Over the past couple decades, astronomers have been discovering a seemingly endless variety of exoplanets orbiting other stars. Some are rather similar to planets in our own Solar System, while others are more like ones depicted in science fiction, ranging from rocky worlds about the size of Earth and larger, to massive, searing hot planets larger than Jupiter orbiting very close to their stars. Tatooine is another well-known example—the desert planet orbiting two suns in the Star Wars films. Now astronomers have found a similar world, using direct imaging, but which orbits within a system of three stars.
The discovery was made using the SPHERE instrument on ESO’s Very Large Telescope in Chile, and led by astronomers from the University of Arizona. Exoplanets orbiting in multi-star systems have been found before, but this one wasn’t expected to be possible in terms of its orbit, but there it is. It was also imaged directly, something which astronomers have only been able to do with a handful of exoplanets so far, due to the tremendous distances and brightness of their host stars.
The planet, known as HD 131399Ab, is 320 light-years from Earth and orbits the brightest of the three stars in the triple-star system. This orbit is the widest known of any planets in similar star systems, and for this reason was very surprising to astronomers. The gravitational interaction between the three stars is always changing, making larger orbits of planets inherently unstable. But that is just what was found, so scientists aren’t quite sure how to explain it just yet; it’s a planet which probably shouldn’t be there, but it is.
“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 Daniel Apai, from the University of Arizona. He is also a co-author of the new paper.
“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 Kevin Wagner, the paper’s first author and discoverer of HD 131399Ab. “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 farther 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.”
The planet itself has a mass four times that of Jupiter and a temperature of about 1,070 degrees Fahrenheit (580 degrees Celsius), which actually makes it one of the coolest exoplanets found so far. It may not be the best candidate by any means in terms of the search for life, but its very existence will help astronomers learn more about planetary formation in general. At an estimated 16 million years old, it is also one of the youngest exoplanets discovered so far. Earth and our Solar System, by contrast, are about 4.6 billion years old. The planet orbits its star at a distance of 80 AU, which is about twice the distance from our Sun to Pluto.
As noted by Wagner, “It is not clear how this planet ended up on its wide orbit in this extreme system, and we can’t say yet what this means for our broader understanding of the types of planetary systems, but it shows that there is more variety out there than many would have deemed possible,” concludes Kevin Wagner. “What we do know is that planets in multi-star systems have been studied far less often, but are potentially just as numerous as planets in single-star systems.”
“If the planet was further away from the most massive star in the system, it would be kicked out of the system,” Apai explained. “Our computer simulations showed that this type of orbit can be stable, but if you change things around just a little bit, it can become unstable very quickly.”
The brightest star in the system, HD 131399A, which the planet orbits, is also orbited by the other two stars (B and C) and is 80 percent more massive than our own Sun. The two smaller stars orbit at a distance of 300 AU, and also orbit each other, at a distance of 10 AU. One AU is equal to the distance between the Earth and Sun. Needless to say, this is a complex system, the likes of which had not been seen before now.
In 2014, an exoplanet was discovered with an Earth-like orbit in a binary star system of two red dwarf stars. The planet, OGLE-2013-BLG-0341LBb, is no more than twice the mass of Earth, with an orbit of approximately 90 million miles, or 0.9 AU. Like HD 131399Ab, the finding of the planet in a binary star system of two red dwarfs was surprising, and was also the first exoplanet discovered with an orbit very similar to Earth’s.
“Binary systems were largely ignored before,” said Dr. David Trilling, Assistant Professor of Astronomy at the Northern Arizona University’s Department of Physics and Astronomy. “They are more difficult to study, but they might be the most common sites for planet formation in our galaxy. There appears to be no bias against having planetary system formation in binary systems. There could be countless planets out there with two or more suns.” Trilling also led a research team in 2006 which discovered stable protoplanetary disks around dozens of binary stars, using data from NASA’s Spitzer Space Telescope.
Another study in 2015, published in The Astrophysical Journal, reassessed the potential habitability of exoplanets in multi-star systems. The study, led by Kimberly Cartier, a graduate student at the Pennsylvania State University’s Department of Astronomy & Astrophysics, found that some planets which were thought to lie within their stars’ habitable zones, actually were not. Other planets, however, thought to have been outside the habitable zone, were actually inside. The habitable zone is the region around a star where temperatures could allow liquid water to exist on the surfaces of rocky planets. Studying exoplanets in double and triple star systems is important, since most stars have companions, unlike our Sun, which is solitary.
The team concluded: “Habitable planets in the canonical sense must not only have the capability for liquid water on the surface, but also have a solid surface on which that water can exist. In short, the planets must be rocky and not gaseous. Using radial velocity measurements coupled with Doppler spectroscopy, high-resolution imaging, and asteroseismology, [previous studies] measured the radii and masses for 65 planet candidates and concluded that only planets with radii less than 1.5 Earth radii are compatible with purely rocky compositions. Planets larger than that must have a larger fraction of low-density material, e.g. hydrogen, helium and water. Our updated planetary radii indicate that none of our potentially habitable planets (Kepler-296Af, Kepler-296Bf, Kepler-296Be, KOI-2626 A.01, KOI-2626 B.01, and KOI-2626 C.01) are small enough to have purely rocky compositions and thus are not habitable in the canonical sense.”
The discovery of an increasing number of exoplanets in multi-star systems supports other findings that planets are very common in the universe. Some research suggests they are even more common than stars. These worlds will be as varied from one another as snowflakes, which makes finding and studying them all the more exciting. Some of them will be habitable, at least by Earthly standards. And some of those, just maybe, are indeed inhabited. Perhaps not on HD 131399Ab, but there may be lifeforms somewhere which gaze up into a sky with two or three suns—an incredible thought.
“This is the kind of discovery that helps us place our own Solar System in the context of the diversity of worlds beyond it, by finding systems that are much different from our own,” said Mary Voytek, senior scientist for astrobiology and program manager of the NExSS network at NASA Headquarters in Washington. “By combining these results with research on the formation of habitable worlds, we will have a better understanding of the systems in which habitable worlds might form. NExSS will ensure such connections are made, within and beyond our NExSS teams.”