One could argue that Dorchester is the first neighborhood to be annexed to Boston when the city took over Dorchester Heights to make South Boston (1804). Maybe again when Washington Village was annexed in 1855 as the third addition after Eastie. Conventionally speaking the date set for the Dot's surrender of sovereignty happened in 1870, the fifth neighborhood to be added to Boston proper. See the timeline maps about midway down this link.
Much like Dorchester, the planetoid Pluto has seen its status downgraded from a full-fledged entity to something a bit less than what it was once regarded. Pluto: the ninth mistress of the Sun. Comparing Pluto to Dorchester is like comparing Hyde Park to Mercury, the Earth, and Jupiter combined. In fact, Hyde Park is the ninth neighborhood annexed to Boston, in 1912 according to wikipedia, and it is the most remote from the golden dome of the State House.
That said, comparing Dorchester to Jupiter is apt. It's the fifth planet to our non-astronomy buffs. The Dot is the biggest part of Boston by far. Comparing Dorchester to the Earth is equally fitting (the third planet in the solar system, equivalent to Washington Village...Does anyone use that name anymore?). The Dot is the most fecund and lively part of the Boston universe. Comparing Dorchester to Mercury? We'll leave that part to Southie, which was the first and is the closest to the heat and flares that radiate from downtown. We aren't suggesting Dorchester's neighbor to the north is an inhospitable, lifeless wasteland by any means by making this comparison. Just pointing out that Southie was the first neighborhood to accrue and be trapped in orbit around the central city's gravity.
Dorchester gave up its independence during a popular vote in the summer of 1869 and its resolution was made binding on January 3rd, 1870. Some thought this day would live in infamy but people adjusted and now, Dorchester is the best part of Boston, adding its vitality and lending its boundless energy to the rest of the city. You don't need to look through a telescope to see Dorchesterites. They are all around our fair city, contributing value, keeping the whole shebang running in good, working order.
Have there been any regrets? Surprisingly few as one intrepid reporter discovered doing his due diligence.