Exoplanets are now almost mundane. Hundreds have been found in just the past few years. What makes Kepler-19c unique is not that it orbits a star other than our sun, but the way it was found. Its presence slightly alters the orbit of a fellow planet that had already been discovered.
The planet Kepler-19b was discovered orbiting the star Kepler-19 by, wait for it, NASA’s Kepler space telescope. The Kepler telescope locates planets using the ‘transit’ method. That is, it looks for the periodic dimming in a star’s light as a planet passes directly in front of that star. Kepler-19b was found by this method. However, Kepler-19b doesn’t transit its star exactly on time. Instead, it crosses its star up to five minutes early or late each time it goes around. That doesn’t sound like a lot, but something has to account for the discrepancy. That something is an unseen second planet (Kepler-19c) whose gravity is ever so slightly tugging on Kepler-19b.
The "invisible" world Kepler-19c, seen in the foreground of this artist's conception, was discovered solely through its gravitational influence on the companion world Kepler-19b - the dot crossing the star's face. Kepler-19b is slightly more than twice the diameter of Earth, and is probably a "mini-Neptune." Nothing is known about Kepler-19c, other than that it exists.
Credit: David A. Aguilar (CfA)
Other than the fact of its existence, little is known about Kepler-19c. It doesn’t transit Kepler-19 from our vantage point, and it has no detectable gravitational effect on that star.
As Daniel Fabrycky of the University of California, Santa Cruz says:
[Kepler-19] could be a rocky planet on a circular 5-day orbit, or a gas-giant planet on an oblong 100-day orbit.
By the way, the planet Neptune was discovered in a similar fashion when astronomers attempted to explain aberrations in Uranus’s orbit.