Exoplanets dubbed "hot Jupiters" — large gaseous planets like Jupiter in our own solar system orbiting closely around their parent stars — have long intrigued astronomers, but now, they have a new puzzle to figure out: one distant system actually has two of these mysterious kinds of worlds.

A team of astronomers from the University of Michigan collaborating with colleagues from the Massachusetts Institute of Technology have found a distant star system that has not just one but a pair of close-in planetary companions, a finding that may reveal new insights into planetary formation and migration.

The mystery is how these large, hot planets ever managed to end up so close to their sun, researchers say.

While around 300 hot Jupiters have been identified over the last 20 years, none have ever been found before orbiting so closely to their star, they point out.

"This is really exciting," says Juliette Becker, a graduate student in the U-M Astronomy Department. "People have looked for these planets and have looked in data that exists for hot Jupiters for years and nothing has come up."

There was a general assumption it was not possible for hot Jupiters to be close-in planetary companions, says Becker, lead author of a study highlighting the discovery.

Large gaseous planets are thought to form in frigid temperatures far from their stars, as in the case of our solar system's gas giant planets beyond the asteroid belt.

However, hot Jupiters in distant systems have been found orbiting their stars at closer distances than Mercury orbits our sun.

"The whole theory of planet formation and migration is not totally understood," Becker says. "Even today there is a lot of active work being done to figure out how Jupiter got where it was. So anything we can discover on how hot Jupiters migrate is useful in understanding planet formation and migration as a whole."

The possibility of companion planets in the hot Jupiter system known as WASP-47 was first noticed in data from the Kepler space telescope by a citizen scientist named Hans Schwengeler, who posted his finding in the public Planet Hunters forum.

That came to the attention of the U-M researchers, who, along with MIT scientists, analyzed the Kepler data and confirmed the presence of not just one but two orbiting close-in planets, a hot gaseous one the size of Neptune and an even closer-in super-Earth sized companion.

"This is really exciting," Becker says.