Real-life Tatooine (Kepler-16b)

[Belisarius]

From the Bad Astronomy Blog at Discover magazine:

Astronomers discover a wretched hive of scum and villainy

If there’s a bright center to the Universe, astronomers have found the planet it’s farthest from. Called Kepler-16b, it’s a Saturn-like world which has the distinction of being the first discovered to orbit both Sun-like stars in a binary system.

OK, Star Wars references aside, this is pretty cool. Most of the planets being found around other stars are orbiting single stars. A very few — like a possible planet orbiting Gamma Cephei — orbit one of the stars in a binary system, and some (like NN Serpentis b and c) orbit both stars, but one of them is a dead star like a white dwarf or a neutron star.

Unlike those, Kepler-16 is a binary where both stars, though dinky, are bona-fide stars like the Sun, and the planet orbits both. Actually, how it was found is pretty nifty. The orbiting Kepler observatory is designed to stare at over 100,000 stars and detect the tell-tale drop in light when a planet transits (that is, from our point of view passes directly in front of) its parent star. Kepler has found a lot of planet candidates this way — well over 1200!

Kepler-16 is one (OK, two) of those stars (hence the name), located about 200 light years from Earth. The two stars are eclipsing binaries, meaning that we are viewing them from Earth in the plane of their orbit. Twice every orbital period, one of the stars blocks the light from the other and we see the total light from the system dip a little bit. We know of a lot of eclipsing binaries, and their properties are pretty well understood.

But Kepler-16 is different. After observing the pair for some time, a third dip in the system’s light was seen happening at odd intervals. It was clearly due to a planet, but if this object orbited one star or the other, the transits would happen at regular intervals. The staggered time between dips, though, indicated it was actually in a wide orbit around both stars: as it orbits, it blocks one star’s light, then the other’s, and the timing between those mini-eclipses changes as the two stars orbit each other.

What a mess! I would’ve loved to have seen the look on the face of the astronomer who first graphed the change in brightness of Kepler-16 over time.

We can actually determine a bit about the planet, called Kepler-16b. The amount of light it blocks tells us its size (bigger planets block more light), and it turns out to be roughly the size of Saturn — about 100,000 km (60,000 miles) across, 8 times the diameter of Earth. Also, as it orbits the stars its gravity tugs on them, and that can be detected by taking very careful measurements of the spectrum of the two stars (basically, breaking up the light from the stars into individual wavelengths, like a rainbow with a hundred thousand colors). The mass of the planet can be found that way, and again it looks a lot like Saturn: about 100 times the mass of the Earth.

And while there are two stars involved in heating the planet, their light is pretty feeble. Even at its distance of a little over 100 million kilometers (65 million miles) from the pair — roughly the same distance at which Venus orbits the Sun — Kepler-16b is cold: the temperature at its cloud tops (assuming it’s a gas giant like Saturn) would be at best -70°C (-100°F).

So any visions you have of Luke Skywalker standing in the desert with his leg resting on a rock while he wistfully watches the two suns set in the west may have to wait. Even if the planet has a big moon (which these observations cannot yet detect) conditions there would be a bit chillier than on Tatooine. More like Hoth.

Still, this is a very interesting discovery. These kinds of planets probably exist in large numbers, but they’re hard to detect: the orbits of all three components have to line up just right for us to see them. I’ll note that if the planet’s orbit were just slightly tilted so that it didn’t appear to pass in front of the two stars, its gravity might still reveal its presence as it tugs on the stars. Finding more planets like that may just be a matter of time as more binaries are observed. I wouldn’t have given that sort of thing much of a chance, but Kepler-16b has given me A New Hope.

Image credits: Kepler 16-b art: David A. Aguilar (CfA); Tatooine: LucasFilms

link:
http://blogs.discovermagazine.com/b...iscover-a-wretched-hive-of-scum-and-villainy/

 

[DwayneHoover]

Here's more info with an animation even:

Go here for 2 animations from Indistrial Light & Magic, one from orbit plane and one from overhead:
http://www.nasa.gov/mission_pages/kepler/news/kepler-16b.html

NASA caption below the 2 images.

Go here for full image caption and links to other resolutions for 1st image:
http://www.nasa.gov/mission_pages/kepler/multimedia/images/Kepler-16_planet-pov-art.html

Go here for full image caption and links to other resolutions for 2nd image:
http://www.nasa.gov/mission_pages/kepler/multimedia/images/Kepler-16_transit-art.html

NOTE: may need to click image for full res

NOTE: may need to click image for full res

The existence of a world with a double sunset, as portrayed in the film Star Wars more than 30 years ago, is now scientific fact. NASA's Kepler mission has made the first unambiguous detection of a circumbinary planet -- a planet orbiting two stars -- 200 light-years from Earth.

Unlike Star Wars’ Tatooine, the planet is cold, gaseous and not thought to harbor life, but its discovery demonstrates the diversity of planets in our galaxy. Previous research has hinted at the existence of circumbinary planets, but clear confirmation proved elusive. Kepler detected such a planet, known as Kepler-16b, by observing transits, where the brightness of a parent star dims from the planet crossing in front of it.

"This discovery confirms a new class of planetary systems that could harbor life," Kepler principal investigator William Borucki said. "Given that most stars in our galaxy are part of a binary system, this means the opportunities for life are much broader than if planets form only around single stars. This milestone discovery confirms a theory that scientists have had for decades but could not prove until now."

A research team led by Laurance Doyle of the SETI Institute in Mountain View, Calif., used data from the Kepler space telescope, which measures dips in the brightness of more than 150,000 stars, to search for transiting planets. Kepler is the first NASA mission capable of finding Earth-size planets in or near the "habitable zone," the region in a planetary system where liquid water can exist on the surface of the orbiting planet.

Scientists detected the new planet in the Kepler-16 system, a pair of orbiting stars that eclipse each other from our vantage point on Earth. When the smaller star partially blocks the larger star, a primary eclipse occurs, and a secondary eclipse occurs when the smaller star is occulted, or completely blocked, by the larger star.

Astronomers further observed that the brightness of the system dipped even when the stars were not eclipsing one another, hinting at a third body. The additional dimming in brightness events, called the tertiary and quaternary eclipses, reappeared at irregular intervals of time, indicating the stars were in different positions in their orbit each time the third body passed. This showed the third body was circling, not just one, but both stars, in a wide circumbinary orbit.

The gravitational tug on the stars, measured by changes in their eclipse times, was a good indicator of the mass of the third body. Only a very slight gravitational pull was detected, one that only could be caused by a small mass. The findings are described in a new study published Friday, Sept. 16, in the journal Science.

"Most of what we know about the sizes of stars comes from such eclipsing binary systems, and most of what we know about the size of planets comes from transits," said Doyle, who also is the lead author and a Kepler participating scientist. "Kepler-16 combines the best of both worlds, with stellar eclipses and planetary transits in one system."

This discovery confirms that Kepler-16b is an inhospitable, cold world about the size of Saturn and thought to be made up of about half rock and half gas. The parent stars are smaller than our sun. One is 69 percent the mass of the sun and the other only 20 percent. Kepler-16b orbits around both stars every 229 days, similar to Venus’ 225-day orbit, but lies outside the system’s habitable zone, where liquid water could exist on the surface, because the stars are cooler than our sun.

"Working in film, we often are tasked with creating something never before seen," said visual effects supervisor John Knoll of Industrial Light & Magic, a division of Lucasfilm Ltd., in San Francisco. "However, more often than not, scientific discoveries prove to be more spectacular than anything we dare imagine. There is no doubt these discoveries influence and inspire storytellers. Their very existence serves as cause to dream bigger and open our minds to new possibilities beyond what we think we 'know.'"

 

[Belisarius]

Thanks for the animations. A little poetic justice in ILM doing those, eh?