Exoplanets and Their Atmospheres

Atmospheres of Distant Worlds

Some planetary systems are fortuitously aligned such that from out perspective on Earth, the exoplanet will pass directly in front of its host star once per orbit. The transit of the exoplanet not only provides a means of detecting the planet and measuring its size, but also enables us to measure starlight passing through (or being blocked) by the exoplanetary atmosphere. After the first such detection of this kind was made using the Hubble Space Telescope of HD209458b (Charbonneau et al. 2002), the instrument that was used failed, and would not be repaired for 5 years. In that time, several attempts were made using ground-based telescopes. Learning from these efforts, I was able to make the first ground-based detection of an exoplanetary atmosphere, of HD189733b, using the Hobby-Eberly Telescope (see figure below).

As we push this technique to smaller and smaller exoplanets, we will be able to probe the detailed atmospheric properties of terrestrial exoplanets. In the coming decades, it is likely to be this kind of work that is used to identify biosignatures and for the first time, identify life elsewhere in our Universe.

Recent Papers On This Topic:

1. Optimal measures for characterizing water-rich super-Earths   Madhusudhan, N., & Redfield, S. 2015, IJAsB, 14, 177

We explore the various issues related to observing the atmospheres of Super-Earth, that is terrestrial planets that are intermediate in size between the Earth and the ice giants. We found that the very hot exoplanet, 55 Cnc e, with a period of only 17.5 hours, is an attractive candidate for studying a cloud-free atmosphere.

2. Probing potassium in the atmosphere of HD 80606b with tunable filter transit spectrophotometry from the Gran Telescopio Canarias   Colón, K.D., Ford, E.B., Redfield, S., Fortney, J.J., Shabram, M., Deeg, H.J., & Mahadevan, S. 2012, MNRAS, 419, 2233

An early detection of potassium in a gas giant exoplanet. This technique uses adjustable narrow bandpasses to isolate the KI line.

3. A Survey of Alkali Line Absorption in Exoplanetary Atmospheres   Jensen, A.G., Redfield, S., Endl, M., Cochran, W.D., Koesterke, L., & Barman, T.S. 2011, ApJ, 743, 203

A "large-scale" survey (at least at the time) of 4 transiting gas giants. Sodium is detected in three targets, but no significant potassium absorption is found. This work was led by Adam Jensen, a postdoctoral researcher working with me at Wesleyan during this time.

4. Sodium Absorption From the Exoplanetary Atmosphere of HD189733b Detected In the Optical Transmission Spectrum   Redfield, S., Endl, M. Cochran, W.D., & Koesterke, L. 2008, ApJL, 673, L87

My first paper on exoplanetary atmospheres! I was inspired while on an observing run at McDonald Observatory taking spectra of nearby stars to study the LISM. I learned that HD209348b, the first known transiting exoplanet around a pretty close star, was going to be transiting during my run. I read Brown (2001) and Seager & Sasselov (2000), learned about previous attempts from the ground, and designed this program with the Hobby-Ebery Telescope. It is the first ground-based detection of an exoplanetary atmosphere, and only the second using any platform (after HD209458b with HST by Charbonneau et al. 2002). This work was featured in Astronomy Now and, surprisingly, in an equivalent kind of magazine in Finland, Tähdet ja avaruus (and I have a copy)!

Extended Atmospheric Structures

Early-on, observations of Lyman-α (the transition from the ground state, n=1 to n=2) indicated that there were extended hydrogen atmospheres around close-in giant planets (Vidal-Madjar et al. 2003). In 2012, we made the first detection of H-α (the transition from n=2 to n=3) in the extended atmosphere of an exoplanet (Jensen et al. 2012). In follow-up observations (which incidentally detected the extended atmosphere in H-α, H-β, and H-γ), we discovered something unexpected. We found substantial absorption hours before the planet passed in front of the star! The observations and a schematic of the geometry is shown in the figure below (Cauley et al. 2015). This could be an extended bow shock or an accretion flow, mediated by the interaction of the exoplanet with its host star.

Such extended structures only become observable by looking at the transit in specific atomic transitions, which requires relatively high spectral resolution. In order to take quick exposures of such a small signal, at high resolution, requires using the largest telescopes in the world, even for host stars that are almost visible to the naked eye.

Recent Papers On This Topic:

1. Variation in the pre-transit Balmer line signal around the hot Jupiter HD 189733b   Cauley, P.W., Redfield, S., Jensen, A.G., & Barman, T. 2016, AJ, 152, 20

A follow-up to the paper below, where we are able to sample the pre-transit time much more densely. We confirm the in-transit absorption and also confirm that pre-transit variability is also detected. However, the pattern of absorption is not consistent with the bow shock model presented in the paper below. Instead, we explore an accretion stream model and also delve into the impact that stellar activity can have on these observations.

2. Optical Hydrogen Absorption Consistent with a Bow Shock Leading the Hot Jupiter HD 189733b   Cauley, P.W., Redfield, S., Jensen, A.G., Barman, T., Endl, M., & Cochran, W.D. 2015, ApJ, 810, 13

This work is the first to redundantly detect extended pre-transit structures, having detected it in the strongest three transitions of the Balmer sequence. If these structures are mediated by the exoplanetary magnetic field, this technique could be used to measure the elusive, but critical planetary property, the strength of the planet's magnetic field. This work was led by Wilson Cauley, a postdoctoral researcher currently working with me at Wesleyan. This work was featured as a highlight on AAS Nova.

3. A Detection of Hα in an Exoplanetary Exosphere   Jensen, A.G., Redfield, S., Endl, M., Cochran, W.D., Koesterke, L., & Barman, T.S. 2012, ApJ, 751, 86

This work is the first detection of Hα in an exoplanetary atmosphere. The ability to observe the extended atmosphere in the visible using ground-based telescopes is critical for when the UV transitions are no longer observable once HST ends its mission. This work was led by Adam Jensen, a postdoctoral researcher working with me at Wesleyan during this time.