Exoplanet or White Dwarf
By Molly Michelson
When is an exoplanet not an exoplanet? When it’s a white dwarf, of course. Well, at least in the case of KOI-256 (Kepler Object of Interest, number 256).
NASA’s amazing exoplanet hunter, the space-based Kepler mission, spotted an object transiting the red dwarf star, KOI-256, using its standard technique—as the object passes in front of its star, Kepler detects a decrease in the star’s light.
But something looked different about this star. In addition to the dip in brightness from the transiting object, the star’s brightness seemed to vary in a way that suggested it was behaving quite oddly. So Phil Muirhead, of the California Institute of Technology, began to explore further.
Muirhead and colleagues first used a ground-based telescope to get another look. Measuring the star’s radial velocity, they discovered that the red dwarf was wobbling around like a spinning top. Because of this, the scientists suspected the object wasn’t an exoplanet after all, but something much more massive—likely a white dwarf.
A white dwarf is essentially what a dead star leaves behind—a hot cinder, incredibly massive for its size. It “weighs” a lot more than an exoplanet, so Muirhead needed to figure out how much mass exists in the KOI-256 system.
To measure the combined mass of the two objects in the binary pair, the researchers used a technique called gravitational lensing: one of the consequences of Einstein’s general theory of relativity is that gravity bends light, so scientists use gravitational lensing to figure out how much mass is bending (or lensing) light from more distant sources. And while the technique has been utilized to measure the mass of galaxies, it’s the first time it has been used to “weigh” a binary star system. Since we know the approximate mass of a red dwarf, we can then estimate the mass of the companion, which indeed turns out to be a white dwarf.
“This white dwarf is about the size of Earth but [with] the mass of the Sun,” says Muirhead. “It’s so hefty that the red dwarf, though larger in physical size, is circling around the white dwarf.”
The red dwarf orbits the white dwarf in just 1.4 days. This orbital period is so short that at an earlier time the stars must have previously undergone a “common-envelope” phase in which the red dwarf orbited within the outer layers of its companion star—a giant star that eventually died and left behind the white dwarf we see today.
The short orbital period also means the red dwarf’s days are numbered. In a few billion years, the intense gravity of the white dwarf will strip material off the red dwarf, forming a hot accretion disk of in-falling material around the white dwarf.