Applying to the planetary science graduate program at Caltech is a little different from most other planetary science programs. Critically, you are admitted to the department rather than being admitted into a specific research group.
It is our belief that scientists-in-training need to be exposed to a wide variety of potential fields and that this exposure best happens by allowing them the freedom to explore their passions. New students have a research fellowship for the first 1.5 years which allows them to work with multiple research groups and find the perfect fit before committing to a thesis area.
Practically, this means that it is absolutely not necessary to contact faculty ahead of time to show interest/convince them to let you work with them/see if they have space/ etc. Students are admitted to the program by a committee (which is the entire faculty) who read your application rather than being admitted by a single faculty member into their group. You are welcome, of course, to contact individual faculty to talk about potential ideas, but it probably neither helps nor hurts your chances.
That said, if you are interested in the details of my group, I would be delighted to talk to you. Please email me at firstname.lastname@example.org. And, if I am distracted (usually by some looming telescope proposal deadline) and don't respond right away, email me again.