Topic based learning is a great way for children to learn. They get engaged quickly and easily, especially if the topics are written specially for children. Unlike more traditional methods of teaching topic based learning allows for multidisciplinary learning since many subjects can be taught using a subject.
Planets and the Solar system is a great example of a topic that can allow for teaching many subjects without "forcing" or "coercion". It's easy to ask students to write some prose about being a space man or perhaps a poem about the coldness of space. If literacy isn't the focus then it's easy to ask about maths. Perhaps calculate the time to travel to the moon or the difference in temperature between the Earth and Saturn. Finally arts and crafts can easily be introduced with the building of a model solar system.
If you think about it the literacy, numeracy and art and craft subjects (along with most of the other subjects) can easily transfer between topics. If you've a source of great topic material then building a suite of lessons that are fun and engaging is pretty simple for an experienced teacher.
A great way to find topics is to use the Internet. The Internet is a gateway to a vast amount of information and a good source of inspiration for topics. You can often find quite easily material on the main subjects that interest children of all levels and capabilities. However often you'll find that the material will need to be downloaded and sorted out before use. The sites that present the material mainly rely on advertising and as a result you'll find the pages are filled with adverts. These kind of pages aren't really the sort of teaching material you want in school (or at home).
When you find sites that are advert filled it's often possible to copy and paste the material into a word processor removing the adverts. This can be sufficient to allow use by children. However you might also find that the material is written more for adults than for children. The material may be written by a university professor, who whilst well meaning and well informed, isn't great at writing material for children.
However despite the difficulties the Internet can still prove an effective source of material for teachers. You might have a fair bit of research and rewriting but it can still be a good way forward.
The other way to tackle topic learning is to use a specialist site like Learning Huddle. Sites like this contain well researched topics that are written just for children and so can significantly reduce the amount of work you need to do to get up and running. Often you'll find that they allow you to focus on the teaching rather than the "searching".