Kepler's Multi-planet Bonanza
Last week at a meeting of the American Astronomical Society in Boston, scientists working with NASA's Kepler mission announced the confirmation of an additional planet in a previously identified planetary system. The abundance of systems showing multiple planets has been one of the early surprises to emerge from the stellar census being conducted by the space telescope.
"We didn't anticipate that we would find so many multiple-transit systems," said astronomer David Latham from the Harvard-Smithsonian Center for Astrophysics. "We thought we might see two or three. Instead, we found more than 100." In a survey of 156,000 stars during just the first four months of the mission, the Kepler team identified more than 700 stars that appeared to host planets. This relatively small fraction (0.5%) was expected, since the technique that is being used to discover the planets is only sensitive to those that pass directly in front of their host star. The orbits of the planetary systems are randomly oriented in the sky, so simple geometry can be used to convert the fraction of stars with observed planets into the fraction of stars that actually host planets. In addition, since the experiment must observe several consecutive orbits to trigger a detection, only planets with the shortest orbital periods have already been identified -- so the number of planets is expected to grow as the mission continues. The big surprise was the relatively large fraction of these stars that appear to have more than one planet (~15%). The planetary orbits within our own solar system are in roughly the same plane, but the slight misalignment would prevent a distant observer from seeing many planets with the technique used by the Kepler mission.
It is exciting enough to learn that multi-planet systems like ours may be more common than we anticipated -- but these systems can also help us measure the masses of the planets, and teach us about how planetary systems form and evolve. In multi-planet systems, not only do the individual planets gravitationally tug on their host star, they also pull on each other and change their orbits over time. Larger planets pull more strongly on other bodies in the system, so the changes in the orbits can be used to measure the masses of the individual planets. The masses are usually determined by measuring the tiny reflex motion of the host star, but many of the planets being discovered by Kepler are far too small -- approaching the size of the Earth -- to make such measurements with currently available technology. So the ability to measure interactions between planets in these distant solar systems represents a huge opportunity to characterize our stellar neighbors. Looking at the multi-planet systems discovered by Kepler so far, one striking fact is that none of them seem to contain planets much larger than Neptune. The gravity of larger planets like Jupiter tend to scatter smaller planets into tilted orbits -- or even eject the planets from the system entirely. So the absence of large planets in the systems that have so far been discovered appears to make sense.
If there is one recurring lesson in the history of astronomy, it is that we have always been conservative about how common other solar systems like ours might be. Kepler has already taught us that tiny planets like ours are even more plentiful around other stars than the big planets that have been discovered over the past two decades. In the coming years, perhaps we will also find that habitable planets -- those that may support life -- are also far more abundant than we ever imagined.
