Authors: Daniel Angerhausen, Georgi Mandushev, Avi Mandell, et al.
First Author’s Institution: NASA Goddard Space Flight Center
“Observing with SOFIA is like riding a horse and trying to shoot a dime.” This fantastic analogy was said by the first author of this paper at the ERES conference this past May. Now, let me explain. There hasn’t been too much talk on astrobites about SOFIA, (Stratospheric Observatory for Infrared Astronomy), although in 2012 there was a great overview article if you want to check that out. The punch line, though, is that SOFIA is a 2.5-meter telescope that is mounted on a massive Boeing 747-SP aircraft, which flies up to 45,000 feet through Earth’s stratosphere. And in today’s bite, Angerhausen et al., successfully used SOFIA to observe a transiting exoplanet system. Aka, they rode the horse and shot the dime. This might sound insane to you but there is some real motivation for why scientists and engineers decided to do this.
It’s no secret that ground-based observations are difficult since you have to look through the Earth’s atmosphere. Water vapor, in particular, causes a lot of headaches because we are also interested in measuring water vapor in exoplanetary atmospheres. Think of holding two different translucent filters, one in front of the other. If the front filter was red and the back one was blue, you just might be able to discern the color of the blue filter. But, if the front one is red and the back one is also red, you might guess that it’s red, but it also could be clear, yellow, or any other light-ish color. Water vapor, in our atmosphere, is akin to the first red filter and, in with respect to exoplanet atmospheres, we are trying to discern the color of the second red filter. SOFIA, doesn’t get us completely through that first red filter (i.e. space), but it does get us most of the way there (i.e. Earth’s stratosphere). This is a great alternative to expensive space based observers.
That being said, SOFIA has its fair share of observational obstacles. Because you are observing in the stratosphere, you still are looking through some of the Earth’s atmosphere. However, at those heights, gases like water vapor, carbon monoxide, and methane are not affected by seasonal variations like on the ground. This makes it easier to remove what we call the, “telluric features” (Earth’s atmosphere) from your observations.
Now, imagine being in an aircraft and having to hold your telescope perfectly still for the duration of an exoplanetary transit (~2 hours!!!). It’s actually worse than shooting a dime from the back of a horse. It’s more like riding a horse while holding a laser pointer and trying to keep the light on the dime for 2 hours. Although this is incredibly difficult and your observation is often limited by how well you remain still while pointing, if you can keep track of exactly how you moved throughout the observation, you can remove those effects from your data. This is like if I drove from DC to NYC and you followed me in a helicopter writing down every turn and bump so that later (if I happened to be blind folded) you could get me back to DC from NYC.
With all this in mind, Angerhausen et al. put these concepts together and did the very first exoplanet observation using the SOFIA. Because this is new, it behooved the observers to look at a planet that was already studied. They chose HD 189733 b, a planet the size of Jupiter, but at the same distance to its parent star that Mercury is to our Sun. We call these hot Jupiters and they are fairly easy to take observations of since they are bright and orbit fairly quickly. Nevertheless, there have been big debates over the discovery of water, methane and carbon dioxide. These debates arise because it is unclear whether or not we do in fact see these species or if we are looking at clouds or hazes in the atmosphere. One group proposed that Rayleigh scattering dominated HD
189733 b’s atmosphere. Rayleigh scattering is the same light scattering phenomenon that makes Earth’s sky blue, as seen in the little cartoon and it affects the blue portion of a planetary atmosphere.
In order to settle some of these debates, Angerhausen et al.’s observations were designed to detect the presence of Rayleigh scattering and/or confirm or reject the notion of the presence of water vapor in the planet’s atmosphere. The following image is the final light curve of the transit. I feel guilty skipping over the analysis that went into producing this precise data, so one should really go and briefly check out what goes into something like this.
The light curve is only half the story, though. In order to detect atmospheric features, we not only need to detect the dip in brightness of the planet crossing in front of the star, we also need to detect small changes in transit depth as a function of wavelength. This will ultimately give us the spectrum we need to confirm or deny Rayleigh scattering and water features. Take a look at this bite if you’d like to learn more about this procedure! The next figure shows the two data points (in black) next to previous observations made with the Hubble Space Telescope, and Spitzer. Right away you will notice the increase in SOFIA HIPO data towards shorter (bluer) wavelengths. This increase in slope can also be seen in Earth’s atmosphere and is a clear indication of Rayleigh scattering. It also matches the red and green HST data previously taken. This tells us that there is some presence of particles, or condensate grains, in the upper atmosphere that is scattering the blue light of the spectrum.
Although there was no water vapor found in these observations, the authors were able to do two incredible things.
- They proved for the first time that SOFIA can be used to do precise exoplanet transit spectroscopy
- They settled the “Rayleigh scattering” debate for HD 189733b
In the end of their paper, Angerhausen et al. argue for a dedicated exoplanet instrument onboard the SOFIA aircraft. If this is realized, you can expect many more groups to take up the challenge of shooting the dime while riding the horse.