[W]e have interesting new work from the European Space Agency’s Herschel space observatory announcing that Gl 581, along with the G-class star 61 Vir, another nearby planetary system, shows the the signature of cold dust at -200 degrees Celsius.
It’s an abundant signature, too, meaning that both these systems must have ten times the number of comets found in our own Solar System’s Kuiper Belt. The two papers on this work grow out of a program called, fittingly, DEBRIS (Disc Emission via a Bias-free Reconnaissance in the Infrared/Sub-mm). What the researchers working these data are suggesting is that the lack of a large gas giant in the two systems may relate to the dense debris cloud. Instead of an era of heavy bombardment triggered by gas giants disrupting the Kuiper Belt, as occurred in our system, these stars may have experienced a much gentler inflow of volatiles.
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An older star like Gl 581 would have had two billion years or so for a substantial amount of water to be delivered to the inner system and, of course, to any potentially habitable worlds that reside there. What we now believe about the planets circling Gl 581 is that they have masses between 2 and 15 times that of the Earth, all located within 0.22 AU of the star, while the debris disk extends from 25 AU to 60 AU. A Neptune-class world further out, however, is a possibility. The researchers believe the large amount of dust Herschel has detected must be the result of cometary collisions, which could be triggered by a planet — perhaps about as large as the close-in planets — orbiting near the debris disk.
So we are looking at larger debris disks around systems where there is no Jupiter-class planet, and far less dense disks around stars where large gas giants are found.