Centauri system was forming, enriching the area around Alpha Centauri with volatile materials. This would be discounted if, for example, Alpha Centauri B happened to have gas giants orbiting Alpha Centauri A (or vice versa), or if Alpha Centauri A and B themselves were able to perturb comets into each other's inner systems as Jupiter and Saturn presumably have done in the Solar System. Such icy bodies probably also reside in Oort clouds of other planetary systems. When they are influenced gravitationally by either the gas giants or disruptions by passing nearby stars, many of these icy bodies then travel star-wards. Such ideas also apply to the close approach of Alpha Centauri or other stars to the Solar System, when, in the distant future, the Oort Cloud might be disrupted enough to increase the number of active comets. To be in the habitable zone, a planet around Alpha Centauri A would have an orbital radius of between about 1.2 and so as to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, the habitable zone is between about 0.7 and . With the goal of finding evidence of such planets, both Proxima Centauri and Alpha Centauri-AB were among the listed "Tier-1" target stars for NASA's Space Interferometry Mission (S.I.M.). Detecting planets as small as three Earth-masses or smaller within two AU of a "Tier-1" target would have been possible with this new instrument. The S.I.M. mission, however, was cancelled due to financial issues in 2010. Circumstellar discs Based on observations between 2007 and 2012, a study found a slight excess of emissions in the 24-µm (mid/far-infrared) band surrounding , which may be interpreted as evidence for a sparse circumstellar disc or dense interplanetary dust. The total mass was estimated to be between to the mass of the Moon, or 10–100 times the mass of the Solar System's zodiacal cloud. If such a disc existed around both stars, disc would likely be stable to 2.8 AU, and would likely be stable to 2.5 AU This would put A's disc entirely within the frost line, and a small part of B's outer disc just outside. View from this system The sky from Alpha Centauri AB would appear much as it does from the Earth, except that Centaurus would be missing its brightest star. The Sun would appear as a white