Washington DC witnessed the year’s American Astronomical Society (AAS) meeting in early January 2010. The meeting spawned a large number of news stories relating to exoplanets so I thought it worthwhile to summarize a few of the more interesting results that could impact apologetics.
Kepler telescope reports first exoplanets. The Kepler space telescope was designed to look for transits of Earth-sized planets in the habitable zone across Sun-like stars. Launched in March 2009, the telescope has detected its first five exoplanets. Each host star is roughly 20 percent more massive than the Sun. All of the detected planets classify as “hot Jupiters,” meaning they are gas giants and orbit close to their host stars. Their masses range from 6 to 200 percent of Jupiter’s mass with radii between 0.36 and 1.5 times that of Jupiter. The planets’ orbital periods are around four days, which means they reside a few million miles from their host stars. Consequently, the temperatures of these planets all exceed 2,000˚ Fahrenheit. Although these exoplanets bear no resemblance to the solar system bodies, they are the most easily detected type of exoplanet. As stated by the mission’s principal investigator, “the discoveries also show that our science instrument is working well. Indications are that Kepler will meet all its science goals.” In other words, results from Kepler over the next few years will go a long way toward answering the question: How unique is our solar system?
Ten percent of planetary systems like ours? The MicroFUN collaboration, headquartered at Ohio State University, reported on a study that indicates that 10–15% of the stars in the galaxy host planetary systems analogous to ours, i.e., with several gas giants in the outer part of the system. This result comes from a statistical analysis of planets detected via microlensing, which is relatively insensitive to the orbital distance of an exoplanet. This technique looks for increases in the brightness of background stars caused by a closer planet system that gravitationally lenses the background star. When looking at all the microlensed planet systems, only one loosely resembles the solar system. In this system the star is half the mass of the Sun and emits about 5% of the light. Additionally, the planets have 80% the mass of Jupiter and Saturn. Nevertheless, the relative masses and distances within this exoplanet system match those in our solar system. So, the 10–15% derived from the statistical analysis of microlensed planets relies heavily on the detection of this single system. While the fraction of systems that match the structure of ours will be much lower, this study provides the first constraints based on actually detected planets.
Lightest planets started as gas giants. The last three discoveries pertain to the lightest and smallest planets yet found. In September 2009, a team of scientists announced the discovery of a definitively rocky exoplanet with the smallest known mass. Designated CoRoT-7b, the planet has a density near Earth’s with a radius roughly 75% larger than Earth’s. However, CoRoT-7b orbits close to its parent star, causing surface temperatures over 2000˚F. Two separate discoveries gave further details on this planet. First, additional analysis revealed that CoRoT-7b has a noncircular orbit. Consequently, it experience strong tidal heating that turns its surface into a volcanic wasteland. Second, a study of its orbital history indicates that CoRoT-7b likely started out as a gas giant with a larger orbit. As the planet migrated closer to its host star, the intense heat and interaction with the material surrounding the star caused the planet to lose most of its mass, which was in gas form. What astronomers detect is simply the remaining core. Given the characteristics of the second smallest known planet, dubbed HD 156668b, CoRoT-7b likely experienced a similar history.
All of these discoveries illustrate the remarkable advances in planet-hunting technology. They also provide more evidence for the unique character of our solar system.