Images of HD 131399Ab, the newly described exoplanet, in orbit around its star. These images were created using ESO’s Very Large Telescope (VLT) in Chile. Credit: Wagner et al (2016)/AAAS.
To hear astronomer Kevin Wagner describe it, you might be shocked to find his latest study in the prestigious pages of the journal, Science. His study, published Friday, documents the direct imaging of an exoplanet in a solar system with three stars—something that is not new to science. “It’s not the first one that’s [been detected] in a triple system,” Wagner explained, “It’s not even the first directly imaged planet that’s in a triple system.”
Though Wagner’s quadruple-Jupiter-sized exoplanet may not be racking up points in terms of firsts, its newsworthiness derives from the absolutely unique solar system it inhabits.
In every other known binary or multiple star system, Wagner told NOW.SPACE, the exoplanet or exoplanets orbit so close to one of the stars that, from an orbital mechanics standpoint, they might as well exist in a system with just one star. But the planet Wagner and his colleagues imaged has a really wide orbit, making it susceptible to the gravitational influence of the other two stars, leaving big questions about how the planet settled into this orbit and how long it will stay there.
This exotic solar system, unassumingly named HD 131399, lies about 320 light-years away in the constellation of Centaurus. Imagine a system with three suns—two of which are really close to each other and orbiting rapidly around one another. The third star, which is the one that the newly imaged exoplanet orbits, itself orbits the duo of stars from a distance at a relatively slower pace, like this:
Computer simulation of HD131399. Credit: ESO/L. Calçada/M. Kornmesser
It’s not that systems with multiple stars like this are rare. In fact, Wagner says, binary and multi-star systems are just as numerous as single star systems. Instead, Wagner’s imaging of the exoplanet challenges the long-standing assumption in the exoplanet world that planets in multi-star systems which stretch far away from their host star would be disrupted and ejected from the system or never form in the first place.
To that end, Wagner and his colleagues are interested not only in finding planets but also in finding them in places that are very different than where we typically search. He is part of a research group applying similar techniques to a range of multi-star systems—HD 131399 is the first discovery to be reported.
The discovery, Wagner said, “shows that these wide-orbit planets and planets in extreme configurations with multiple stars are possible outcomes of the planet formation and migration process,” something that challenges prevailing logic.
You don’t need crazy, non-Keplerian physics to be in awe of this Jovian iconoclast.
“In the past, people kind of myopically focused on single star systems because that’s what we’re used to with thinking about the Sun,” said Justin Crepp, an expert in exoplanet detection who recently published a paper about an exoplanet found in a triple star system. He argues that these kinds of solar systems are probably much less rare than we think. “Now we’re starting to finally branch out, and this University of Arizona team is systematically searching them, and lo and behold they found something right away.”
A stable orbit, like the ones the planets of our solar system enjoy, is one that is locked into place for perpetuity, unbothered by the influence of other planets. Our uncertainty about Wagner’s exoplanet’s orbit results from a lack of data. “We have an estimate for it, but we don’t know what it is exactly, and because of that, we don’t really know how stable it is yet.” That estimate puts it on an elliptical orbit that takes around 500 years to complete. All Wagner and his colleagues know for certain is that a stable orbit is possible based on the data they have now.
The team modeled a variety of other possibilities, too. The most exciting one, in Wagner’s opinion, was that the planet could theoretically be captured into an orbit around the other two stars before being flung out into the dark, lonely emptiness of interstellar space. It could also, again theoretically, be flung off into space without being captured by the other stars at all.
But the likelihood of something that dramatic happening is fairly low. That’s because this solar system is only about 16 million years old—super young by Earth standards—but based on what we know about how solar systems form, it’s a bit too old for a planet in an unstable orbit to be sticking around, Wagner said.
Still, you don’t need crazy, non-Keplerian physics to be in awe of this Jovian iconoclast. If somehow you found yourself on its surface, you’d be struck with some pretty wild differences compared to life in our own solar system. The length of the year (which is longer than many human lifetimes) might be different each trip around its main sun, and, depending on the season, you might experience constant daylight or enjoy triple sunrises and sunsets each day.
Wagner and his colleagues sought to find out what happens to planets that form in solar systems with multiple stars. By documenting the existence of a 16 million-year-old planet in this complex triple star system, they have found that some seemingly improbable orbits can form, and potentially persist.
“Basically, what we’re gathering,” Crepp said, “is if you can dream up a scenario for an exoplanet existing it probably has happened.”