While searching a planet software I only found planetplanet, which is written in python at a first glance. As a PHP freak, this is not an option for me.
After a while of hard googeling I found the combination serendipity plus the aggregator plugin, which claims to solve the problem in PHP.
Installing serendipity and the aggregator plugin is really easy, also configuring the aggregator to collect the feeds of the attending blogs is easy. But than the hard work starts.
The look and feel of a standard serendipity installation as planet is worse. After some more hard googeling I thought, that there is no other way as to hack the templates and to investigate the needed variables by searching in the code and database. It took me a few hours to find all the stuff in the template and core code to build a nice planet.
I was shortly before the point of frustration to code my own PHP planet. After finishing work, I took a look into the aggregator plugin itself to understand how a certain parameter makes it way into the database.
And here the surprise comes.All the documentation about how to build a planet with this plugin is in the code:
// LAYOUT NOTE:
// For best "planet experience" you are advised to create your own template and
// disable s9y-specific options which do not make sense in a planet environment.
// A suggestion is to display the originating feed URL inside entries.tpl via:
//
// {$entry.properties.ep_aggregator_feedname}
// {$entry.properties.ep_aggregator_feedurl}
// {$entry.properties.ep_aggregator_htmlurl}
// {$entry.properties.ep_aggregator_articleurl}
// {$entry.properties.ep_aggregator_author}
//
// See, eg. suggestions in file:
//
// [serendipity]/plugins/serendipity_event_aggregator/theme-patch.diff
It’s quite interesting that there information about this is so rearly spread over the net. Hopefully google finds this post and saves you from hours of code greping.

I think the reason why you didn’t find so much information is because Google’s pageranking sucks. I know there are tons of conspiracies about google, but one I personally believe 100% is that they have commercialised their pagerank. You can buy your pagerank with Adwords and Adsense. And that is also why catalogueing the rest of the web is not that important anymore for google.
Hi Nethanja,
I woun’t be too shure about that. Just do a googeling with:
serendipity aggregator planet
just one day after my post.
And honestly: I have not Adwords and/or Adsense
Google searches websites, so it’s an bad idea to cemment stuff in code like they do in the aggregator plugin. If they had commented it as phpdoc tags and build an api doc as website, one would have had a chance.
The best of course is, to build a normal website about the topic, than google has a chance to find it.