An orbit for Nemesis.

Astronomers disagree forcefully over the existence of a companion star to the sun, usually called Nemesis, which is postulated to send showers of comets into the inner solar system every 26 million years (SN: 4/21/84, p. 250). Supposing, however, that Nemesis does exist, Armand Delsemme of the University of Toledo in Ohio has used a new approach to calculate its orbit.

His method begins with a statistical survey of what he calls "new" comets arriving in the inner solar system. Astronomers generally agree that the source of comets is the Oort cloud, a band or spherical shell of cometary lumps orbiting the sun about 2 light-years away. New comets are those that left the Oort cloud within the 20 million years or so. Delsemme did a statistical study of the motions of 126 of these -- not a large number, he admits, but still he claims 95 percent statistical confidence.

He determines that the majority of such comets move in a retrograde direction -- that is, opposite to the motion of nearly all planetary motions. From the directions of the comets' momenta he calculates that the Oort cloud received a gravitational impulse from some other object less than 20 million years ago. Neither a fast-moving object passing by the solar system nor the passage of the solar system through one of the interstellar gas clouds fits, he says -- thus disposing of two hypotheses of those who don't believe in Nemesis. However, a slow-moving object with a speed of 0.2 or 0.3 kilometers per second would fit, he says. "Nemesis is a good explanation of this."

From the dynamics, Delsemme calculates that the 26-million-year orbit of Nemesis should be almost perpendicular to the ecliptic, or the plane of the earth's orbit. (The orbits of the other planets and their natural satellites are inclined at most a few degrees to the ecliptic.) Delsemme further calculates that Nemesis should now be near its aphelion point, its farthest distance from the sun, and its direction should be about 5[deg.] from the pole of the ecliptic.