systemic

In Search of Planet Vulcan — The Ghost in Newton’s Clockwork Universe, by Richard Baum and William Sheehan, is one of my favorite astronomy books. It certainly has one of the best overviews of the momentous events and controversies surrounding the discovery of Neptune in September 1846. I’ll take the liberty to quote Baum and Sheehan’s recounting of the exact moment of Neptune’s discovery.

On September 18, Le Verrier wrote to Johann Gottfried Galle, then an obscure astronomer at the Royal Observatory in Berlin. A year earlier, Galle had sent Le Verrier his doctoral dissertation, which concerned observations made by 17th-century Danish astronomer Olaus Roemer. Belatedly, Le Verrier wrote to acknowledge it. Among other things, he queried Galle about Roemer’s Mercury observations, but then came quickly to his point:

“Right now I would like to find a persistent observer, who would be willing to devote some time to an examination of a part of the sky in which there may be a planet to discover… You will see, Sir, that I demonstrate that it is impossible to satisfy the observations of Uranus without introducing the action of a new Planet, thus far unknown; and, remarkably, there is only one single position in the ecliptic where this perturbing Planet can be located… The actual position of this body shows that we are now, and will be for several months, in a favorable situation for the discovery.

Galle indeed proved to be his man. He received Le Verrier’s letter on September 23, and at once sought permission from the observatory’s director, Johann Encke, to carry out the search. Encke was skeptical but nonetheless acquiesced: “Let us oblige the gentleman in Paris.” A young student astronomer, Heinrich Ludwig d’Arrest, begged to be included, and joined Galle as a volunteer observer. That night, they opened the dome to reveal the observatory’s main instrument, a 9-inch Fraunhofer refractor aimed at the spot assigned by Le Verrier. Recalculated for geocentric coordinates, its position was at right ascension 21 h, 46 min, declination -13 deg 24 min, very close to the position occupied by another planet, Saturn.

The question arose: What maps were available? At first they could think of none but “Harding’s very insufficient Atlas.” D’Arrest then suggested “it might be worth looking among the Berliner Akademische Sternkarten to see whether Hora XXI was among those already finished. On looking among a pile of maps in Encke’s hall [Vorzimmer], Dr. Bremiker’s map of Hora XXI [already engraved and printed at the beginning of 1846 but not yet distributed] was soon found.” As d’Arrest later recalled, “We then went back to the dome, where there was a kind of desk, at which I placed myself with the map, while Galle, looking through the refractor, described the configurations of the stars he saw. I followed them on the map one by one, until he [Galle] said: and then there is a star of the 8th magnitude in such and such a position, whereupon I immediately exclaimed, that star is not on the map!”

Neptune’s moment of discovery, at 11 PM Berlin local time on September 23, 1846, corresponded to 22:07 UT, or JD 2395563.4215. The period of Neptune is 60,190.03 days, or 164.79132 years. The first “Neptunian anniversary” of the discovery is therefore CE 2011 July 10 22:49:26.4 UT Sunday, that is, right now.

In 1846, photography was still in its very earliest stages, and it would be nearly two decades until the publication of Jules Verne’s De la Terre à la Lune. The fact that we greet the completion of one orbit in the possession of photographs of a crescent Neptune is a marvelous indeed.

Image Source.

Certainly, an occasion for celebration! On Friday, I got an invitational e-mail from Gaspar Bakos, who is hosting a Neptune-at-One cocktail party in Cambridge, Massachusetts. I briefly perused airfares before sadly having to decline.

A well-known theorem states that there’s no such thing as a free lunch. A corollary is that interesting discoveries tend to be made at the ~3-4 sigma level of confidence, and this is especially true if the supporting data is drawn from the public domain. If a signal is stronger than 4-sigma, then someone else has invariably pointed it out. If it’s weaker than 3-sigma, it’s probably wishful thinking.

With those rules of thumb in mind, I’m very optimistic that Kevin Schlaufman has obtained a genuinely important insight into how the planet formation process works:

The plot shown is above is from a paper that Kevin and I submitted soon after the 1,235 Kepler planet candidates were announced last Spring. After going through review, it was accepted by the ApJ, and it was posted to astro-ph last week.

I wrote about the underlying details of the plot in this post from several months ago. The basic idea is as follows: The 997 Kepler planet candidate host stars are divided up into two groups — (i) the less numerous group of stars that host a candidate with R_pl>5 Earth radii (red), and (ii) the more numerous group of stars that only host a planet (or planets) with R_pl<5 Earth radii (blue). The two groups of stars, along with a control sample of 10,000 non-candidate-bearing dwarf stars from the Kepler field (gray), are plotted in a color-color diagram (and then binned to create the diagram above):

The y-axis corresponds to the magnitude difference between a given star’s green (Sloane g filter) and red (Sloane r filter) colors. The x-axis charts the differences between the 2Mass J and H infrared colors for each star. Metal-rich stars tend to have redder optical colors than metal-poor stars, whereas the J-H index sorts the stars in terms of their overall temperatures (with cool stars to the right and warm stars to the left of the plot). Metal-rich stars thus lie along the upper part of the main-Sequence locus.

The binned version of the plot provides a confirmation of several trends that were already very well known. First, among host stars with masses similar to the Sun that harbor giant planets, there’s a strong preference for metal-rich stars. This is the classic planet-stellar metallicity effect. Second, among low-mass stars, there’s a dearth of giant planet candidates. This is the known giant planet-stellar mass effect. Finally, among the solar mass stars that host low-mass planets, there’s no discernible metallicity correlation.

The new result pertains to low-mass planets orbiting low-mass stars. The diagram shows that for this subset, there’s strong evidence for a metallicity correlation — At masses less than ~0.8 solar masses, higher metallicity stars are more likely to host low-mass planets. We take this as direct evidence regarding the overall bulk efficiency of planet formation for planets that aren’t required to bulk up via rapid gas accretion. Take a 0.7 solar mass with twice the Sun’s metal content and a typical 0.02 solar mass disk. The entire planet-forming disk contains about 150 Earth-masses worth of stuff heavier than hydrogen and helium. Kevin’s result is effectively saying that a good fraction of the time, a good fraction of this total burden of metals winds up in planets.

We had to be careful. There are a lot of systematic “gotchas” that can potentially throw a wrench into the exciting big-picture conclusions, and so much of the paper is devoted to considering potential show stoppers in turn. I think that the result is robust, and that it will hold up as the planet catalog continues to grow.