Atmospheric Modification Plant
The atmospheric converter (Atmospheric Modification Plant) is a large facility that will, after construction, begin converting the atmosphere of the planet to one breathable by whichever species is in the majority on the planet. There is one major caveat: Gas giants cannot be converted to atmosphere type 'none'.
Note that after an atmosphere is successfully converted, the facility can be scrapped with no further consequence. If you leave it on the planet, however, the atmosphere will change again if the balance of species changes to favour a different atmosphere type.
Note that the race with the highest population present will be the one that decides the atmosphere, not the breather-type. For example, suppose you have a world with 100 million Terrans (oxygen-breathers), 100 million Piundon (Also oxy-breathers) and 105 million Phong (CO2-breathers). In this case the planet will be converted to Co2, because the Phong represent the largest single ethnic group. The fact that there are almost twice as many oxy-breathers as CO2-breathers is not taken into account.
The facility is researched by progressing along the Tech Tree Planet Utilization, accessable after researching Level 1 Planetary Engineering. Each facility costs 15000 of each resource to construct. The Atmospheric Modification Plant I requires Tech Level 7 in Planet Utilization, and requires 3 years to convert a planet's atmosphere. Atmospheric Modification Plant II is gained at Level 8, and requires 2.5 years. The third plant is accessed at Level 9, and requires only 2 years to convert a planet's atmosphere.
Note that if you upgrade an atmosphere converter while it is converting the atmosphere, it will convert at the new facility's time rather than restart. So if its been 2 years since you installed an atmosphere converter I (3.0 Years), and upgrade to atmosphere converter II(2.5 Years), the atmosphere will change 2.5 years after the first one was built.