Astronomers have outdone themselves by doing the (seemingly) unimaginable: Discovering an invisible exoplanet.

Using NASA's Kepler spacecraft, the astronomers have observed a planet that runs alternately late and early in its orbit because a second, 'invisible' world is tugging on it.

This invisible planet makes itself known by its influence on the planet we can see, said Sarah Ballad, a Harvard student with the Harvard-Smithsonian Center for Astrophysics, or CfA.

Writing in the Astrophysical Journal, Ballard, and several colleagues, said the new exoplanet is orbiting a star about 650 light-years from Earth, in the constellation Lyra, whose presence was revealed by a gravitational pull on another planet.

It's like having someone play a prank on you by ringing your doorbell and running away, Ballard said. You know someone was there, even if you don't see them when you get outside.

The exoplanet, named Kepler-19c, was discovered by Kepler by measuring the decrease in light from its sun as the planet passes in front of it. The greater the dip in light, the larger the planet relative to its star is. Kepler-19b transits its star every nine days and seven hours, orbiting the star at a distance of 8.4 million miles, where it is heated to a temperature of 900 degrees Fahrenheit. Kepler-19b has a diameter of 18,000 miles, making it slightly more than twice the size of Earth.

Scientists discovered the planet Neptune in the same way, through its effects on Uranus.

This method holds great promise for finding planets that can't be found otherwise, said David Charbonneau, one of Ballard's co-author's.

The astronomers know little about the new exoplanet, but say it is too small to be able to measure its mass.

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 in an aim to gather more information about Kepler-19c. Future ground-based instruments will attempt to measure the mass of Kepler-19c.

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