Just how good is Kepler at finding planets? We’re getting a pretty good idea. In his talk yesterday at the AAS meeting in Washington, William Borucki (NASA Ames) showed a plot of the lightcurve for previously known planet HAT-P-7. The signature of the planetary transit is unmistakable in these data, a well defined dip in the starlight as HAT-P-7 makes the star just a little dimmer by its passage. Kepler’s sensitivity is apparent.
But the plot is more fascinating still, for in addition to the well defined signature that denotes the dip in starlight caused by the planet moving across the face of the star, Kepler also saw a second dip. That one was caused by the light of the planet being blocked by the star itself. It’s a tiny dip, but one readily demonstrated in Borucki’s chart, and it tells us that Kepler is living up to expectations in terms of finding faint signals. We all hope, of course, for a future finding, the faint signal of a terrestrial world, preferably one in the habitable zone of a star not so different from our own.
Not that the first Kepler planets weren’t fascinating in their own right. What Borucki announced yesterday were more ‘hot Jupiters,’ planets of high mass and extreme temperatures, the five ranging from something similar to Neptune to larger than Jupiter, with estimated temperatures between 1200 and 1650 degrees Celsius. They’re hot, bright, and extremely close to their stars. Kepler 7b is one of the least dense planets yet discovered.
All of these worlds orbit stars larger and hotter than the Sun, and in that regard all are more or less what we expected in early Kepler results, the low-hanging fruit that turns up readily while smaller planets with longer period orbits take longer to discern and be confirmed. Right now, after all, we’re working with no more than six weeks of data collected since May. Note this from Greg Laughlin’s systemic site:
The Kepler planets are primarily orbiting high-metallicity, slightly inflated, slightly evolved stars. These particular parent stars were likely selected for high-priority confirmation observations because their abundant, narrow spectral lines should permit maximally efficient, cost-effective Doppler-velocity follow-up.
We also have an unusual find, a system where the light curve from the apparent planet dips more strongly during the occultation than the transit, suggesting it is hotter than the parent star. What has astronomers puzzled is that early indications are the object is too big to be a white dwarf, and Borucki noted that this wasn’t the only example Kepler had found of such an object. All in all, we’re seeing good things for Kepler, adding Kepler 4b, 5b, 6b, 7b and 8b to our exoplanet list, and finding unexpected things the explanation of which should further refine our planetary formation models.
You can get a look at numerous preprints for Kepler’s newly revealed work here (this is the arXiv page for new submissions, so it will be changing), and I note especially Latham et al., “Kepler-7b: A Transiting Planet with Unusually Low Density,” citing a world whose mass is less than half that of Jupiter, but whose radius is fifty percent larger (preprint). We’re looking at a density of 0.17 g/cc for a planet orbiting a star considerably larger than the Sun, one presumed to be near the end of its life on the Main Sequence.