness ratio of 2.5:1 and a magnitude difference of 5 is a brightness ratio of 100. With a 10-inch diameter (250mm) telescope, the dimmest stars visible are magnitude 15. The brightest Plutinos have magnitudes of about 17 or 18, so they are very faint indeed. You'd need a telescope the size of a minibus to see one of them visually.