Hundreds of ‘Exocomets’ Around Nearby Star Come in Two Flavors

By Charles Q. Choi | October 22, 2014

The best look yet at nearly 500 comets around a distant star reveals these alien snowballs come in very different flavors, just like comets around the sun, researchers say.

The discovery of these distant “exocomets” could shed light on what the solar system was like in its tumultuous youth, scientists added.

In the past 20 years or so, researchers have confirmed the existence of more than 1,700 exoplanets, or worlds circling distant stars. Recently, scientists have begun detecting alien comets as well. Comets are icy bodies that give birth to giant tails of gas and dust when they approach stars. [Alien Comets of Beta Pictoris Explained (Infographic)]

This artist’s impression shows exocomets orbiting the star Beta Pictoris. Credit: ESO/L.

Astronomers investigated Beta Pictoris, a relatively young star located about 63 light-years from the sun. At only about 20 million years of age — far younger than the 4.6-billion-year-old sun — Beta Pictoris is surrounded by a huge protoplanetary disk. A disk of gas and dust just like this one encircled the sun in its youth.

For nearly 30 years, scientists have detected subtle changes in the light from Beta Pictoris that researchers thought were caused by exocomets traveling in front of the star. This is because the gas and dust from comets can absorb some of the light passing through them.

To study these exocomets around Beta Pictoris, the research team examined more than 1,000 observations gathered between 2003 and 2011 with the HARPS (High Accuracy Radial velocity Planet Searcher) instrument on the European Southern Observatory’s 3.6-meter (11.8 feet) telescope at the La Silla Observatory in Chile. They analyzed the orbits of 493 different exocomets.

This composite image shows the environment close to star Beta Pictoris as seen in near infrared light. Image released Nov. 21 2008 Credit: ESO/A.-M.

The scientists discovered two very different families of exocomets around Beta Pictoris. One is apparently old and under the gravitational sway of a giant planet in the system, while the other is apparently young and the result of a recent breakup of far larger objects. Different families of comets also exist in the solar system.

The first group of exocomets generated relatively low amounts of gas and dust, suggesting they’re old enough to have mostly exhausted their supplies of ice during multiple passages close to their star. The shape and orientation of their orbits vary, but a giant planet seems to gravitationally influence all of them. This planet, Beta Pictoris b, orbits about 620 million miles (1 billion kilometers) from Beta Pictoris and was one of the first exoplanets that astronomers photographed directly.

The second family of exocomets generates far more gas and dust. These comets are also on nearly identical orbits to one another, suggesting they all have the same origin, probably as remnants of one or more larger icy objects that might have grazed the star Beta Pictoris. This makes the comets similar to the fragments of Comet Shoemaker-Levy 9, which collided spectacularly with Jupiter in 1994.

“Our study confirms that the evaporating bodies around Beta Pictoris are analogous to the comets in the [Earth’s] solar system,”study lead author Flavien Kiefer, an astronomer at the Institute of Astrophysics of Paris in France and of Tel Aviv University in Israel,told Space.com.

“They are icy planetesimals, most probably the remnants of the processes which formed the planet Beta Pictoris b,” Kiefer added. “Now they are pushed towards the star, maybe by Beta Pictoris b itself, and become star-grazing evaporating bodies, or comets. This is very similar to the action of Jupiter on the Kuiper Belt objects.”

This composite shows the reflected light on the dust disc in the outer part of the surroundings of star Beta Pictoris, with the observations of the planet Beta Pictoris b, obtained in 2003 and autumn 2009 in the center. Image released Oct. 22, 2014. Credit: ESO/A.-M. Lagrange

The Kuiper Belt is the ring of frigid objects beyond the orbit of Neptune. Pluto is the most famous of these faraway bodies.

The scientists said they expect to see similar cometary behavior in most, if not all, distant planetary systems younger than about 100 million years of age — when they can get a good look at the protoplanetary disks, Kiefer said.

“Our work shows that processes known to take place in the solar system also take place in young planetary systems,” Kiefer said. “This reinforces the feeling that when looking to the Beta Pictoris and its environment, we are observing a somewhat younger version of our sun, when it was 10 million to 20 million years old and it just formed its planets.”

The scientists plan to continue collecting data on Beta Pictoris using HARPS, which should shed light on how Beta Pictoris b influences cometary orbits, among other effects. The researchers detailed their findings in the Oct. 23 issue of the journal Nature.

Info-graphic:

At only 23 million years old, the Beta Pictoris star system is very young compared to Earth’s solar system. Beta Pictoris is still undergoing the initial condensation of its planets from smaller bodies, a process called accretion. The system is 63.4 light-years away from Earth in the constellation Pictors, the Painter’s Easel.

Source: Space.com