Short-lived nuclides , now extinct in the solar system , are expected to be present in interstellar matter ( ISM ) .
Grains of ISM origin were recently discovered in the inner solar system and at Earth orbit and may accrete onto Earth after ablation in the atmosphere .
A favorable matrix for detection of such extraterrestrial material is presented by deep-sea sediments with very low sedimentation rates ( 0.8–3 mm/kyr ) .
We report here on the measurement of Pu isotopic abundances in a 1-kg deep-sea dry sediment collected in 1992 in the North Pacific .
Our estimate of ( 3 \pm 3 ) \times 10 ^ { 5 } ^ { 244 } Pu atoms in the Pu-separated fraction of the sample shows no excess over the expected stratospheric nuclear fallout content and under reasonable assumptions sets a limit of 0.2 ^ { 244 } Pu atoms/cm ^ { 2 } yr for extra-terrestrial deposition .
Using available data on ISM steady-state flux on Earth , we derive a limit of 2 \times 10 ^ { -11 } g- ^ { 244 } Pu/g-ISM for the abundance of ^ { 244 } Pu in ISM .