Mapping an Alien World

Jonathan Fortney, a planetary scientist and Astrobiologist from the Carl Sagan Center may well have something in common with Claudius Ptolemy. They are both pioneers in the world of map making, despite lacking more data than they actually possess. Ptolemy, working a couple of millennia ago, created some of the earliest known maps of planet earth based on rudimentary knowledge of less than one quarter of the globe. Jonathan and his team have modeled a rudimentary temperature map of an alien world, the first of its kind, with an even more limited data set. Still, this new map represents our first real insight into an extra-solar planet, any extra-solar planet, beyond knowing that it exists or doesn't. The map is pictured here (Figure 1).

The search for extra-solar planets, those around stars outside our solar system, has been stepping up over the last decade or so. Since the discovery of 51 Pegasi (Figure 2), announced in late 1995, roughly 216 giant extra solar planets have been detected orbiting their parent stars scattered throughout the Milky Way. To date, these alien worlds are Jupiter-like gas giants, not necessarily because giants abound out there, but because, like the guy looking for his lost keys under the lamppost, with current detection methods, giants are what we are able to find.

Dr. Fortney specializes in "hot Jupiters". These are giant gas planets much like our Jupiter but orbiting very close to their parent stars and whizzing around them in days, rather than the years (11.9 for Jupiter and 29.5 for Saturn, 84 for Uranus, and 165 for Neptune) that it takes our own. Their orbits are small and their speeds rapid because they are so close in to their stars. This proximity and the gravitational wobble that closeness causes to their parent stars, is often what helps us detects these planets. The closeness also accounts for the heat. In addition, such "hot Jupiters" are inevitably tidally locked to their parents. That is, their orbital periods are identical to their rotations, and they always present a single face to their parent stars, much as our moon always faces the earth. This happens for known reasons due to the unstable distorting tug of gravity of the larger body on the smaller, close in body. This instability is resolved through tidal locking.

Planet HD 189733b was detected by NASA's Spitzer Space Telescope, an infrared space based observatory looking at heat energy in the universe (Figure 3). Looking from space, Spitzer was able to use an alternative detection method to discover this new planet. The method is called transit photometry. As a planet circles its parent star it crosses in front of that star, reducing the total luminosity, or light output, slightly. In effect, the planet eclipses the star. If you continue to observe such a system, the reduced luminosity will be repeated, and you learn the orbital period of the orbiting body. Spitzer has discovered 3 new planets using this detection method without having been specifically designed to do so. In 2008, NASA's Kepler Mission, specifically designed and dedicated to detecting planets through transit photometry, will launch and gaze at 100, 000 stars. Kepler's sensitivity is such that it will be capable of detecting small, inner, rocky planets like earth in addition to gas giants.

Jonathan and his team successfully competed to get 33 continuous hours of observing time out of Sptizer's precious schedule to get the data set that became the temperature map, the first true map of any extra solar planet. HD 189733b is 63 light years away and orbits its parent star in 50 hours. Previous models and temperature data of gas giant planets seemed to suggest that such bodies would be very hot on their sun facing sides and significantly cooler on their far sides. What Forney and his team found was a smaller temperature range than might have been predicted plus an intriguing displacement of the "hot spot". What accounts for their results is almost certainly the presence of high winds, some kilometers per second that transport the energy to the far side of the planet. The team is continuing to refine their model to understand these winds and the transport mechanisms involved.

Hot Jupiters are interesting for several reasons. They are extremely luminous when they are formed, 1000 time more luminous than the 4.5 billion year old planets in our solar system. We may someday be able to directly image such planets from space. We don't truly understand their formation and evolution and atmospheres yet. We know gas giants are largely composed of hydrogen and helium, the most abundant elements in the universe. In our solar system gas giants form far enough from the sun that these volatile elements don't just burn off.  What is happening with these newly detected close in giants? Saturn displays "excess" luminosity and appears to be powered by an additional interior energy source that Jupiter may not have.  We don't understand this yet, but the Cassini Mission, still orbiting Saturn continues to provide insight. Understanding the formation, evolution and atmospheres of gas giants will help us understand the structure and evolution of solar systems in general and will eventually give us insight into how rocky planets form and evolve. Rocky planets, like earth, are capable of holding liquid water and could support life. Now, that's really interesting.