Three Star Ballet, With Exoplanet

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This artist's impression shows a view of the triple-star system HD 131399 from close to the giant planet orbiting in the system. The planet is known as HD 131399Ab and appears at the lower-left of the picture. Located about 320 light-years from Earth in the constellation of Centaurus (The Centaur), HD 131399Ab is about 16 million years old, making it also one of the youngest exoplanets discovered to date, and one of very few directly imaged planets. With a temperature of around 580 degrees Celsius and an estimated mass of four Jupiter masses, it is also one of the coldest and least massive directly-imaged exoplanets.
An artist’s impression of the triple-star system HD 131399 from close to the giant planet orbiting in the system. The planet is known as HD 131399Ab and appears at the lower-left of the picture. Located about 320 light-years from Earth in the constellation of Centaurus (The Centaur), HD 131399Ab is about 16 million years old, making it also one of the youngest exoplanets discovered to date, and one of very few directly imaged planets. (ESO/Luis Calcada)

It hardly seems possible, but researchers have detected a planet in apparently stable orbit within a three star system — a configuration now known as a trinary.

The ubiquity of binary stars has been understood for some time, and the presence of exoplanets orbiting around and within them is no longer a surprise.  But this newest planet detected — four times the mass of Jupiter — is most unusual because trinary systems are not known to be particularly conducive to keeping planets in orbit, and especially not a planet in an extremely wide (i.e., 550 year) orbit.

Yet this planet has found the sweet spot between the stars where it balances the gravitational pulls of the three.  The system is a relative toddler at 16 million years old, and so the researchers involved in its detection say it may later be ejected from the system.  But for now, it is the only known planet of its kind.

The discovery, reported in the journal Science, was made using the European Southern Observatory’s Very Large Telescope (VLT) in Chile’s Atacama desert.  The team was from the University of Arizona in Tucson and was led by Daniel Apai, an assistant professor of Astronomy and Planetary Sciences who leads a planet finding and observing group.  That team includes research doctoral student Kevin Wagner, the first author on the paper.

“It is not clear how this planet ended up on its wide orbit in this extreme system — and we can’t say yet what this means for our broader understanding of the types of planetary systems — but it shows that there is more variety out there than many would have deemed possible,” Wagner said.

This new planet is a gas giant and definitely not habitable, but the possible universe of exoplanets that just might meet some of the basic criteria for habitability may well have grown.

“What we do know is that planets in multi-star systems have been studied far less often, but are potentially just as numerous as planets in single-star systems,” Wagner said.

An artist's rendering of the Kepler-16 system, showing the binary star being orbited by Kepler-16b
An artist’s rendering of the Kepler-16 system, showing the binary star being orbited by Kepler-16b.  The planet was the first confirmed to have a circumbinary 0rbit — traveling around both stars in 229 days.  (NASA)

Astronomers estimate that about half of the stars we see in the sky are actually two stars or more.  Stars are created when massive clouds of gas and dust collapse in on themselves, and sometimes that results in a fragmenting into multiple stars.

Two of the stars in the HD 131399 system are close together, twirling around each other like a spinning dumbbell.  The third and far brightest star, which is located about 300 times further away than the distance between Earth and the sun,  is orbited by the new gas giant planet HD 131399Ab. With a temperature of around 580 degrees Celsius and an estimated mass of four Jupiter masses, it is one of the coldest and least massive directly-imaged exoplanets.

The three-star system and the planet were found using direct imagining of thermal emissions, as opposed to the traditional techniques of searching for the effects of an exoplanet on the host sun and other planets.

This planet discovery was a first for SPHERE, the Spectro-Polarimetric High-Contrast Exoplanet Research Instrument, which took a decade to build.  The instrument is sensitive to infrared light and is capable of detecting the heat signatures of young planets picked up by the Very Large Telescope’s mirrors.  SPHERE has a coronograph to block out the otherwise blinding light of the host star, and new capabilities to  correct for disturbances caused by features of our atmosphere.

Apai said that detecting a planet in such a triple system was both surprising and “really cool.”

“We’re now are going back and take a careful look at all the other triple systems that haven’t been observed because we didn’t think planets could be there. I’m very curious to continue to study this system to figure out whether the planet formed in that odd orbit or if it moved there after encountering another planet or one of the double stars.”

Here is a wonderful video animation of the choreography of the stars and planet, the work of the ESO’s Luis Calçada and Martin Kornmesser:

As is so often the case, the discovery involved substantial serendipity;  it was not at all what the group was looking for.

Rather, Apai’s team has been looking to prove, or disprove, a confounding pattern in exoplanet discoveries:  most of the (very few) planets discovered via direct imaging were found around stars with masses about twice of the sun’s mass, while only one planet has been found around a sun-like star.  Most were also short orbital period planets, unlike the one with a 550-year orbit just discovered. If this pattern of detections were found to be a feature of the galaxy as a whole, it would challenge some components of the basic planet formation model.

“Strangely, most of the eight or so planets discovered via direct imaging were found around stars with masses about twice of the Sun’s mass, while – up to now – around Sun-like stars only one Jovian planet was discovered,” he said in an email.

So the group set out to study about 100 stars more massive than the Sun to determine how many of them have giant planets that can be imaged — with the goal of seeing whether they are really more common than around Sun-like stars, or if it was just a coincidence.

While that campaign will continue for another year finding the three star exoplanet may well become their most significant finding.

Wagner, who first saw the presence of the exoplanet in the SPHERE data, confirmed the finding during observations a half year later.

“For much of the planet’s year the stars appear close together, giving it a familiar night-side and day-side with a unique triple-sunset and sunrise each day,” he said. “As the planet orbits and the stars grow further apart each day, they reach a point where the setting of one coincides with the rising of the other – at which point the planet is in near-constant daytime for about one-quarter of its orbit, or roughly 140 Earth-years.”

The precise orbit of the planet has actually not yet been determined, and doing so is one of Wagner’s next tasks.  The team knows that the planet is 80 times further from the A star than the Earth is from our sun, and that the A and BC stars are 300 times further away from each other. Asked if that orbit could change dramatically, even becoming circumtrinary around all three stars, he said it was quite unlikely but that it couldn’t be ruled out.

Wagner used an apt quote to describe his reaction to the discovery, one attributed to many great astrophysicists but originally probably from Arthur Stanley Eddington,  who helped confirm Einstein’s General Theory of Relativity in the early 1900s.

“Not only is the universe stranger than we imagine,” he said, “it is stranger than we can imagine.”

This annotated composite image shows the newly discovered exoplanet HD 131399Ab in the triple-star system HD 131399. The image of the planet was obtained with the SPHERE imager on the ESO Very Large Telescope in Chile. This is the first exoplanet to be discovered by SPHERE and one of very few directly-imaged planets. With a temperature of around 580 degrees Celsius and an estimated mass of four Jupiter masses, it is also one of the coldest and least massive directly-imaged exoplanets. This picture was created from two separate SPHERE observations: one to image the three stars and one to detect the faint planet. The planet appears vastly brighter in this image than in would in reality in comparison to the stars.
This picture was created from two separate SPHERE observations: one to image the three stars and one to detect the faint planet. The planet appears vastly brighter in this image than in would in reality in comparison to the stars. (ESO)
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