Unlike the Cassini Saturn orbiter, which we looked at yesterday in the context of cryovolcanism on Titan, the Kepler spacecraft has but a single scientific instrument. It’s a photometer based on a Schmidt telescope design with a 95 cm aperture and a field of view larger than 100 square degrees. Measuring brightness variations for over 100,000 stars, Kepler is the first mission that should be able to detect Earth-size planets in the habitable zones of their stars.


That made yesterday’s news conference an eagerly anticipated event, but we have to remember that it’s going to be a while before we start talking about terrestrial planet detections. It takes multiple transits and much data analysis to make that possible, and a transiting world at roughly Earth-like distance from its star will demand several years of work. Kepler’s baseline mission is three and a half years, more than enough to make such detections, and the good news is that the instrument works.

Image: Magnified Kepler measurements of the planet HAT-P-7b showing transits and occultations. Credit: NASA.

The lightcurve of the planet HAT-P-7b shown at the news conference yesterday was dramatic proof. It was based on a mere ten days of test data collected during Kepler’s commissioning period, before science operations officially began. And even before the instrument has been fully calibrated and its data analysis software fine-tuned, it was able to detect HAT-P-7b’s atmosphere. The level of exactitude in these measurements has everyone talking. Here’s William Borucki, Kepler principal science investigator:

“When the light curves from tens of thousands of stars were shown to the Kepler science team, everyone was awed; no one had ever seen such exquisitely detailed measurements of the light variations of so many different types of stars.”

The paper on HAT-P-7b is being published in Science today, describing work on a planet that is roughly a thousand light years from Earth. It was a useful early target for calibration given that this gas giant orbits in a mere 2.2 days, a ‘hot Jupiter’ some 26 times closer to its star than Earth is to the Sun. That makes for numerous transits in short order, and in this case offers observations of a planet that is hot enough to be glowing like the burner of a stove (more in this news release).

Both initial transit and occultation were clearly visible as the planet first passed in front of, then behind the star as seen from Kepler’s vantage point. What we learn is that HAT-P-7b’s atmosphere has a dayside temperature of more than 2350 degrees Celsius (4310 degrees Fahrenheit). And here’s the key: The observed brightness variation is a mere one and a half times what would be expected from a terrestrial planet transit.


The prospects for terrestrial planet detections, in other words, have never looked better. The NASA image above gives us an idea how the story will play out. Here are planets graphed by mass and orbital distance, with plentiful representation at the high end and no planets of Earth mass or lower yet detected in the habitable zones of their stars. We should be able to offer a significantly different chart within just a few years.

The paper is Borucki et al., “Kepler’s Optical Phase Curve of the Exoplanet HAT-P-7b,” Science Vol. 325. no. 5941 (7 August 2009), p. 709 (abstract).