A comet spotted orbiting the Sun may be an alien intruder from another star system, scientists believe. That is because it has a very unusual chemical make-up that is different to other known comets.

The comet, called 96P/Machholz 1, orbits every five and a quarter years in an elliptical path that brings it closer to the Sun than the planet Mercury.

Now a US astronomer is suggesting that it did not originate in our own solar system but was instead captured from another star.

Belts of comets have been detected around other nearby stars including Epsilon Eridani, which has planets too.

The alien explanation is proposed by David Schleicher of Lowell Observatory in Arizona. He studied the composition of 150 known comets and Machholz 1 stood out as being completely different because it is very low in carbon.

Comets are icy bodies born in the outer reaches of the solar system and are thought to be pristine examples of the building blocks that formed planets. They contain organic materials and are believed to have brought water and possibly even life itself to Earth.

Comet Machholz 1 may be so different because it comes from a star system that formed from a disk of dust that was low in carbon materials.

Dr Schleicher said: “A large fraction of comets in our own solar system have escaped into interstellar space, so we expect that many comets formed around other stars would also have escaped.

“Some of these will have crossed paths with the Sun, and Machholz 1 could be an interstellar interloper.”

Dr Schleicher makes no comment on the notion that comets carry the ingredients of life. But the idea that they can cross from one solar system to another raises intriguing possibilities for the spread of life through the universe.

Dr Schleicher says alternative explanations for the comet’s strange chemistry could be either that it formed further out in our own solar system than other comets or that its chemistry was altered when it was “cooked” by the Sun in a close approach. The solar wind has been seen to have a dramatic effect on a comet.

Picture: A photo of Comet 96P/Machholz 1 from the Soho space observatory. (NASA).

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