A planet, in the context of the indieweb, and blogs/feeds for even longer, is a site that aggregates feeds/updates from a variety of sources, typically focused on a particular topic or community.
Similar to a public timeline, a planet provides an easy way to get an overview or introduction to a topic or community.
One way to build an indieweb-style planet is to use a reader that allows public access to view the page. This can be done using dobrado, where an account is required to manage the subscription list but the page containing the feed list can be made public.
Question Sven Knebel 2017-03-24: How should posts on a planet be marked up? Should the planet repost, or just produce an h-feed reproducing the posts (and u-uid and u-url pointing to the canonical source?
- repost if full content is reproduced, h-cite otherwise, per IRC discussion.
- per later discussion, u-uid = original_post should be enough. also doesn't show up as repost on every post and is still detectable via Webmention. 
Atom specified a
source property to indicate the original source of a post it republished. 
(accidential) interaction with the planet post instead of the original could be redirected by the planet’s webmention endpoint -> just re-send it to the original post’s, if wants to receive them it can add the URL of the syndicated copy to the list of URLs it accepts webmentions for.
- Aggregates a tag from each included blog
- Subscribes to all feeds found from members of the community
There have been various brainstorming discussions about planets, in particular, about the potential of building an IndieWebCamp Planet site, based on signups on this page:
Welcome to the IndieWebCamp Planet sign-up page.
Are you a member of the IndieWebCamp community and want your personal site feed aggregated into http://planet.indieweb.org/ ? Add your personal site below (please don't add others' sites) and the aggregator will discover your feed and do the rest.
Sites to aggregate into an IndieWebCamp Planet, sorted alphabetically by domain name:
- Add yourself here… (see this for more details)
Chris Aldrich maintains an OPML file which includes a number of active IndieWeb members' sites and RSS/Atom links for easy subscription. http://boffosocko.com/wp-links-opml.php?link_cat=1521
Additional IndieWebCamp shared-community resources: