Paris, Sept 26 : The ESA/NASA Solar and Heliospheric Observatory (SOHO) has discovered its first officially periodic comet - which flies by the Sun at regular intervals. SOHO had already spotted this comet twice before.

The find is important, as this is the first one conclusively proven and officially declared by SOHO as a periodic comet.

Astronomers have seen thousands of comets but classified only around 190 as periodic. Many more are proposed to be periodic, but they only gain this classification officially if they are seen to follow their orbits around the Sun more than twice, and have orbital periods of less than two hundred years.

The most famous periodic comet is Halley's comet, which returns every 76 years, with its last close pass to the Sun taking place in 1986.

For the first time, SOHO's Large Angle and Spectrometric Coronagraph Experiment (LASCO) has found a rare type of comet called a periodic comet.

SOHO's new catch has a much smaller orbit though, taking approximately four years to travel once around the Sun.

It was first seen in September 1999, and then again in September 2003. In 2005, German PhD student Sebastian Hoenig realised that the two comets were so similar in orbit that they might actually be the same object.

To test his theory, he calculated a combined orbit for the comet, and consequently predicted that it would return on September 11 2007.

Sebastian's prediction proved to be extremely accurate – the comet reappeared in SOHO's LASCO camera right on schedule, and has now been given the official designation of P/2007 R5 (SOHO).

There is a puzzling aspect, however, as the comet does not look exactly like a comet.

It has no visible tail or coma of dust and gas. Initially, some scientists wondered if it were actually an asteroid, a chunk of space-rock rather than a chunk of space-ice. However, P/2007 R5 (SOHO) did exhibit some cometary characteristics.

However, as it passed to within 7.9 million kilometres of the Sun, around five percent of the distance from the Earth to the Sun, astronomers observed it brighten by a factor of around a million, a common behaviour for a comet.

Karl Battams, who runs SOHO's comet discovery programme, said P/2007 R5 (SOHO) seems to behave like a comet, even though it doesn't really look like one.

“It is quite possibly an extinct comet nucleus of some kind. Extinct comets are those that have expelled most of their volatile ice and have little left to form a tail or coma. They are theorised to be common objects amongst the celestial bodies orbiting close to the Sun,” said Battams.

He further said the comet faded as quickly as it brightened, and soon became too faint for SOHO's instruments to see it.

Scientists now estimate that P/2007 R5 (SOHO) is probably only 100-200 metres in diameter. (With inputs from ANI)