Triple Sunset: Planet Discovered in 3-Star System
By Michael Schirber
Staff Writer
posted: 13 July 2005
01:00 pm ET

A newly discovered planet has bountiful sunshine, with not one, not two, but three suns glowing in its sky.

It is the first extrasolar planet found in a system with three stars. How a planet was born amidst these competing gravitational forces will be a challenge for planet formation theories.

"The environment in which this planet exists is quite spectacular," said Maciej Konacki from the California Institute of Technology. "With three suns, the sky view must be out of this world -- literally and figuratively."

The triple-star system, HD 188753, is located 149 light-years away in the constellation Cygnus. The primary star is like our Sun, weighing 1.06 solar masses. The other two stars form a tightly bound pair, which is separated from the primary by approximately the Sun-Saturn distance.

"The pair more or less acts as one star," Konacki told SPACE.com.

The combined mass of the close pair is 1.63 solar masses.

Using the 10-meter Keck I telescope in Hawaii, Konacki noticed evidence for a planet orbiting the primary star. This newfound gas giant is slightly larger than Jupiter and whirls around its central star in a 3.5-day orbit. A planet so close to its star would be very hot.

Although other so-called hot Jupiters have been found in such close-in orbits, the nearby stellar pair in HD 188753 likely sheared off much of the planet making material in the disk that would likely have existed around the primary star in its youth. Since this proto-planetary disk holds the construction materials for planets, there does not appear to be any safe place for this far-off world to have been assembled.

Snow line and migration

The heat coming from a nearby star frustrates the initial stages of giant planet formation -- the gluing together of planetary seeds, called cores. Therefore, the typical hot Jupiter is thought to form farther out -- beyond a theoretical limit called the snow line.

"Past about 3 AU, it is cold enough to form ices and other solid material for building cores," Konacki said. An AU is the distance between the Sun and the Earth -- about 93 million miles.

Once a sufficiently large core is built outside the snow line, the planet can start accreting gas and -- if the conditions are right -- migrate toward its sun.

Although this scenario appears to work in most stellar systems, it has difficulty explaining the newly-discovered planet in HD 188753. Of all the planet-harboring stars known, this is the closest that a stellar companion has ever been found.

"The problem is that the pair is a massive perturber to the system," Konacki said. "Together, these two stars are more massive than the main star."

Moreover, the pair goes around the primary along an oblong orbit that stretches from 6 AU out to 18 AU over a 26 year period. This eccentricity increases the instability of the disk around the primary. Konacki estimates that due to the gravitational perturbations from the pair, the proto-planetary disk was truncated down to 1.3 AU, far within the snow line.

"How that planet formed in such a complicated setting is very puzzling. I believe there is yet much to be learned about how giant planets are formed," Konacki said.

Targeting multiple stars

Konacki hopes to find more planets around stars with companions. About 30 extrasolar planets have been found around double-star systems, or binaries. This is a small percentage of the total number of extrasolar planets, even though multi-star systems outnumber single star systems.

The reason for this disparity is that the main technique for locating planets -- the radial velocity method -- is not well-suited for finding planets with more than one star.

"Single stars are much easier to work with, since the shape of the spectrum stays the same," Konacki explained.

By watching for wobbles in a star’s spectrum, astronomers can infer the gravitational tug from a nearby planet. But when there is a companion star, its light competes with that of the main star. Konacki has developed a method to extract the planet wobbles from this messy, combined spectrum.

He found this triple-sun planet in the first 20 stars he looked at. He plans to survey about 450 stars in the future.

The discovery is reported in the July 14 issue of Nature. Animations created by JPL's PlanetQuest show the orbital motion of the system, as well as what it looks like from a hypothetical moon.

How the hell does this thing stay in orbit?

That's exactly the weird thing about the whole discovery. Once again it seems that our theories about the universe were too simple.

But.... if it can't have a circular orbit, then what? Triangular?

I'm just drawing this out... I'd guess on conventional theory that it would resemble an octagon but the whole discovery doesn't make any conventional sense in the first place.

Just, wow.

It's like that movie "Pitch Black"

There is so much we don't know about the universe. In the case of this 3 star solar system, there could be some colliding gravitational forces that create some stable orbital force that we can't even conceive of yet.

I so hope that I am alive to see the first humaned deep space mission. But I am in serious doubts that that will happen.

Sometimes I think it will take a capitalist reason to catapult our current space program into what American minds are truly capable of: that meaning someone will discover some new (and profitable) something that can only be created/built in space.

In other words, I think greed will ultimately be the impetus to true space exploration.

*goes back to watching star trek*

No, no. There's no such thing as triangular orbit - basically, orbits with "edges" are not possible, or at the very best are not stable and therefore no celestial body can have such an orbit for a long time.

The problem with the triple star system is that up to this point scientist thought planets cannot exists in such systems because their orbits would be unstable (they would either be catapulted out of the system, or would collide with one of the stars).

In other words, the more stars in the system the more gravitational interfernce they cause and the more 'chaotic' the whole system becomes.

Here's a nice animation showing the discovered planet in its system (mov file - you'll need the QuickTime plugin):
http://planetquest.jpl.nasa.gov/pq_movi ... th_web.mov

Oh I was just thinking out loud - the whole thing just blows my mind that I didn't make any sense

Even the octagon I drew was smooth at the edges. I was assuming the three stars were in the centre and the planet was orbiting around all three of them.

The animation suggests two of those stars orbit around the sun and the planet.

But now a new set of questions:

- If the gravitational pull of the centre sun is strong enough to keep in orbit, not one, but two other stars... how the hell does that planet not get completely sucked into its sun?

- How can a star have another star as its "moon"

and...

the two orbiting suns supposedly orbit at the same speed as the planet in the centre?

Well, first of all I think the animation may be a bit wrong.

The article says that the two stars are heavier than the "center" star. It would suggest that they actually don't orbit the "center" star like the animation shows, but rather the "center" star and the "twins" orbit around their center of gravity which lies somewhere in space - about half a distance between them.

The other thing is, no matter how massive a star is, if an orbiting object travels around it in a stable orbit it won't get sucked into it. The faith of plunging into the center object or orbiting around it is decided mostly by the satellite's trajectory and not the mass of the objetcs involved.



See above

Basically, if the center star was, say, ten times as massive as its 'moon' star, such system would be a lot like Earth and Moon.

Generally, two objects always orbit each other around their center of gravity, and if one of the objects has a huge mass compared to the other, the center point happens to be located "inside" the heavier object making it seem like the lighter of the two revolves it.

The fact that the celestial objects orbit around the center of their gravity is actually used to detect distant planets.

If you take our Solar System - the nine planets don't exactly orbit the Sun, but rather the Sun and the planets orbit the center of gravity (located inside the Sun). This causes the Sun to wobble. And this wobble can be detected from a great distance thus making it possible to indirectly detect planets invisible through a telescope.

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You. Rule.

All of that made complete sense.

I find this stuff incredibly fascinating. I wish I'd pay more attention to it. Alas, my chosen field of study and would-be profession is far more human science. Not that it's a bad thing, I love it, but this stuff makes me feel like my life is insignificant and meaningless -- and I love that. Gives me much freedom.

(please don't make me explain --- suffice to say, I've read too much Nietzsche)

When they discover a planet in a three-star system that has intelligent life capable of cross-mating and wiping each other out at the same time, then will I be impressed.