Unseen planet uncovered

Artist's conception of exoplanet Kepler-19c

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—seen as a 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.

USUALLY, RUNNING FIVE MINUTES LATE is a bad thing since you might lose your dinner reservation or miss out on tickets to the latest show. But when a planet runs five minutes late, astronomers get excited because it suggests that another world is nearby.

NASA’s Kepler spacecraft has spotted a planet that alternately runs late and early in its orbit because the gravity of a second, “invisible” world is tugging on it. This is the first definite detection of a previously unknown planet using this method. No other technique could have found the unseen companion.

“This invisible planet makes itself known by its influence on the planet we can see,” said astronomer Sarah Ballard of the Harvard-Smithsonian Centre for Astrophysics (CfA). Ballard is lead author on the study, which has been accepted for publication in The Astrophysical Journal.

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

Found in transit

Both the seen and unseen worlds orbit the Sun-like star Kepler-19, which is 650 light-years from Earth.

Kepler locates planets by looking for a star that dims slightly as a planet transits the star—that is, passing across the star’s face from our point of view.

Transits give one crucial piece of information—the planet’s physical size. The greater the dip in light, the larger the planet relative to its star.

However, the planet and star must line up exactlyfor us to see a transit.

Artist's impression of the Kepler spacecraft

Artist's impression of the Kepler spacecraft

In this case, the known planet, called Kepler-19b, transits its star every 9 days and 7 hours. It orbits the star at a distance of 13.5 million kilometres, where it is heated to a temperature of about 480 degrees Celsius.

Kepler-19b has a diameter of 29,000 kilometres, making it slightly more than twice the size of Earth. It may resemble a “mini-Neptune,” however its mass and composition remain unknown.

Given away by gravity

If Kepler-19b were alone, each transit would follow the next like clockwork. Instead, the transits come up to five minutes early or five minutes late.

Such transit timing variations show that another world’s gravity—dubbed Kepler-19c—is pulling on Kepler-19b, alternately speeding it up or slowing it down.

Historically, the planet Neptune was discovered similarly. Astronomers tracking Uranus noticed that its orbit didn’t match predictions. They realised that a more distant planet might be nudging Uranus and calculated the expected location of the unseen world. Telescopes soon spotted Neptune near its predicted position.

Multiple p

“This method holds great promise for finding planets that can’t be found otherwise,” stated Harvard astronomer and co-author David Charbonneau.

Very little known

So far, astronomers don’t know anything about the invisible world Kepler-19c, other than that it exists. It weighs too little to gravitationally tug the star enough for them 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,” said co-author Daniel Fabrycky of the University of California, Santa Cruz (UCSC).

The Kepler spacecraft will continue to monitor Kepler-19 throughout its mission. Those additional data will help nail down the orbit of Kepler-19c.

Future ground-based instruments will attempt to measure the mass of Kepler-19c. Only then will we have a clue to the nature of this invisible world.

Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics. Graphics by David A. Aguilar (CfA) and NASA.

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