Kepler finds first multi-planet system around a binary star

Aug 28, 2012
This is an artist's rendition of the Kepler-47 system. Credit: NASA/JPL-Caltech/T. Pyle

NASA's Kepler mission has found the first multi-planet solar system orbiting a binary star, characterized in large part by University of Texas at Austin astronomers using two telescopes at the university's McDonald Observatory in West Texas. The finding, which proves that whole planetary systems can form in a disk around a binary star, is published in the August 28 issue of the journal Science.

"It's Tatooine, right?" said McDonald Observatory astronomer Michael Endl. "But this was not shown in Star Wars," he said, referring to the periodic changes in the amount of daylight falling on a planet with two suns. Measurements of the star's orbits showed that daylight on the planets would vary by a large margin over the 7.4-Earth-day period as the two stars completed their mutual orbits, each moving closer to, then farther from, the planets (which are themselves moving).

The in question is called Kepler-47. The primary star is about the same mass as the Sun, and its companion is an M- one-third its size. The is three times the size of Earth and orbits the binary star every 49.5 days, while the is 4.6 times the size of Earth with an orbit of 303.2 days.

The outer planet is the first planet found to orbit a binary star within the "," where liquid water could exist and thus create a home for life. However, the planet's size (about the same as Uranus) means that it is an icy giant, and not an abode for life. It's a tantalizing taste of discoveries waiting to be made.

The combination of observations from the and McDonald Observatory allowed astronomers to understand the characteristics of Kepler-47's two stars and two planets.

The 2.7-meter (107-inch) Harlan J. Smith Telescope at The University of Texas at Austin's McDonald Observatory in West Texas. Credit: Marty Harris/McDonald Observatory/UT-Austin

The looks for minute dips in the amount of light coming from a star that might indicate a planet is passing in front of it, an event called a "transit." The is also adept at identifying eclipsing binary stars, in which two stars pass in front of each other as they each other. In the case of Kepler-47, they found both stellar eclipses and planet transits in one system.

So Kepler astronomers Jerome Orosz (lead author on the study) and William Welsh of San Diego State University flagged the Kepler-47 system as worthy of follow up from the ground. They asked the McDonald Observatory Kepler team to work with them.

Endl studied the binary star with the 9.2-meter Hobby-Eberly Telescope (HET, one of the world's largest telescopes), as well as the 2.7-meter Harlan J. Smith Telescope at McDonald.

The Hobby-Eberly Telescope (HET) at The University of Texas at Austin's McDonald Observatory in West Texas. Credit: Marty Harris/McDonald Observatory/UT-Austin

"The challenging thing is that this is a very faint star," Endl said, "about 6,000 times dimmer than can be seen with the naked eye."

He was taking spectra of the system—looking for characteristics in its light to indicate the motions of the primary star. (The secondary star is too faint to measure.) The McDonald observations enabled astronomers to calculate the mass of the primary star.

These values, along with the Kepler eclipse and transit timings, were plugged into a model that calculated the relative sizes of all the bodies involved, Endl said.

The Kepler team at McDonald Observatory also includes Bill Cochran (a co-Investigator of the Kepler mission), research scientist Phillip MacQueen, graduate students Paul Robertson and Eric Brugamyer, and recent graduate Caroline Caldwell.

"This is the type of research where McDonald Observatory really excels," Cochran said. "We have excellent scientific instruments on our telescopes, and the queue-scheduled operation of the HET allows us to obtain spectra at the optimal times when they will give us the best information about the stars."

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User comments : 15

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jibbles
5 / 5 (4) Aug 28, 2012
actually, while the outer planet (being an icy giant)itself may be a poor candidate for harboring life, it may well harbor life on its (likely numerous)rocky moons
Kafpauzo
not rated yet Aug 28, 2012
Good point, jibbles. Articles about planet-hunting make it sound like planet-hunters are always forgetting this possibility. That's quite odd, they ought to be very well aware of it.

I wonder if our planet-hunting technology will get to a point where we can get atmospheric spectra from such exoplanetary moons, and tease out the spectra of individual moons from the various different atmospheres. A very tall order.

In any case, seeing that there are probably several moons, it sounds like it would be a good idea to point SETI at this target, to see if there are intelligent radio signals.

A space-faring civilization in such a moon system would have some fascinating possibilities, with several moons within easy reach.
Mastoras
not rated yet Aug 29, 2012
Um, I didn't get it. Do they mean that the planets orbit *both* of the two stars? or do they orbit only the primary one?
-.
cdt
5 / 5 (2) Aug 29, 2012
I don't understand how being 4.6 times the mass of earth makes a planet icy. Indeed, that seems to contradict its being characterized as occupying a zone in which liquid water can exist. Uranus is icy not just because of its size but because of its location. Pluto is probably icy as well -- it's certainly cold. Bring either of them to where earth is and the temperature would be very different.

Mastoras, the two stars orbit each other very closely. The planets orbit both stars much further out from the center. (You can think of the sun and Mercury as an extreme model of these two suns: If there were only the sun, Mercury and Earth, Earth would orbit the center of mass of the sun and Mercury. That point would always be very close to the center of the sun, of course, but making Mercury into a star-massed object wouldn't change the fact that Earth would continue to orbit around the center of mass of that object and the Sun.)
Skxawng
not rated yet Aug 29, 2012
Another point is as the planet is bigger, hydrogen gas would not escape the atmosphere and the planet would be inhospitable to life.
jsdarkdestruction
4 / 5 (4) Aug 29, 2012
Another point is as the planet is bigger, hydrogen gas would not escape the atmosphere and the planet would be inhospitable to life.

as we know it....
Birger
not rated yet Aug 29, 2012
The way I see it, it means both planets orbit the barycentre of the two dwarf stars, who are always "inside" the planets.
Mastoras
not rated yet Aug 29, 2012
@cdt, @birger,

Thanks for the clarification.
-.
El_Nose
not rated yet Aug 29, 2012
we can't detect moons -- granted moons probably exist - but just because the super giants in our solar system have them doesn't mean that these inner giants would.
NotAsleep
not rated yet Aug 29, 2012
@cdt, I agree that it would be difficult to tell whether or not a planet is icy based on size and location alone. I suspect there's more info they used to make the "icy" determination and it just didn't make it into the article.
GSwift7
5 / 5 (5) Aug 29, 2012
I don't understand how being 4.6 times the mass of earth makes a planet icy. Indeed, that seems to contradict its being characterized as occupying a zone in which liquid water can exist.


Planets in this size range are often called watery or icy, meaning that they are thought to have more volatiles than a true Earth surrogate. It doesn't imply that the water would actually be frozen, though the terminology is unfortunately confusing. They're just using the term "icy" to describe the elemental composition, ratio of various elements, and density most likely to exist there. Anything from about 2 Earth masses up to a gass planet, would be like this (according to our current best guess). Even if the water is liquid, the composition should be more like Neptune than Earth. Higher gravity and atmospheric pressure would change the chemistry in the atmosphere as well (boiling points, solubility, etc.)
GSwift7
5 / 5 (1) Aug 29, 2012
we can't detect moons -- granted moons probably exist - but just because the super giants in our solar system have them doesn't mean that these inner giants would.


Based on the only example we can look at, our own solar system, inner rocky planets do not have moons. Our own moon is likely to be an exception because its density is appears to be uncommon for anything this close to its star. In our own solar system and in the case of exoplanets we have observed, density appears to correllate inversely with distance from the star (less dense planets are farther out). Our moon sticks out like a sore thumb in terms of density compared to everything else in our solar system, and no other innner planet here has a moon except Mars. However, the moons of Mars are clearly captured asteroids rather than true moons.
Torbjorn_Larsson_OM
5 / 5 (2) Aug 29, 2012
@ Kafpauze:

They are aware, The Habitable Exoplanet Database currently lists more predicted habitable exomoons (30) than potential unconfirmed habitable exoplanets (27). [ http://phl.upr.ed...-catalog ]

***********
Btw, if any of you are interested in habitables, you may want to check the site later today: "The Habitable Exoplanets Catalog will present new results today, August 29 @ 10:30 PM (EDT)"!
**********

@ cdt:

To pitch in, it comes from a proposed new classification of planets, especially giants, that the surveys results in. Gas giants are another name for Jupiter massed and ice giants for Neptune massed. Mass is easier to get at.

The old Sudarsky classification has 5 classes of giants, depending on atmosperic conditions. It is used when you know that. [ http://en.wikiped...fication ]
Torbjorn_Larsson_OM
5 / 5 (1) Aug 29, 2012
@ GSwift7:

I think the main theory of Mars moons right now is that they are impact generated moons like ours. It is very hard to predict corotation, equatorial plane and other orbit elements else for two moons and an elliptical impact scar at the equator that can only be an earlier deorbited moon.

The difference is that the ejected mass was too little to coalesce into one large moon. Many similar impact generated moons are known throughout the system, from asteroids with moons to Kuiper objects like Pluto-Charon.

The density of our Moon just got its prediction from a "hit-and-run" impact regime that can generate our Earth-Moon system at times. (They need to work on the likelihood.) Turns out most remaining moon material then originates from Earth, while the rest escapes. Could predict the enstatites that still inhabits the inner asteroid belt.

Venus is counterrotating, slowly. Simplest prediction is a similar hit-and-run, but the orbital remains already deorbited.
GSwift7
not rated yet Aug 30, 2012
Torbjorn

I think the main theory of Mars moons right now is that they are impact generated moons like ours. It is very hard to predict corotation, equatorial plane and other orbit elements else for two moons and an elliptical impact scar at the equator that can only be an earlier deorbited moon


Sure. Once again, terminology becomes a barrier. If they are rocks that were thrown into space by an impact, then don't they become an asteroid? That's what we call them when we find Mars rocks here on Earth. Only in the case of the two 'moons' of Mars they were captured by Mars gravity, right?

I see your point about a potential Mars moon, but current best guess says that there should have been several more rocky planets in the early solar system. They would have all been in the ecliptic, so any collisions should have been near that plane too.

As it stands, there's aren't any inner moons now.

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