upside down planet kepler telescope
An image of the Sun used to simulate what the sun-like star in a self-lensing binary star system might look like Nasa

What was initially seen as an "upside-down" planet has revealed a new method for studying binary star systems.

A student astronomer from the University of Washington has confirmed the first "self-lensing" binary star system, one in which the mass of the closer star can be measured by how powerfully it magnifies light from its more distant companion star.

The discovery confirms an astronomer's prediction in 1973, based on stellar evolution models of the time, that such a system should be possible.

Astronomers detect planets too far away for direct observation by the dimming in light when a world passes in front of, or transits, its host star. Ethan Kruse was looking for transits that others may have missed in data from the Kepler Space Telescope when he saw something in the binary star system KOI-3278 that stood out.

"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."

The two stars of KOI-3278, which is situated approximately 2,600 light-years away in the Lyra constellation, take turns in being closer to Earth as they orbit each other every 88.18 days. They are about 43 million miles apart and the white dwarf, a cooling star believed to be in the final stage of life, is around 200,000 times larger than Earth.

That increase in light, rather than the dip Kruse expected, was the white dwarf bending and magnifying light from its more distant neighbour through gravitational lensing.

Eric Agol, an astronomer at the university, added: "The basic idea is fairly simple. Gravity warps space and time and as light travels towards us it actually gets bent, changes direction. So, any gravitational object - anything with mass - acts as a magnifying glass. You really need large distances for it to be effective.

"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," Agol added. "And instead of getting a dip now you get a brightening through the gravitational magnification."

The finding improves on 2013 research by the California Institute of Technology, which detected a similar self-lensing effect without the brightening of the light because the two stars being studied were much closer together.

"The effect in this system is much stronger," said Agol. "The larger the distance, the more the effect."

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.

Yet the process had only been used in the fleeting instances of a nearby and distant star. Agol said: "The chance is really improbable. As those two stars go through the galaxy they'll never come back again, so you see that microlensing effect once and it never repeats. In this case, though, because the stars are orbiting each other, it repeats every 88 days."

White dwarfs can be used to estimate age within a galaxy. As the embers of burnt out stars, they cool off at a specific rate over time. Using this information, astronomers can estimate what its mass and temperature are.

By expanding their understanding of white dwarfs, astronomers take a step closer to learning about the age of the galaxy. The team said they would examine data from the Hubble Space Telescope to study KOI-3278 in more detail, and to see if there are other such star systems waiting to be discovered in the Kepler data.