One possible explanation is that the Hubble observations were more sensitive to the perpetual dark nightside of the planet where the atmosphere is slightly colder and the photochemical mechanisms that destroy methane are less efficient than on the dayside. (solstation.com)
A molecule containing three oxygen atoms, computer models suggest that ozone on Venus is formed when sunlight breaks up carbon dioxide molecules and releases oxygen atoms, which are swept around to the planet's nightside by winds where they can then combine to form unstable, three - atom ozone molecules, as well as much more stable, two - atom molecules essential for animals. (members.nova.org)
The extreme difference between day - and nightside temperatures suggests 55 Cancri has no appreciable atmosphere, because winds should redistribute heat between the two hemispheres. (scientificamerican.com)