The Kepler orrery

If you haven’t heard by now, the Kepler mission has opened up a firehose of exoplanet (candidate) detections. We’re up to more than 2300 candidates found by the mission, with more to come.

I’ve just discovered an awesome animation that Dan Fabrycky created to visualize systems discovered by Kepler that have more than one detected transiting exoplanet. (Note: this includes unconfirmed planet candidates as well.)

“There are 885 planet candidates in 361 systems. In this video, orbits are to scale with respect to each other, and planets are to scale with respect to each other (a different scale from the orbits). The colors are in order of semi-major axis. Two-planet systems (242 in all) have a yellow outer planet; 3-planet (85) green, 4-planet (25) light blue, 5-planet (8) dark blue, 6-planet (1, Kepler-11) purple.

I could stare at this for hours. Wow.

Astrotagging and Milky Way orbits

I attended an excellent talk today by David Hogg, a cosmologist at NYU, that was titled “A Comprehensive Model of All Astronomical Imaging Ever Taken.” Here I highlight just two of the interesting and thought-provoking topics that appeared in his talk.

Astrotagging: His group developed astrometry.net, a service that will analyze digital images of the night sky and automatically annotate them with identifiable stars. You can access this impressive service by uploading a photo to flickr and adding it to the group “astrometry,” as in this example. Automatically, astrometry.net analyzes all new images added to this group and adds a comment with all of the stars that were found, as well as marking them on the image itself. Clever, reliable, and useful! Nice work!

Milky Way orbits: Kepler deduced planetary orbits based on repeated observations of planetary positions. We know that our Sun, and the rest of the stars in our galaxy, also orbit around the Milky Way’s core. But those orbits are perturbed by the presence of dark matter, something we can’t observe directly, and anyway, it would take hundreds of millions of years just to observe one orbit. Could there be a short cut? If you look up at the sky in the right location you can find a “stream” of stars that mark out one such orbital tracks, where clumps of stars formed together but moved slightly apart, along their shared orbit. Hogg and colleagues came up with a 6-D description of an orbit fit to those observations, concluding that the best fit is “an eccentric orbit in a flattened isothermal potential.” For more details, see their paper: “Constraining the Milky Way Potential with a Six-Dimensional Phase-Space Map of the GD-1 Stellar Stream”. I wonder what this line of inquiry will end up telling us about that dark matter distribution?

Earth’s quasi-satellites

How many moons does the Earth have? Just one, of course. But I recently learned that right now the Earth also has five natural quasi-satellites. The companion bodies orbit the Sun with the same period that the Earth does, but with a different eccentricity. Our five quasi-satellites are 3753 Cruithne, 2002 AA29, 2003 YN107, 2004 GU9, and 2010 SO16.

Here is a depiction of 3754 Cruithne’s orbit:

These orbits are not stable over the long term (with respect to the Earth) because they lie outside the Earth’s Hill sphere, its region of moon attraction. Eventually they’ll move on to other orbits. But for now, we have these five extra companions in our journey around the Sun.

Control a robotic telescope

The other day, I came across Observing With NASA, a site that lets anyone submit requests to a network of robotic telescopes. You pick a target and some simple observational settings, then submit your job — and the next day, you get an email with your results.

I had to try this out.

On March 17, I submitted a request to observe the moon, which seemed a good target since it would be nearly full. The next day, I received the excellent news that my image, that’s right, MY IMAGE OF THE MOON, was ready for accessing. Is it not beautiful?

You can also request images of the planets, stars, nebulae, and galaxies. I’m full of praise for this endeavor — what better way to let the public get involved with astronomy than by letting them select which observations to make? The website is easy to use and the results are rewarding. (You can download a FITS file with your data if you’d like to do more analysis, for which tools are also provided.)

Want to take your own picture with the Robotic Telescope Network? Click here!

Kepler’s challenges

The Kepler mission has already reported a slew of fascinating discoveries, including new planets and new kinds of planetary systems, and there is every expectation that in the final two years of observations it will continue to reveal more and more planetary treasures. However, no mission or instrument functions exactly as expected, and Kepler has had its share of challenges in collecting and processing its data. “Overview of the Kepler Science Processing Pipeline” by Jenkins et al. (2010) provides a fascinating behind-the-scenes look at some of these obstacles and their solutions.

Kepler consists of a one-meter telescope that has been staring at the same patch of sky for two years. Its goal is to measure the brightness of 156,000 stars every 29.4 minutes (“long-cadence” observations) and a smaller set of 512 stars ever 58.85 seconds (“short-cadence”). Each star generates a light curve of its brightness as a function of time. Exoplanets are detected as slight drops in the brightness while the planet transits in front of the star. For this light curve to be usable for detecting planets, Kepler needs two things: 1) a stable pointing so that the stars don’t bounce around or smear, and 2) a stable sensitivity so that any perceived brightening is due to an actual change in the stars.

During the first few months of observations, the first requirement was challenged. Kepler uses a set of “guide stars” to help fine-tune its pointing, and unfortunately it turned out that one of the guide stars selected in advance was an eclipsing binary. Whenever it would eclipse (so one star hid the other one), its brightness dropped and Kepler lost lock on it. As a result, the pointing was slightly off for 8 hours every 1.7 days (!). Kepler only downlinks its data once a month, so it took a few months to notice and correct this. The eclipsing binary star was eliminated from the guide star list and this problem has gone away.

The telescope is very sensitive to thermal conditions, any changes in which can wreak havoc with its focus. One of Kepler’s RWAs (reaction wheel assemblies, used to point the spacecraft, e.g., to pivot it towards Earth for data downlink and back to resume looking at the stars) has a heater that inadvertently modifies the telescope’s focus by about 1 micron every 3.2 hours. There’s no way to fix this, so it just has to be modeled and removed from the data in processing. Likewise, the spacecraft has experienced two “safing” events in which most of its systems shut down, which cools the entire assembly; each time when operations resumed, it took five days for the thermal effects to disappear from the data.

Perhaps most challenging is an artifact that manifests as “Moiré patterns caused by an unstable circuit with an operational amplifier oscillating at ~1.5 GHz.” Luckily, the actual impact on the data values is very small, generally only perturbing them by a single increment, but it is virtually impossible to adequately model and remove, so no doubt a source of at least minor frustration:

“Given that the Moiré pattern noise exhibits both high spatial
frequencies and high temporal frequencies, the prospect of reconstructing a high-fidelity model of the effects at the pixel level with an accuracy sufficient to correct the affected data appears unlikely. We are developing algorithms that identify when these Moiré patterns are present and mark the affected CCD regions as suspect on each affected LC.”

And finally, there was a curious overall brightening (termed “argabrightening”) observed in early phases of the mission. About 15 times per month, the background brightness of the entire field increased dramatically for a short time. The current hypothesis is that this was caused by remnant dust particles coming loose from Kepler and floating off, then reflecting sunlight back into the telescope. Detecting and removing affected observations was crucial for yielding consistent light curves. Fortunately, the rate of these events has decreased over time (Kepler might be running out of dust).

I look forward to more fascinating news from this great mission! And I hope they keep sharing the interesting challenges and lessons learned from operating a telescope from so very far away.

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