Kepler-19c exists, but scientists have never seen it.
It seems the planet can only be detected by the effect it has on another planet in the system, Kepler 19b. According to the press release, NASA's Kepler spacecraft spotted a delay in Kepler 19b's orbit, caused by the gravitational pull of another planet in its vicinity.
This is the first time this method has been used to detect a new planet. However, a similar process was used in the 19th century to help discover Neptune.
"This invisible planet makes itself known by its influence on the planet we can see," said astronomer Sarah Ballard of the Harvard-Smithsonian Center for Astrophysics said in a press release. "It's like having someone play a prank on you by ringing your doorbell and running away. You know someone was there, even if you don't see them when you get outside."
The entire solar system lies 650 light-years from Earth in the constellation Lyra.
According to the CfA, Kepler-19b's short orbit, just 9 days and 7 hours, makes it easy to detect the effect of a nearby planet. A five minute delay in its orbit signals a significant outside gravitational pull.
Scientists know little about this unseen world, but have a few clues to its nature. "Kepler-19c has multiple personalities consistent with our data. For instance, it could be a rocky planet on a circular 5-day orbit, or a gas-giant planet on an oblong 100-day orbit," said co-author Daniel Fabrycky of the University of California, Santa Cruz.
NASA scientists have been finding a number of unusual planets, as of late. Recently, scientists discovered the darkest planet known to date, as well as a diamond planet with a mass equivalent to that of about Jupiter.
Clarification: A previous version of this article did not make light of the similarity between the process used to make this discovered, and the discovery of Neptune.