Using NASA's Kepler spacecraft, they've observed a planet running alternately late and early in its orbit because a second, 'invisible' world is tugging on it.
Both orbit the sun-like star Kepler-19, which is located 650 light-years from Earth in the constellation Lyra.
"This invisible planet makes itself known by its influence on the planet we can see," says astronomer Sarah Ballard of the Harvard-Smithsonian Center for Astrophysics (CfA).
Kepler discovered the visible planet, Kepler-19b, by measuring the decrease in light from its sun as the planet passes in front of it.
Kepler-19b, transits its star every nine days and seven hours, orbiting at a distance of 8.4 million miles.
If it were alone, each transit would follow the next like clockwork - but, instead, the transits come up to five minutes early or five minutes late. This shows that another world's gravity is pulling on Kepler-19b, alternately speeding it up or slowing it down.
The planet Neptune was discovered similarly, through its effects on Uranus.
"This method holds great promise for finding planets that can't be found otherwise," says Harvard astronomer and co-author David Charbonneau.
Kepler-19c weighs too little to tug the star enough for astronomers to measure its mass. And Kepler hasn't detected it transiting the star, suggesting that its orbit is tilted relative to Kepler-19b.
"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," says co-author Daniel Fabrycky of the University of California, Santa Cruz.
The Kepler spacecraft will continue to monitor Kepler-19 throughout its mission, with the additional data helping to nail down the orbit of Kepler-19c. Future ground-based instruments like HARPS-North will attempt to measure the mass of Kepler-19c.