The Planetary Society Blog
By Emily Lakdawalla
Searching for extrasolar planets with Deep Impact
Nov. 3, 2006 | 10:39 PST | 18:39 UTC
EDIT: I goofed in part of my interpretation of the science of this earlier, so there are changes in the third paragraph below. Thanks to Dr. Deming for the corrections.
When the Discovery program selections were announced on Monday, everybody I talked to was surprised and intrigued by one of the "Missions of Opportunity" on the list. The EPOCh mission, which stands for Extrasolar Planet Observations and Characterization, would use Deep Impact's high-resolution camera to search for Earth-mass planets around other stars. First of all, the idea of using a mothballed spacecraft for an observation as tricky as the discovery of Earth-mass extrasolar planets was a surprise in itself. Second of all, the Deep Impact high-resolution camera has a flaw that makes its pictures blurry, so it was quite surprising to me that this particular camera would be seized upon for an extrasolar planet search.
I sent an email to EPOCh's Principal Investigator, Drake Deming, at Goddard Space Flight Center, to ask him how the search would work. It turns out that he plans to turn Deep Impact's flaw to his benefit. He explained: "EPOCh will do photometry of giant planets transiting several nearby bright stars (it's like Kepler in that respect, but Kepler is locked-in to a particular field in Cygnus, and won't look at nearby bright transiting systems). We exploit the fact that the Deep Impact telescope is out of focus (allows us to get better photometry). We are sensitive to terrestrial planets via their perturbations on the transit times of the giant planets (which we measure very precisely)."
That's what I call making lemonade from lemons. The camera blur spreads the point-source light from stars out over several pixels on the camera's CCD. Deming explains here how that helps (I tried to explain earlier, but I had the wrong impression of how it worked, so I'll leave it to the expert to explain for me): "We are doing very precise photometry, measuring the brightnesses of the stars. As the giant planets pass in front of them ("transit" the stars) we will see the dip in the star's light. This dip lasts for several hours, but we want to time its occurrence very precisely (to about 1 second accuracy). It's those changes in the time of the giant planet transits that will indicate the presence of terrestrial planets. That means we have to measure the stellar brightness very accurately, so that the curve of brightness versus time is very "smooth", i.e. has high signal-to-noise ratio, and we can find the center time accurately. But to get high signal to noise, we have to collect lots of photons from the star. That's where the defocus helps. Each pixel of the CCD has a limited capacity to collect photons before it saturates. With a defocused image, we have about 75 pixels collecting light for us, so we can collect lots of photons in each exposure without saturating, and that gives us the high signal-to-noise ratio that we need."
Kudos to Deming for the idea -- I hope that NASA decides to go ahead with this mission, and that a spacecraft considered beyond its design life can make a contribution to a completely different field of planetary science than the one for which it was built. Deep ImpactAn artist's depiction of the Flyby craft after releasing the Impactor (left) in the vicinity of comet Tempel 1. Credit: NASA / JPL |
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