A ‘comet graveyard’ has been discovered in the main belt of asteroids located between the orbits of Mars and Jupiter, by researchers at the University of Anitoquia, Medellin, Colombia. Some of the objects found in this ‘asteroid belt’ are in fact comets which have become inactive — sometimes for millions of years — but have the potential to return to life if the energy that they’re receiving from the Sun increases by only a few percent. The researchers have named these objects ‘Lazarus comets’.
Some background — comets are, essentially, small objects, usually only a few kilometers across, composed mostly of a frozen rock/ice/gas mixture. When their orbits bring them close to the Sun, these frozen materials begin to melt and the gas begins to sublimate, this gas is then swept back by the light of the Sun and the solar wind, producing the characteristic tail of gas and dust that trails most comets.
As most comets possess highly elliptical orbits, its relatively rare (from our perspective) for them to approach the Sun. “Some of these so-called long period comets take thousands of years to complete each orbit around our nearest star. There is also a population of about 500 short period comets, created when long period comets pass near Jupiter and are deflected in orbits that last anything between 3 and 200 years. Although uncommon events, comets also collide with Earth from time to time and may have helped bring water to our planet.”
The focus of the new research was a third and distinct region of the Solar System — the main belt of asteroids found between the orbits of Mars and Jupiter. “This volume of space contains more than 1 million objects ranging in size from 1 m to 800 km. The traditional explanation for asteroids is that they are the building blocks of a planet that never formed, as the movement of the pieces was disrupted by the strong gravitational field of Jupiter.”
Over just the last ten years, though, twelve active comets have been found within this region. These somewhat surprising discoveries have offered something of a challenge with regard to the traditional explanation, so the researchers set out to find out more — investigating the origin of these comets. And their research yielded results.
“We found a graveyard of comets,” states Professor Ignacio Ferrin, an astronomer at the University of Anitoquia. “Imagine all these asteroids going around the Sun for aeons, with no hint of activity. We have found that some of these are not dead rocks after all, but are dormant comets that may yet come back to life if the energy that they receive from the Sun increases by a few percent.”
The Royal Astronomical Society continues:
Surprisingly, this can happy fairly easily, as the orbits of many objects in the asteroid belt are nudged by the gravity of Jupiter. The shape of their orbits can then change, leading to a decrease in the minimum distance between the object and the Sun (perihelion) and a slight increase in average temperature.
According to this interpretation, millions of years ago the main belt was populated by thousands of active comets. This population aged and the activity subsided. What we see today is the residual activity of that glorious past. Twelve of those rocks are true comets that were rejuvenated after their minimum distance from the Sun was reduced a little. The little extra energy they received from the Sun was then sufficient to revive them from the graveyard.
Professor Ferrin explains: “These objects are the ‘Lazarus comets’, returning to life after being dormant for thousands or even millions of years. Potentially any one of the many thousands of their quiet neighbours could do the same thing.”
The new research was just published in the journal Monthly Notices of the Royal Astronomical Society.