Astronomers Detect a Swarm of Tiny Objects Orbiting an Alien Sun

September 16, 2018

There are tiny comets orbiting foreign suns. Humans can detect them.

Six times, about 800 years ago, dark things passed between the bright yellow dwarf star KIC 3542116 and Earth. They are very small in the universe, about 330 billion tons (300 billion metric tons). That’s about the same size as Halley’s Comet, or just one 245 millionth of the mass of Earth’s moon.

But they’re big enough. They blocked a small fraction of the light streaming outward from that star. Eight hundred years later, the sensitive lens of the Kepler Space Telescope – a piece of precision-cut glass nearly a meter wide floating in the darkness of space – detected the kind of dimness that KIC 3542116’s ancient light had when it reached this solar system.

Between 2009 and 2013, the star seemed to dim quickly, though almost imperceptibly, as the dark little thing passed in front of it six times (from Earth’s perspective). During those four years, it dimmed deeply three times and faintly three times, at irregular intervals.

It’s a familiar signal to astronomers, and that same dimness has led them to most of the 3,728 exoplanets discovered as of Feb. 2. But these little dark things only behaved like asteroids at the beginning of their trek. It took about a day for the stars to slowly regain their brightness as they continued through the planes of the stars.

In Kepler’s view, exoplanets (basically great symmetrical spheres) are not like this. But this is how a comet, with a long dusty tail, would have appeared. In fact, back in 1999, a team of astronomers predicted what such a comet passerby would look like.

In a study to be published Feb. 21 in the journal Monthly Notices of the Royal Astronomical Society (and first published on arXiv in 2017), a team of researchers reported that the dark objects were the first-ever discovery of an “exocomet,” or a comet from another star system.

The team wrote that they weren’t sure exactly how many comets there were, casting shadows on Kepler’s lens during that time. It could be six individuals, each making a close pass of its star and showing up in Kepler’s data. Or it could be a smaller cluster of stars, with some comets making multiple passes.

Perhaps only one comet orbited its star very closely, they suggested – though they couldn’t quite figure out the orbit of one comet, resulting in six irregular time shadows.

It took the astronomers more than five months of hunting through more than 50 Kepler images of 2012 to find the six shadows, and in that time they found only one other comet shadow that might have crossed another star.KIC 11084727, also a yellow dwarf, is faintly dimmed once, as is KIC 3542116, where the six shadows were found.

The two stars are “near twins,” the astronomers wrote. Both stars are very bright and similar in size and magnitude. And they are somewhat unusual in the Kepler dataset, which tends to target “cooler, sun-like stars,” they write. Perhaps, they suggest, comets (or at least comet transits visible from Earth) are more common around this type of star.

Regardless of where more comets might be found in the future, these comets are the smallest objects that humans have detected in alien solar systems. Previously, the authors write, the smallest thing ever found to pass in front of a star was Kepler-37b. That tiny exoplanet is only 2,400 miles (3,860 kilometers) wide, or a little larger than Earth’s moon.