Reconnaissance of the HR 8799 System with Project 1640

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An outline of the paper: Reconnaissance of the HR 8799 Exosolar System. I. Near-Infrared Spectroscopy by Oppenheimer et al.

At Exoclimes, we’re often biased towards transiting planets because until now they’ve provided the most observational data on atmospheres and they’re the systems that we typically focus on in our own research. However, this paper by Ben Oppenheimer and collaborators gives a nice update on the state-of-the-art in direct imaging, where light from the planet is spatially resolved from that of the star.

The paper details observations of the famous HR 8799 planetary system made using a high-resolution, high-contrast imaging instrument known as “Project 1640” on the 5m Hale Telescope at Palomar Observatory. In a nutshell, Project 1640 consists of four main components: a deformable mirror for adaptive optics correction; a Lyot coronograph to block out as much of the star light as possible; an interferometer to calibrate residual wavefront errors; and an integral field spectrograph that allows spectra to be obtained for multiple objects in the field simultaneously. Using this instrument, the team spent a mere 77 minutes observing the HR 8799 system, spread over two nights in June 2012. With the data in hand, they had to clean away the “speckles” in the images, which are caused by imperfections in the instrument’s wavefront correction. From the cleaned images, they were able to extract the following spectra for the four known companions b, c, d, and e:

Measured spectra for the four known planetary companions around HR 8799, with solid lines indicating well-characterised substellar spectra that provide the closest match. Companions 'b', 'c', 'd' and 'e' orbit at distances of 68AU, 43AU, 27AU and 15AU, respectively, and all have estimated masses in the 5-10MJ range.
Measured spectra for the four known planetary companions around HR 8799, with solid lines indicating well-characterised substellar spectra that provide the closest match. Tentatively identified molecular absorption features are labelled. Companions ‘b’, ‘c’, ‘d’ and ‘e’ orbit at distances of 68AU, 43AU, 27AU and 15AU, respectively, and all have estimated masses in the 5-10MJ range.

The points with errorbars are the new data and the solid lines show the closest matches to previously-measured substellar objects that could be found. The authors tentatively attribute the peaks and troughs in the spectra to absorption by molecules like water, ammonia and methane, and possibly acetylene (C2H2) and carbon dioxide. Each of the spectra are different – like the members of our own solar system, these planets appear to be a diverse bunch. What’s more, there are significant discrepancies between the data and the best-match spectra plotted above, suggesting that we’re dealing with a new breed of objects.

With less than 1.5 hours of observing time, an entire planetary system has been characterised in one go. As more systems are characterised in a similarly efficient manner perhaps we’ll start to see patterns emerge, despite the view that planet formation is a messy process.

Feature Image: An example image of HR 8799 taken using the Project 1640 instrument, with a log colour scale to accentuate high-contrast features. The coronograph successfully blocks out a large portion of the stellar light, but there are still many speckles remaining due to imperfections in the instrument’s wavefront correction. These must be removed as part of the data post-processing in order to isolate the light from the faint planetary companions.

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About Author

I'm a PhD student at the University of Oxford. My work focuses on transiting exoplanets and, in particular, what we can learn about the atmospheres of these systems. A large part of this involves getting a better handle on the various instrumental systematics that contaminate the small signals we're trying to measure, and devising methods to remove them from the data. I'm also investigating ways of correcting for the effect of star spots on planetary transmission and emission spectroscopy measurements. My supervisor is Suzanne Aigrain.