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.
> 
> 
>
Personally, I don't think they want to explore the worlds. I think they want to meet the people. Humans are social creatures. We saw several times in ST:ENT where Archer chose to go with an inhabited world instead of a astronomical oddity. When they explored that star that was close to supernova, they seemed more interested in meeting the people who were studying it.