Astronomers on the hunt for planets with real star quality have found themselves a triple threat: On Thursday, a paper in Science announced the discovery of planet with three suns.
Wait, you say, I've heard this one before. And you have! This isn't the first triple-star system found to contain exoplanets. But the gas giant HD 131399Ab does have a few stand-out qualities, even among the class of worlds that put Tatooine to shame: For starters, it's one of a few exoplanets that have been directly imaged — as opposed to being inferred based on the flickering light of its sun(s) as it orbits. It's also on an orbit so far from its host stars that scientists were surprised it hadn't been flung out into open space.
Oh, and then there are its insane seasons — ones that make the mysterious "winter" on "Game of Thrones" seem like a cakewalk: They last longer than human lifetimes and feature either triple sunrises and sunsets or constant sunlight.
In this case, direct imaging doesn't mean scientists snapped a high-res photo but, rather, that they were able to collect data straight from the world itself instead of relying on the analysis of changes in starlight, as is the case for most exoplanet detections.
It's the first exoplanet detected using SPHERE, short for Spectro-Polarimetric High-Contrast Exoplanet Research Instrument. SPHERE, which is part of the European Southern Observatory's Very Large Telescope, is tuned to infrared light in order to seek out the heat signatures of young planets. It's designed to filter out the bright light of a host star (or three), allowing scientists to directly detect these infant worlds.
At four times the mass of Jupiter and 580 degrees Celsius, HD 131399Ab sounds pretty extreme. But it's actually one of the smallest, coolest exoplanets ever imaged directly. It's also one of the youngest planets detected, period — it's just 16 million years old. For reference, scientists have found 100 million-year-old bees. Bees! This planet is, like, 84 million years younger than bees.
"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," study author Dániel Apai of the University of Arizona said in a statement.
Apai and his colleagues aren't positive about the planet's orbital trajectory, but they think it goes something like this: At the center of the system sits the biggest star, one 80 percent more massive than our own sun dubbed HD 131399A. The other two stars, B and C, orbit the larger one, sitting about 300 times the distance between Earth and our sun. B and C are separated from one another by a distance about equal to that between our sun and Saturn. They twirl as if connected around a common point, like a spinning dumbbell, orbiting HD 131399A in tandem. Here's what that looks like:
As a result of this complex configuration, the Jupiter-like world doesn't orbit all three stars; it orbits only HD 131399A. At times, it sits on one side of A, with B and C twirling around on the other. But it sometimes passes in between A and the dance of the smaller pair.
That makes for pretty crazy seasons.
"For about half of the planet's orbit, which lasts 550 Earth-years, three stars are visible in the sky, the fainter two always much closer together, and changing in apparent separation from the brightest star throughout the year," Kevin Wagner, a first-year doctoral student in Apai's research group and the paper's first author, who discovered HD 131399Ab, said in a statement. "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. 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."
Those seasons are so long because the planet's orbit is long: It takes about 500 Earth years for this little world to orbit its host star once. According to the researchers' computer models, that orbit strains the limits of stability. If it were much longer, the sun would lose its gravity grip on the planet and lose it to interstellar space. The researchers are fairly certain the planet couldn't have formed so far from its host, because planet-making materials tend to stay closer to the center of the system. It's possible that HD 131399Ab was pushed out into a long orbital period by some bigger, badder planet that has yet to be detected, the researchers report. The planet could also have formed around the binary system of stars B and C and then been pulled into an orbit around their larger companion star. It could even have formed around any one of the stars before the three moved in unison, then been jostled around by the stellar interactions that made them a family.
However it came to be, that far-flung orbit makes the world a perfect candidate for direct imaging methods such as SPHERE: More traditional exoplanet-hunting tools such as Kepler get information about a planet only when it passes in front of its host star from Earth's perspective (which we call a transit). If a planet takes 500 years to orbit its star, we're not getting more than one transit every half a millennium.
Multi-star systems are pretty common — probably as common as those with single stars — so scientists are always intrigued by the unique ways these alien neighborhoods come together, weird orbits included. Understanding the many environments in which an exoplanet can form could help researchers better understand the probability of rocky, Earth-like world that fall in the habitable zones of their stars — ones where life as we know it could evolve.
The researchers hope to get a closer look at young planets like HD 131399Ab when the James Webb Space Telescope launches in 2018.
Have more questions about this weird little world? Some of the researchers who discovered it will be hosting a Reddit Ask Me Anything on Thursday from 5 p.m. Eastern until 7 p.m.