A Planet by Any Other Name

Remember a few years back when there was a brouhaha over whether Pluto should be considered a planet? The International Astronomical Union, the body that decides, among other things, on the official names of astronomical objects and features, has been working on a new definition of the word “planet”. The results of their deliberations should be known by the end of September.

The reason there is some confusion is because we have discovered a population of icy worlds orbiting beyond Neptune that have much more in common with Pluto than Pluto does with any of the other planets. In fact, some of these new worlds (prosaically called Kuiper Belt Objects or KBOs), are in the same special orbital relationship with Neptune that Pluto is. These have been dubbed Plutinos. Recently a KBO was discovered that is bigger than Pluto, but on an orbit that is even more unusual (as far as planet orbits go, which is to say, nearly circular) than Pluto’s. The matter is complicated by the discovery over the course of the past decade or so of scores of extrasolar planets, that is, objects orbiting stars other than the Sun. Some of these blur the line between planet and star just as some of the KBOs blur the line between planet and ice cube. To make things even more fun, when KBOs occasionally get scattered into the inner solar system they start to evaporate as they near the Sun, and we call these objects comets. So should we call KBOs comets? Should we call Pluto a KBO? Should we call of them planets? Or come up with another name for the KBOs (for one thing, there are certainly KBOs around other stars, and it would be a bit peculiar naming all of them after our own backyard Kuiper Belt).

Personally, I favor demoting Pluto from the ranks of the planets, and I expect that the IAU definition will do just that. There is simply no sensible physical definition of a planet that includes Pluto and does not also include countless other objects in the outer solar system. Let’s reserve “planet” for the big guys. But, Pluto by any other name is still just as interesting a place to explore, and when the New Horizons spacecraft goes zipping by in 2015 it will undoubtedly teach us a lot about the origins of our solar system, planets, ice cubes, and all.

Finding Other Earths

When I was a graduate student, not that long ago (it seems), there were no confirmed worlds orbiting other stars. Now there are over 100. Most of these are larger than Jupiter and frequently closer to their stars than Mercury is to the Sun. Both factors make these strange new worlds intriguing, but inhospitable to say the least. New telescopic missions on the horizon, such as the Kepler mission, promise to expand our list of distant planets to include Earth-like worlds. In the meantime, a study of a multi-planet system suggests that one of its planets may be habitable: not too warm, not too cold, and with a solid surface beneath a reasonable atmosphere. Sean Raymond, whose office is just next door to mine, participated in this study and does computer simulations of the formation of habitable planets around other stars. While his research suggests that only a small fraction of the currently discovered extra-solar planetary systems may have habitable planets, the total number in the Milky Way, even in our neck of the woods, could still be, well, astronomical.

Saturn’s Rings: Prediction vs Observation

Last night I got my first new measurement of Saturn’s rings from Cassini in almost a year. A couple of weeks ago I posted a prediction about what that measurement would look like. Here are the results. The black curve (new observation) shows excellent agreement with the prediction, particularly in the outer half of the A ring (right half of the plot). There is some disagreement in the inner half of the ring. This may be due to inaccuracies in the model. Another possibility has to do with instrument calibration. That gets down to the nitty-gritty work. Overall, this observation confirms our model by the good match with the prediction.

Lakes on Titan

The strongest evidence yet for large lakes on Saturn’s moon Titan are in these RADAR images of the north polar regions of the large moon. At the very cold temperatures of Titan, these lakes are liquid methane or ethane. Water is hard as a rock at Saturn. These lake-like features may be lake beds, or marshy areas. They are certainly smooth, but may not currently be liquid. If they are, it would make Titan the only object other than Earth to have liquids on the surface at the present time.

It’s Spoke Season in Saturn’s Rings

One of the most intriguing features of Saturn’s rings is the occasional appearance of ghostly “spokes”. Gone for several years, we got a glimpse of them from Cassini only just before the spacecraft went into orbits in Saturn’s equatorial plane from which the rings are all but invisible. Now that Cassini is able to see the rings again, the Cassini cameras have seen spokes again.

These features are so named because they cut radially across the rings, like the spokes of a wheel. Unlike bicycle spokes, however, they are ephemeral, appearing in a matter of only a few minutes (or less), and rarely (if ever) lasting more than one orbit around the planet (around 10 hours). (That orbital time period is pretty amazing by itself: it takes the Earth’s Moon about 27 days to orbit the Earth along a path that is only about four times longer than the orbits of Saturn’s ring particles. These particles are really zipping around Saturn, though their speeds relative to each other are literally at a snail’s pace.)

The spokes, first observed by the Voyager spacecraft in the early 1980s, are made up of particles roughly the same size of smoke particles. Particles this small are easily pushed around by electric fields. This is why your computer screen, even though it is vertical, is probably coated with a significant amount of dust right now. Saturn’s spinning magnetic field produces an electric field that can explain the radial nature of the spokes. But what makes them come and go? And why did they disappear for years? The spokes may be caused by meteoroid impacts onto the rings. Some theories connect the spokes to thunderstorms on Saturn. Their disappearance may be due to the changing seasons of Saturn. Direct sunlight on the rings in summer and winter may make the environment near the rings inhospitable to spokes. Now that spring is approaching on Saturn (Saturn’s year is 29 years long, so seasons run over 7 years on Saturn), the environment appears to be favorable for spokes again. Keep your eyes on the Cassini imaging team’s site and the Saturn web site for spectacular images and movies in the months ahead.

Titan’s Xanadu Seen by Cassini’s Radar

The Cassini spacecraft shines radio waves towards its large moon Titan on some of the craft’s dozens of close flybys of Titan. The Cassini RADAR experiment measures the reflected radio waves to produce a radio wave picture. Check out this newly released RADAR view of Titan’s fancifully named Xanadu region. Titan may have seeping creeks or rivers and possibly lakes of liquid methane (also known as natural gas – the stuff cow’s produce and that you use to cook spaghetti) in the dark, smooth areas in Xanadu.

Ode to Eccentricity

It seems like scientists have a different meaning for perfectly ordinary words. To an astronomer, for example, “eccentricity” does not mean an inclination to strange behavior. Nor does “inclination” refer to a tendency. There are also the anomalies: true anomaly, mean anomaly, and eccentric anomaly. None of these are anomalous or eccentric. And don’t get me started on wakes. “Eccentricity” is one we deal with all the time. It simply describes how much an object’s orbit differs from a circular orbit. I have noticed an interesting trend among my younger colleagues the last few years: they pronounce “eccentric” either “eh-sentric” or “ee-sentric”. Call me old-fashioned, or maybe eccentric, but I can’t stand by and see a perfectly good double-c get the s-treatment. This ode is dedicated to the eccentrics.

Ode to Eccentricity
by Josh Colwell

“Eccentricity” has two c’s
like “occipital” and “accidentally”.
“Eccentricity” has two c’s
sandwiched between two short e’s.

Syllablically speaking,
If one can do such a thing,
That’s an “ek” then a “cen”:
Two c’s: no redundancy.
To skip one of those c’s
While maybe a breeze
Fills the ears and the brain with a bit of a quease.

Do our cars A-celerate when we step on the gas?
Only acceleration allows us to pass.
Howard Hughes’ bottled pee
Was no sign of E-centricity.

But we don’t need to rely on our O-cipital lobes,
Don’t take it from me,
Check the OED
The word is pronounced “EK-sen-TRISS-ity”

While soft c sounds may be palatable,
“E-centricity” is just not A-ceptable.

A Prediction About Saturn’s Rings

One of the cool things I’ve done with data from Cassini is analyze self-gravity wakes in Saturn’s rings. These are long, finger-likeshaped clumps of particles in the rings (a few tens of meters across and perhaps 100 or more meters long) that are the result of the particles running into each other and trying to accrete through gravity, but getting pulled apart by tides from Saturn. We published a paper earlier this year (Geophysical Research Letters, Vol. 33, L07201, doi:10.1029/2005GL025163 for those interested in the gory details) using our first measurements of Saturn’s A ring to derive the size and shape of the clumps in the ring. I have used that model to predict what we will see with our next measurement on July 25, 2006. You can see the prediction (purple curve) here. The other curves on the plot show the range of values from different measurements to date. Once I get the data I will post a comparison of the observation with the prediction. This is a fun opportunity to see the scientific method in action:

1. Observations by Cassini led to a theorymodel (the theory was proposed more than 30 years ago and has been developed by a number of researchers since then; we developed a particular mathematical model based on that theory and applied it to Cassini data;) of self-gravity wakes;
2. Application of that theorymodel to the data produced a prediction for a future observation;
3. July 25 comes the test of the theorymodel which will either confirm or refute the theory.

My guess is that there will be good, but not perfect, agreement between prediction and observation. This would then lead to some modification of the self-gravity wake parameters that I have previously calculated. We will be back at step 1 with an improved model of the ring. Check out the prediction and check back in a couple of weeks to see how well I did.

Saturn’s Rings in Psychedelic Ultraviolet

For the first post in my blog it’s only fitting that I explain the image in my header. I created this image around 3:00 a.m. July 2, 2004 from a series of observations of Saturn’s rings made by the Ultraviolet Imaging Spectrograph on the Cassini spacecraft. The image took on a life of its own as it was featured in many newspapers around the world, was selected as one of Time magazine’s pictures of the year of 2004, and (my favorite) was shown on The Daily Show with Jon Stewart. The image shows the outer third of Saturn’s ring system, with the Cassini Division in red at the left, and the turquoise A ring across the rest of the image. The thin red band at the right is the Encke Gap, a ~350 km wide gap that is home to a small moon, Pan, and a set of faint narrow rings not visible in this image.

There are two caveats that need to go with this image: (1) the original data is actually only a narrow strip across Saturn’s rings, and I stretched it azimuthally to make a larger and more ring-like picture. The implicit assumption of circular symmetry in this step is valid at the ~150 km resolution of the data; and (2) the red color represents the Lyman-alpha glow from interplanetary Hydrogen gas throughout the solar system shining through gaps and transparent regions in the rings. So, where it is red there is less ring material. The space beyond the edge of the ring is black instead of red because we had no data beyond the edge of the ring.