A comet orbiting our Sun may be an interloper from another star system. Comet Machholz 1 isn't like other comets. David Schleicher of the Lowell Observatory in Flagstaff, Arizona, measured the chemical makeup of 150 comets, and found that they all had similar levels of the chemical cyanogen (CN) except for Machholz 1, which has less than 1.5% of the normal level. Along with some other comets, it is also low on the molecules carbon2 and carbon3. Schleicher suggests three possible explanations. The simplest is that Machholz 1 could have formed in an extremely cold region of the solar system....