DPS meeting: Saturday: Studying extrasolar planets with planetary spacecraft
Posted by Emily Lakdawalla
13-10-2008 13:15 CDT
I'll be writing quite a bit about the annual meeting of the Division of Planetary Sciences (DPS) of the American Astronomical Society this week; see this post for an introduction.
One of the first sessions at DPS was on results from the EPOXI mission, which is what they are calling the Deep Impact extended mission. There are two components to the extended mission: Deep Impact is on a long cruise toward a second cometary encounter, with Hartley 2 in 2010; but in the meantime, astronomers are using Deep Impact's defocused high-resolution imager to study known transiting exoplanets, and also to study Earth as though it were an exoplanet. What's a transiting exoplanet? It's a planet that orbits another star, which has been detected from Earth by the way it dims the light from its star as it crosses in front of the star on its orbit. In order for a planet to be detected this way, we have to be looking at the star system edge-on, so it's only possible for us to detect such exoplanets around the small fraction of stars whose planetary disks just happen to be oriented that way.Sarah Ballard showed some initial results of the EPOCh mission, which generally indicate that the mission is producing valuable data on transiting exoplanets. She explained why Deep Impact is so useful for such a study: "EPOCh takes advantage of the defocused point-spread function [of the high-resolution imager]. It spans about 4 arcseconds on the sky. That defocus is an advantage to us because we spread the light out on more pixels, thus enabling us to increase our exposure time and extend our duty cycle since we have very limited onboard memory. Each target was observed nearly continuously for several weeks at a time, stopping only for the spacecraft to downlink data back to Earth." They captured data from January to August of this year, observing seven different transiting exoplanets. They were looking to see if they could detect rings and moons, or if they could detect other planets in the same systems. She didn't have any such detections to report, but she did show a very nice-looking data set on a relatively small exoplanet, the only known transiting "hot Neptune" (a Neptune-sized planet in a tight orbit about its star), GJ 436. Ballard then performed a statistical analysis indicating that if there were an Earth-sized planet in the same system as the hot Neptune, then they'd have about a 2/3 chance of detecting it and deducing its diameter to within 10% of the actual value. So stay tuned for Deep Impact to detect smaller planets in these systems. Timothy Livengood gave a presentation on Deep Impact's work studying Earth as though it were an extrasolar planet, which was mostly just to show that they have a good data set -- actually three separate sets of observations, conducted from one equinox to one solstice. One of the three included a transit of the Moon, which is pretty cool (and which I've written about before). His talk contained such humorous lines as "I just thought I'd let you know that the Earth is blue. You may not have known that." He showed data from Deep Impact's spectrometer, in which he could detect water (no surprise there) but also carbon dioxide, as well as some tiny features from methane, molecular oxygen, and ozone.
One thing Livengood searched for in the Deep Impact data is the so-called "red edge." This refers to the shape of the spectrum of chlorophyll, the green pigment in photosynthetic plants. Chlorophyll is generally pretty dark, slighly more reflective in green wavelengths than blue and red (duh, that's why leaves look dark green). And that makes sense, because, after all, chlorophyll is what allows green plants to absorb solar energy to perform photosynthesis. But if you look at green plants in wavelengths of light slightly longer than the ones we can see, suddenly those leaves are brilliantly reflective. So one common way to study the photosynthetic productivity of a landscape from space is to look at Earth using a near-infrared filter; through such a filter, agricultural land, forests, and jungles glow brilliantly, while rocks, soil, water, and manmade surfaces are much darker. It's also common to make false-color images of Earth using three filters that are shifted slightly toward the infrared -- instead of using red, green, and blue-filter images to compose a true-color image, you use infrared, red, and green images to compose a false color image. In such images, vegetated areas appear bright red. The sudden jump in reflectivity of plants in the infrared is called the "red edge," and it's considered one way to detect Earth-like plant life from a long way away. Livengood demonstrated its presence in the Deep Impact data.
David Grinspoon presented the results of a similar experiment being performed by Venus Express. At Venus Express' distance from Earth, its highest-resolution VIRTIS instrument can't even resolve Earth -- it's always much smaller than one pixel -- which is actually a better analogue to future studies of extrasolar planets. If VIRTIS can't see Earth as a disk, it does get excellent data on Earth's spectrum, which is fun to study from a distance because of all of the different sources of variability: the spectrum changes as Earth rotates, bringing more continents or more oceans into view; it changes as weather patterns increase or decrease cloud cover and change their distribution; it changes with the seasons, as more or less ice covers the northern hemisphere (the southern hemisphere, having much less continent, shows much less seasonal ice cover variability); and so on. Again, the presentation didn't contain any real surprises; more than anything else it demonstrated that VIRTIS has captured a valuable, real-world data set that will be useful in developing models of what might be causing spectral variability on distant worlds that we will someday study with powerful telescopes.There were a few more talks on actual exoplanets, but I think they were too technical for me to cover here. On to the next session...
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