Alpha is a planet orbiting the largest star in the Alpha Centauri system. Alpha Centauri is cited by Lord Dorwin as one of the solar systems where humankind potentially originated. The others are Sol, Sirius, 61 Cygni and Arcturus. Dorwin gives the description of the planet almost no information other than it was in the Sirius Sector. The planet is covered entirely by ocean, with the exception of a single island that's 15,000 square kilometers. The inhabitants call it New Earth and live a simple lifestyle that of women and men are completely shirtless, weather permitting, and the men engage in long sea voyages to fish. They also infect visitors with viruses.
The planet was terraformed by the Galactic Empire, with the only available land mass being the small island which the Alphans live on. It was all put together by the Empire laboring people to create land which the last refugees of Earth could live from the radiation of their home planet. The project was canceled, however, when the funds and the Empire's interest in the project and the rest of the plans for the other continents were scrapped. The planet was left alone for a millennia with advanced weather biotechnology. Some estimates say that it is a normal planet covered by water from the melted polar caps, which would predate the Imperial Era. However, it wasn't implemented properly in the database, so most of the information wasn't recorded.
Alpha first appears in Foundation and Earth, which confirms a merger between the Spacer and Foundation series. In the Spacer series, the planet Aurora in the Tau Ceti system was the first extrasolar colony, suggesting that Alpha Centauri had no habitable worlds. This fits with the late and expensive settlement of Alpha.
Less easily understood is the belief in the early Foundation era that Alpha Centauri might have been the original solar system; a terraformed planet with a very small land area would not be plausible. However, as Alpha is depicted as being solely aquatic with the exception of its single island, it could feasibly have been a planet whose ice caps had melted, covering what might have originally been a larger landed area. Such would likely have been the reasoning of someone like Lord Dorwin. This could just as easily be explained (as it is in the series itself) by the deterioration in standards of academic research in the late Imperial Era, since Lord Dorwin explained that rather than going to places and gathering firsthand evidence, he prefers to pore through available archives from which by this stage much was missing; he refers to this as the "scientific method". (In addition, R. Daneel Olivaw had sent agents to remove information about Earth from archives, although we don't know exactly when). Alpha and Sol were therefore not worlds Dorwin knew by experience, but only by vague report.