Bizarre Alien World With 3 Stars Directly Imaged

An exoplanet has been found orbiting far from its star in a triple star system, prompting questions as to why it hasn't been sling-shotted into oblivion.

An exoplanet in a triple star system has been directly imaged by the world's most advanced visible light observatory. Though this feat is impressive, the discovery has sent orbital dynamics experts into a spin.

Exoplanets have been discovered orbiting stars in multi-star systems before, proving that the science fiction star system of Tatoo (Luke Skywalker's home system in "Star Wars: A New Hope") is actually a fairly common occurrence in our galaxy. But what is weird about HD 131399Ab, it has a really wide -- and apparently stable -- orbit about one of the stars, making astronomers wonder how it formed and how the heck it wasn't sling-shotted into oblivion eons ago.

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The star system of HD 131399, located 320 light-years away, is the epitome of extreme. It has a binary pair of smaller stars HD 131399B and C orbiting one another with a tight separation of only 10 AU -- the approximate distance between our sun and Saturn. The HD 131399BC binary then orbits a larger and brighter star called HD 131399A at a distance of around 300 AU -- around eight times the sun-Pluto distance. It is about HD 131399A that the directly-imaged exoplanet (HD 131399Ab) orbits at a distance of around 80 AU (twice the sun-Pluto distance).

The gas giant exoplanet has an estimated mass of four Jupiter masses and a temperature of around 580 degrees Celsius (1080 degrees Fahrenheit), allowing astronomers to put an estimate on its age -- approximately 16 million years old. This makes it the smallest, coolest and youngest directly-imaged exoplanet found to date.

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Usually, exoplanets are discovered as they pass in front of their star, creating a slight dip in starlight. This is known as the "transit method" and is used by NASA's Kepler space telescope to detect surprisingly tiny worlds. Another technique, known as the "radial velocity method," is commonly employed by ground-based observatories to detect the very slight gravitational tug of massive planets orbiting distant stars, causing them to wobble.

However, discovering exoplanets using direct imaging is a tougher undertaking as it requires extremely precise imaging techniques, the most powerful telescopes and the exoplanets themselves need to be far enough away from their parent stars to avoid the glare of starlight drowning out the faint reflected light. This is why HD 131399Ab is so special; it joins the ranks of only a handful of these worlds that orbit far enough away from their star, but are big enough (and bright enough) to be seen from afar.

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"HD 131399Ab is one of the few exoplanets that have been directly imaged, and it's the first one in such an interesting dynamical configuration," said astronomer Daniel Apai, of the University of Arizona, in a statement.

"For about half of the planet's orbit, which lasts 550 Earth-years, three stars are visible in the sky; the fainter two are always much closer together, and change in apparent separation from the brightest star throughout the year," added Ph.D. student Kevin Wagner, also of the University of Arizona and discoverer of HD 131399Ab.

On an extra-special note, the direct imaging of HD 131399Ab is the first exoplanetary discovery for the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument that is attached to the ESO's Very Large Telescope (VLT) located in Chile.

Now it has been found, more observations are needed to understand the exoplanet's orbital trajectory.

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The exoplanet's wide orbit around HD 131399A takes it a third of the way to the orbit of the HD 131399BC binary. The constant gravitational turbulence caused by this 3-star configuration should be enough to knock the exoplanet off-kilter. And yet there it is, apparently stable and merrily going about its strange orbit.

"If the planet was further away from the most massive star in the system, it would be kicked out of the system," said Apai. "Our computer simulations have shown that this type of orbit can be stable, but if you change things around just a little bit, it can become unstable very quickly."

This discovery serves to remind us that multi-star planetary systems may be a common occurrence in our galaxy; just because there's more stars orbiting one another, it doesn't necessarily mean the gravitational dynamics will make planetary formation and orbital stability impossible -- it simply adds a whole new layer of exoplanetary possibilities.

This research has been published in the journal Science.

Source: ESO