Astronomers have discovered a planet unlike any other ever found.

The giant planet, called HD 131399Ab, loops widely around one star that is locked in a gravitational embrace with two others in a triple-star system, creating a curious celestial ballet.

The findings, published last week in the journal Science, challenge current notions about what makes a planetary system viable.

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With three stars in the system, the massive planet would experience triple sunrises and triple sunsets during one season and all daylight in another. Since the planet’s orbit is very long, each season lasts hundreds of years.

Planet HD 131399Ab loops widely around one star:

“Depending on which season you were born in, you may never know what night time is like,” lead researcher Kevin Wagner of the University of Arizona said.

HD 131399Ab is about four times bigger than Jupiter, the largest planet in our solar system, and is orbiting in a three-star system about 340 light years from Earth in the Centaurus constellation.

Scientists say HD 131399Ab orbits its parent star about twice as far as Pluto circles our sun, needing 550 years to complete a single orbit.

Young planet

Astronomers have previously discovered planets in multi-star systems, but never one that circles a parent star with such a wide berth. It also is one of the few extrasolar planets – those outside our solar system – to be directly imaged by telescope.

The planet HD 131399AB is about four times the size of Jupiter, pictured. Photo: NASA

The planet’s orbit is akin to the distance more typically seen when a star orbits another star, not when a planet orbits a star.

“This is the first planet that we’ve found with an orbit that is comparable to that of the stars,” Wagner said.

If HD 131399Ab’s orbit was a bit wider, computer simulations show it could be gravitationally elbowed out of the system by the pair of smaller stars that orbit each other and the main star, which is about 80 per cent bigger than our sun.

Though the planet is relatively young, around 16 million years old compared to the 4.5-billion-year-old Earth, it likely has had an eventful life.

Scientists suspect it may have started off in a much closer orbit around two parent stars before it was gravitationally bounced to its extreme distance.

Scientists plan additional observations to determine if the planet’s orbit is stable.

“It is not clear how this planet ended up on its wide orbit in this extreme system … but it shows there is more variety out there than many would have deemed possible,” Wagner said.

The planet was detected using the European Southern Observatory’s Very Large Telescope in northern Chile.

– AAP