How an "Upside-Down" Planet Gave New Insight into Gravitational Lensing

Sometimes it’s the strange, accidental discoveries that teach us the most. Like the so-called "upside-down” planet astronomers found last week. It’s not really upside-down; what astronomers actually found is a very interesting case of gravitational lensing within a binary star system.

The unique arrangement is opening a new way of looking at dual star systems.

University of Washington astronomer Eric Agol and doctoral student Ethan Kruse discovered the upside-down planet while looking at exoplanet data the way so many astronomers do: by looking for dips in starlight as a planet passes between the star and the Earth.

Kruse was on the lookout for missed transits in old data from NASA’s exoplanet-hunting Kepler Space Telescope when he saw something really strange in the binary star system KOI-3278. Instead of a dip in starlight, there was an increase in the star’s brightness associated with a planetary transit.

“I found what essentially looked like an upside-down planet,” Kruse said in a release. “What you normally expect is this dip in brightness, but what you see in this system is basically the exact opposite—it looks like an anti-transit.”

The pair of stars that make up the KOI-3278 system lie about 2,600 lightyears from Earth in the constellation Lyra. And being a pair they switch positions relative to our planet as they orbit one another, one being closest to Earth first then the other. Their arrangement is handy, allowing astronomers to measure the mass of one star by looking at how powerfully it magnifies the light from its companion.

And interestingly, Kruse’s discovery of this “anti-transit” is in line with a prediction made more than 40 years ago. In 1973, astronomers predicted that self-lensing binary systems should exist, not immediately, but later in the stars’ life cycle.

For now, we know a little about the binary system. The pair of stars orbit one another every 88.18 days, and they sit about 43 million miles apart (roughly the distance from the Sun to Mercury). We also know that one of the stars is a white dwarf, a cooler star in the final stage of its life with about 200,000 times the mass of the Earth.

The increase in light in Kruse saw—the anti-transit—was the white dwarf of the pair bending and magnifying the light from its companion star though gravitational lensing.

Gravitational lensing is an interesting phenomenon wherein gravity actually warps light as it travels from, say, a distant star to an observatory on Earth or in Earth orbit. The light being warped bends and changes direction, and the result is magnification. A gravitational object effectively acts like a magnifying glass, though a weak one. Only across really large distances can astronomers see the effects of gravitational lensing.

“The cool thing, in this case, is that the lensing effect is so strong, we are able to use that to measure the mass of the closer, white dwarf star,” said Agol. “And instead of getting a dip now you get a brightening through the gravitational magnification.”

Gravitational lensing isn’t a new tool for astronomers. It’s been used for decades in exoplanet detection and was the first method to confirm Albert Einstein’s general theory of relativity. It works on both big and small scales, with distant galaxies and within our own Milky Way; when the gravitational lens is in the Milky Way it’s called microlensing.

Gravitational lensing has typically been used for fleeting meetings (from our Earthly perspective) of galaxies and stars. But the lensing found in this binary system isn’t going away. Because the stars orbit one another, astronomers will be able to see the gravitational lens effect every 88 days.

There is a lot of good packed in this discovery. That one star is a white dwarf is a bonus. These old remnants of the stars they once were are great indicators of the age of the galaxy because they cool at a known and specified rate. With regular gravitational lensing, astronomers could learn a lot about these old stars, and by extension learn a lot about the age of the Universe.

Even finding this self-lensing is exciting because it’s a new discovery in old Kepler data. The telescope may be done planet hunting, but this is a clear example that there’s no reason to write off the mission just yet. “If everyone’s missed this [binary pair], then there could be many more that everyone’s missed as well,” said Kruse.