Astronomers have found a group of comets that have risen from the dead. The asteroidal belt comets - or ABCs for short - lie in the main asteroid belt between Mars and Jupiter, report astronomers on the on the pre press website ArXiv.org. Dr Ignacio Ferrin, Dr Jorge Zuluaga and Pablo Cuartas from Columbia's University of Antioquia, say the group of eleven objects behave like comets, but have asteroidal orbits. They propose the objects they've dubbed 'Lazarus comets' are extinct comets that have been rejuvenated when their orbits changed.
"The asteroidal belt contains an enormous graveyard of ancient dormant and extinct rocky comets, that [are rejuvenated], in response to a diminution of their perihelion distance [closest orbital position to the Sun]," the authors write.
The findings, which are accepted for publication in the Monthly Notices of the Royal Astronomical Society, blurs the line between comets and asteroids.
Comets become dormant when they no longer emit volatile gases - they are then called asteroids. But previous research has shown these dormant comets can be rejuvenated into comets after collisions with asteroids, meteors or other comets, as well as high energy particle impacts.
Ferrin and colleagues now suggest a new comet rejuvenation theory.
The researchers argue that planetary gravitational perturbations can move comets into new orbits bringing them closer to the Sun.
Orbital change allows more heat from the Sun to penetrate further into the comet, causing additional water ice and other volatiles located deeper in the comet nuclei to turn into gas, they suggest. This gas is then emitted, turning asteroids back into comets.
Ferrin and colleagues calculated the temperature of each comet, compared to its distance from the Sun.
They then developed a mathematical model to work out the thickness of crust encasing each comet, based on the time it took the comet to begin emitting gases after its orbit changed.
Comets can develop a thick crust of carbon rich compounds and dust, left behind as the comet's ices evaporate.
The researchers found the crustal thickness estimates achieved for these comets were more accurate than previous estimates calculated for other comets and asteroids.
By understanding how much crust there is covering the comet's ice, Ferrin and colleagues were able to model the behaviour and release of that ice, as the comet's orbit moves closer to the Sun.
The research has energised debate on this issue, says astronomer Dr Brad Carter of the University of Southern Queensland.
"What we're seeing here is further evidence for the fact there is really a blurred division between asteroids and comets," says Carter.
"These objects contain a mixture of ices and more rocky material and at some point you arbitrarily change the name of the object from asteroid to comet depending on what the orbit might look like... and how much ice is there compared to rock and dust."