Kepler Watches White Dwarf Warp Spacetime

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The Kepler space telescope’s prime objective is to hunt for small worlds orbiting distant stars, but that doesn’t mean it’s not going to detect some extreme relativistic phenomena along the way.

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.

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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 technique 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 there — 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.

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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.

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 be slightly brighter than expected. This is counter-intuitive to how transits work, but KOI-256 is anything but intuitive.

As the white dwarf passed in front of the red dwarf, its extreme gravitational field was causing spacetime to bend, focusing the light from the red dwarf, enhancing the starlight. As the white dwarf passed behind the red dwarf, there would be no gravitational disruption of starlight and therefore no starlight enhancement. This finding will be published on April 20 in the Astrophysical Journal.

“Only Kepler could detect this tiny, tiny effect,” said Doug Hudgins, Kepler program scientist at NASA Headquarters, Washington. “But with this detection, we are witnessing Einstein’s theory of general relativity at play in a far-flung star system.”

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Indeed, this relativistic effect, known as “microlensing,” has been used to detect exoplanets before, but this is one of the first examples of a binary stellar partner being detected through the analysis of the light of its lower-mass sibling.

Microlensing is a transient event, but large-scale lensing events have been recorded in deep space. For example, the Hubble Space Telescope has identified arcs of light surrounding massive galactic clusters as distant light being bent around warped spacetime. This light is often from distant galaxies behind the galactic clusters, magnifying and focusing the ancient galaxy’s light.

These are nature’s natural cosmic magnifying lenses, and with this new white dwarf discovery, Einstein has teamed up with Kepler to reveal a fascinating twist in our hunt for extrasolar planets (and, now, white dwarfs).

Image: Artist’s impression of the white dwarf warping spacetime and bending the light of the red dwarf. Credit: NASA/JPL-Caltech