Phantom Menace Bullies Alien World

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In a star system, far, far away, an exoplanet is being menaced by the gravitational pull of another world, causing it to slow down and speed up as it orbits its parent star…

And yes, this is a real phenomenon, and not just a butchered opening for a Star Wars spin-off.

Some 650 light-years from Earth in the constellation Lyra, an exoplanet orbits a sun-like star called Kepler-19. The world, Kepler-19b, has a very strange orbit in that sometimes it speeds up, completing its 9 day orbit 5 minutes fast, and at other times, it will complete an orbit 5 minutes slow.

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Needless to say, orbiting bodies don’t do this — Johnannes Kepler’s laws of planetary motion don’t allow for celestial laziness — unless they are being acted on by another body. In this case, it would appear there is a second world orbiting Kepler-19, its gravity tugging on Kepler-19b, causing the bullied world to sometimes slow down and sometimes speed up.

“This invisible planet makes itself known by its influence on the planet we can see,” said astronomer Sarah Ballard of the Harvard-Smithsonian Center for Astrophysics (CfA) who analyzed Kepler Space Telescope observations to make the discovery. Ballard is lead researcher on the study that has been accepted for publication in The Astrophysical Journal.

This is the first time that an “invisible” exoplanet has been discovered through its gravitational influence on another exoplanet.

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Kepler spots exoplanets as they pass between the star and Earth. Their physical size can then be estimated by measuring the slight dip in starlight being received by Kepler’s sensitive instruments. Also, the orbital periods of these worlds can be accurately measured, so when the CfA scientists noticed slight changes in the orbital period of Kepler-19b, they grew suspicious.

Interestingly, planets in our solar system have been detected through a similar method.

In 1781, British astronomer Sir William Herschel noticed something strange about Uranus’s orbit. By 1821, French astronomer Alexis Bouvard surmised that Uranus was being perturbed by the gravity of another massive planet in the outer solar system. There had to be something out there tugging at the 7th planet from the sun.

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Then in the 1840′s, English and French astronomers John Couch Adams and Urbain Le Verrier independently went on to calculate where this mystery planet should be in the night sky by purely measuring these little ‘wobbles’ in Uranus’ path.

55 years after Herschel noticed Uranus’ perturbations, the distant planet was officially discovered by German astronomer Johann Galle in the location predicted by Couch Adams and Le Verrier. It was named Neptune.

As Kepler-19b is roughly twice the size of Earth, it may actually resemble a “mini-Neptune,” according to the CfA team.

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

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As the Kepler-19c “phantom” world has yet to be seen, its characteristics are unknown. The only thing we know about this invisible exoplanet is that it is big enough to act like a gravitational bully. On the Kepler mission discoveries page, the only entry under Kepler-19c is an approximation of its mass, less than 31.6 Earth masses.

“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 in the hope of better understanding Kepler-19b’s perturbations. Also, with the help of ground-based instruments, the mass and orbit of Kepler-19c will be refined.

But for now, Kepler-19c will remain a phantom amongst exoplanets.

Image credit: David A. Aguilar (CfA)