April 4, 2013
While monitoring a red dwarf star — designated KOI-256 — astronomers detected a dip in starlight in the Kepler data. The NASA space telescope is constantly on the lookout for these dips as they can be an indicator of an extrasolar planet passing in front of the star’s disk. This event is known as a “transit” and Kepler has the unprecedented sensitivity to detect sub-Earth-sized worlds drift in front of their host stars.
When a transit was detected in the KOI-256 system, researchers led by Caltech’s Phil Muirhead thought they’d just witnessed a massive planet orbiting the star. However, something was very strange about this particular transit.
“We saw what appeared to be huge dips in the light from the star, and suspected it was from a giant planet, roughly the size of Jupiter, passing in front,” said Muirhead.
Using the ground-based Palomar Observatory in California, Muirhead’s team applied another exoplanet-hunting method to KOI-256. The “radial velocity method” can detect worlds in orbit around other stars through the careful analysis of the spectrum of starlight. If an exoplanet is in orbit, the mass of the world will gravitationally “tug” on the host star. This tugging creates a slight wobble, generating a red- and blue-shifting of light; a tell-tail sign that a planet is there.
Radial velocity measurements of KOI-256, however, revealed that something else was in orbit —and it certainly wasn’t an exoplanet. The star was found to be wobbling “like a spinning top” meaning something way more massive is in orbit — a compact white dwarf star, the stellar husk of a burned-out star.
Although red dwarfs are small, white dwarfs are even smaller, but very, very dense. The white dwarf in the KOI-256 binary is about the size of Earth and yet packs the mass of the sun. “It’s so hefty that the red dwarf, though larger in physical size, is circling around the white dwarf,” added Muirhead.
Most of the stars in our galaxy are binary stars; two stars in a tight cosmic dance is not a rarity. But in the case of KOI-256, something was very strange. With the help of another NASA space observatory, the Galaxy Evolution Explorer (GALEX), which analyzes the ultraviolet light of the stars in Kepler’s field of view, the researchers noticed that as the white dwarf passed behind the red dwarf, the starlight would dim, but when the white dwarf passed in front, the light would brighten. This is counter-intuitive to how transits work, but KOI-256 is anything but intuitive.
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