Debris discs hold clues to solar system evolution
Posted: 18 April 2011
Complex debris discs of dust have been spotted around a multitude of stars, giving insights into the evolution of other solar systems, according to work described today at the Royal Astronomical Society’s National Astronomy Meeting (NAM) in Llandudno, Wales.
DEBRIS – Disc Emission via a Bias-free Reconnaissance in the Infrared and Sub-millimetre – is a survey of nearly 450 stars being conducted by the European Space Agency’s Herschel Space Telescope. The goal is to search for stars that emit excess radiation at infrared and sub-millimetre wavelengths – the smoking gun of the presence of a disc of dust, like an asteroid belt, created by planetesimals and asteroids colliding with each other. “We want statistics to be able to put all these different objects into context,” says Bruce Sibthorpe, who presented the preliminary results from the survey at NAM today.
The debris disc observed in infrared wavelengths around HR 8799 by the Spitzer Space Telescope. Image: NASA/JPL–Caltech/K Su (University of Arizona).
The survey is detecting debris discs with different zones of various temperatures (‘warm’ corresponds to about –123 degrees Celsius, or 150 kelvin, and ‘cold’ refers –228 degrees Celsius, or 45 kelvin), and even some with eccentric ring-like belts that wrap around the star, going as close as two astronomical units (about 300 million kilometres from the star; one AU is equivalent to the distance between Earth and our Sun) and as far as 65 astronomical units (9.7 billion kilometres). Various types of star – spectral classes A, F, G, K and M (A stars are hotter, whilst M stars are cooler red dwarfs) – have been targeted in the survey. In particular Dr Nathalie Thureau from the University of St Andrews has been analysing the data from the hot, but shorter lived, A-type stars. She found no decline in emission from the debris discs with the age of the star, meaning that the discs can survive the entire length of the star’s life. “This means we’re going to find discs around even the oldest stars in the sample,” says Thureau.
In another example of a debris disc, Dr Jenny Patience from the University of Exeter revealed new observations of the planetary system around the young (60 million year old) A-type star HR 8799. This system is notable because it is one of just a few for which planets have been directly photographed, and the debris discs optically resolved. It has an inner disc, with a radius between six and fifteen astronomical units (897 million to 2.2 billion kilometres); a planetesimal disc between 90 and 300 AU (13.4 billion to 44.8 billion kilometres) and an extended halo reaching out to 1000 AU (150 billion kilometres). But new observations made by the SCUBA-2 instrument on the world’s largest submillimetre wavelength telescope, the James Clerk Maxwell Telescope in Hawaii, have revealed a mysterious clump in the planetesimal disc. “It stood out,” says Patience, “so we looked to investigate further, asking our theoretical colleagues to run a numerical simulation of this disc.”
The simulation implied that a clump can form within the disc in a resonance with a more massive outer planet. “It showed that migration [of a planet into the outer system] was necessary to scatter dust into a 2:1 resonance,” says Patience. In other words, for every one orbit the planet makes, the dust makes two orbits, and the gravitational tug of the planet as it nears that part of the disc every orbit shepherds the dust into a clump. The fact that a planet needed to migrate from its original orbit means that the observations “give us a glimpse into the past history of the system,” according to Patience.
There’s one other possibility. The clump could actually be a low mass planet, similar in size to Earth, in the process of forming out of the disc. Further observations at greater resolution – currently beyond the limit of our detectors – will be the only way to determine which is the case.
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