Large long-period comets are much more common than expected

Illustration of the method to estimate a comet's size (Image NASA/JPL-Caltech)
Illustration of the method to estimate a comet’s size (Image NASA/JPL-Caltech)

An article published in “The Astronomical Journal” describes a research on long-period comets concluding that they’re more common than expected. A team of researchers led by James Bauer of the University of Maryland used data collected by NASA’s WISE Space Telescope to discover that those at least one kilometer (0.6 miles) across are more common than expected and are, on average, twice as large as those of the Jupiter family.

Comets are different from the asteroids for their water ice content, although there are limit cases of almost exhausted comets that are almost asteroids but that’s another story. Typically comets have very elliptical orbits with the consequence that for short periods pass more or less close to the Sun and at that time their surface warms up enough for water to sublimate forming their characteristic tail. Most of a comet’s life, however, is spent far away from the Sun but this time depends on how far it is from our star.

Comets with such an eccentricity that their orbital period is over 200 Earth’s years are called long-period comets. They’re difficult to study because they can get many billions of kilometers away from the Sun even reaching the Oort cloud, the ouperiodost area of ​​the solar system. If the Kuiper belt is still little known because of its distance of a few billion kilometers from the Sun, the Oort cloud is still a theoretical concept because right now it’s impossible to observe objects so far away from the Earth.

The WISE Space Telescope is currently engaged in its NEOWISE mission, with the target of looking for and keep under contro control what are called Near Earth Objects (NEOs), asteroids and comets that cross the Earth’s orbit and might become dangerous. However, during its primary mission WISE could discover more distant objects such as long-period comets that might have arrived from the Oort cloud even millions of years ago.

The infrared surveys conducted using the WISE Space Telescope showed that in eight months a quantity between three and five times longer than predicted of long-period comets passed by the Sun. The observations made it possible to assess that an amount seven times greater than expected of long-period comets are at least one kilometer across. On average they’re twice as large as those of the Jupiter cometary family, composed of short-period comets with an orbital period of less than 20 Earth’s years.

The estimate of the size of long-period comets was a problem because their comas formed by a cloud of gas around their nuclei obscure their surface. The WISE Space Telescope made it possible to see beyond the coma thanks to its infrared sensitivity giving the researchers data on 95 comets of the Jupiter family and 56 long-period comets.

Amy Mainzer, one of the article’s authors and NEOWISE mission’s principal investigator, explained that this study gave a rare glance to objects thrown out of the Oort cloud, objects that haven’t changed much since the time when the solar system was formed. It makes sense that their size is greater than that of the comets of the Jupiter family because they lost much less of the materials that make them passing by the Sun.

As in the case of asteroids, the study of comets is partly motivated by checking if the trajectory of some of them can cross that of the Earth. However, the long-period ones are particularly important for the clues they contain about the history of the solar system and the Oort cloud, an area that is yet to be discovered.

Artist's illustration of the Kuiper belt and the Oort cloud (Image NASA/JPL)
Artist’s illustration of the Kuiper belt and the Oort cloud (Image NASA/JPL)

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