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.

New Scientist offers an animation of the two stars currently in action (with a rockin’ soundtrack). The research is published in the Astrophysical Journal.

Image: NASA/JPL-Caltech

This morning, NASA announced the confirmation of an exoplanet in its star’s habitable zone—the region around a star where liquid water, and life, could potentially exist on an Earth-sized planet.

The newly confirmed planet, Kepler-22b, is the smallest yet found to orbit in the middle of the habitable zone of a star similar to our sun. The planet has a radius about 2.4 times that of Earth. Scientists don’t yet know if Kepler-22b has a predominantly rocky, gaseous, or liquid composition, but its discovery brings us one step closer to finding Earth-like planets.

Previous research hinted at the existence of near-Earth-size planets in habitable zones, but clear confirmation proved elusive. Two other small planets orbiting stars smaller and cooler than our sun recently were confirmed on the very edges of their habitable zones, with orbits more closely resembling those of Venus and Mars.

Kepler discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars to search for planets that cross in front, or “transit,” the stars. Kepler requires at least three transits to verify a signal as a planet.

“Fortune smiled upon us with the detection of this planet,” says Bill Borucki, Kepler principal investigator at NASA Ames Research Center, who led the team that discovered Kepler-22b. “The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season.”

In addition, the Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to confirm planet candidates the spacecraft finds. In this case, Spitzer’s infrared instruments confirmed the new exoplanet’s existence.

Kepler-22b lies 600 light years away from Earth. While the planet is larger than Earth, its orbit of 290 days around a sun-like star resembles that of our world. The planet’s host star belongs to the same class as our sun, called G-type, albeit slightly smaller and cooler.

Of the 54 habitable zone planet candidates reported in February 2011, Kepler-22b is the first to be confirmed. This milestone will be published in the Astrophysical Journal.

The confirmation also provided our friends at SETI with very good news today. As of 6:18 a.m., SETI received renewed funding for its Allan Telescope Array, specifically to target Kepler’s discoveries, like Kepler-22b. From a SETI press release:

“This is a superb opportunity for SETI observations,” said Jill Tarter, the Director of the Center for SETI Research at the SETI Institute. “For the first time, we can point our telescopes at stars, and know that those stars actually host planetary systems—including at least one that begins to approximate an Earth analog in the habitable zone around its host star. That’s the type of world that might be home to a civilization capable of building radio transmitters.”