The transiting exoplanet WASP-12b is located some
870 light-years from Earth. This extremely irradiated planet orbits
its Sun-like, G0 parent star in just over 26 hours, resulting in a
dayside temperature of ~2,700 K (4,400 F). The tidal forces
exerted by its star are sufficiently strong to distort the exoplanet
into a prolate shape and cause mass loss at a rate of 1 Earth-mass
every ~30,000 years. WASP-12b is the hottest planet with a
measured atmospheric composition.
In the December 9th, 2010 issue of Nature,
Madhusudhan et al. report that WASP-12b has a carbon-to-oxygen (C/O)
ratio > 1, the first of what may be many carbon-rich exoplanets.
The formation, interiors, and atmospheres of these planets are likely
completely different from planets with a solar C/O ratio (~0.57).
The interior of WASP-12b could be dominated by pure-carbon rocks,
possibly in the form of graphite or diamonds. The atmosphere is
found to have a surprising amount of methane, a key fact that
supports the large C/O ratio.
The primary constituents of hot exoplanetary
atmospheres are hydrogen (H2), helium (He), carbon
monoxide (CO), carbon dioxide (CO2), methane (CH4),
and water vapor (H2O). Assuming C/O < 1 and
chemical equilibrium, we would expect to find significant amounts of
CO and very little CH4 in a 2,700 K atmosphere.
However, when C/O > 1, the excess carbon has no oxygen to form CO
so it must form CH4 instead.
The Spitzer Space Telescope observed WASP-12b as it
passed behind its parent star, as seen from Earth. The Infrared
Array Camera (IRAC) measured two secondary eclipse events, the first
at 3.6 and 5.8 μm and the second at 4.5 and 8.0 μm. Joseph
Harrington, a UCF professor and member o f the Planetary Sciences
Group, is the principal investigator on this project. He and
his team of graduate and undergraduate students led the Spitzer
observations and data analysis portions. An associated paper on
the orbit of exoplanet WASP-12b has been submitted to ApJ by UCF
undergradute student Christopher J. Campo. Three ground-based
secondary eclipse measurements were also used in the atmospheric
WASP-12b spectrum with an artist's concept.