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: 
One reason is philosophical: *Star Trek* was always as much about entertainment and social commentary as about telling science fiction stories -- actually possibly more so. As such, production staff felt that there always needed to be ways to ensure the audience had just enough familiarity with what was going on to feel comfortable and to be able to draw parallels to the real world easily. That, in turn, meant keeping the environments Earthlike-but-weird.

Another reason is pseudoscientific: At the time *Star Trek*, the original series, was being developed, there was a "theory" that Roddenberry felt had some merit (and touts in all the material he used to sell the series to the studios, and in the early writers' bibles) suggesting that the universe was filled with parallel Earths. This is not parallel-universe theory as we now tend to see it played out both in later *Star Trek* and in other stories like *Sliders*. This is the notion that *in our universe* are multiple, separately evolved planets that nevertheless have people and histories remarkably similar to our own. Several stories clearly play on this, most notably "Bread and Circuses" (in which the Roman Empire survived to see technology equivalent to our 20th Century) and "The Omega Glory", which depicts a world in which the Cold War yielded to actual nuclear war. 

And the final reason is budgetary: all those Earth-like worlds meant they could use existing Paramount back-lots, with their pre-built faux-towns and faux-cities that could be redressed to match a variety of needs (or in some cases had been built to mirror a particular time and place for some movie or other and now needed to be used).
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.
> 
> 
>