A day in exoplanet HD 131399Ab is unlike any other.

This strange and unique exoplanet, which was detected just 340 light-years away from Earth in the constellation Centaurus, enjoys three sunsets and sunrises each day, based on the season.

How did that happen? According to Kevin Wagner of the University of Arizona, the exoplanet HD 131399Ab orbits three suns for 550 Earth years. It's an incredible cosmic phenomenon.

Young Exoplanet

In recent years, astronomers have detected several exoplanets that, like the fictional planet Tatooine, actually orbit around two suns. These exoplanets are known as circumbinary planets.

But researchers from the University of Arizona did not find two suns, they discovered three. And these suns are the parent stars of a relatively young exoplanet.

Known as HD 131399Ab, the three-sun planet is considered young by astronomical standards as it is only 16 million years old.

The exoplanet weighs at about four times the mass of the planet Jupiter, and its temperature is about 580 degrees Celsius (1,070 degrees Fahrenheit).

At this temperature, HD 131399Ab is one of the least massive and coldest directly imaged exoplanets, researchers said.

Three Sunsets, Three Sunrises

All of HD 1313199Ab's three parent stars are visible in the planet's sky. The fainter two appear closer to each other and change in visual separation from the brightest star throughout one year.

For much of the year, all three stars appear close together. This gives the planet a familiar night-side and day-side, but with a twist: the planet experiences triple sunsets and triple sunrises every day.

Wagner says that as the exoplanet moves around its suns, the trio of stars grows farther apart each day. Because of this, the suns can reach a point where one sunset coincides with the sunrise of the other.

At this point, HD 131399Ab is in near-constant daytime for one-quarter of its orbit around its suns, which is equivalent to 140 years on Earth.

This means that the seasons on the exoplanet can last longer than the lifetime of a human being. This also means that the planet faces extreme conditions every day.

Why Studying Multi-Star Systems Is Important

Planets such as HD 131399Ab are of special interest to planetary scientists and astronomers because they offer an example of how planet formation works in these extreme scenarios.

While multi-star systems seem exotic compared to our solitary star system, these multi-star systems are actually just as common as single star-systems.

Wagner says it is not yet clear how HD 131399Ab ended up on its current orbit in an extreme system. He also says that they cannot conclude what the existence of the three-sun planet means for a broader understanding of planetary systems.

However, Wagner says that the findings do confirm that there is much more variety out there than what we currently know.

He adds that what they are certain of is that multi-star system planets are much less explored, but they are possibly just as numerous as single-star systems.

Details of the study are published in the journal Science.

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