Though it had long been believed that planets existed outside our solar system, astronomers did not confirm their presence until 1992, when at least two were found to be circling a pulsating radio star, or pulsar. Since then, more than 3,000 additional exoplanets have been located. Very few of these have been observed directly; instead, most are discovered when a passing planet causes a star to dim slightly or when its gravitational pull causes a star to wobble.
In 2014, with an eye toward directly observing and photographing more exoplanets, the intergovernmental European Southern Observatory installed SPHERE, a piano-size instrument, at its so-called Very Large Telescope facility in Chile’s Atacama Desert. Even with the help of SPHERE, astronomers can only directly image hot, bright planets with a wide orbit that won’t get lost in the glare from their parent stars. Last summer, a research team began a survey of about 100 stars it felt might contain direct imaging candidates. Staff astronomers in Chile took observations and electronically sent the data to Kevin Wagner, a PhD student at the University of Arizona and lead author of the recent Science paper, who said in an interview that he had the “strange and surreal experience of exploring other solar systems while sitting on my couch.”
By sheer chance, the first exoplanet Wagner found happened to inhabit a triple-star system roughly 320 light years away. Though a handful of other planets are likewise known to have three suns, this one, which the paper named HD 131399Ab, stands out for its wide orbit. Located a whopping 7.6 billion miles from its primary star, more than twice as far as the average distance from Pluto to our sun, it is purportedly just close enough to avoid being ejected from its current path. “It is in a stable orbit,” Wagner said, “unless we’re catching it at a very, very unlikely moment.” He added that they would know for sure with a few more years of observations.
Wagner and his colleagues describe HD 131399Ab as being only about 16 million years old—a baby compared to Earth, which is about 4.5 billion years old. Four times more massive than Jupiter, its atmosphere consists largely of hydrogen and helium, though water and methane have been detected as well. “In contrast to most of the directly imaged exoplanets, this planet also has a clear atmosphere, or at most a partly cloudy atmosphere,” Wagner said, pointing out that with temperatures exceeding 1,000 degrees Fahrenheit, liquid iron rain could be expected.
The planet’s orbit lasts 550 Earth years. For much of that time, the Science paper explains, all three suns are visible together, and any visitor would witness triple sunsets and sunrises (and hence experience nighttime). At another point, however, it apparently endures about 140 straight years of near-constant daylight. The planet’s primary star dominates the sky. Estimated to be nearly twice as massive as our sun, it’s easily visible from Earth with binoculars or a telescope. The other two stars, meanwhile, are further away from HD 131399Ab and much less bright. Located only as far apart as our sun and Saturn, they rapidly whirl around each other “like a spinning dumbbell,” according to the paper. In turn, all three stars orbit the system’s center of mass.
No other known exoplanet, Wagner said, has “this extreme of a configuration in a multi-star system.” Yet he acknowledged it may not actually be that unusual. “We’re really just starting to explore what type of exoplanet systems are out there,” he said. “Basically, if it can exist, it will exist, is what we seem to be finding.”