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The Planet Characterizer

Today we have a guest post from Seth Redfield. Seth is an Assistant Professor at Wesleyan University in Middletown, CT. Before Wesleyan, he was a postdoc in Austin, TX, and a graduate student in Boulder, CO. He is an avid hiker, and an oboe player (with a degree from the New England Conservatory of Music in Boston), but these days spends any free time with his kids (4 and 1 years old) and sleeping.

To date, I have working on studying known exoplanets rather than finding them. Instead of a “planet hunter”, you could call me a “planet characterizer” (which doesn’t have quite the same ring to it). Perhaps the most well-studied planet, and one of my personal favorites, is HD189733b. This is because it is a transiting exoplanet, orbiting one of the brightest stars that we know has a planet. The fact that it transits, allows us to use spectroscopy of the starlight from HD189733 while the planet is transiting to look for wavelength-dependent effects that reveal interesting properties of the planet. For example, we can measure the composition, temperature, and even wind speeds in the atmosphere of the planet. The fact that HD189733b orbits a bright star, makes all these measurements “easier”, meaning that they are still incredibly difficult and require careful observations using the world’s largest and most sophisticated telescopes, but nonetheless are possible.

Because transiting planets are so useful, I follow with excitement all the searches of transiting planets, hoping they will find one around a bright, nearby star. However, the strategy for searching is directly at odds with finding one around a bright star. In order to find the rare planetary system that is edge-on, and therefore transits, one must observe many tens of thousands of stars. Bright stars tend to wash out large sections of our detectors and make it difficult to see the multitude of fainter stars around them. For this reason, all the searches largely avoid the bright stars.

Indeed, HD189733b was not discovered first by its transit, but by the radial velocity method of observing the host star orbit the center of mass of the system. It is for this reason, that I feel that the planets that will become household names, meaning the planets whose names will be known by school kids around the world, have yet to be discovered. These will be small, Earth-like planets, for which we can just barely detect using the radial velocity method, but which will also transit a bright host star and thereby make it possible for us to probe the characteristics of the planetary atmosphere.

So, as this young field of exoplanet research matures, I see this clear synergy between the detection of exoplanets and characterization of those exoplanets. Obviously, exoplanets must be detected in order to be characterized. The handful of exoplanet atmosphere detections to-date have uncovered a diverse collection of atmospheres that appear to be influenced by a myriad of planetary and stellar phenomena (such as planetary composition, stellar flares, etc). So, the characterization of exoplanets motivates us to find more exoplanets with new and extreme properties. I feel like we are at a similar point to astronomers 150 years ago when spectroscopic observations of stars were being made for the first time. Every discovered exoplanet is amazing, but it is likely that the planets we are talking about now will not be the planets we will be obsessing over in twenty years.

One final note, is that the brightest stars being observed by Kepler are almost as bright as HD189733, so let me take this opportunity to make a plug for searching the brightest stars in the Kepler field. Anything found to be transiting those stars will certainly be of interest to the “planet characterizers” out there.

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