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article imageTwo families of comets found orbiting nearby star

By Robert Myles     Oct 22, 2014 in Science
Paris - Such has been the huge advances in the field of space telescopes in recent years that astronomers are now turning their attention to comets in orbit around stars other than our Sun.
A case in point is the most complete census yet of comets around a nearby star thanks to the European Southern Observatory’s (ESO) HARPS instrument at ESO’s La Silla Observatory in Chile.
A French team of astronomers studied almost 500 individual comets orbiting the star Beta Pictoris, a relatively young star located about 63 light-years from the Sun.
Beta Pictoris is a youthful 20 million years old, compared with our Sun, now entering middle age, at about 4.6 billion years old.
The French astronomers found Beta Pictoris shepherds two distinct flocks of exocomets. The first comprises old exocomets that have been around their parent star multiple times. In contrast, another group of comets are much younger and may result from the recent breakup of one or more larger objects.
Beta Pictoris is the epicentre of a solar system in formation. It’s surrounded by a huge disc of material, in essence an embryonic planetary system where gas and dust are the result of comets evaporating and asteroid collisions.
Composite image representing the close environment of Beta Pictoris as seen in near infrared light. ...
Composite image representing the close environment of Beta Pictoris as seen in near infrared light. This very faint environment is revealed after a very careful subtraction of the much brighter stellar halo. The outer part of the image shows the reflected light on the dust disc,the inner part is the innermost part of the system.
ESO/A.-M. Lagrange et al.
Beta Pictoris has been under detailed observation for about 30 years. During that time, astronomers noted subtle changes in the light emitted by the star believed to be caused by comets passing in front of their parent star.
In cosmic terms, comets are tiny objects some just a few kilometers across. Much of their composition is in the form of ices so that when they near their parent star, these ices evaporate. This gives comets their distinctive tails of gas and dust, much easier to spot than comets themselves.
These comet tails can absorb some of the light passing through them but the dim light from the exocomets is swamped by the brilliant light of the star so they cannot be imaged directly from Earth.
The team used over 1,000 observations taken by the HARPS instrument between 2003 and 2011 to study Beta Pictoris’ exocomets.
Their sample consisted of 493 different exocomets, with some of these comets observed several times and for a few hours.
Detailed analysis gave measurements for the speed and the size of the gas clouds. In some cases, the orbital properties of these exocomets, including the shape and orientation of their orbit and distance to the star, could also be deduced.
Such an analysis of several hundreds of exocomets in a single exo-planetary system has never been attempted until now. It demonstrated that two distinct families of exocomets orbit Beta Pictoris.
The movement of the older exocomets is governed by a massive planetary body, an exoplanet known as Beta Pictoris b. This huge planet, with a mass estimated at between four and 11 times that of Jupiter, and a radius around 65 percent greater than Jupiter's, orbits at about a billion kilometers from its parent star. That would place Beta Pictoris b roughly halfway between the orbits of Jupiter and Saturn in our own solar system.
Beta Pictoris other family of exocomets probably arose from the recent breakdown of one or a few bigger objects.
The family of older exocomets were found to have a variety of orbits. They were also only weakly active in giving off gas and dust. Such an analysis suggests these comets had just about exhausted their supplies of ices, as a result of these ices being burned off due to multiple passages close to Beta Pictoris.
It turned out the orbits of these older comets, both in their eccentricity and orientation, were exactly as predicted for comets trapped in orbital resonance with a massive planet — in other words a massive planet, much like a sheepdog penning sheep, was keeping this flock of exocomets in hemmed into predetermined paths.
From that the astronomers could deduce that this planet in resonance must be located roughly 700 million kilometers from Beta Pictoris, close to where the planet Beta Pictoris b was discovered.
The second family of exocomets are much more active but they’re also on nearly identical orbits, suggesting they all had the same origin, in all probability the breakdown of a larger object, the fragments of which are now in an orbit that grazes Beta Pictoris.
These younger exocomets are comparable to the comets of the Kreutz family of Sun-grazing comets in our Solar System, or the fragments of Comet Shoemaker-Levy 9, which smashed into Jupiter in July 1994.
Lead author of the study, Flavien Kiefer of the Institute of Astrophysics in Paris (Institut d’astrophysique de Paris), commented, “For the first time a statistical study has determined the physics and orbits for a large number of exocomets. This work provides a remarkable look at the mechanisms that were at work in the Solar System just after its formation 4.5 billion years ago.”
The French team’s results are presented in a paper entitled "Two families of exocomets in the Beta Pictoris system" in the journal Nature Oct. 23, 2014.
The ESO is Europe’s foremost intergovernmental astronomy organisation, a leader in its field of ground-based astronomical observations.
The 15 country venture currently operates from three unique astronomical observation locations at La Silla, Paranal and Chajnantor in Chile.
ESO is engaged as European partner of a revolutionary astronomical telescope, the Atacama Large Millimeter/submillimeter Array or ALMA for short, the largest astronomical project in existence.
The Atacama Desert will also be the home of the planned ESO 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which, when constructed, will become the world’s biggest “eye on the sky”.
More about Beta Pictoris, exocomets, Comets, origins of solar system, Astronomy
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