Monthly Archives: April 2006

The ninth planet

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The frigid outer reaches of the solar system are generating a lot of activity. Pluto, Charon, Sedna, Quaoar, and 2003 UB313 all clamor for attention on the pages of the New York Times. The glamour to be gained from discovering these strange cold orbs has produced skulduggery of the highest caliber: the hacking of internet observing logs, the computation of an orbit from a series of telescope pointings, a hasty search of a guilty patch of sky. This is the stuff of thrillers. I’ve enjoyed it from the sidelines.

I have no stake and little interest in the “Is Pluto a Planet?” debate, but one point does seem clear. I seriously doubt that New Horizons would currently be on its way to the edge of the Solar System if Pluto had been stripped of it’s planetary status in 1978 when its tiny mass was finally revealed by the discovery of Charon. An unexplored outer planet can captivate the imaginations of congressional staffers. The 2nd-largest known member of Colonel Edgeworth and Dr. Kuiper’s belt just doesn’t have the same effect.

And I’d certainly pay my ~ $2.50 share to see a close-up picture of 2003 UB-313 as well…

2-color reflectivity map of pluto

Surprisingly, the image of Pluto shown above is not a photograph in the usual sense. Rather, it’s the two-color reflectivity map of Pluto’s sub-Charon surface that was obtained by (Young, Binzel & Crane 2001) with photometric transit observations. From 1985 through 1990, Charon’s orbital plane with respect to Pluto was close to alignment with the line of sight from Pluto to the Earth. This allowed a map of Pluto’s surface to be constructed by keeping careful track of the brightness of Pluto as Charon transited different chords across Pluto’s face. Measurements of the brightness through two different filters (B and V) allowed a two-color map to be produced. It’s not clear what causes the surface of Pluto to vary in reflectivity. One possibility is that we are seeing patches of methane frost.

Here’s a stop-action movie of Pluto and Neptune during the course of three Neptune orbits. Due to the 3:2 resonance between Pluto and Neptune, Pluto executes close to 2 orbits during the time it takes Neptune to go around the Sun three times. The animation was produced by integrating the two planets with a computer, and then plotting their positions at equally spaced time intervals on a sheet of paper. Peppercorns are then placed on the paper to represent the positions of Pluto and Neptune, and a Kumquat is placed at the position of the Sun. The peppercorns are then “integrated” through their motion using stop-action photography, and the resulting .jpg frames are processed into .mp4 and .mov format animation files.

frame from the pluto-neptune animation

pluto_and_neptune.mp4: If the .mp4 file won’t load in your browser, try this small version: pluto_and_neptune.mov.

two for the show

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As I’m writing this, it’s about 22:08 UT, April 2, 2006. (JD 2453828.4226). The midpoint of the most recent predicted transit window for GL 581 b occurred a few hours ago, at 15:46 UT. That was in broad daylight in both the United States and Europe, but it was in the middle of the night in Australia and Japan. Hopefully, the Australian and Japanese participants in Transitsearch.org had clear weather at their observing sites.

what exactly is it?

As dicussed in previous posts, GL 581 “b” has a minimum mass of 17.8 times the Earth’s Mass (very close to the mass of Neptune), and orbits with a 5.366 day period around a nearby red-dwarf star. The a-priori geometric probability that GL 581 b can be observed in transit is 3.6%. Because the orbit of the planet has been well-characterized with the radial velocity technique, we can make good predictions of the times that transits will occur if the plane of the planet’s orbit is in close enough alignment with the line of sight to the Earth. The star can then be monitored photometrically during the transit windows to look for a telltale dimming lasting a bit more than an hour as the planet crosses the face of the star.

If GL 581 b is found to transit, then we will have a scientific bonanza on our hands. The size of the planet, and hence its transit depth, is highly dependant on the planet’s overall composition. If it is an “ice giant”, with a similar overall composition and structure to Neptune, then it should have a radius about 3.8 times larger than Earth, and it should block out about 1.7% of the star’s light at the midpoint of a central transit. If, however, the planet is a giant version of the Earth, with an iron core and a silicate mantle, then it will be considerably smaller and denser, with a radius only ~2.2 times that of the Earth. If the planet is a super-Earth, then the transit depth will be much smaller, and only about 0.6% of the star’s light will be blocked. A 0.6% transit depth is tough to detect, but it’s nevertheless possible for skilled amateur observers to reach this precision.

Here are some cutaway diagrams showing the internal structure and relative sizes of Jupiter, and of GL 581 b in each of the two possible configurations:

Core comparisons

Why would it be a big deal if we could determine the internal structure of GL 581 b? If the planet is a Super-Earth (that is, if the transit depth is small), then we would know that it accreted more or less in situ, using water-poor grains of rock and metal. The existence of such a structure would strongly suggest that habitable, Earth-like planets are very common in orbit around the lowest-mass M dwarf stars. That is, it would verify that high surface densities are a ubiquitous feature of the innermost disks of low-mass stars. On the other hand, if the planet turns out to be similar in size and composition to Neptune, then we will know that it is made mostly of water-rich material, and that it had to have accreted at a larger radius, beyond the so-called snowline of GL 581’s protoplanetary disk.

hd 20782 oct 20, 2006 (3.6%)

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As advertised in yesterday’s post, three newly published radial velocity data sets have just been added to the system menu of the Systemic Console, and to the www.transitsearch.org candidates list. The data set for HD20782, published by Jones et al. of the Anglo-Australian Planet Search, is definitely the most interesting of the trio. Let’s work the HD 20782 velocities over with the console, and see what they have to say.

sunset

First, fire up the console. (If you use Firefox on Windows, and you’ve had success getting the console to work with that particular line-up, please post a response in answer to Vincent’s comment on yesterday’s post. All of Aaron’s oklo.org Java development has been done on Mac OSX using Safari. Also, we’ve had many reports that the console works well with Internet Explorer on Windows, so if Firefox won’t run the Java, give IE a try. And could someone ask Mr. Bill G. to send me a check for that plug?)

At any rate, the HD 20782 radial velocity data set has one data point that sticks down like a sore thumb:

velocities

Activation of one planet and a little bit of fooling around with circular orbits shows that even when the discrepant point is ignored, the waveform of the planet is not at all sinusoidal. The points contain an almost sawtooth-like progression:

circular orbit fit

Because of the non-sinusoidal nature of the velocities, the periodogram (obtained by clicking the periodogram button) is rather uninformative. There’s a lot of power in a lot of different peaks, and it’s not immediately clear what is going on planet-wise:

periodogram

Aaron has been working very hard on console development, and we will soon release an updated version with a number of absolutely bling features. Ever wondered what your fits sound like? One new feature is a “folding window”, which allows the data to be phased at whatever period one likes. The folding window is very useful for data-sets of the type produced by HD 20782. It quickly reveals that something like a 600 day periodicity brings out the overall shape of the planetary waveform:

folding window

Using 600 days as the basis for a 1-planet fit, activating eccentricity, and using a combination of slider work, 1-d minimization, and Levenberg-Marquardt, eventually produces excellent fits to the data that look like this:

fit to hd20872

Jones et al., for example, in their discovery paper, report an orbital period of P=585.86 days, an eccentricity, e=0.92, a mass (times the sine of the unknown orbital inclination) of Msin(i)=1.8 Jupiter masses, and a longitude of periastron of 147 degrees.

This planet is one bizzare world, and seems to be very similar to HD 80606 b (another oklo.org favorite). The orbital period is 1.6 years. The planet spends most of it’s time out at ~2.6 AU. In our solar system, this distance is out beyond Mars in the inner asteroid belt. Once per orbit, however, HD 20782 b comes swinging in for a steamy encounter with the star. The periastron distance is a scant 0.11 AU, roughly half Mercury’s distance from the Sun. The planet is likely swathed in turbulent white water clouds. Raindrops vaporize as the star looms larger and larger in the sky.

Stars that loom large in alien skies are good news for transitsearch.org, and in the case of HD 20782 b, we here on earth are particularly fortunate. HD 20782 b’s line of apsides lies within about 60 degrees of alignment with the line of sight to the Earth. This raises the a-priori geometric probability of having a transit observable from Earth to a relatively high 3.6%. (The a-priori probability of transit for a planet with a 1.6-year period and a circular orbit is only ~0.3%).

oribital figure