Comets Detected Outside Our Solar System for the First Time

An artist's interpretation of what one of the newly identified exocomets might look like. Danielle Futselaar

Comets are so tiny compared with planets that scientists had never managed to spot one in distant solar systems. Now that's all changed with at least two, and potentially as many as seven, different hunks of ice identified orbiting other stars.

A team of professional and amateur astronomers reported the discovery in a new paper that will be published in the Monthly Notices of the Royal Astronomical Society journal. The finding came about through the Planet Hunters project, which enlists citizen scientists to pore through data recovered by the Kepler mission. The mission has spent about four years staring at 150,000 stars around the constellations Cygnus and Lyra to watch for tiny flickers in their light.

Typically, that data has been used to spot exoplanets, planets just like those in our own solar system orbiting other stars. The mission's data has already led to the discovery of 2,335 confirmed exoplanets, with nearly 2,000 more candidates still waiting to be verified.

Scientists never actually see those exoplanets directly; instead, they spot them by looking for the small, repetitive dips in a star's brightness that are caused by the exoplanet passing between Kepler's sensors and the star.

That's the same technique the current team used to spot the exocomets—but comets are much, much smaller than planets. The astronomers also know these brightness dips were caused by comets because they could see their characteristic tails made up of gas and dust. Where exoplanet dips are smooth and symmetrical, these new exocomet dips started abruptly, then gradually let the star return to its normal brightness.

Planet Hunters participant Thomas Jacobs had spent five months combing through more than 200,000 images produced by Kepler data when he spotted three of these unusual dips in data from a star known as KIC 3542116, which is about 800 light-years away.

Because they knew how quickly comets travel, the team was able to estimate that the comets were orbiting fairly close to the star. One comet may be responsible for three separately observed brightness dips in the star, but the team isn't sure yet if that's the case or if three separate comets created the pattern. A second pattern of three smaller dips poses the same puzzle, so the team isn't yet sure if they've spotted two exocomets, or six, or something in between. The team estimated that the comets responsible would be about the same size as Halley's Comet, which is about four miles across.

And as if the first reported exocomets weren't impressive enough, the team also threw in observations of another possible exocomet, orbiting a star called KIC 11084727.