The study of extrasolar planets now proceeds via a wide range of techniques, including radial
velocity monitoring, microlensing, direct imaging with adaptive optics, and transit monitoring.
This latter technique has an additional advantage in that it provides a measurement of the
physical size of the planet, by virtue of the magnitude of the drop in flux when the planet
passes in front of the star.
There was a great flurry of activity in this field when the Spitzer Space Telescope was launched, using the IRAC cameras to measure both transits and secondary eclipses (when the planet passes behind the star and the small planet contribution to the overall system light is eclipsed). This has led to various claims about the compositions and thermal profiles in the atmosphere, but some of these have proven controversial. To move forward properly, we really require proper spectrscopic information.
To address this deficiency, my student Ian Crossfield , now a Sagan Fellow at the University of Arizona, and collaborator Travis Barman , and I have used a variety of telescopes to perform ground-based spectroscopy of systems during transit, using a variety of techniques. The picture at the right shows Ian and I at the Subaru telescope on the summit of Mauna Kea, Hawaii. Our observations have been primarily on this island, although Ian has now accumulated data from five different telescopes on the mountain (both Keck telescopes, Subaru, Gemini North and the NASA InfraRed Telescope Facility).
The plot at left shows our observations of the transit of the planet GJ3470b, an example of this class of object,
using the new MOSFIRE spectrograph on the Keck Telescope. We find that the depth of the transit is very similar
as a function of wavelength. The `flatness' of the spectrum allows us to place constraints on the abundance of
methane in this object as we are observing at wavelengths where we expect strong spectral features from this molecule.
Overall, these observations support the idea that this planet, and others like it, are not strictly Neptune analogues,
but possess small, Hydrogen-rich/Ice-poor surface layers over a rocky core, and thus suggest a different evolutionary
origin than the similar mass planets in our solar system. The relevant paper can be found
Ian is now also pursueing observations of these systems with the Hubble Space Telescope, which offers the opportunity to make similar measurements at shorter wavelengths (which are obscured from the ground by the atmosphere). Those wavelengths can be used to probe for evidence of water and so the combination of ground-based and space-based observations can be used to search for both the atmospheric Carbon and Oxygen reservoirs.
Although these studies are forefront science in their own right, the greatest potential payoff lies in the future, as
our observational capabilities extend to the detection of genuine earth-analogue planets and studies of their atmospheres.
In our present projects we are developing techniques and experience that will allow us to perform similar measurements
under the even more challenging conditions that will arise in that case.
Our present day project in this regard is being led by Ian again, to follow up potential planetary systems unearthed by the K2 mission, using the resurrected Kepler satellite. We intend to use the Keck telescope to image candidate planet systems to validate those most interesting for further follow-up. The first exciting results from this project can be found here .