This latest announcement brings the number of planets discovered by Kepler to a whopping 4,034, 50 of which have been confirmed as being Earth-sized and located within the habitable zone.
"This carefully-measured catalog is the foundation for directly answering one of astronomy's most compelling questions - how many planets like our Earth are in the galaxy?" said Susan Thompson, Kepler research scientist for the SETI Institute in Mountain View, California, and lead author of the catalog study.
"Understanding their frequency in the galaxy will help inform the design of future Nasa missions to directly image another Earth".
The 1,284 newly discovered exoplanets were only announced as such after being identified with 99% confidence, but as NASA admits, that leaves out 1,327 other likely exoplanets that couldn't reach that 99% certainty.
With Monday's addition of planets, the original Kepler mission now has resulted in the identificatoin of 4,043 planet candidates.
The Kepler team found that planets which are about 1.75 times the size of Earth and smaller tend to be rocky, while those two- to 3.5 times the size of Earth become gas-shrouded worlds like Neptune.
In February, NASA announced the discovery of seven new planets that "could have liquid water - key to life as we know it - under the right atmospheric conditions". Of those, 2,335 have been verified as exoplanets.
The Kepler team has also discovered that small planets typically occur in two very distinct sizes - Earth-size planets and gaseous planets smaller than Neptune. Then, they added data that appear to come from a planet, but were actually false signals, and checked how often the analysis mistook these for planet candidates.
This artist's vision shows what a Kepler exoplanet might look like. But while Kepler looked at one portion of the sky for stars that were farther away for a longer time, TESS will observe the entire sky and focus on the brightest and closest stars.
A follow-up study, described today by astronomer Benjamin Fulton of the University of Hawaii at Manoa in Honolulu, fine-tuned the size measurements for some 2000 Kepler planets using the Keck telescopes in Hawaii.
NASA is getting better at identifying Earth-sized exoplanets in other ways, too.
Scientists were even able to estimate the size and density of the planets. This work told them which types of planets were overcounted and which were undercounted by the Kepler team's data processing methods.
Including other telescope surveys, scientists have confirmed the existence of almost 3,500 planets beyond the solar system.