The people who build and run the hardware we send into space probably experience a lot of sleepless nights. It's possible to design multiple redundant systems and test everything on the ground, but that's no guarantee that things will operate as you expect them to once they get sent into space (witness the originally distorted optics of the Hubble). For the people running the Kepler observatory, which is designed to detect planets that have orbits and sizes which approximate Earth's, it's apparently time to breathe easily. Scientists have now calibrated its camera against a known planet, and found that its sensitivity is up to its intended task.

The Kepler was launched in early March and went operational in the middle of May, so these results have come very quickly. That's partly the result of the planet it has observed, HAT-P-7b. The planet is what's called a "hot Jupiter" that orbits close to its host star, and completes one orbit (a HAT year?) in a brisk 2.2 days. As a result, the folks running Kepler were able to observe multiple orbits in just 10 days of work that took place during the commissioning period. The results are published in today's issue of Science, and NASA has hosted a press conference to describe them.

Finding a planet we know is there isn't much of a breakthrough, but the data obtained provided an important test of both the Kepler hardware and the data processing pipeline, as well over 50,000 stars were imaged during this testing. After extracting the data from the host star, HAT-P-7, the authors tested various models to explain the data. The best fit was to a model for a planet that partly occludes its host star, and then is entirely occluded by the star as it orbits behind it from the perspective of our solar system.

The real test of Kepler comes from the precision of this fit. By combining the data from multiple orbits, he authors calculated an orbital period of 2.204802 days, ?0.000063—that's give or take five seconds.

Zooming in on the HAT-P-7 data obtained by Kepler.

But what's really striking is how clean the data is. A couple of slides were shown at the press conference, which are combined here. The top shows what the light from HAT-P-7 looks like from Earth-based telescopes, with the dip occurring as the planet passes in front of the star. In contrast, the variability in the measurements from Kepler, shown immediately below, is impossible to detect at this resolution. Some noise becomes apparent when you zoom in by a factor of 7 but, more importantly, a small, secondary dip is also apparent on the right of the graph.

Zooming in further, that dip becomes more apparent, and it's clear that it's surrounded by a slight rise. This is the product of HAT-P-7b sliding behind the star. Since the planet is very hot, radiation from it combines with reflections of the light from the host star as the planet's orbit brings it close to the star from our perspective, explaining the increase in light. Once it goes behind the star, however, you get the drop, caused by the star obscuring HAT-P-7b entirely.

It's the size of the dip that's important here. It's on the order of 122 parts-per-million in the total signal. We'll need to get down to about 84ppm to spot an Earth-like planet, and the data show that Kepler is up to the task.

As this data was being processed, Kepler had already gotten down to work, staring down a star-rich arm of our Milky Way galaxy, looking for variations in the light of 100,000 of these stars that would indicate the presence of planets. As time goes on, it should be able to detect multiple transits of planets that take longer and longer to orbit, which means planets that are further and further out. As the hosts of the press conference put it, check back in 2011 if you want to know about more Earth-like planets.

NASA has placed animations from the press conference on the Kepler site.

Science, 2009. DOI: 10.1126/science.1178312