Astronomers detect comets outside the Milky Way

The discovery could offer new insights into ways solar systems evolve.

By Brooks Hays
An artist's rendering shows a trio of exocomets circling a faint star. Photo by Danielle Futselaar/MIT News
An artist's rendering shows a trio of exocomets circling a faint star. Photo by Danielle Futselaar/MIT News

Oct. 27 (UPI) -- An international team of researchers has identified six extra-galactic comets, or exocomets, outside the Milky Way. The discovery was assisted by amateur astronomers.

The distant ice balls, roughly the size of Halley's Comet, were spotted orbiting a small star 800 light-years from Earth. They were documented using transit photometry -- the first time the technique has been used to identify comets.


Transit photometry describes the measurement of light dimming as objects pass across the face of a luminous object, like a star. The dimming can reveal the presence of planets and other orbital objects.

In recent years, the improved precision of astronomical instruments has allowed scientists to measure even the slightest dimming of distant stars. The six comets -- detailed this week in the journal Monthly Notices of the Royal Astronomical Society -- were revealed by their tails, which blocked out just 1 percent of their host star's light.


"It's amazing that something several orders of magnitude smaller than the Earth can be detected just by the fact that it's emitting a lot of debris," Saul Rappaport, professor emeritus of physics at MIT's Kavli Institute for Astrophysics and Space Research, told MIT News. "It's pretty impressive to be able to see something so small, so far away."

The data that revealed the light-dimming caused by the tails of distant exocomets was recorded by NASA's Kepler Space Telescope. The unusual dimming was first identified by an amateur astronomer, Thomas Jacobs, who enjoys surveying Kepler data for irregularities in his spare time.

"Looking for objects of interest in the Kepler data requires patience, persistence, and perseverance," Jacobs told MIT. "For me it is a form of treasure hunting, knowing that there is an interesting event waiting to be discovered. It is all about exploration and being on the hunt where few have traveled before."

Scientists have designed computer algorithms to parse transit data and pick out patterns caused by passing planets, but Jacobs set out to find dimming signatures missed by the computers -- specifically, one-time, or single, transits, dimming that does not repeat with regularity.

Jacobs found three such transits and alerted scientists, including researchers at MIT. When they looked closer, they found six, and the light signatures they discovered surprised them.


Instead of a U-shaped dimming pattern caused by a planet passing across the face of the star, scientists found sharp dips followed by a gradual rise. The signatures suggested the presence of disintegrating planets with tails of debris.

But disintegrating planets would create a repeating dimming pattern. Instead, they found isolated signatures.

"We thought, the only kind of body that could do the same thing and not repeat is one that probably gets destroyed in the end," Rappaport said. "The only thing that fits the bill, and has a small enough mass to get destroyed, is a comet."

The discovery could offer new insights into ways solar systems evolve. Scientists believe water and other ingredients essential to life were delivered by comets early in Earth's history. The discovery of six comets passing close to their host star in the span of four years suggest bombardment eras may be common in young solar systems.

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