Before the Voyages: The Early Reach of United Earth In Star Trek

 The voyage of the NX-01 Enterprise was a historic event in Human history in Star Trek - the initial voyage in 2151, as the first Warp 5 Earth ship, made a number of first contacts and explorations that eventually led to the formation of the Federation itself.  

However, very little is said about what United Earth knew about prior to the voyage.  There are only a handful of references to other Earth colonies, which are never seen onscreen (apart from within the Sol system or the lost colony at Terra Nova).  So I decided to go through Enterprise and see what I could find reference-wise to worlds and species known prior to the start of the show.

Most of the references come from the first two seasons - the Enterprise remains mostly within the Delphic Expanse for the third season, and in the fourth season we do not see much in the way of exploration apart from a handful of episodes.  

             Earth's colonies:

The initial attempt at extrasolar colonization, as seen in the 1st season, was in 2069 - only 6 years after first contact with the Vulcans! - at which time it is stated the only colonies in the Sol system were New Berlin on Luna, the Utopia Planitita colony on Mars, and some isolated asteroids.  

Terra Nova - In the Eta Cassiopeia system, 19.32 ly.  Established in 2078, with contact lost in 2083.

Alpha Centauri -  4.34 ly.  Our nearest neighbor; also presumed to be the Proxima colony.  This is likely the first successful Earth colony (and it's not clear why this would not have been the first attempt).  Since Zephram Cochrane moved here in 2102, we can presume its settlement was between 2078 and 2102.  (Stellar Cartography places its founding in 2085).

Vega - 25 ly.  Major colony that is mentioned a number of times.  Around since at least 2126.

Deneva - In the Kappa Fornacis.  74 ly.  Likely one of the newer colonies; the original series mentioned it as being established in the mid-22nd century, and was definitely around in 2151.

Alpha - Unknown distance away, but the original series cites the "Statutes of Alpha III" as being a foundational legal document.  Early sources call it "Terra Four".  There is also an Alpha V, aka Earth Colony 5 or Terra Five, possibly in the system - but see Sirius.

Sirius - 8.6 ly.  The animated series mentions the Treaty of Sirius ending the Earth-Kzinti Wars, apparently the earliest of Earth's conflicts.  The issue here is the problem with the whole Kzinti species having an odd canonicity due to being really part of a separate universe created by Larry Niven.  However, the original Star Trek TTRPG placed "Alpha III" in the Alpha Canaris system - and almost all different sources have interpreted the fictional 'Canaris' constellation (first mentioned in the original series, coincidentally in the episode with Zephram Cochrane) as shorthand for the real 'Canis Majoris' constellation - and Alpha Canis Majoris is Sirius!  Since Alpha III has a separate position in Stellar Cartography, one solution would be to place Alpha V at Sirius.

    Known worlds

A handful of worlds were apparently known to Earth prior to 2151.  It was established in Enterprise that no known Earth vessel went beyond 90 ly.

Vulcan - 40 Eridani system.  16.3 ly.   While the Vulcans are first interstellar race to officially meet Humans, it is unknown when Earth first visited this world, but the United Earth embassy implies it had been there a while.

Denobula - Seems to be two separate systems; the colony at Denobula Triaxa, at Iota Bootis (96 ly), and Denobula itself at 83 Leonis (59.23 ly).  Note that Denobula Triaxa is beyond the 90 ly limit and would not have been visited by Earth vessels prior to 2151.  

Draylax - Epsilon Indi system.  12 ly.  Homeworld of the Draylaxians, an early trade partner with Earth, at least since 2126.

Trillius Prime - Unknown distance.  Homeworld of the Trills, although knowledge of the Symbionts were unknown at the time.  Known since at least 2137.

Teneebian moons - Alpha Comae Berenices system.  58 ly.  Visited by Earth traders.

Neubilia Prime - Unknown distance.  Inhabited world, visited by Earth traders.

Trelkis III - Unknown distance.  Known to Earth for years.

Loracus Prime - Unknown distance.  Home to a large variety of termite species.  Visited as early as 2136.

        Alien races

The ECS has traded with Demerians, Orions, and Deltans.  Tellarites and Tarkaleans are also likely to have been known prior.

Little is mentioned about Earth's attempts to explore the galaxy.  We know that the Vulcans were reluctant to share their star charts; and a number of nearby systems were never visited until the Enterprise did.  Most of the actual encounters with alien worlds and species, in fact, seems to have been commercial, through the Earth Cargo Service; this was reinforced in the 4th season where at least one ECS captain was upset that the onset of the exploration by NX-class vessels were shifting crew away to them, because likely the only reasonable way you could visit alien worlds as a Human before 2151 was through the ECS - with the exception of interstellar diplomatic services, and medical doctors that participated in the Interspecies Medical Exchange program.

We do know there was some attempt at exploration, which often seemed to be heading out into deep space - the SS Valiant, launched in 2065, made the ambitious attempt to breach the Great Barrier around the galaxy (likely by going straight up or down in the z-axis rather than heading for the rim), the SS Bonadventure, lost in 2079, and the SS Hokule'a, launched in 2102.  They also launched interstellar unmanned warp probes, such as the Friendship One in 2067, which they kept contact with until 2248.  The Earth Cargo Service (which has been around since at least 2102) predated the founding of Starfleet itself, according Johnathan Archer (who said he had considered joining the ECS prior to Starfleet's founding).

One last item of interest - the Ficus Sector.  The colony ship SS Mariposa was launched from Earth in 2123 to go to the Ficus Sector,  Two groups were to settle here, on Bringloid V and Mariposa; it appears to be a distance away and/or out of normal space lanes, because Earth lost the records of these colonies being established and did not find them again until the 24th century - implying that they were outside normal space lanes, and that the worlds were deemed unsuitable for colonization in general (otherwise one would presume that there would have been other attempts).  There is the suggestion that the colonization attempt was privately funded, rather than an official United Earth mission.