Earlier this week, we described a brief announcement from the team behind the Kepler space observatory, designed to spot the transit of planets in front of their host stars. That announcement was followed up by a paper in Thursday’s issue of Science, which provided a few more details on some of the planets and other things that have been spotted within its field of view. But that paper was really a vehicle for a massive information dump; it contains information about 22 papers submitted to the Astrophysics Journal, many of which have been posted on an arXiv preprint server.
Many of these describe the instrument aboard the Kepler in greater detail, and others the scientific pipeline that handles the data it returns—eliminating false positives and coordinating follow-up observations are central to the process. One paper lists the follow-up resources available to the team, which include the Hubble and Spitzer in space, and Hawaii’s Keck telescope back on Earth. As of the end of 2009, there were already 177 items that were in or had been through the planetary pipeline. Five of these have already been confirmed to be signals from planets, and another 52 appear to be promising candidates. Another 65 are still under observation, status unknown.
Some of the planets are rather unusual, at least based on the bodies in our solar system. One, Kepler-4b is quite similar to Neptune (nearly the same size, and 1.4 times the mass). But, as the authors note in a fit of understatement, “A major difference between Kepler-4b and Neptune is the irradiancelevel for Kepler-4b is over 800,000 times larger.” If Kepler-4b and Neptune had the same composition, then that much heat (its equilibrium temperature is 1650K) would have caused the planet to swell dramatically. So, although the paper’s title refers to the planet as “Hot Neptune-like,” there seem to be some substantial differences in composition.
Another planet, Kepler-6b, does seem to have undergone some temperature-related bloating. Despite being only about two-thirds the mass of Jupiter, at 1,500K, it’s bloated up to a radius of 1.3 times Jupiter’s. That leaves it with a density of 0.35g/cm3 (for the metrically unaware, water’s value is 1). Although this seems odd based on our familiar planets, the authors describe these features as “fairly typical.” Kepler-8b, however, falls off the far end of typical. It’s got a density of 0.26g/cm3, making it one of the lowest density planets known.
Returning to a known planet, HAT-P-7 (which Kepler used to validate its instruments, scientists were actually able to detect the fact that the massive planet, orbiting so close to its host star, induced disturbances in the star’s surface. That’s good news for the observatory’s future. “The Kepler light of HAT-P-7 curve reveals ellipsoidal variations with an amplitude of approximately 37 ppm,” the authors of that paper note. “This is the first detection of ellipsoidal variations in an exoplanet host star, and shows the precision Kepler is capable of producing even at this early stage. For comparison, a transit of an Earth-analog planet around a Sun-like star would produce a signal depth of 84 ppm, a factor of 2 larger than this effect.”
Some of the other papers point out that, although Kepler’s primary mission is to hunt planets, it’s unusual in that it does so by staring intently at a the same stars, day in and day out (the ESA’s CoRoT does something similar). That is allowing researchers to do some asteroseismology, tracking the variations in stars over the short term, and allowing them glimpses into the processes that drive stellar evolution.
They’re also using the data to observe the orientation of the orbits of these planets in order to constrain our understanding of the development of planetary systems. These hot Jupiters can’t possibly form so close to the host star, but this the data will help identify whether they are typically dragged inwards by that star’s gravity, or shoved inwards by interactions with other planets.
All that in the first six months that Kepler has been operational.
Science, 2009. DOI: 10.1126/science.1185402