On 9 January 1992, astronomers Alex Wolszczan and Dale Frail announced they had discovered the first two exoplanets, or planets outside our solar system, while working at the Arecibo Observatory in Puerto Rico.

The two planets orbit a pulsar, a neutron star, 2,300 light-years away, in the constellation Virgo.

We now know of the existence of nearly 6,000 exoplanets, but Poltergeist and Phobetor were the first to be confirmed.

Rachel Naylor speaks to Alex Wolszczan.

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(Photo: An artist's impression of the pulsar planet system discovered by Alex Wolszczan in 1992. Credit: NASA)