With Hubble’s Space Telescope Imaging Spectrograph now out of commission, the study of exoplanetary atmospheres becomes a bit more problematic. But Seth Redfield (University of Texas at Austin) has now used a ground-based instrument to detect the atmosphere of a planet orbiting the star HD189733, some 63 light years away in the constellation Vulpecula. Discovered in 2004, this transiting world is about twenty percent more massive than Jupiter, orbiting its parent ten times closer than Mercury orbits our Sun.
Working from the ground is tricky but the odds go up when you observe more than a single transit. Redfield worked with eleven transits observed over the course of a year, using the Hobby-Eberly Telescope (HET) at McDonald Observatory in Austin. Studying the chemical composition of a distant atmosphere involves taking a spectrum during a transit and another when no transit is occurring. Working with the difference and comparing results over multiple transits helps you put together the atmospheric spectrum.
Image: The dotted line shows the planet’s orbit around the star HD189733. The planet orbits the star once every 2.2 Earth days, crossing the face of the star well below its equator. The small circles indicate the planet’s location during each of Seth Redfield’s more than 200 HET observations over the course of one Earth year. The red circles indicate observations during transit; the rest of the circles denote out-of-transit observations. Credit: S. Redfield/T. Jones/McDonald Obs.
None of which is easy. The light blocked out by the planet amounts to only 2.5 percent of the star’s total light as seen from Earth, and Redfield figures another 0.3 percent for the planetary atmosphere. But gradually the pieces of the puzzle come together. Redfield says this about his method:
“Each time the planet passes in front of the star, the planet blocks some of the star’s light. If the planet has no atmosphere, it will block the same amount of light at all wavelengths. However, if the planet has an atmosphere, gases in its atmosphere will absorb some additional light.”
Which gets interesting indeed when you look at the planet at wavelengths corresponding to specific transitions of the sodium atom. The presence of sodium means that the planet absorbs more starlight at those wavelengths, making the planet appear larger by about six percent than at other wavelengths. With sodium now detected, the search can move to other atmospheric constituents. Hundreds of observations went into this result, along with the necessary task of filtering contamination to the data from Earth’s atmosphere.
So precise was the work that the transmission spectrum achieved from this distant transit via ground methods was higher in resolution than previous Hubble work on exoplanetary atmospheres. Honing this technique will get us into even more interesting territory as we start extending our methods to planets more amenable to life. It will also give us a head start as we wait for the next generation of space-based observatories to provide data at higher levels of precision.
The paper is Redfield, Endl et al., “Sodium Absorption From the Exoplanetary Atmosphere of HD189733b Detected in the Optical Transmission Spectrum,” accepted for publication in Astrophysical Journal Letters (abstract).