Two of the techniques used by astronomers to find planets orbiting other stars (exoplanets) have been brought together for the first time. By combining data from the Kepler Space Telescope and gravitational lensing a planet has been found twice as far from us as any other world discovered using Kepler. By coincidence, it happens to be a remarkable match for Jupiter in the features we can measure.
Kepler operated by making frequent checks of the light from thousands of stars. Professional and amateur astronomers then sifted through the data seeking regular dips in brightness indicative of planets blocking a star's light. However, most stars beyond about 10,000 light-years are too faint for such dips to be detectable in Kepler's data.
Eventually, more powerful telescopes may enable us to find planets close to the galactic center, but meanwhile, University of Manchester PhD student David Specht experimented with an alternative approach. In a forthcoming paper in Monthly Notices of the Royal Astronomical Society (preprint on ArXiv.org, not yet peer-reviewed) Specht and co-authors report success, proving the concept and adding a new world to our databases.
Gravitational lensing uses the fact that light bends around massive objects. When a large enough object is suitably placed, it can act like a lens, focusing light from something more distant on Earth, just as a badly placed mass can distort the view.
Astronomers have used gravitational lenses created by galaxies to allow us to peer much deeper into space than we would be able on our own. They've also discovered exoplanets through a process known as microlensing. When a star passes in front of more distant stars from our perspective and creates a temporary gravitational lens, it is often preceded or followed by a much smaller lens, indicating a planet's presence.
Microlensing has revealed planets to giant telescopes on Earth, but Kepler also spent a lot of time looking towards the galactic center where stars are thickly clustered.
"The chance that a background star is affected this way by a planet is tens to hundreds of millions to one against. But there are hundreds of millions of stars towards the center of our Galaxy. So Kepler just sat and watched them for three months,” said co-author Dr Eamonn Kerins of Jodrell Bank in a statement.
Buried in the data Kepler collected in 2016, Specht, Kerins, and dozens of co-authors found five examples that might represent planets. Combining Kepler's observations (made while almost as far from the Earth as we are from the Sun) with ground-based data the team are confident one of these, K2-2016-BLG-0005Lb, is real. "The difference in vantage point between Kepler and observers here on Earth allowed us to triangulate where along our sight line the planetary system is located,” Kerins said.
The other telescopes were necessary for confirmation, but Kepler provided the bulk of the data, being free from daylight, clouds, and atmospheric interference.
K2-2016-BLG-0005Lb is 17,000 light-years away and only marginally more massive than Jupiter, while orbiting its star at a quite similar distance, but the star itself is about 40 percent less massive than the Sun. Sadly, we can't get any more information about K2-2016-BLG-0005Lb with existing instruments, but the find represents a rare example of the detection of a planet so far from its star, where gas giants are thought to form initially. Existing methods greatly favor finding those with tighter orbits.
The forthcoming Nancy Grace Roman space telescope is designed to find 1,400 planets towards the galactic center using microlensing, including 100 of Earth-like mass. Before that, the Euclid space telescope, while built primarily for other purposes, is also much better suited to find planets this way than Kepler. The fact that Kepler could find even one increases astronomers' confidence that both future telescopes will find many more.
1