What I Learned Today

The EPOXI mission was born out of the desire to make use of the Deep Impact spacecraft, after it successfully hit Comet Tempel 1 with a separate smaller spacecraft. Two missions were selected to make use of Deep Impact: EPOCh (Extrasolar Planet Observations and Characterization) and DIXI (Deep Impact eXtended Investigation). If you stick DIXI and EPOCh together, you get… EPOXI. The proposers got kudos from NASA HQ for this acronym.

EPOCh has just finished its main investigation, which involved observing seven stellar targets that were believed to have planetary companions. I recently attended an excellent talk summarizing the results by Dr. Drake Deming, the deputy PI for EPOCh. They used Deep Impact’s camera to watch for the characteristic dip in stellar brightness when a planet transits across it. Since the camera was not designed for observing distant stars, it had no automatic stabilization, and the star would appear to wander all over the CCD. Tracking the star in the data once it was downlinked to Earth, and applying a different correction for each pixel in the CCD, makes ground processing challenging. However, they’ve been able to analyze this data and extract some interesting findings.

• They studied a Neptune-sized planet (radius about 4 times that of Earth) orbiting the red dwarf star GJ 436. It has an eccentric orbit that is likely to be influenced by a second, smaller planet. EPOCh has searched industriously for a signal from this smaller planet, so far not yet finding it (down to 1 Earth radius, the limit of what they can see with this instrument).
• A secondary transit happens when a planet goes behind its host star, from our perspective. This also causes a (smaller) dip in total brightness because the planet no longer reflects light from the star. This dip can help provide an upper bound on the albedo (brightness) of the planet. (Neat!)
• They also observed the Earth from Deep Impact, treating it as if it were an exoplanet and trying to see if they could accurately infer its properties. These observations serve as the perfect validation set to help us do a good job of interpreting similar observations of other planets, when we get to the point of having them. More details will appear in a paper on this subject of how an alien observer would view planet Earth.

Even better, the data collected by EPOCh will be released to the public in the spring. So you can try your hand at analyzing it, too!

And of course, stay tuned for news from the Kepler mission, set to launch on March 6. It will stare at (relatively) nearby stars specifically seeking Earth-sized planets in the “habitable zone” (where liquid water is stable). It will survey so many stars that even a null result (if they don’t find any Earth-sized planets) would make an interesting statement about the distribution of planets in the galaxy. It’s far more likely, though, that they will find such planets. We live in such exciting times!