Comets are among the smallest objects in the solar system, typically a few kilometers across and composed of a mixture of rock and ice. If they come close to the Sun, then some of the ice turns to gas before being swept back by the light of the Sun and the solar wind to form a characteristic tail of gas and dust.
Most observed comets have highly elliptical orbits, meaning that they rarely 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 anywhere between three and 200 years. Although uncommon, comets also collide with Earth from time to time and may have helped bring water to our planet.
The new work looked at a third and distinct region of the solar system, the main belt of asteroids between the orbits of Mars and Jupiter. This volume of space contains more than 1 million objects ranging in size from 3 feet to 500 miles (1 meter to 800km). The traditional explanation for asteroids is that they are the building blocks of a planet that never formed because the movement of the pieces was disrupted by the strong gravitational field of Jupiter.
In the last decade, 12 active comets have been discovered in the asteroid main belt region. This was something of a surprise, and the Medellin team set out to investigate their origin. The team, made up of Ferrin and his colleagues Jorge Zuluaga and Pablo Cuartas, now thinks they have an explanation.
“We found a graveyard of comets,” said Ferrin. “Imagine all these asteroids going around the Sun for eons 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.”
Surprisingly, this can happen 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.
“These objects are the ‘Lazarus comets,’ returning to life after being dormant for thousands or even millions of years,” said Ferrin about the 12 active comets. “Potentially any one of the many thousands of their quiet neighbors could do the same thing.”