Exoplanet Kepler-19b orbits its star in ways that violate the laws of physics, speeding up and slowing down its orbit for no apparent reason. The only explanation is a second, hidden planet...making it the first "phantom" exoplanet ever found.
Discovered by (and named after) NASA's exoplanet-hunting Kepler Space Telescope, Kepler 19-b is located about 650 light-years from Earth, and it completes one orbit every nine days and seven hours, give or take. That "give or take" bit is the problem - sometimes it will complete the orbit five minutes faster than it should, and other times it will be five minutes slow.
That may not seem like much of a difference, but that amount of variance flies completely in the face of the laws of planetary motion, first formulated by Johannes Kepler - which is strangely appropriate, considering it's his namesake that found this problem planet.
The explanation for this physics-defying anomaly is the presence of a nearby companion planet. The problem is that we've yet to find any direct sign of this planet, designated Kepler-19c. That makes this invisible exoplanet the first ever that is only known from its gravitational effect on another planet. Still, Kepler-19c is in good company - it was the gravitational anomalies of Uranus that led to the discoveries of Neptune and, albeit somewhat unintentionally, Pluto.
Since we can't see Kepler-19c, we don't have any solid idea what it actually looks like. We do know that it isn't massive enough to tug at its star, which means its mass can't be more than about 30 times Earth's mass, and the fact that we haven't detected it transiting in front of its star suggests an unusually oblong orbit. Other than that, it's anyone's guess, as UC Santa Cruz astronomer Daniel Fabrycky explains:
"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."
For now, Kepler-19c will have to just remain known as the galaxy's only phantom planet. It's a temporary title, to be sure, but still not too shabby, as these things go.
Via the Harvard-Smithsonian Center for Astrophysics. Image by David A. Aguilar (CFA).