Data collected by the Kepler space observatory has led to the discovery of ten previously unknown comets orbiting other stars. Some of these comets were found in mature star systems, while one was detected near a red giant. Until recently, it was believed that cometary activity fades over time.

How Exocomets Are Discovered

The first exocomets were detected back in 1987. Spotting comets as they pass in front of their parent star is actually easier than finding exoplanets: a comet leaves behind a tail that can be wider than the star itself. Even though the material is sparse, modern instruments are sensitive enough to detect these traces.

To date, several dozen exocomets have been identified. Most are found by analyzing the unique features in the star’s light spectrum: a comet’s tail absorbs and re-emits light in a distinctive way. Additionally, the cloud of material from a comet creates a characteristic dimming of the star during transit, after which the star’s brightness gradually returns to normal.

New Discoveries with Artificial Intelligence

While analyzing Kepler data—which from 2009 to 2018 was focused on searching for exoplanets—researchers used a neural network. Artificial intelligence processed observations of more than 201,000 stars and identified 56,000 potential exocomet candidates. After further filtering, 1,349 likely exocomet transit events remained, which scientists then reviewed manually. This resulted in a list of the 17 most probable events.

Of these, ten were previously unknown, while the rest had already been identified as exocomets. Five known exocomet transits were observed near the same star—a yellow-white dwarf named KIC 3542116, located 738 light-years from Earth. Comets in this system have been observed since 2018. If so many comets have passed in front of the star over just a few years of observation, it raises questions about the possible total number of such objects in the system, especially considering the diversity of their orbits.

Exocomets in Young and Mature Systems

The star KIC 3542116 is considered young. Most previously discovered exocomets have been found in young systems, which matched astronomers’ expectations: many comets in the inner part of a system are expected only in the early stages of its evolution. In our Solar System, hundreds of short-period and dozens of long-period comets are known, but compared to the total number of celestial bodies, this is relatively few. The Sun is 4.6 billion years old.

The discovery of exocomets around mature stars came as a surprise to astronomers: some of these stars are estimated to be over a billion years old. One example is a red giant—a star that has completed the main stage of its evolution. Now, it is eight times larger than the Sun, though in its youth it was only slightly bigger in size and mass. Such stars can live for billions of years. These findings suggest that the behavior of comets in different stellar systems is still not fully understood.