In a chance discovery, astronomers have spotted an 'upside-down' planet in the first ever "self-lensing" binary star system confirmed about 2,600 light-years away.
A self-lensing binary star system is one in which the mass of the closer star can be measured by how powerfully it magnifies light from its more distant companion star.
Working with University of Washington astronomer Eric Agol, doctoral student Ethan Kruse confirmed an astronomer's prediction in 1973, based on stellar evolution models of the time, that such a system should be possible.
Like so many interesting discoveries, this one happened largely by accident, researchers said.
Using data from the planet-hunting Kepler Space Telescope, Kruse saw something in the binary star system KOI-3278 that didn't make sense.
"I found what essentially looked like an upside-down planet," Kruse said.
"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," said Kruse.
The two stars of KOI-3278, about 2,600 light-years away in the Lyra constellation, take turns being nearer to Earth as they orbit each other every 88.18 days.
They are about 43 million miles apart, roughly the distance the planet Mercury is from the Sun. The white dwarf, a cooling star thought to be in the final stage of life, is about Earth's size but 200,000 times more massive.
That increase in light, rather than the dip Kruse thought he'd see, was the white dwarf bending and magnifying light from its more distant neighbour through gravitational lensing, like a magnifying glass.
"The basic idea is fairly simple. Gravity warps space and time and as light travels toward us it actually gets bent, changes direction. So, any gravitational object — anything with mass — acts as a magnifying glass," Agol said.
"You really need large distances for it to be effective." said Agol.
"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.
Gravitational lensing is a common tool in astronomy. It has been used to detect planets around distant stars within the Milky Way galaxy, and was among the first methods used to confirm Albert Einstein's general theory of relativity.
Lensing within the Milky Way galaxy, such as this, is called microlensing.