Both the planets, known as Kepler-19b and Kepler-19c, were detected by NASA`s Kepler space telescope which was launched in 2009 with an aim to hunt for alien worlds.
First, Kepler spotted 19b as it passed in front of, or transited, its host star. Then, scientists inferred the existence of 19c after observing that 19b`s transits periodically came a little later or earlier than expected. The gravity of 19c tugs on 19b, changing its orbit.
The discovery of Kepler-19c, the researchers said, marks the first time this method, known as transit timing variation or TTV, has robustly found an exoplanet.
"My expectation is that this method will be applied dozens of times, if not more, for other candidates in the Kepler mission," said lead researcher Sarah Ballard of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts.
The Kepler spacecraft typically hunts for alien worlds by measuring the telltale dips in a star`s brightness caused when a planet crosses the star`s face from the telescope`s perspective, blocking some of its light.
It has been incredibly successful using this so-called transit method, spotting 1,235 candidate alien planets in its first four months of operation. That`s the way it detected Kepler-19b -- a world 650 light-years away from Earth in the constellation Lyra.
The researchers, who reported their findings in The Astrophysical Journal, said Kepler-19b has a diameter about 2.2 times that of Earth, and orbits 13.5 million km from its parent star. It likely has a surface temperature around 900 degrees Fahrenheit (482 degrees Celsius).
Kepler-19b transits its host star once every nine days and seven hours. But that number isn`t constant, Ballard and her team found; transits can occur up to five minutes early or five minutes late.
That variation told them another planet was tugging on 19b, alternately speeding it up and slowing it down. Researchers know little about Kepler-19c at the moment. It takes the alien world 160 days or less to zip around its host star, and 19c`s mass could range from a few times that of Earth to six times that of Jupiter, the researchers said. But 19c should start coming into clearer focus soon.
"It`s a mystery world, but of course we don`t expect it to remain a mystery," study co-author David Charbonneau, also of the Harvard-Smithsonian Center for Astrophysics, said.
"Kepler, and large ground-based telescopes, should help us figure out its true identity soon enough!"