Applying to (STEM) Ph.D. Programs from Undergrad by Eric Gagliano, a Civil & Environmental Engineering Grad Student
This is an in-depth guide to the entire application process, from how to decide to apply for grad school to sending thank you notes after deciding where you're going! He provides fantastic, specific examples such as screenshots of emails to professors as well.
You can download the pdf of his comprehensive guide here. This guide includes topics such as advice for freshmen/sophomore, GRE advice, letters of recommendation, talking to faculty, fellowships, application components, visiting schools, etc.
With an unconventional background in the US Navy and as a middle school teacher before grad school, Logan is super accomplished and shares heartfelt advice.
Useful to look at for both the NSF Fellowship and for grad school apps, you can find a table of research and personal statements of successful applicants here.
Some programs have interviews, others don't. For smaller programs it's more typical for one-on-one meetings with potential advisors. Here's a list of questions they might ask so you'll be prepared.
Spreadsheet of fellowships, grad funding programs, undergrad scholarships, internships, etc. Not sure if this is updated regularly, but gives you a good idea of what's out there.
Spreadsheet of which schools are/aren't requiring the GRE. Geo GRE spreadsheet is available here, though it's not as robust.
Planetary Science Specific
Every year Sarah Horst from Johns Hopkins puts out a Tweet for planetary science professors to add themselves to a spreadsheet if they have a grad student opening in their research team. Note: planetary science is a relatively small field, I'd highly recommend emailing the professors directly. If they don't reply, it's probably because they forgot so just send them a follow-up. :)
Insights from the Process
Don't hesitate to reach out to grad students at the schools you're interested in! They will provide the most honest and detailed breakdown of their advisor and program. Also, since they have gone through the application process as well, they have excellent advice. :)