NASA Kepler: 'Invisible' planet discovered with new technique

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.

For the first time, scientists have definitively discovered an "invisible" alien planet by noticing how its gravity affects the orbit of a neighbouring world, a new study reports.

NASA's Kepler space telescope detected both alien planets, which are known as Kepler-19b and Kepler-19c.

Kepler spotted 19b as it passed in front of, or transited, its host star. Researchers then 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 marks the first time this method — known as transit timing variation, or TTV — has robustly found an exoplanet, researchers said. But it almost certainly won't be the last.

"My expectation is that this method will be applied dozens of times, if not more, for other candidates in the Kepler mission," said study lead author Sarah Ballard of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Finding two new planets
The Kepler spacecraft launched in March 2009. It 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.

Kepler 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.

Kepler-19b has a diameter about 2.2 times that of Earth, researchers said, and orbits 8.4 million miles (13.5 million kilometers) from its parent star. The planet 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.

In our own solar system, scientists used similar methods to predict the existence of the planet Neptune. Astronomers noticed that Uranus did not orbit the sun exactly as expected, and surmised that an unseen planet was pulling on it. This prediction was borne out when telescopes confirmed Neptune's existence in 1846.

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, 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, told Space.com in an email. "Kepler, and large ground-based telescopes, should help us figure out its true identity soon enough!"

The study will be published in The Astrophysical Journal.