Question: Starfleet is supposed to "explore *strange* new worlds and boldly go where no man has gone before", yet most of the time they explore worlds that are more Earth-like than not, and are inhabited by human-like species.

I understand that the out-of-universe reason is that it is difficult to tell a story when the characters are always in space suits, but couldn't the writers have found some other way, rather than skipping all the interesting non-humanoid life forms? I remember the animated series characters using a skin-tight force field (a life-belt?) to explore inhospitable environments.

Or, are there really not that many interesting life forms on other classes of planets?

Answer: 
In Universe:
------------

The [original studio pitch](http://50yos.tumblr.com/post/79244934260/original-star-trek-pitch-1964) offered by Gene Roddenberry confirmed that Captain Robert April's standing orders were to investigate Class-M planets due to **the limitations of his ship** (landing craft, standard sensor packages, etc).

> 
> **Nature and duration of command:**
> 
> 
> *Galaxy exploration and Class-M investigation: 5 years*
> 
> 
> You will patrol the ninth quadrant, beginning with Alpha Centauri and
>  extending to the outer Pinial Galaxy limit.
> 
> 
> You will conduct this patrol to accomplish primarily: 
> 
> 
> (a) *Earth security, via exploration of intelligence and social systems
>  capable of a galaxial threat, and*
> 
> 
> (b) *Scientific investigation to add to the earth's body of knowledge
>  of life forma and social systems, and*
> 
> 
> (c) *Any required assistance to the several earth colonies in this
>  quadrant, and the enforcement of appropriate statutes affecting such
>  Federated commerce vessels and traders as you might contact in the
>  course of your mission.*
> 
> 
> **Consistent with the equipment and limitations of of your cruiser-class
>  vessel, you will confine your landings and contacts to planets
>  approximating earth-Mars conditions, life, and social orders.**
> 
> 
> 

Out of Universe
---------------

**It's cheaper.** 

> 
> Some format and budget considerations . . . 
> 
> 
> SETS: Our format is tailored to practical production and cost factors.
>  Use of stage sets, backlot and other locations are simplifled by
>  Captain April's "Class-M" orders. And our own "Parallel Worlds"
>  concept. The majority of story premises listed can be accomplished on
>  such sommon studio backlot locales and sets such as Early 1900 Street,
>  Oriental Village, Cow town, Border Fort, Victorian Drawing Room,
>  Forest and Streamside.
> 
> 
>
First your premise is wrong. Starfleet has multiple missions and purposes, but it's the Enterprise specifically that:

> 
> Space: the final frontier. These are the voyages of the starship Enterprise. Its five-year mission: to explore strange new worlds
> 
> 
> 

The Enterprise, both TOS and TNG, has a duty to explore strange new worlds. But that's only part of the mission.

> 
> to seek out new life and new civilizations, to boldly go where no man has gone before.
> 
> 
> 

It is being sent to search for new sentient, sapient life. People it can trade and learn from, or from who it should prepare to defend from if they are a threat.

Second, strange has multiple meanings. While the first means different and weird, hard to understand, the second meaning is unknown, foreign, ***alien***. This is in line with the Enterprise mission, seeking new people. It's a first contact mission.

Third, apparently in universe, the Enterprise mission statement comes from Cochrane, from the Enterprise Pilot.

> 
> On this site, a powerful engine will be built. An engine that will someday help us to travel a hundred times faster than we can today. Imagine it – ***thousands of inhabited planets at our fingertips...*** and we'll be able to explore ***those strange new worlds***, and seek out new life and new civilizations. This engine will let us go boldly... where no man has gone before. *emphasis mine*
> 
> 
> 

Out of universe, the intention was the same.

Finally, the mission is first contact and planetary charting. Strange planets, ones that don't have any sentient life or ruins detectable, are still noted, scanned, and mapped, **for future research** and follow up by dedicated long term teams. Case in point, Wrath of Khan, a research team returns to the sister planet that Khan was dumped on. Ceti Alpha VI was not expected to have any sentient life. Ceti Alpha V was "barely" Class M as it was too, no sentient life either. Both were cataloged by the Enterprise.

As a parting note, there were only a handful of classification for planets used on screen. Only some of these are considered to have life of any kind. Some classes are Gas Giants and the Demon worlds. Some are life less, atmosphere less rock planets or moons. Class L only had vegetable life. Class h is "generally uninhabitable". Class K needs Terra forming or pressure domes. The Enterprise can scan planets from space for life signs, or signs of civilization like ruins, buildings, man made structures. If a non Class M planet has no signs of life, they catalog it and move on.